Materials Included In Kit

Additional Materials Required

Prelab Preparation

Magnetic Domains

1. Fill each of the bottle preforms about ⅔–¾ of the way with iron filings and close with cap.

Magnetic Field

1. Prepare individual iron-filing trays for each group by adding approximately 5 g of iron filings to each weighing dish.

Induced Fields

1. Cut 3 separate 20 cm long pieces of insulated wire.
2. Strip both ends of each piece of wire so there is about 2 inches of bare wire on each end.

Gravity vs. Magnetism 

1. Stand the device on a flat surface on its two feet. The T-nut should be in the left block and the magnet in the right block.
2. Thread an eyebolt into the T-nut so that the loop end (eye) of the eyebolt is facing the inside of the assembly.
3. Turn the eyebolt in until it just starts to come out the far side of the T-nut.
4. Cut the fishing line into three pieces of 16" each. Tie one end of one piece of fishing line to a paper clip.
5. Tie the fishing line to the loop end of the eyebolt so that the paper clip is just touching the magnet and the line is tight.
6. Adjust the eyebolt by turning it so that the paper clip is not touching, but so that it appears to “float.”
7. Attach the ruler (on the self-adhesive stock) onto the front of the back wood piece. The zero mark should be at the very edge on either side; the white edge of the label on either side needs to be cut off. When positioning the ruler, be sure that the ruler markings run perpendicular to the tabletop, and that the ruler markings match up with the taut string when observing the string at eye level. This will allow for the numbers to be readable from both above and below the string.
8. The Magnetic Levitation Device is now assembled and ready for use.

Safety Precautions

The materials in this lab are considered nonhazardous. Iron filings can be messy and it is important to neatly collect the iron filings and place them back into the container after the experiment. Students should wear safety glasses when performing this experiment. Advise students to wash their hands with soap and water when this experiment is complete. The round ceramic magnets are fragile; be careful to not let them collide together too forcefully as they may break. Do not leave the battery connected for a long period of time; the battery, wire and paper clips will get hot if the circuit is complete for too long. Keep the magnetic levitation device away from cell phones and computers—the strong magnet can create errors on computer disks and interfere with electronic equipment. Please follow laboratory safety guidelines.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The iron filings may be collected and saved in the original bottle for future use. Iron filings may be placed in the regular trash according to Flinn Suggested Disposal Method #26a.

Lab Hints

• These laboratory activities can be completed in two 50-minute class periods. Prelab Questions may be completed before lab begins the first day. 
• In order to manage time at each activity, it is important to offer students guiding questions to help them understand concepts that may be unfamiliar to them while still keeping an efficient pace with their lab work.
• Set up each lab station before class. Students should leave the stations as they found them before moving on to the next lab station.
• When students are working on Gravity vs. Magnetism, make sure students do not allow the floating clip to touch the magnet during data collection. Sometimes students don’t notice it is touching and no longer floating.
• Regarding the Magnetic Fields activity, if the magnetic field of the magnet is too strong and making it difficult to observe the many different magnetic field lines, weaken the magnetic field by placing a thicker piece of cardboard between the magnets and the index cards. For each group, cut a 3" x 5" piece of cardboard from a shipping box or from the back of a paper note pad, if necessary. Place this thicker cardboard on the magnets and then place the index cards on top of the cardboard.
• A pinch technique can be used to sprinkle the iron filings in a more controlled manner. Pinch the iron filings between the thumb and index finger and then sprinkle them evenly on the index card. However, the iron filings will leave a metal dust smudge on fingers that may easily transfer to clothing or other objects. Make sure students are aware of this stain before suggesting this technique, and that they thoroughly wash their hands after completing this portion of the experiment. You may wish to have latex or nitrile gloves available for students to wear while handling the iron filings.
• The batteries in Magnetic Fields may wear out quickly if left in a closed circuit for too long. Be sure students are aware that they need to manage the battery use.

Teacher Tips

• This lab can be done as an introduction to magnetism as a whole or after magnetism has been introduced in class.
• If the bar magnets are sufficiently strong, the magnetization of the compass needles may become reversed; compass direction should be checked regularly.
• The terms ferromagnetic, paramagnetic and diamagnetic have not been defined. These may be useful to go over before or after the lab activities in order to supplement the abstract concept of magnetic domains.

Further Extensions

Opportunities for Inquiry
If magnetic sensors are available, explore magnetic properties by quantitatively measuring the magnitudes of the magnetic fields of various magnet combinations.

Alignment to the Curriculum Framework for AP^® Physics 2

Enduring Understandings and Essential Knowledge
An electric field is caused by an object with electric charge. (2C)
2C4: the electric field around dipoles and other systems of electrically charged objects (that can be modeled as point objects) is found by vector addition of the field of each individual object. Electric dipoles are treated qualitatively in this course as a teaching analogy to facilitate student understanding of magnetic dipoles.

A magnetic field is caused by a magnet or a moving electrically charged object. Magnetic fields observed in nature always seem to be produced either by moving charged objects or by magnetic dipoles or combinations of dipoles and never by single poles. (2D)
2D1: The magnetic field exerts a force on a moving electrically charged object. That magnetic force is perpendicular to the direction of velocity of the object and to the magnetic field and is proportional to the magnitude of the charge, the magnitude of the velocity, and the magnitude of the magnetic field. It also depends on the angle between the velocity and the magnetic field vectors. Treatment is quantitative for angles of 0°, 90°, or 180°and qualitative for other angles.
2D2: The magnetic field vectors around a straight wire that carries electric current are tangent to the concentric circles centered on that wire. The field has no component toward the current-carrying wire.
2D3: A magnetic dipole placed in a magnetic field, such as the ones created by a magnet or the Earth, will tend to align with the magnetic field vector.
2D4: Ferromagnetic materials contain magnetic domains that are themselves magnets.

All forces share certain common characteristics when considered by observers in inertial reference frames. (3A)
3A2: Forces are described by vectors
3A3: A force exerted on an object is always due to the interaction of that object with another object.
3A4: If one object exerts a force on a second object, the second object always exerts a force of equal magnitude on the first object in the opposite direction.

At the macroscopic level, forces can be categorized as either long-range (action-at-a-distance) forces or contact forces. (3C)
3C3: A magnetic force results from the interaction of a moving charged object or a magnet with other moving charged objects or another magnet.

The electric and magnetic properties of a system can change in response to the presence of, or changes in, other objects or systems. (4E)
4E1: The magnetic properties of some materials can be affected by magnetic fields at the system. Students should focus on the underlying concepts and not the use of vocabulary.

Learning Objectives
2C4.1: The student is able to distinguish the characteristics that differ between monopole fields (gravitational field of spherical mass and electrical field due to single point charge) and dipole fields (electric dipole field and magnetic field) and make claims about the spatial behavior of the fields using qualitative or semiquantitative arguments based on vector addition of fields due to each point source, including identifying the locations and signs of sources from a vector diagram of the field.
2D1.1: The student is able to apply mathematical routines to express the force exerted on a moving charged object by a magnetic field.
2D2.1: The student is able to create a verbal or visual representation of a magnetic field around a long straight wire or a pair of parallel wires.
2D3.1: The student is able to describe the orientation of a magnetic dipole placed in a magnetic field in general and the particular cases of a compass in the magnetic field of the Earth and iron filings surrounding a bar magnet.
2D4.1: The student is able to use the representation of magnetic domains to qualitatively analyze the magnetic behavior of a bar magnet composed of ferromagnetic material.
3A2.1: The student is able to represent forces in diagrams or mathematically using appropriately labeled vectors with magnitude, direction, and units during the analysis of a situation.
3A3.2: The student is able to challenge a claim that an object can exert a force on itself.
3A3.3: The student is able to describe a force as an interaction between two objects and identify both objects for any force.
3A4.1: The student is able to construct explanations of physical situations involving the interaction of bodies using Newton’s third law and the representation of action-reaction pairs of forces.
3A4.2: The student is able to use Newton’s third law to make claims and predictions about the action-reaction pairs of forces when two objects interact.
3A4.3: The student is able to analyze situations involving interactions among several objects by using free-body diagrams that include the application of Newton’s third law to identify forces.
3C3.1: The student is able to use right-hand rules to analyze a situation involving a current-carrying conductor and a moving electrically charged object to determine the direction of the magnetic force exerted on the charged object due to the magnetic field created by the current-carrying conductor.
3C3.2: The student is able to plan a data collection strategy appropriate to an investigation of the direction of the force on a moving electrically charged object caused by a current in a wire in the context of a specific set of equipment and instruments and analyze the resulting data to arrive at a conclusion.
4E1.1: The student is able to use representations and models to qualitatively describe the magnetic properties of some materials that can be affected by magnetic properties of other objects in the system.

Science Practices
1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain.
1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.
1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
2.2 The student can apply mathematical routines to quantities that describe natural phenomena
2.3 The student can estimate numerically quantities that describe natural phenomena.
4.2 The student can design a plan for collecting data to answer a particular scientific question.
5.1 The student can analyze data to identify patterns and relationships.
6.1 The student can justify claims with evidence.
6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understanding and/or big ideas.

Answers to Prelab Questions

1. In your own words, describe what a compass does and how it works.

   A compass is a thin magnetic needle that is free to rotate on a pivot. The compass needle will align itself with the external magnetic field it is placed in. The red tip of the compass needle used in this lab is the north pole of the magnet and points in the direction of the magnetic field, therefore it always points towards a magnetic south pole. The compass is known to point to geographic North Pole because that is near where the Earth’s magnetic south pole is located.

2. Refer to Figure 2 in the Background. Compare and contrast the geometry of the respective electric and magnetic fields around the electric dipole configuration and bar magnet configuration.

   The field vectors appear to be very similar in their orientation in space, emanating outward from one end and looping around to the opposite end. In the case of the electric field, the vectors emanate out from a positive charge and end at a negative charge; the magnetic field is similar in that it flows from one pole to the other, however, there is no “end” and is actually a continuous loop that carries through the bar magnet itself.

3. How is the direction of the magnetic field defined?

   The direction of the magnetic field at a particular location is defined as the direction the north pole of a compass needle points at that location.

Answers to Questions

Guided-Inquiry
 
Magnetic Domains

6. Record observations about the number of north poles and south poles that result from breaking the stack magnet.

   Students should note there is always both a north and south pole for each magnet “fragment” every time a magnet is “broken.”

9. Approach the north and south ends of the compass needle with the end of the tube. Record observations.

   This step is repeated for an unmagnetized tube, a magnetized tube, and a shaken tube after being magnetized. Students will observe no interaction by the compass when the tube is not magnetized. When the tube is magnetized, the compass needle will deflect toward the tube and one pole will be attracted while the other is repelled. After being shaken, the compass needle will weakly deflect toward the tube.

Analyze the Results

• Under which condition did the compass deflect the most?

  The compass deflected the most after the tube was stroked with a magnet because the individual magnetic fields of the iron filings became aligned, producing a larger magnetic field.
  

• In your own words, describe the net magnetic field of the tube before being magnetized, after being magnetized, and after being shaken.

  Before being magnetized, the net magnetic field of the tube was zero. After being magnetized, the observed field was large as most of the iron filings were aligned and contributed their individual fields to a larger field. After being shaken, the iron filings were randomly aligned and the field strength of the tube decreased.

• Did the magnetic field of the tube increase or decrease after shaking? Why?

  The magnetic field of the tube decreased because the shaking randomized the alignment of the individual iron filings. This would cause some of the “magnetic domains” to cancel each other out, decreasing the overall strength of the field.

• Dropping a magnet is not under the warranty of almost all magnet manufacturers. What is the reason for this?

  Dropping a magnet could misalign the magnetic domains inside the magnet and thus cause the magnetic field of the magnet to become negligible.

• If the stack magnet was broken into 10 pieces, how many north and south pole pairs could be detected? What if it was broken into 20 pieces?

  There would be 10 pairs of magnetic poles if the stack magnet was broken into 10 pieces. If the magnet was broken into 20 pieces, there would be 20 pairs of poles.

• In your own words, why are some objects able to be magnetized but not others?

  Some objects are able to be magnetized because they have more magnetic domains that are more susceptible to be aligned with a magnetic field. Objects that do not have many magnetically susceptible domains are less likely to become magnetized.

Magnetic Field

2. How do the south poles interact when facing each other? How do the north poles interact when facing each other? How do the north and south poles interact when facing each other?

   The south poles repelled each other when facing, the north poles repelled each other when facing, and the north and south poles attracted each other when facing.

5. Describe the behavior of the compass needles. Do they behave as if in a magnetic field? What can account for this behavior?

   After being shaken, the compass needles settle and align pointing in the same general direction. They behave as if in a magnetic field with the red end always pointing in the same general direction. The needles align with the Earth’s magnetic field and point to the magnetic south pole of the Earth.

6. Place one bar magnet flat on the table. Using the compass, design a procedure that might determine the direction of the magnetic field vectors around the bar magnet. Is there a way to determine the magnitude of the field with just a compass?

   Yes there is. One can observe how quickly the compass “locks” into place when near the bar magnet at different locations for a qualitative measure of the strength of the field.

Analyze the Results

• How does the magnetic field of a single magnet compare in shape to that of an electric dipole? What is different?

  The magnetic field due to a single magnet looks very similar in shape as that of the electric field of an electric dipole. It curves from the north pole to the south pole much like the electric field curves from the positive charge to the negative charge. However, the magnetic field appears to have no end compared to the electric field of an electric dipole. In the electric dipole configuration, the field lines start at the positive charge and end at the negative charge; in the bar magnet, the magnetic field lines are continuous, externally pointing towards the south pole and inside the bar magnet the lines continue on from south pole to north pole.

• How does the direction of the compass needle change as the compass is moved along a magnetic field line?

  The direction of the compass needle is equivalent to that of a vector tangent to the magnetic field line. The needle is always aligned tangent to the magnetic field lines and only points directly toward the magnet when it is near the poles.

• Where is the magnetic field the strongest? How can you tell?

  The magnetic field is strongest at the poles, both north and south. The force field strength is felt when the magnets are pulled toward or pushed away from each other. The force is felt the most when the magnets’ poles are positioned close to each other. The magnetic field lines are more closely spaced near the poles compared to the middle of the magnet or far away from the magnet poles. Therefore, the stronger the magnetic field, the closer the field lines are (more lines per unit area) in that region.

Induced Fields

3. Using a compass, explore the region around the bar magnet. How does the compass needle behave near the north pole of the magnet? How does the compass needle behave near the south pole of the magnet? How is the direction of the magnetic field defined?

   The red end of the needle is attracted to the south pole of the magnet and repelled from the north pole of the magnet. The direction of the magnetic field at a particular location is defined as the direction the north pole of a compass needle points at that location.

4. Using a compass, explore the region around the hanging wire. Does the wire have any effect on the compass?

   The wire does not appear to have any effect on the compass needle/

7. What is the direction of the current in the wire?

   This depends on how students have connected the battery. The current will run from the positive terminal to the negative terminal. The following answers will assume the current flows from left to right initially, meaning the positive terminal is clipped on the left hand side paperclip and the negative terminal is clipped on the right hand side paperclip.

8. Approach the current-carrying wire from underneath with the compass and observe the orientation of the needle. Does it behave as if in a magnetic field? What is the direction of the field at the location of the compass?

   The needle does behave as if in a magnetic field. The north pole of the compass points away from the observer.

9. Approach the current-carrying wire from above with the compass and repeat the observations in step 8.

   The north end of the compass points toward the observer.

10. Reverse the current on the wire and repeat steps 8 and 9.

    When the current is reversed, it will run from right to left. Underneath the wire, the compass will point toward the observer. Above the wire, the compass will point away from the observer.

11. Open the circuit.
12. Approach the wire from underneath with the north pole of the bar magnet (see Figure 8). Is the wire attracted to or repelled by the magnet? Make note of all observations.

    There is no attraction while the circuit is open.

13. Close the circuit and repeat step 12.

    Student answers will vary. The current is assumed to be running from right to left now. If the north pole of the magnet is brought near the wire, the wire will move away from the observer. It will appear to be repelled by the magnet.

14. Repeat step 12 for the south pole of the magnet.

    When the current-carrying wire is approached by the south pole of the magnet from below, the wire moves toward the observer.

15. Approach the current-carrying wire from above with each pole of the bar magnet. Record all observations.

    When approached from above with the north pole of the magnet, the wire moves towards the observer. When approached from above with the south pole of the magnet, the wire moves away from the observer.

16. Reverse the current on the wire and repeat steps 13–15.

    The current is running from left to right. When approached from below by the north and then south pole of the magnet, the wire moves toward the observer and then away from the observer, respectively. When approached from above by the north and then south pole of the magnet, the wire moves away from the observer and then toward the observer, respectively.

Analyze the Results

• Due to experimental observations, a pattern is deduced regarding the direction of the magnetic field lines around a current-carrying wire. The method for determining the orientation of the field around a current-carrying wire is known as the right-hand rule for the magnetic field. Imagine grasping the wire in your hand with your thumb pointing in the direction of the current. Your fingers curl around the wire in the direction of the magnetic field vectors. Do your experimental observations agree with the described method?

  Student answers will vary.

• Using the right-hand rule, determine the direction of the field at point A in Figure 9. Is the orientation of the field vectors into the page or out of the page?
  {14015_Answers_Figure_9}

  The magnetic field vectors are coming out of the page.

• Sketch a diagram that shows the wire, the direction of the current running through it, and the direction of the magnetic field below the wire at the location of the compass in step 8.
  {14015_Answers_Figure_12}
• Consider step 13. When the current-carrying wire was approached from below by the north end of the magnet, what was the observed motion of the wire? What was the motion of the wire when approached from above?

  The current is assumed to be running from right to left (due to it being reversed in step 10). When the north pole of the magnet was brought near the wire from below, the wire moved away from the observer. When approached from above with the north pole of the magnet, the wire moved toward the observer.

• Sketch a diagram that shows the wire, the direction of the current through it, and the direction of the bar magnet’s magnetic field when the north pole is brought near the magnet from below.
  {14015_Answers_Figure_13}
• Sometimes a magnet exerts a force on a current-carrying wire, and at other times it does not—this depends on the orientation of the field relative to the current in the wire. When perpendicular to the current, the magnetic field is found to exert a force on a wire. If the field is parallel to the direction of the current, no force is exerted on the wire. The right-hand rule for the magnetic force can illustrate the direction of the magnetic force that a magnetic field exerts on a current-carrying wire. Hold your hand flat with your thumb extended from your fingers (see Figure 10). Orient your hand so that your fingers point in the direction of the magnetic field and your thumb points along the direction of the current. The direction of the magnetic force exerted by the magnetic field on the current is the direction your palm faces—perpendicular to both the direction of the current and the direction of the field. Do your observations agree with the right-hand rule for the magnetic force?
  {14015_Answers_Figure_10}

  Student answers will vary.

• Using the right-hand rule for the magnetic force, does the wire in Figure 11 experience a force into the page or out of the page?
  {14015_Answers_Figure_11}

  The wire will experience a force into the page.

Gravity vs. Magnetism

• Consider Figure 2 in the Background section. What would the magnetic field vectors look like for the magnet in the wooden frame?

  The field vectors would come out from the north pole of the magnet and into the south pole. The poles of the magnet in the frame can only be determined with a labeled magnet or a compass.

• If a piece of cork was tied to the string, instead of a paper clip, would the cork also “float”? Give a reason for your answer.

  It would not. The cork is not attracted to the magnet because its magnetic domains are not susceptible enough to align themselves with the external magnetic field. This is most likely due to all electrons being paired at the atomic level of the cork.

• Draw a free body diagram of the forces acting on the paperclip. What is opposing the force of gravity? What is opposing the force of tension in the string?
  {14015_Answers_Figure_14}
• How does the force between the “floating” paper clip and the magnet change as the “floating” paper clip is moved away from the magnet?

  As the paper clip is moved farther away from the magnet, the force between the two objects decreases. This seems to follow a pattern similar to that observed with the force of gravity.

• It is found that the measured magnitude of the magnetic field produced by a current-carrying wire follows an inverse-square law. Does the magnetic field produced by the magnet also follow an inverse-square law? How does the data support your answer?

  Yes, the magnitude of the force due to the magnetic field produced by the magnet seems to follow an inverse-square law. The graph supports this statement because a parabolic relationship is represented that would correlate with F ∝ 1/d².

Review Questions for AP^® Physics 2

1. A charged rod is brought near cups A and B. Then the charged rod is replaced with a bar magnet and brought near cups a and b. When the charged rod is used, the paper clip does not move, but the pith ball is attracted to the rod; when the magnet is used, the paper clip is attracted to the magnet but the pith ball does not move. Explain, in terms of the electric and magnetic fields and respective magnetic domains, the movement of the paper clip and the pith ball in each scenario.
   {14015_Answers_Figure_15}
   1. The paper clip does not move because of the aluminum lining. The aluminum shields the inside of the cup from external electric fields and therefore the electric field in the cup is zero—no electric force is felt by the paper clip. When the magnet is brought near, the magnetic domains of the paper clip align with the external magnetic field and are attracted to the magnet. The aluminum has no shielding effect on the magnetic field.
   2. When the charged rod is brought near the pith ball, the pith ball is attracted to the rod because the electrons in the ball will come towards the rod or away (depending on the charge of the rod). The separation of charges in the ball is due to the electric field provided by the charged rod. There is no aluminum lining; therefore there is no shielding of the electric field. When the magnet is brought near, the pith ball does not move because it does not have a magnetic field or magnetically susceptible domains.
2. Electrons are emitted from an electron source with velocity, v, and enter a region of uniform magnetic field B that is perpendicular to the page. The electrons then leave the magnetic field at point P.
   {14015_Answers_Figure_16}
   1. On this figure, sketch the path of the electrons from when they enter the region of uniform magnetic field B to point P.
   2. Indicate whether the magnetic field is directed into or out of the page. Explain your reasoning.

      The magnetic field is directed into the page. Using the right-hand rule for magnetic force but considering the charge being negative, not positive, (therefore the thumb points in the opposite direction of the velocity of the moving electron) one orients the palm so that it pushes the electrons to the left and one’s fingers are oriented into the page.

   3. If the electron’s velocity is 5 x 10⁶ m/s, what is the magnetic force on the electron due to the magnetic field B if the field has a magnitude of 2 T?

      Using F = qvBsinθ where q = –1.602 x 10^–19 C, v = 5 x 10⁶ m/s, B = 2 T, and sinθ = 1:
      F = –1.602 x 10^–12 N

3. A conducting rod hangs from two conducting springs with their upper ends fixed at points X and Y. The rod is in a uniform magnetic field, B, that is directed out of the page. A battery is connected between points X and Y as shown, which results in a current, I, in the rod. The rod is displaced downward and eventually comes to equilibrium.
   {14015_Answers_Figure_17}
   Which point, X or Y, is connected to the positive terminal of the battery? Justify your answer.

   The positive terminal is connected to point X. This can be deduced by finding the direction of the current with the righthand rule for magnetic force. The fingers correlate with the direction of the magnetic field coming out of the page. The palm correlates with the direction of the force, which is downwards. This leaves the thumb pointing to the right, indicating that the current flows from left to right, from point X to point Y; therefore the positive terminal must be at point X.

4. Consider the following figure.
   {14015_Answers_Figure_18}
   Rank, in order of increasing strength, the magnitude of the magnetic field at points A–D.

   A, C, B, D

5. Two long current-carrying wires are placed parallel to each other.
   1. What is the direction of the magnetic field at point P produced by wire A? By wire B?

      The direction of the magnetic field produced by the wires at point P is out of the page for both wires A and B.

   2. Are the wires attracted to or repelled from each other? Explain your reasoning.

      The wires are repelled from each other. An object cannot exert a force on itself, therefore only the magnetic field produced by the opposing wire is of interest when using the right-hand rule. The field produced by wire A is coming out of the page. The palm of the right hand faces to the right when orienting your thumb with the direction of the current in wire B and orienting your fingers out of the page. The field produced by wire B is coming out of the page. The palm of the right hand faces to the left when orienting your thumb with the direction of the current in wire A and orienting your fingers out of the page. The forces face away from each other meaning that the wires are repelled and move away from each other.

      {14015_Answers_Figure_19}

References

AP^® Physics 1: Algebra-Based and Physics 2: Algebra-Based Curriculum Framework; The College Board: New York, NY, 2014.

Introduction

The Greek philosopher, Thales (624–546 BCE) was among the first to study attractive forces systematically. It had been observed that certain naturally occurring ores of iron attracted other iron-containing materials but nothing else. Thales lived in a town located on the coast of modern-day Turkey and the sample of iron ore that was studied originated from the nearby town of Magnesia. The material was dubbed “the Magnesian rock” and such iron-attracting materials are now called magnets, with the phenomenon of these interactions known as magnetism. Why are some materials magnetic and not others? What is the magnetic field, the mechanism, behind all magnetic interactions?

Concepts

• Magnetic field
• Magnetic force
• Magnetic domains
• Dipole vs. monopole
• Right-hand rules

Background

Field theory is used to explain what appear to be action-at-a-distance forces. For example, the force of gravity works within a gravitational field, and electric field vectors are used to explain the attractive and repulsive forces of electric charges. Magnetic fields are similarly defined for magnetic interactions. Field vectors are used to designate the magnitude and direction of a force field at specific points in space.

Magnets have north and south poles. The terms “north” and “south” originated from the discovery of the compass. A compass is a thin magnet able to rotate freely around its midpoint. One end of the compass needle consistently points in the direction of geographic north, the other end points south, thus each end of a magnet is labeled “north” and “south,” respectively. The north pole of a compass needle (traditionally the red end) always points in the direction of the magnetic field it is in (see Figure 1). Any material observed to have magnetic properties has always been found to have a north and south pole.

All magnetic fields observed in nature are caused by dipoles (two poles) or a combination of dipoles, never by a single pole (monopole). A compass can be used to detect the direction of the magnetic field at a particular location. Figure 2 compares the field vectors of a dipole charge set up and a bar magnet. The magnetic field is a continuous vector loop whereas electric field vectors begin on positively charged objects and end at negatively charged objects. If a vector is drawn from the south pole to the north pole of a compass placed next to the bar magnet in Figure 2, that vector would correspond exactly with a magnetic field vector.

Analogous to the fundamental properties that are mass and electric charge, certain matter has a property called magnetic spin. Electrons have this property and can be considered to be microscopic magnets themselves. The electron cloud around the nucleus of an atom accounts for the magnetic properties of certain materials. At the atomic level, electrons are paired in their respective orbitals and their magnetic fields cancel each other out. Unpaired electrons, however, provide a magnetic field that is not cancelled out by another electron. Metals such as iron, nickel and cobalt exhibit magnetic properties because atomic-scale magnets (the unpaired electrons) are aligned; each individual magnetic field of each individual unpaired electron contributes to the net magnetic field that can be observed. These regions of aligned microscopic magnetic fields are called magnetic domains (see Figure 3). A magnetic domain can be considered to be a tiny magnet (composed of even smaller magnets). In unmagnetized materials, the magnetic fields of domains cancel each other out, resulting in no net magnetic field. Magnetic fields have also been found to be produced by moving electrical charges. When electrons travel in a conducting wire to create an electric current, the moving electrons produce a magnetic field that surrounds the wire. The magnitude of the magnetic field produced by the moving charges is directly proportional to the magnitude of the current running in the wire. If a magnet is brought toward a moving charged object (like the electrons producing a current), that object will experience a magnetic force due to the magnetic field of the external magnet. The magnetic force is proportional to the magnitude of the charge, the magnitude of the velocity of the object, and the magnitude of magnetic field. It also depends on the angle between the velocity and magnetic field vectors (Equation 1). where

F = magnetic force
q = magnitude of charge
v = velocity of charged object
B = magnitude of magnetic field
θ = angle between magnetic field and velocity vector

Experiment Overview

This advanced inquiry lab uses an activity-stations approach for an in-depth investigation of magnetic properties. Four selfcontained guided-inquiry activities focus on different topics. The magnitudes of the forces of gravity and magnetism are quantitatively compared. Induced magnetic fields and the right-hand rules are studied. Magnetic domains and the magnetic field are explored.

Materials

Prelab Questions

1. In your own words, describe what a compass does and how it works.
2. Refer to Figure 2 in the Background. Compare and contrast the geometry of the respective electric and magnetic fields around the electric dipole configuration and bar magnet configuration.
3. How is the direction of the magnetic field defined?

Safety Precautions

The round ceramic magnets are fragile; be careful to not let them collide together too forcefully as they may break. Follow all laboratory safety guidelines. The materials in the Magnetic Field lab are considered nonhazardous. Iron filings can be messy, and it is important to neatly collect all the iron filings and place them back into the container after the experiment. Do not leave the Induced Fields battery connected for a long period of time; the battery, wire and paper clips will get hot if the circuit is complete for too long. Keep the magnetic levitation device away from cell phones and computers. The strong magnet can create errors on computer disks and interfere with electronic equipment. Wear safety glasses when performing this laboratory. Wash hands thoroughly before leaving the laboratory. Always follow laboratory safety guidelines.

Procedure

Guided-Inquiry Activity Station: Magnetic Domains

1. Obtain 10 round ceramic magnets and stack them together.
2. Lay the stack of magnets horizontally on a table.
3. With a compass, observe the orientation of the field vectors of the magnetic field around the magnet stack by placing the compass at various points around the magnet stack.
4. Repeat step 3 for the bar magnet.
5. Pull the stack of magnets apart into two stacks of 5 magnets. Use a compass to observe the orientation of the field vectors at the “breaking point” of each stack.
6. Record observations about the number of north poles and south poles that result from separating the magnet stack.
7. Obtain the plastic tube with iron filings.
8. Lay a compass flat on the table.
9. Approach the north and south ends of the compass needle with the end of the tube. Record observations.
10. Stroke the tube 50 times with the north end of the bar magnet (see Figure 4).
    {14015_Procedure_Figure_4}
11. Repeat step 9.
12. Shake the tube vigorously for 60 seconds and repeat step 9.
13. Attempt to magnetize miscellaneous objects at your disposal and record all observations. Note: Do not attempt to magnetize cell phones or computers as the magnet can potentially produce errors in computer disks and interfere with electronic devices.

Analyze the Results

• Under which condition did the compass deflect the most?
• In your own words, describe the net magnetic field of the tube before being magnetized, after being magnetized and after being shaken.
• Did the magnetic field of the tube increase or decrease after shaking? Why?
• Dropping a magnet is not under the warranty of almost all magnet manufacturers. What is the reason for this?
• If the stack magnet was broken into 10 pieces, how many north and south pole pairs could be detected? What if it was broken into 20 pieces?
• In your own words, why are some objects able to be magnetized but not others?

Guided-Inquiry Activity Station: Magnetic Field

1. Using the compass and two bar magnets, design and carry out a procedure to determine the north and south poles of each magnet.
2. How do the south poles interact when facing each other? How do the north poles interact when facing each other? How do the north and south poles interact when facing each other?
3. Move both compasses far away from the magnets and other potentially magnetic objects. Note: The compasses themselves are magnetic and at least 10 cm of space should separate them.
4. Shake the compasses and place them on a flat surface.
5. Describe the behavior of the compass needles. Do they behave as if in a magnetic field? What can account for this behavior?
6. Place one bar magnet flat on the table. Using the compass, design a procedure that might determine the direction of the magnetic field vectors around the bar magnet. Is there a way to determine the magnitude of the field with just a compass?
7. Obtain two index cards, 5 g of iron filings in a weighing dish, and both bar magnets.
8. Lay one magnet flat on the table and place the index card over the magnet (see Figure 5).
   {14015_Procedure_Figure_5}
9. Carefully sprinkle the iron filings on the index card. Sprinkle a small amount of iron filings to cover the entire card at first. Then, add more iron filings where lines appear to form. Observe the lines that form as the iron filings line up with the magnetic field of the magnet. Draw the magnetic field lines produced by one magnet.
10. Carefully lift the index card off the magnet, keeping the card horizontal to prevent spilling the iron filings. Note: Do not allow the iron filings to contact the bar magnet directly. The iron filings will be difficult to remove once they “stick.”
11. Neatly pour the iron filings back into the weighing dish.
12. Obtain the second bar magnet.
13. Line the two magnets up, north pole facing south pole, about 2 cm apart as shown in Figure 6. Transparent tape may be used to secure them to the table if necessary.
14. Place two index cards over the magnets so that the index cards overlap (see Figure 6).
    {14015_Procedure_Figure_6}
15. Carefully sprinkle the iron filings on the index cards. Sprinkle a small amount of iron filings to cover the entire area of the cards at first. Then, add more iron filings where lines appear to form. Observe the lines that form as the iron filings line up with the magnetic field of the magnets. Draw the magnetic field lines produced.
16. Carefully lift the index cards off the magnet, keeping them horizontal to prevent spilling the iron filings. Note: Do not allow the iron filings to contact the bar magnet directly. The iron filings will be difficult to remove once they “stick.”
17. Neatly pour the iron filings back into the weighing dish
18. Rotate one of the magnets 180 degrees so that both north poles or both south poles are facing each other. Space them about 2 cm apart. Transparent tape may be used to secure them to the table if necessary.
19. Repeat steps 14–17.

Analyze the Results

• How does the magnetic field of a single magnet compare in shape to that of an electric dipole? What is different?
• How does the direction of the compass needle change as the compass is moved along a magnetic field line?
• Where is the magnetic field the strongest? How can you tell?

Guided-Inquiry Activity Station: Induced Fields

1. Horizontally clamp the wooden dowel to the support stand (see Figure 7).
   {14015_Procedure_Figure_7}
2. Hang the insulated wire with stripped ends from the paper clips as shown in Figure 7. Note: Do not connect the wires to the battery until told to do so.
3. Using a compass, explore the region around the bar magnet. How does the compass needle behave near the north pole of the magnet? How does the compass needle behave near the south pole of the magnet? How is the direction of the magnetic field defined?
4. Using a compass, explore the region around the hanging wire. Does the wire have any effect on the compass?
5. Connect the battery to the paper clips using the alligator clip leads.
6. What is the direction of the current in the wire?
7. Approach the current-carrying wire from underneath with the compass and observe the orientation of the needle. Does it behave as if in a magnetic field? What is the direction of the field at the location of the compass? Note: Open the circuit after every observation in order to prevent the batteries from running down too quickly.
8. Approach the current-carrying wire from above with the compass and repeat the observations of step 7. Note: Open the circuit after every observation in order to prevent the batteries from running down too quickly.
9. Reverse the current on the wire and repeat steps 7 and 8.
10. Open the circuit.
11. Approach the wire from underneath with the north pole of the bar magnet (see Figure 8). Is the wire attracted to or repelled by the magnet? Make note of all observations.
    {14015_Procedure_Figure_8}
12. Close the circuit and repeat step 11. Note: Open the circuit after every observation in order to prevent the batteries from running down too quickly.
13. Repeat step 11 for the south pole of the magnet. Note: Open the circuit after every observation in order to prevent the batteries from running down too quickly.
14. Approach the current-carrying wire from above with each pole of the bar magnet. Record all observations. Note: Open the circuit after every observation in order to prevent the batteries from running down too quickly.
15. Reverse the current on the wire and repeat steps 12–14.

Analyze the Results

• Due to experimental observations, a pattern is deduced regarding the direction of the magnetic field lines around a current-carrying wire. The method for determining the orientation of the field around a current-carrying wire is known as the right-hand rule for the magnetic field. Imagine grasping the wire in your hand with your thumb pointing in the direction of the current. Your fingers curl around the wire in the direction of the magnetic field vectors. Do your experimental observations agree with the described method?
• Using the right-hand rule, determine the direction of the field at point A in Figure 9. Is the orientation of the field vectors into the page or out of the page?
  {14015_Procedure_Figure_9}
• Sketch a diagram that shows the wire, the direction of the current running through it, and the direction of the magnetic field below the wire at the location of the compass in step 9.
• Consider step 14. When the current-carrying wire was approached from below by the north end of the magnet, what was the observed motion of the wire? What was the motion of the wire when approached from above?
• Sketch a diagram that shows the wire, the direction of the current through it, and the direction of the bar magnet’s magnetic field when the north pole is brought near the magnet from below (see Figure 8).
• Sometimes a magnet exerts a force on a current-carrying wire, and at other times it does not—this depends on the orientation of the field relative to the current in the wire. When perpendicular to the current, the magnetic field is found to exert a force on a wire. If the field is parallel to the direction of the current, no force is exerted on the wire. The right-hand rule for the magnetic force can illustrate the direction of the magnetic force that a magnetic field exerts on a current-carrying wire. Hold your hand flat with your thumb extended from your fingers (see Figure 10). Orient your hand so that your fingers point in the direction of the magnetic field and your thumb points along the direction of the current. The direction of the magnetic force exerted by the magnetic field on the current is the direction your palm faces—perpendicular to both the direction of the current and the direction of the field. Do your observations agree with the right-hand rule for the magnetic force?
  {14015_Procedure_Figure_10}
• Using the right-hand rule for the magnetic force, does the wire in Figure 11 experience a force into the page or out of the page?
  {14015_Procedure_Figure_11}

Guided-Inquiry Activity Station: Gravity vs. Magnetism

1. Obtain the magnetic levitation device.
2. Use the eyebolt to adjust the “floating” paper clip so it is as close to the magnet as possible without touching it. Measure the distance in millimeters between the paper clip and the magnet. Record this measurement as the first, or “minimum,” distance.
3. Carefully hang one paper clip on the fishing line 18 cm away from the loop end of the eyebolt. Hang additional paper clips onto the first paper clip, slightly unbending each one so it can be easily hung. Make sure the first clip remains at the 18-cm point. Count the additional paper clips as they are added onto the first clip. Continue adding paper clips until the magnetic force is broken and the “floating” paper clip falls. Note: Add the paper clip masses slowly and carefully.
4. Record the total number of paper clips (the force) needed to make the floating clip fall. Do not count the “floating” clip. The mass of the “floating clip” and the mass of the string will be considered constants in the experiment. Note: This force is the force required at the “minimum” distance.
5. Determine the maximum possible distance at which the “floating” paper clip will “float” by turning the eyebolt out slowly (with no hanging paper clips on the line.) Read the exact distance at which the magnetic force is broken and the “floating” paper clip falls. Record the measurement as the maximum distance and the minimum force (force of zero).
6. Repeat the experiment for at least 9 other distances, choosing distances equally spaced between the maximum and minimum and always hanging the clips at the 18-cm mark. Record the distance (between the “floating” clip and the magnet) versus the force in number of paper clips required to make the floating clip fall. Conduct the 9 other trials, increasing the distance by turning the eyebolt.
7. Prepare a graph comparing distance and force.

Analyze the Results

• Consider Figure 2 in the Background. What would the magnetic field vectors look like for the magnet in the wooden frame?
• If a piece of cork was tied to the string instead of a paper clip, would the cork also “float”? Give a reason for your answer.
• Draw a free body diagram of the forces acting on the paperclip. What is opposing the force of gravity? What is opposing the force of tension in the string?
• How does the force between the “floating” paper clip and the magnet change as the “floating” paper clip is moved away from the magnet?
• It is found that the measured magnitude of the magnetic field produced by a current-carrying wire follows an inversesquare law. Does the magnetic field produced by the magnet also follow an inverse-square law? How does the data support your answer?

Student Worksheet PDF

14015_Student1.pdf