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ELECTROMAGNETIC EFFECTS. THE MOTOR EFFECT. 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 = Bilsin θ This is the basis of electric motors and analogue electric meters. CATHODE RAY TUBES.

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the motor effect
THE MOTOR EFFECT
  • 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.

free charges also experience a force in a magnetic field
Free charges also experience a force in a magnetic field.
  • This is the basis of the old cathode ray tube TV’s and computer monitors (and oscilloscopes). F = qvBsinθ
electromagnetic induction
Electromagnetic induction
  • 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

slide13

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.
slide17

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

slide21

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.