Physics 9

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Lab 9: Faraday’s Law

Objectives In this lab you will use PhET’s simulation Faraday’s Law to study electromagnetic induction.

Theory Electric fields and magnetic fields are not independent physical properties. One of the phenomena

that highlights the relationship between the two types of field is the phenomenon of

electromagnetic induction in which a changing magnetic field creates an electric field. In this lab we

will study, qualitatively, electromagnetic induction by observing and describing the effect of the

induced electric field on electric charges (electrons) moving through metal wires forming an

electric circuit.

In the context of this lab, Faraday’s law states that the voltage induced in a coil is proportional to

two factors: total area enclosed by the coil and the rate of change of the magnetic field.

The total area enclosed by a coil is the product of the number of loops and the area of one loop. In

this simulation, the area of individual loops cannot be changed and, therefore, the only way to

change the total area is by changing the number of loops. The simulation allows us to connect one

or two coils with different numbers of loops.

Similarly, the simulation doesn’t allow us to change the strength of the magnet. Instead, adjusting

the rate of change of the magnetic field inside the coils will be achieved by adjusting the speed at

which the magnet is moved relative to the coil (you will be responsible for moving the magnet into

the coil at various speeds).

Part 1: Dependence on the Speed of the Magnet 1. Check the “Field lines” box to display the magnetic field lines.

2. How many magnetic field lines emerge from the north pole? How many converge into the

south pole?

3. Select the single coil option.

4. Drag the magnet so that its axis aligns with the axis of the coil and it is far enough from the

coil that no more than four magnetic field lines go through the coil.

5. With a very slow and steady motion, push the magnet into the coil, north-pole first, until it

exits through the other side completely clearing the coil so that no more than four magnetic

field lines are left inside the coil. Make sure the speed of the magnet is slow enough that the

needle of the voltmeter never passes beyond half the left and/or right half of the scale; this

might take repeated attempts. Observe the motion of the needle and the brightness of the

bulb very carefully.

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a. Describe the motion of the needle of the voltmeter: In what direction does it move

first (- or +)? When does the needle change direction in relation to the position of

the magnet relative to the coil? When is the maximum deflection achieved by the

needle, on either side, in relation to the position of the magnet?

b. Describe the change in the brightness of the bulb: When is the brightness at a

maximum in relation to the position of the magnet relative to the coil? Can you see a

correlation with the motion of the needle?

6. Repeat step 5 at a faster speed; this time the needle of the voltmeter may cover the full scale

of the voltmeter. Describe any differences from your observations in step 5.

7. Now flip the poles of the magnet.

8. Repeat step 5. Describe any differences from your observations in step 5.

9. Are your observations in this part consistent with Faraday’s law?

Part 2: Dependence on the Number of Loops 1. Check the “Field lines” box to display the magnetic field lines.

2. Select the double coil option.

3. Drag the magnet so that its axis aligns with the axis of the upper coil, and it is far enough

from the coil that no more than four magnetic field lines go through the coil.

4. With a slow and steady motion, push the magnet into the coil, north-pole first, until it exits

through the other side completely clearing the coil so that no more than four magnetic field

lines are left inside the coil. Observe the brightness of the bulb and the motion of the needle

very carefully.

5. Repeat step 4 for the lower coil and with approximately the same speed as in step 4.

Observe the brightness of the bulb and the motion of the needle very carefully.

6. Describe the difference in brightness of the bulb and the motion of the needle between the

upper and lower coils.

7. Are your observations in this part consistent with Faraday’s law?