Magnetic Induction

Current can be induced to flow in a wire by an external magnetic field. We will explore how that occurs using coils of wire. The device provided for the first two procedures consists of two coaxial coils. The primary coil is wound on a solid cylinder and can slide into the secondary coil, which is wound on a hollow cylinder. The last procedure will use two identical coils that can be placed side by side.

1. Current Induced by a Moving Permanent Magnet.

You will detect the flow of current in the secondary (hollow) coil using a galvonometer connected to the terminals of the coil. You will need to establish the direction that the needle of the galvonometer deflects by quickly tapping the terminals of a weak battery to the terminals of the galvonometer. Connect the coil to the galvonometer and carefully inspect the wire to determine the direction the wire is wrapped around the cylinder. Your instructor will show you how to sketch the coil to minimize confusion. (See images below.)

1. Move the North pole of the magnet rapidly towards one end of the coil and observe the galvonometer deflection. Carefully record the results, indicating the direction of magnet movement and resulting induced current flow in a sketch. Now move the magnet quickly away from the coil and record the results.
2. Repeat with both movements with the South pole of the magnet.
3. Repeat the previous steps at a variety of speeds for the magnet. Record qualitatively the effect of magnet speed on the induced current.

• Sketch a picture of the magnet and coil for each action taken,
  • with the poles of the magnet labeled,
  • the direction of motion of the magnet indicated,
  • and the direction of the subsequent current flowing in the coils. (Note: You do not need to indicate the needle deflection of the galvonometer.)

• Is the flow of current consistent with Lenz's law?
• What effect does the speed of magnet have on the current?
• Note the effect of the magnet's speed, and consider how current, charge and time are related. What quantity would you surmise might depend only upon the net change in magnetic flux? (Consider that the coil and galvonmeter have a total resistance R, hence the emf generated at any instant will create a current given by emf = IR.)

2. Current Induced by a Moving Coil.

Connect the primary coil in series with the power supply, rheostat and deadman switch. (The rheostat has already been adjusted to provide a reasonable current in the primary.) Note the direction that the current will flow and determine which pole (N or S) is present at the end of the primary. Confirm your determination with a compass.

1. Close the switch and repeat step 1 in the previous section, using the primary coil as you did the permanent magnet and moving it towards and away from the secondary coil.
2. Reverse the leads to the power supply and repeat.

 Report

Make at least one sketch indicating the direction of current in the primary, the resulting N and S poles associated with that current, and the direction of the subsequent current in the secondary. Is there any discernible subjective difference between using the permanent magnet and using the primary coil?

3. Transformer Action.

It is possible to induce a current without any actual movement of the coils. By orienting two coils such that the magnetic flux of one coil passes through the loops of the other, a current can be induced. Place the two free standing coils back-to-back. Connect the terminals of one coil to the galvonometer and the other to the power supply circuit.

1. Press and hold the deadman switch for a few seconds and record your observations.
2. Release the switch and observe what happens. Be sure to explain your results in terms of Lenz's Law.
3. Place the coils at right angles and repeat steps 1 and 2. Describe and explain the results.
4. Return the coils to the back-to-back position. Place each of the metal cylinders through both coils and repeat step 1, observing only the magnitude of the induced current. (The cylinders are labeled. There should be one each made of copper, brass, soft iron, stainless steel, aluminum and zinc.) From your results determine which of these metals are ferromagnetic and which are paramagnetic.

Here are two coil images that you may use in your write-up.