- Motor effect
- Current flows in the wire/coil.
- This creates a magnetic field around the wire/coil.
- This magnetic field interacts with the field from the permanent magnet.
- This produces a force on the wire/coil which moves the wire/coil.
- The split-ring commutator changes the direction of the current every half turn as it spins. This reverses the direction of the forces, allowing the coil to continue spinning.
- Fleming's left hand rule can tell us the diirection of the motion
- Thumb - motion
- First finger - field (north to south)
- Second finger - current (tall to short)
- F = BIl
- Induced potential difference: caused by a change in the magnetic field around a conductor.
- Alternators are
- Dynamos are used to generate direct current (dc) from kinetic energy:
- Coil is rotated within a magnetic field
- As it turns the coil cuts the magnetic field lines.
- This induces a voltage (or current) in the coil.
- How a transformer works
- alternating current in the primary coil means current is constantly changing direction.
- this means that magnetic field is constantly changing.
- (which is connected to the secondary coil by the iron core)
- which induces a voltage in the secondary coil
- which induces a current in the second circuit.
- Why does a step-up transformer have more turns in the secondary coil?
- more potential difference is induced in the secondary coil
- so therefore we induce less current
- since power is held constant and P = IV
- How to build a step-up transformer
- wind two coils of wire around an iron core
- the secondary coil should have more turns
- connect the primary coil to an a.c. supply
- Step up transformers increase voltage: more turns in secondary coil.
- Step down transformer decrease voltage: less turns in secondary coil.
- Microphone - converts pressure variations of sound waves into variations in current
- Loudspeakers - opposite (same as motor effect except use an alternating current)
- An alternating current from source passes though coils in speaker
- This creates a magnetic field around the coil
- This field interacts with the magnetic field from the permanent magnets
- Creating a constantly changing force on the coil.
- This causes the coil to vibrate which we hear as sound waves.