Electricity and Magnetism
Unit 4 of Cambridge IGCSE Physics 0625. Circuits, current, voltage, resistance, series and parallel, magnetic fields, the motor effect, electromagnetic induction and transformers.
What you need to know
- Current is the same at every point in a series circuit. In parallel the current splits between the branches and adds back up.
- Voltage is shared between components in series. In parallel every branch has the same voltage.
- Resistors in series add. In parallel the total resistance is always less than the smallest single resistor.
- An ammeter goes in series and has very low resistance. A voltmeter goes in parallel and has very high resistance.
- The motor effect needs a current and a magnetic field. Use Fleming left hand rule for the direction of the force.
- Induction needs relative movement between a conductor and a magnetic field, or a changing field. No change means no induced voltage.
Equations
- V = IR
- I = Q / t
- P = IV
- P = I^2 R
- E = IVt
- Vp / Vs = Np / Ns (transformer)
Electricity and Magnetism questions students ask
Is the current the same everywhere in a series circuit?
Yes. There is only one path, so the same current passes through every component. Current is not used up as it goes round.
How does current behave in a parallel circuit?
It splits between the branches at the junction and adds back together after. The total current from the supply equals the sum of the branch currents.
Where do you connect an ammeter and a voltmeter?
An ammeter goes in series with the component and has very low resistance. A voltmeter goes in parallel across the component and has very high resistance.
Why is the total resistance smaller when resistors are in parallel?
Adding a parallel branch gives the current another path, so more current flows for the same voltage. The total resistance is always less than the smallest single resistor.
How does a transformer work?
Alternating current in the primary coil makes a changing magnetic field in the iron core. That changing field induces an alternating voltage in the secondary coil.
Why does the national grid use high voltage?
High voltage means low current for the same power. Power wasted as heat in the cables is I squared R, so halving the current cuts the loss to a quarter.
What is the motor effect?
A wire carrying a current in a magnetic field experiences a force. Use Fleming left hand rule with first finger for field, second finger for current, thumb for force.
How do you induce a voltage in a wire?
Move the wire so it cuts magnetic field lines, or change the magnetic field through a coil. No movement and no change means no induced voltage.
What is the difference between voltage in series and in parallel?
In series the supply voltage is shared between the components. In parallel every branch gets the full supply voltage.
How do you calculate electrical power?
Power equals current times voltage. You can also use I squared R, or V squared over R, depending on what the question gives you.
Mistakes that cost marks
- Saying current gets used up as it goes round a circuit. It does not.
- Mixing up where the ammeter and voltmeter go.
- Adding parallel resistances as if they were in series.
- Forgetting that a transformer only works on alternating current.
- Writing "the magnet makes electricity" instead of describing the changing magnetic field cutting the conductor.
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