Electromagnetic Induction
Inquiry Lab Kit for AP® Physics 2
Materials Included In Kit
Cardboard tubes, 8 Connector cords with alligator clips, 16 Galvanometer Iron nails, 8 LEDs, red, 16
Magnet wire, 22 gauge Magnet wire, 30 gauge, 8 Neodymium magnets, 16 Plastic jars, 60-mL, 8 Sandpaper sheet, 9" x 11"
Additional Materials Required
(for each lab group) Galvanometer
Scissors Transparent tape
Prelab Preparation
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Asking questions and defining problems Developing and using models Planning and carrying out investigations Engaging in argument from evidence Obtaining, evaluation, and communicating information
Disciplinary Core Ideas
MS-PS2.A: Forces and Motion MS-PS2.B: Types of Interactions HS-PS2.A: Forces and Motion HS-PS2.B: Types of Interactions
Crosscutting Concepts
Patterns Cause and effect Scale, proportion, and quantity Systems and system models Energy and matter
Performance Expectations
MS-PS2-3. Ask questions about data to determine the factors that affect the strength of electric and magnetic forces MS-PS2-5. Conduct an investigation and evaluate the experimental design to provide evidence that fields exist between objects exerting forces on each other even though the objects are not in contact HS-PS2-5. Plan and conduct an investigation to provide evidence that an electric current can produce a magnetic field and that a changing magnetic field can produce an electric current.
Answers to Prelab Questions
- 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.
- 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
- 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.
- Draw the magnetic field around a bar magnet.
{14016_PreLabAnswer_Figure_7}
Sample Data
Introductory Activity
{14016_Data_Table_1}
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
- Measure the length of the cardboard tube and mark the center.
- 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.
- Make a short (0.5 cm) diagonal cut at the top of the tube to hold the wire.
- Unwind a small length from the coil of magnet wire (about 10 cm), and hook it into the cut.
- 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}
- 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.
- Make another diagonal cut on the opposite end of the cardboard tube generator and secure the wire end in it.
- Use the sandpaper to remove the enameled coating on the two ends of the wire coiled around the generator.
- 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.
- 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}
- 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).
- 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}
- Wind each shaved end of the wire around the LED’s leads, ensuring the leads do not touch each other.
- 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
- 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.
- 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
- 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}
- Predict whether current will flow through the wire of the loop in each of the following cases. Explain your reasoning.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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}
- 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.
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