Electricity and Magnetism
What this domain tests
Electricity and Magnetism tests circuits, Ohm's law, electrical energy, electromagnetism, motors, electromagnetic induction and transformers.
Core rules
- Current I = Q/t is measured in amperes with an ammeter in series. Potential difference V = W/Q is measured with a voltmeter in parallel.
- Ohm's law: V = IR at constant temperature.
- Series circuits: same current; R_total = R₁ + R₂ + …; voltages add. Parallel circuits: same voltage; 1/R_total = 1/R₁ + 1/R₂ + …
- Power P = VI = I²R = V²/R; energy E = VIt; 1 kWh = 3.6 MJ.
- An electromagnet is a coil with current and an iron core; reversing the current reverses the poles.
- Force on a current-carrying conductor F = BIL when the conductor is perpendicular to the field; a motor uses this force.
- Transformer: Vp/Vs = Np/Ns; an ideal transformer has input power equal to output power.
Worked example
A 4 Ω resistor and a 6 Ω resistor are in series across 12 V. R_total = 10 Ω. I = V/R = 12/10 = 1.2 A. Voltage across the 4 Ω resistor = IR = 1.2 × 4 = 4.8 V; across the 6 Ω resistor = 7.2 V; total 12 V. Power in the 4 Ω resistor = I²R = 1.44 × 4 = 5.76 W.
Traps
Do not place an ammeter in parallel. Do not add parallel resistances directly. In a transformer, voltage steps down while current steps up; power is not multiplied. Transformers require alternating current.