1. current-carrying wires in an external magnetic field experience a force, dependent on b, i, l and...
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ELECTROMAGNETIC EFFECTS
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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
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CATHODE RAY TUBES
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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.
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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
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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.
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All together,
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These concepts of changing flux associated with a wire are used in electric generators.
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The voltage produced (and so also the current) will be an alternating current (AC).
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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
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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.