Electricity and Magnetism
Circuits
SEC Physics tests current, potential difference, resistance, series and parallel circuits, electrical power and the magnetic effects of current.
- Current I = Q/t (ampere = coulomb per second); conventional current is from positive to negative.
- Potential difference V = W/Q (volt = joule per coulomb).
- Ohm's law: V = IR for a conductor at constant temperature; resistance R = V/I.
- Series: same current, voltages add, R_total = R₁ + R₂ + ...; parallel: same voltage, currents add, 1/R_total = 1/R₁ + 1/R₂ + ...
- Power: P = VI = I²R = V²/R. An ammeter is connected in series; a voltmeter in parallel.
Magnetism and induction
A current creates a magnetic field; field lines run from north to south outside a magnet. A current-carrying conductor in a magnetic field experiences a force (motor effect); moving a conductor in a field induces an emf (generator effect). Transformers change ac voltage: V_s/V_p = N_s/N_p.
Worked example
A 12 V supply is connected to 4 Ω and 6 Ω resistors in series. Find the current and the voltage across the 6 Ω resistor.
R_total = 4 + 6 = 10 Ω.
I = V/R = 12/10 = 1.2 A.
V_6 = IR = 1.2 × 6 = 7.2 V.
Traps
Placing an ammeter in parallel or a voltmeter in series; adding series resistances as reciprocals; thinking parallel resistance is larger than the smallest resistor; reversing conventional current direction; using peak voltage instead of rms in power calculations when required.