ELECTROMAGNETIC EFFECTS

1. Current-carrying wires in an external magnetic field experience a force, dependent on B, I, l and the angle between wire and B field.

F = Bilsinθ

This is the basis of electric motors and analogue electric meters.

THE MOTOR EFFECT

CATHODE RAY TUBES

This is the basis of the old cathode ray tube TV’s and computer monitors (and oscilloscopes). F = qvBsinθ

Free charges also experience a force in a magnetic field.

Moving a wire in a magnetic field can produce a voltage in the wire, and thus a current. Moving a magnet around a wire does the same thing.

Key concept: if the flux (amount of magnetic flow or amount of magnetic field lines) changes around a conductor, then a voltage is produced across the ends of the conductor.

Φ = B A cosθ

Electromagnetic induction

The faster the flux changes, the higher the voltage.

There will be opposition to flux change. As we change flux, a voltage (emf) is generated in the wire, that opposes the change we are making. (Lenz’ Law).

Every loop of wire exposed to a changing flux experiences its own emf across its ends. If the loops are in series, we add the emfs.

All together,

These concepts of changing flux associated with a wire are used in electric generators.

The voltage produced (and so also the current) will be an alternating current (AC).

Generators must supply power to distant places. How is this done? To reduce heat losses, it is better to send the electric current with a low I and high V, rather than high I and low V. To do this a transformer is needed.

Vp/Vs = Np/Ns

VpIp = VsIs

A last look at electric motors:When a motor starts up, lights often dim.

Why is this? When the motor starts, there is a maximum

current flow due to the high emf across the motor. As the motor speeds up, a back emf is induced (coils rotating in a magnetic field) in the coils, that opposes the forward emf. The back emf reaches a maximum when the motor is turning at full speed. This reduces the overall emf across the motor. The current is also reduced, and the lights elsewhere get more current again.