what rings does a d.c. motor use?
split rings
what rings does an a.c. generator use?
slip rings
what induces an electromotive force?
a conductor moving across a magnetic field
electromotive force definition
the work done or energy per unit charge around the whole circuit by an energy source, such as a battery
Faraday’s law
a wire close to a changing magnetic field will experience an induced electromotive force
inducing an e.m.f: (2 points)
- moving a magnet so that its field lines are cut by a wire
- moving a wire across a magnetic field
current diagram (dot/circle)
current out
current diagram (cross)
current in
Lenz’s law
the direction of the induced e.m.f. opposes the change that creates it
current definition
rate of flow of charges (electrons) around a circuit
alternating current
electrons continuously change directions
direct current
electrons flow in one direction only
A.C. GENERATOR - magnets
provide a constant magnetic field across the coil
A.C. GENERATOR - coil
usually made from many turns of wire, it is rectangular so that its sides are perpendicular to the magnetic field
A.C. GENERATOR - slip rings
cylindrical conductors that make constant contact with the coil during rotation. They allow the direction of the induced electromotive force (e.m.f.) to alternate and therefore cause an alternating current.
A.C. GENERATOR - carbon brushes
make an electrical connection between the rotating coil and a circuit, avoiding the wires becoming twisted
At a certain point, the output of an alternating current generator is +V. What will the output be 180 degrees later? (+ explanation)
-V. After half a rotation, the output electromotive force will be equal in size but will have an opposite direction
When is a magnetic field created?
when an electric current (charge) flows through a wire
to increase the strength of the magnetic field you can: (2 points)
- increase the current through the solenoid
- increase the number of turns of wire in the solenoid
magnetic effects of a current in relays
when the electromagnet is supplied with a current, a magnetic field is generated. The field attracts the iron bar, which is pivoted and allows the conducting contacts to touch.
magnetic effects of a current in loudspeakers
when an a.c. is supplied to the coil, the speaker cone oscillates left and right. The magnetic field due to the a.c. in the coil either attracts or repels a permanent magnet around, resulting in the vibrations necessary for sound.
to increase the size of the force: (4 points)
- increase the number of individual wires
- increase the current in the wire
- increase the strength in the magnetic field
- increase the length of the wire in the magnetic field
D.C. MOTOR - to make the motor spin more quickly (3 points)
- Increase the strength of the magnets and thus the magnetic field
- increase the number of turns of wire in the coil
- increase the current to the coil from the power supply
D.C. MOTOR - coil
rectangular and often made up of lots of turns of current-carrying wire
