Electricity and magnetism covers DC circuits, electrical measurements, safety, permanent magnets, electromagnetism, and electromagnetic induction — one of the most heavily tested sections.
Current I = Q/t (A). Potential difference V = W/Q (V). Resistance R = V/I (Ω). Ohm\'s law: V ∝ I for constant temperature. Series: same current, voltages add, R_total = R₁ + R₂. Parallel: same voltage, currents add, 1/R_total = 1/R₁ + 1/R₂. Variable resistors, thermistors (resistance decreases with temperature), LDRs (resistance decreases with light).
Power P = IV = I²R = V²/R (W). Energy E = Pt = IVt (J). Kilowatt-hour (kWh) for billing: E = P(kW) × t(h). Cost = kWh × price per unit. Safety: fuses (protect against overcurrent), earth wire (safety path for fault current), circuit breakers, double insulation. Live, neutral, earth wire colours and functions.
Permanent magnets: north and south poles (like repel, unlike attract). Magnetic field lines: from N to S outside the magnet. Earth\'s magnetic field. Electromagnets: coil of wire carrying current around a soft iron core. Strength increased by: more turns, larger current, iron core. Applications: relays, electric bells, motors, circuit breakers.
EMF induced when magnetic flux through a circuit changes (Faraday\'s law). Methods: moving a wire through a magnetic field, moving a magnet into/out of a coil. Lenz\'s law: induced current opposes the change. AC generator: rotating coil in magnetic field produces alternating current. Transformer: V_p/V_s = N_p/N_s. Step-up and step-down transformers. National Grid uses high voltage transmission to reduce energy losses (P = I²R).
Power transmitted = voltage × current (P = IV). For a given power, increasing voltage allows current to decrease. Since energy lost as heat in cables = I²R, lower current means much less energy loss. So the National Grid uses step-up transformers to increase voltage (and decrease current) for transmission, then step-down transformers near homes to reduce voltage to a safe level (230V in UK, 110V in US).
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