Electricity and Magnetism
Electricity and Magnetism covers electric current, potential difference, Ohm's law, resistance combinations, heating effects, and basic magnetic effects of current. Numerical questions often combine Ohm's law with series or parallel circuits.
Core ideas
- Current I = Q/t, measured in amperes. Conventional current is opposite to electron flow.
- Potential difference V = W/Q, measured in volts. It is the energy transferred per unit charge.
- Ohm's law: V = IR at constant temperature. Resistance depends on length, area, material and temperature.
- Series: R = R1 + R2 + ...; current is the same, voltage divides.
- Parallel: 1/R = 1/R1 + 1/R2 + ...; voltage is the same, current divides. Parallel resistance is less than the smallest branch.
- Heating effect: H = I^2 Rt. Electric power P = VI = I^2R = V^2/R. Energy in kWh is power in kW times time in hours.
- A current-carrying wire has a magnetic field; a solenoid behaves like a magnet and can make an electromagnet.
Worked example
A 6 V battery is connected across 6 ohm and 3 ohm resistors in parallel. Find the total resistance and current. 1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6, so R = 2 ohm. I = V/R = 6/2 = 3 A. Reasoning: in parallel the equivalent resistance is smaller than either resistor, and the battery supplies the total current.
Traps that cost marks
Ammeters go in series and voltmeters in parallel. Do not add parallel resistances directly. In series, the same current flows through each resistor, not the same voltage. For kWh, use kilowatts and hours, not watts and seconds.