Tasks

Predict and test the direction that current will be induced in a wire loop when:

1. Moving the north or south end of a bar magnet toward or away from the loop
2. Moving the wire loop toward or away from each end of a solenoid with current running through it.
3. The wire loop is near the end of a solenoid when the current is turned on or turned off.

Resources

• Power supply
• Large Solenoid
• Small wire loop
• Permanent magnet
• Galvanometer
• Compass
• Battery
• Resistor (~10K)
• Wires

Background

Magnetic fields can exert force on moving charged particles. In certain situations, they can cause charges to flow in wires, producing a voltage or electromotive force (EMF). A magnetic field will produce an EMF in a loop of wire if the total amount of magnetic field passing through the loop changes over time. This can be because the wire loop moves toward or away from the source of the magnetic field, or vice versa. The amount of EMF is given by \mathcal{EMF}=-N\frac{d}{dt}\Phi_B=-\frac{\Delta B}{\Delta t}NA\cos\theta where \Phi_B is the magnetic field flux, B is the magnetic field, A is the area of the wire loop, N is the number of turns in the wire loop, and \theta is the angle between the axis of the loop and the magnetic field direction.

Physically, the induced current in the wire moves in a direction such that it creates a magnetic field that opposes the direction of change of the external magnetic field through the loop, as shown by the minus sign in the equation above.