Generating ElectricityOxford AQA IGCSE Physics: Revision notes
Section 1
What is the generator effect?
The generator effect is the induction of a potential difference (pd) across a conductor when it moves relative to a magnetic field, or when it is in a changing magnetic field.
- If a permanent magnet is moved into or out of a coil, a pd is induced across the coil
- The same effect happens if the coil is moved relative to a stationary magnet — what matters is the relative movement between conductor and field
- If the conductor is part of a complete circuit, this induced pd causes a current to flow
State that it is relative motion between conductor and field that matters — either can move, or the field can change, and induction still occurs.
Section 2
What direction does the induced current flow?
When a current is induced in a conductor as part of a complete circuit, the induced current's magnetic field opposes the field of the permanent magnet that produced it (this opposes the motion causing the induction, resisting change — a consequence of energy conservation).
Reversing the direction of motion of the magnet, or reversing its polarity, reverses the polarity of the induced pd and the direction of the induced current.
Don't say the induced current 'has no direction' — always link reversing motion or polarity to a reversal of the induced pd/current.
Section 3
What factors increase the size of the induced pd?
The size of the induced pd can be increased by:
- Increasing the speed of movement (faster relative motion between coil and magnet)
- Using a stronger magnetic field
- Increasing the number of turns on the coil
- Increasing the area of the coil
Moving a bar magnet more quickly into a coil produces a larger induced pd than moving it slowly.
Section 4
How do alternators and dynamos differ?
Power stations use turbines to turn coils of wire between magnets (or turn magnets within coils), generating electricity via the generator effect.
- An alternator generates alternating current (ac) — the pd against time graph is a smooth wave that regularly reverses direction (a sine wave)
- A dynamo generates direct current (dc) — the pd against time graph stays on one side of the axis (though it may vary in size, it does not reverse direction)
Must Know
- The generator effect: a pd is induced across a conductor moving relative to a magnetic field, or in a changing magnetic field
- If part of a complete circuit, the induced pd drives a current, whose field opposes the magnet's field
- Reversing motion direction or magnet polarity reverses the induced pd and current direction
- Induced pd increases with: faster movement, stronger field, more coil turns, larger coil area
- Power stations use turbines to turn coils between magnets to generate electricity
- Alternators generate ac; dynamos generate dc
That's the notes covered.
Carry on to the next subtopic.