Electric Current and Its Effects / Electromagnetism
The paper combines Ohm's law, series and parallel circuits, the heating effect, electric power, and the magnetic effects of current including the motor and generator.
Ohm's law and resistance
Current I = Q/t, measured in amperes. Ohm's law states V = IR at constant temperature. Resistance R = ρl/A, so it rises with length and falls with thickness, and rises with temperature in metals. Resistivity is a property of the material, not of the sample.
Series and parallel
- Series: the same current flows through each component, voltages add, R = R1 + R2 + ...
- Parallel: the same voltage is across each branch, currents add, 1/R = 1/R1 + 1/R2 + ...
- An ammeter is connected in series and has low resistance; a voltmeter is connected in parallel and has high resistance.
Heating, power and magnetism
Joule's law gives heat H = I²Rt. Power P = VI = I²R = V²/R, in watts; domestic energy is billed in kWh. A current-carrying conductor has a magnetic field around it, found by the right-hand thumb rule. A solenoid behaves like a bar magnet and is used to make electromagnets. A current-carrying conductor in a magnetic field experiences a force, given by Fleming's left-hand rule, which is the principle of the motor; a changing magnetic field induces a current, given by Fleming's right-hand rule, the principle of the generator. A fuse of suitable rating is placed in the live wire, and earthing gives safety.
Worked example
Resistors of 6 Ω and 3 Ω are joined in parallel across a 6 V battery. Then 1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω. Total current I = 6/2 = 3 A. The 6 Ω branch carries 1 A and the 3 Ω branch 2 A, and total power = VI = 18 W.
Traps
Series shares current and parallel shares voltage; the parallel combination is always smaller than the smallest resistor; the fuse rating must slightly exceed the normal current.