Effects of Electric Current and Magnetism
Effects of Electric Current and Magnetism tests Ohm's law, series and parallel circuits, electrical power and heating, magnetic fields due to current, force on a conductor, electric motor, electromagnetic induction and generator. Numerical circuit questions are common.
Core rules
- Current I = Q/t. Ohm's law V = IR for a conductor at constant temperature. Resistance depends on length, area, material and temperature.
- Series: R = R1 + R2 + ...; current is the same. Parallel: 1/R = 1/R1 + 1/R2 + ...; voltage is the same.
- Power P = VI = I^2R = V^2/R. Heat produced H = I^2Rt.
- A straight current-carrying wire has concentric circular magnetic field lines; a solenoid behaves like a bar magnet. Fleming's left-hand rule gives the force on a current-carrying conductor in a magnetic field.
- Electric motor converts electrical energy to mechanical energy. Electromagnetic induction produces current when magnetic flux through a coil changes; a generator converts mechanical energy to electrical energy.
Worked example
A 6 V supply connects 6 ohm and 3 ohm resistors in parallel. First 1/Rp = 1/6 + 1/3 = 1/6 + 2/6 = 3/6, so Rp = 2 ohm. Total current I = V/Rp = 6/2 = 3 A. Branch currents are 6/6 = 1 A and 6/3 = 2 A, which add to 3 A. Total power P = VI = 6 x 3 = 18 W. The parallel resistance is less than the smallest resistor, as expected.
Traps
Ammeters go in series and voltmeters in parallel. In parallel, total resistance is less than each branch resistance. Do not confuse motor and generator energy conversions. Conventional current is opposite to electron flow. Fuse rating must match current, not voltage alone.