Electricity and electromagnetism (AS91173)
Electricity and electromagnetism (AS91173)
This domain tests DC circuit analysis, electrical power, magnetic fields and electromagnetic induction. NCEA questions often combine a circuit calculation with a magnetic-force or transformer context, so keep electrical and magnetic models separate.
Circuit rules
- Current is conserved at a junction. In series, current is the same and voltage is shared; in parallel, voltage is the same and current splits.
- Ohm's law V = IR applies to ohmic resistors at constant temperature. Lamps and diodes are non-ohmic; use their V-I graph.
- Series resistance adds: R_eq = R1 + R2 + ... . For parallel: 1/R_eq = 1/R1 + 1/R2 + ... .
- Power P = VI = I²R = V²/R. Energy E = Pt.
- With internal resistance r, terminal voltage V_T = EMF - Ir and current I = EMF/(R + r).
- A current-carrying wire has a magnetic field; use the right-hand grip rule. Force F = BIL sinθ, direction by the right-hand (FBI) rule. A moving charge feels F = qvB sinθ.
- Changing magnetic flux induces an EMF (Faraday's law); Lenz's law says the induced effect opposes the change. For an ideal transformer, V_s/V_p = N_s/N_p and input power equals output power.
Worked example
A 12 V battery of internal resistance 0.50 Ω is connected to a 5.5 Ω resistor. I = EMF/(R + r) = 12/(5.5 + 0.50) = 2.0 A. Terminal voltage V_T = 12 - (2.0)(0.50) = 11 V. Power in the external resistor: P = I²R = (2.0)²(5.5) = 22 W.
Traps
Do not ignore internal resistance when it is given. Do not treat current as used up around a series circuit. In parallel, take the reciprocal of the total, not the sum. For induction, the induced current opposes the change in flux, not the flux itself.