Electricity, Magnetism and Electronics
Electricity, magnetism and electronics tests circuit analysis, electromagnetic induction and the behaviour of semiconductor devices. Expect calculations with internal resistance and simple operational-amplifier or logic circuits.
DC circuits
Ohm's law V = IR; resistivity R = ρL/A. Series: R = R₁ + R₂; parallel: 1/R = 1/R₁ + 1/R₂. Kirchhoff's laws: the sum of currents at a junction is zero, and the sum of EMFs around a loop equals the sum of p.d.s. For a cell, terminal p.d. = EMF − Ir.
Capacitors
Q = CV; energy = ½CV². Capacitors block steady direct current but pass changing signals; time constant τ = RC.
Magnetism and induction
A current in a magnetic field experiences F = BIL sinθ. Faraday's law: induced EMF = −dΦ/dt; Lenz's law gives the direction opposing the change. A transformer obeys Vₛ/Vₚ = Nₛ/Nₚ.
Electronics
A silicon diode conducts above about 0.7 V in forward bias. A transistor can act as a switch or amplifier; an ideal op-amp has infinite input impedance and voltage gain set by feedback resistors. Logic gates implement AND, OR and NOT combinations.
Worked example
A cell of EMF 12 V and internal resistance 1.0 Ω is connected to a 5.0 Ω resistor. I = E/(R + r) = 12/6.0 = 2.0 A. Terminal p.d. = E − Ir = 12 − 2.0 = 10 V.
Traps
Ignoring internal resistance, using series rules for parallel resistors, and assuming a capacitor behaves as a short circuit to DC once charged.