Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Measure and cut eight 4-m lengths of 22-gauge magnet wire and eight 1-m lengths of 30-gauge magnet wire; each group receives one of each.
2. Cut eight 1" x 4" pieces from the sandpaper sheet.

Safety Precautions

The materials in this lab are considered nonhazardous. Care should be taken when wrapping and unwrapping the wire. The pointed ends of the wire are dangerous to eyes. Neodymium magnets are very strong and will accelerate towards each other and other metal objects very quickly. Care should be taken to avoid unexpected and significant pinches of skin. Remind students to wear safety glasses. Please follow all normal laboratory safety guidelines.

Disposal

All materials may be saved and stored for future use. Do not store magnets near electronics.

Lab Hints

• This laboratory activity can be completed in two 50-minute class periods. It is important to allow time between the Introductory Activity and the Guided-Inquiry Design and Procedure for students to discuss and design the guided-inquiry procedures. Also, all student-designed procedures must be approved for safety before students are allowed to implement them in the lab. Prelab Questions may be completed before lab begins the first day.
• When building the AC generator in the Guided-Inquiry Design and Procedure, it may be beneficial to test the coil using a multimeter’s “continuity beeper” function to ensure you have a complete circuit. If the test indicates you do not have a complete circuit, remove more enamel from the ends of the wire. Check the resistance to ensure the total resistance is less than 100 Ω.
• The magnet wire can be reused for additional classes as long as it is unwrapped carefully so that there are no kinks in the wire. When removing the magnet wire from the round form (plastic containers), pull the magnet wire firmly and allow the round object to spin between your fingers as the wire unwraps. This will typically produce relatively straight wire with no kinks. Holding the round object secure while unwrapping the wire generally results in curled wire and can easily produce kinks.
• To facilitate winding the coils, spools can be set up on a stand or stabilized rod so it can be held in place but rotate freely.
• The magnets are fragile and may crack or chip if they are dropped or allowed to snap together with excessive force. Use caution when handling.
• In order to preserve the coil in the instance that the wire is cut too short to light the LED (due to a lack of coils), sand the ends of the coiled wire and the wire spool, and twist them together. Test on the edges of the shaved region to make sure you have a circuit through the twisted wires, and continue spooling.
• Dimming the lights in the room may make it easier to observe the flickering LED when the magnets are being spun.
• Taping the magnets to a stirring rod or long pencil may help in handling the magnet for making observations in the Introductory Activity.

Teacher Tips

• The Introductory Activity is a qualitative exploration of the effect of changing magnetic flux on a coil’s area. The investigation and subsequent observations do not require a quantitative approach and students should focus on the orientation of the magnet and the specific movements that register a needle deflection on the galvanometer. The activity provides a hands-on introduction to the phenomenon of induced current in a coil due to changing the magnetic flux relative to a coil’s area.
• If available, a compass can be used to designate the poles of the magnets used in the Introductory Activity so students know the orientation of the field lines due to the magnets when exploring induced current with the galvanometer.
• A complete circuit is essential for success in the Guided-Inquiry Activity. If a voltage drop over the wire is not produced, the LED will not light. Verify that the ends of the wires are well sanded and not touching each other to ensure good conductivity.
• Multimeters can be used to quantitatively test the amount of AC current and voltage generated by the spinning magnets in the coil.
• Upon completing the Introductory Activity students should be able to design a procedure in the Guided-Inquiry Design and Procedure where the spinning of the magnets inside the coil are the source of emf for the LED. It is important to survey each group and ask probing questions to guide them to this discovery.

Further Extensions

Opportunities for Inquiry
With the use of the galvanometer or other means of detecting current, design a procedure to quantify the effect of each variable on the induced current.

Alignment to the Curriculum Framework for AP^® Physics 2 

Enduring Understandings and Essential Knowledge
The electric and magnetic properties of a system can change in response to the presence of, or changes in, other objects or systems. (4E)
4E2: Changing magnet flux induces an electric field that can establish an induced emf in a system.

1. Changing magnetic flux induces an emf in a system, with the magnitude of the induced emf equal to the rate of change in magnetic flux.
2. When the area of the surface being considered is constant, the induced emf is the area multiplied by the rate of change in the component if the magnetic field is perpendicular to the surface.
3. When the magnetic field is constant, the induced emf is the magnetic field multiplied by the rate of change in area perpendicular to the magnetic field.
4. The conservation of energy determines the direction of the induced emf relative to the change in the magnetic flux.

Learning Objectives
4E2.1: The student is able to construct an explanation of the function of a simple electromagnetic device in which an induced emf is produced by a changing magnetic flux through an area defined by a current loop (i.e., a simple microphone or generator) or of the effect on behavior of a device in which an induced emf is produced by a constant magnetic field through a changing area. [See Science Practice 6.4]

Science Practices
1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.
4.3 The student can collect data to answer a particular scientific question.
6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.

Answers to Prelab Questions

1. What is magnetic flux?

   Magnetic flux is defined as the number of magnetic field lines through an area, such as the area of a coil.

2. A magnetic field of 2 T is perpendicular to a 5 cm x 5 cm square coil. What is the magnitude of magnetic flux through the coil?

   Φ = BAcosθ
   B = 2 T
   A = 5 cm x 5 cm = 0.05 m x 0.05 = 0.0025 m²
   θ = 0° because the field is perpendicular to the coil
   Φ = 2 x 0.0025 x cos(0)
   Φ = 2 x 0.0025 x 1
   Φ = 0.005 Wb

3. What are the units for magnetic flux? Hint: Refer to Equation 2.

   Since the units for a magnetic field B is T, the tesla, and the units for area is m², then multiplying the two quantities results in T x m², the units for magnetic flux.

4. Draw the magnetic field around a bar magnet.
   {14016_PreLabAnswer_Figure_7}

Sample Data

Introductory Activity

Analyze the Results

• Does the galvanometer needle deflect more when the magnet approaches the coil rapidly or slowly?

  The needle deflects more when the coil is approached rapidly by the magnet.

• How does the orientation of the magnet affect the observed current?

  When the poles of the magnet are parallel to the plane of the coil, no induced current is observed when approaching the magnet. However, when the poles of the magnet are perpendicular to the plane of the coil, an induced current is clearly observed.

• Does the needle deflect when the magnet is at rest?

  No, the needle does not deflect when the magnet is at rest, regardless of the orientation.

• Does rotating the magnet induce a current in the coil?

  Yes, when the magnet is rotated, a current is induced in the coil.

• When the magnet is oscillated to and from the coil, the needle moves from side to side indicating a current that changes direction. In your own words, why does the current change direction? Consider Lenz’s law in your explanation.

  The current changes direction depending on whether the external magnetic flux is increasing or decreasing. According to Lenz’s law, the induced current in the coil opposes the change in magnetic flux through the coil’s area. If the flux is increasing, the induced current will generate a magnetic field that opposes the direction of the external magnetic field that is the cause of the changing flux. When the magnet is pulled away from the coil, the current reverses and generates a magnetic field in the direction of the decreasing external magnetic field. This is a consequence of the conservation of energy.

Guided-Inquiry Activity

Sample Design Solution

1. Measure the length of the cardboard tube and mark the center.
2. Push the tip of the nail through the cardboard tube at the center mark and drop the nail straight down. Poke a hole through the other side as well.
3. Make a short (0.5 cm) diagonal cut at the top of the tube to hold the wire.
4. Unwind a small length from the coil of magnet wire (about 10 cm), and hook it into the cut.
5. Wind the wire around the cardboard tube at least 300 times (see Figure 9). Be cautious to not pull too tightly, as the thin wire may snap. Note: The more coils, the easier it will be to light the LED.
   {14016_Answers_Figure_9}
6. Once you have the desired length, spool a bit of excess and cut the wire. Note: This step would be skipped if the student intends to vary the number of turns.
7. Make another diagonal cut on the opposite end of the cardboard tube generator and secure the wire end in it.
8. Use the sandpaper to remove the enameled coating on the two ends of the wire coiled around the generator.
9. Widen each hole by inserting the nail through one hole at a time and then wiggle the nail using circular motions until the nail can spin easily.
10. Test the nail’s ability to spin by holding the nail steady, then spinning the cardboard tube (see Figure 10). The tube should spin easily, without much friction to slow it down. If not, repeat step 10. Leave the nail in its slots.
    {14016_Answers_Figure_10}
11. Carefully detach the magnets from each other. Slowly bring one inside the generator, gripping it loosely so it will attach to the nail on its own. The magnet should attach by a flat circular end (one of its poles).
12. Reach in with your fingers and grip the magnet attached to the nail to hold it in place. Grasp the other magnet firmly, and slowly and carefully insert it into the other end of the tube. If opposite poles are facing each other, a pulling force will be felt. If a force of repulsion is felt, flip the magnet around. Gripping both magnets firmly, bring the second magnet as close as possible before releasing it, allowing it to attach to the other side of the nail (see Figure 11).
    {14016_Answers_Figure_11}
13. Wind each shaved end of the wire around the LED’s leads, ensuring the leads do not touch each other.
14. Turn off or dim the lights in the room and spin the nail with both hands. Increase the spinning speed until you can see the LED flicker on and off (see Figure 12).
    {14016_Answers_Figure_12}

Answers to Questions

Guided-Inquiry Activity  

1. What variables affect the value of an induced emf in a coil of wire?

   The variables that affect the value of an induced emf in a coil of wire are the number of wire loops in the coil, the rate of change in the strength of the magnetic field, the rate of change of the coil’s area, the rate of change of the angle (angular velocity of a spinning magnet or coil) that the magnetic field lines make with the plane of the loop.

2. If a magnet is spinning inside a coil of wire, how would an increasing rate of spin relate to the induced current?

   An increasing rate of spin means an increase in the rate of change of the angle that the magnetic field lines make with the plane of the loop. This means an increase in the rate of change in magnetic flux. An increase in the rate of change of magnetic flux directly increases the magnitude of the induced current.

Review Questions for AP^® Physics 2

1. Recall that a current-carrying wire produces a magnetic field. The magnetic field of a long line of current-carrying wire loops is analogous to the field of a bar magnet. The diagram below shows a copper wire loop near a solenoid, a long line of wire loop. The switch in the circuit is initially open.
   {14016_Answers_Figure_13}
   1. Predict whether current will flow through the wire of the loop in each of the following cases. Explain your reasoning.
      1. Just after the switch has been closed.

         Yes, current will flow in the wire because there will be an increase in magnetic flux through the area of the loop.

      2. A long time after the switch has been closed.

         No, there will be no current flow because there is no changing magnetic flux to induce a current.

      3. Just after the switch has been reopened.

         Yes, current will flow in the wire due to a decrease in magnetic flux but will flow in the opposite direction as to when the switch was closed.

      4. A long time after the switch has been reopened.

         No, there will be no external changing magnetic field to induce a current in the wire loop.

2. An induction stovetop has a smooth surface. When on high, the surface does not feel warm, yet ramen noodles are quickly cooked in a metal bowl. However, when an attempt is made to cook the noodles in a ceramic bowl, the noodles remain cold. Explain how the stovetop works.

   The stovetop must generate a changing magnetic field beneath the surface. This changing magnetic field penetrates through the metal (ferromagnetic) pan. The changing magnetic flux through the metal pan induces currents on the metal like it does in a wire. This in turn will generate heat and cook the food.

3. The magnetic flux through three different coils is changing as shown in the following figures. For each situation, draw a corresponding graph showing qualitatively how the induced emf changes with time.
   {14016_Answers_Figure_14}
4. A transformer consists of two coils, each wrapped around an iron core. The core confines the magnetic field produced by the electric current in one coil so that it passes through the second coil without spreading outside. The primary coil in each situation below is connected to identical emf sources.
   {14016_Answers_Figure_15}
   Which ammeter would register the higher secondary emf output? Explain your reasoning.

   The ammeter in situation B would register the higher emf output. If the magnetic field produced by the primary coil completely passes through the secondary coil, then the resulting magnetic flux through the secondary coil would induce a current in that coil. The only difference between the secondary coils in each scenario is the number of turns. When analyzing Equation 1, the more turns in a coil of wire, the larger the induced emf. Since all other variables are kept constant, the secondary coil in situation B must have a higher induced emf because it has more turns than the coil in situation A.

References

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

Introduction

Electricity and magnetism are just two parts of the unifying concept of electromagnetism. Once it was known that electric currents produce magnetic fields, it was natural to wonder if magnetic fields could produce electric currents. Devices, such as AC generators, microphones and MRI machines, all became possible due to an understanding of electromagnetic induction.

Concepts

• Electromagnetic induction
• Magnetic flux
• Faraday’s law
• Lenz’s law

Background

A magnetic field is produced by moving charged particle, such as an electron. A current-carrying wire, in which many moving charged particles are the source of current, produces a magnetic field that surrounds the wire. When current travels around a wire loop, the magnetic field produced by the current is strongest at the center of the loop. The direction of the magnetic field produced by a looping current is perpendicular to the plane of the loop (see Figures 1 and 2).

When many current-carrying loops are grouped together in a line, such as with a coil of wire, the current travels in the same direction in all the loops and the ensuing superposition of the magnetic field produced by each loop provides a stronger net magnetic field. This phenomenon of an induced magnetic field was discovered by Hans Christian Oersetd in 1819 when he noticed the deflection of a compass needle when a circuit was connected near the compass. This was the first observation of the connection between electricity and magnetism, whose combined interactions are known as electromagnetism.

In 1831, Michael Faraday (1791–1867) discovered through careful experimentation that “any change in the magnetic environment of a coil of wire will cause a voltage or emf to be ‘induced’ in the coil.” The relationship states that the average magnitude of the induced emf, ε_in, in a coil with N loops is the magnitude of the ratio of the magnetic flux change through the loop ΔΦ to the time interval Δt during which that flux change occurred, multiplied by the number of loops. This is known as Faraday’s law. The number of magnetic field lines through a two-dimensional area, most commonly the area of a coil, is known as magnetic flux. The induced emf in a coil due to changing magnetic flux is given by where

emf = electromotive force or voltage (V)
N = number of turns in a coil of wire
ΔΦ = change in magnetic flux (weber or Wb)
Δt = time interval.

Magnetic flux is given by where

Φ = magnetic flux
B = magnitude of magnetic field (T, tesla)
A = area the field lines pass through (m²)
θ = angle between the magnetic field lines and a vector perpendicular to the area.

Magnetic flux can change if the magnetic field or area changes. If the field and area are held constant, magnetic flux can still change if the angle between the perpendicular area and magnetic field lines changes.

Henrich Lenz (1804–1865) further investigated Faraday’s law and discovered that the induced current in a coil travels in a direction that aligns with the principle of conservation of energy—the induced magnetic field opposes the change in magnetic flux through the coil’s area. If the increasing magnetic flux through a loop led to an induced magnetic field in the same direction as the external field, then the magnetic flux would continue to increase, leading to a greater induced current and so on. Energy would not be conserved and the wire would melt. Instead, if the magnetic flux through a coil is increasing, the direction of the induced current’s magnetic field would lead to a decrease in the flux. If the magnetic flux through a coil is decreasing, then the direction of the induced current’s magnetic field would lead to an increase in flux. This is known as Lenz’s law.

Experiment Overview

The purpose of this advanced inquiry investigation is to understand the unity of electricity and magnetism through a discovery of electromagnetic induction. An introductory activity uses a coil of wire, galvanometer and magnet to investigate how magnetic flux induces a current on a coil of wire. The guided-inquiry activity is a challenge to design a mechanism using available materials that will light an LED through an AC generator.

Materials

Prelab Questions

1. What is magnetic flux?
2. A magnetic field of 2 T is perpendicular to a 5 cm x 5 cm square coil. What is the magnitude of magnetic flux through the coil?
3. What are the units for magnetic flux? Hint: Refer to Equation 2.
4. Draw how the magnetic field would look around a bar magnet.

Safety Precautions

The materials in this lab are considered nonhazardous. Care should be taken when wrapping and unwrapping the wire. The pointed ends of the wire are dangerous to eyes. Neodymium magnets are very strong and will accelerate towards each other and other metal objects very quickly. Care should be taken to avoid unexpected and significant pinches of skin. Wear safety glasses. Please follow all normal laboratory safety guidelines.

Procedure

Introductory Activity

1. Obtain 4-m and 1-m lengths of magnet wire, a 60-mL plastic jar, a piece of sandpaper, two neodymium magnets, two connector cords, some transparent tape and a galvanometer.
2. Use sandpaper to completely sand off about 2 cm of red enamel at both ends of each piece of magnet wire to expose the shiny copper underneath.
3. Leaving about 3 cm of magnet wire free to be connected with the alligator clips later, use a small amount of transparent tape to secure one end of one of the 4-m magnet wire piece to the outside of the 60-mL plastic jar (see Figure 4).
   {14016_Procedure_Figure_4}
4. Remove the lid from the jar and begin wrapping the wire just below the lip of the jar so the wire stays in place. Tightly and neatly wrap the wire around the jar as shown in Figure 4.
5. Leave about 3 cm of magnet wire at the end and use a small amount of tape to secure the loose end of the magnet wire to the jar. Make sure the free ends of the magnet wire are accessible and can be connected to the alligator clips.
6. Attach alligator clips to the free ends of the magnet wire and connect the cords to the inputs on the galvanometer.
7. Obtain the neodymium magnets. They should be “stuck” together.
8. Rapidly move the magnet toward the coil and observe the galvanometer needle. Repeat the process while moving the magnet slowly. Record observations of the galvanometer needle.
9. Hold the magnet stationary inside the coil and observe the galvanometer needle. Record any observations.
10. From inside the coil, pull the magnet away from the coil and observe the galvanometer needle. Compare the direction of the deflection to what was observed in step 8. Record your observations.
11. Move the magnet to and from the coil, varying the speed (see Figure 5). Record your observations.
    {14016_Procedure_Figure_5}
12. Rotate the magnet 90°. Observe if the galvanometer registers a current during the rotation. Note: Keep the magnet the same distance away from the coil while rotating.
13. With the magnet now oriented so its poles are parallel to the plane of the coil (see Figure 6), oscillate the magnet back and forth and record your observations.
    {14016_Procedure_Figure_6}
14. Remove the coil from the jar.
15. Repeat steps 3–6 for the 1-m length of magnet wire.
16. Repeat steps 8–13 for the 1-m length of coil. Record your observations and compare the “magnitude” of the deflection of the galvanometer needle.

Analyze the Results 

• Does the galvanometer needle deflect more when the magnet approaches the coil rapidly or slowly?
• How does the orientation of the magnet affect the observed current?
• Does the needle deflect when the magnet is at rest?
• Does rotating the magnet induce a current in the coil?
• When the magnet is oscillated to and from the coil, the needle moves from side to side indicating a current that changes direction. In your own words, why does the current change direction? Consider Lenz’s law in your explanation.

Guided-Inquiry Design and Procedure
Form a working group with other students to discuss the following questions.

1. What variables affect the value of an induced emf in a coil of wire?
2. If a magnet is spinning inside of a coil of wire, how would an increasing rate of spin relate to the induced current?
3. Using the following materials, design a device that can turn mechanical energy into electrical energy and light an LED: cardboard tube, iron nail, LED, 30 gauge magnet wire, two neodymium magnets, sandpaper, transparent tape, ruler and scissors.
4. Before carrying out the desired procedure, submit your design plan to your instructor for approval.

Analyze the Results 

• In your own words, write a detailed summary of any errors and challenges you encountered. Include a list of variables that could affect the emf generated and explain what variable(s) you focused on in your experimental design and why.

Student Worksheet PDF

14016_Student1.pdf