Key facts & rules

• Current: flow of charge (electrons in metals). Unit: ampere (A). Measured with ammeter (in series).
• Voltage (pd): energy transferred per unit charge. Unit: volt (V). Measured with voltmeter (in parallel).
• Resistance: opposition to current. Unit: ohm (Ω).
• Series circuit: same current everywhere; voltage splits; resistance adds.
• Parallel circuit: same voltage across each branch; current splits; total resistance less than smallest.
• Direct current (DC): current always in same direction (batteries).
• Alternating current (AC): current reverses direction periodically (mains electricity, 50 Hz in UK).
• Magnetic field: caused by moving charges/current. Right-hand rule for solenoid.
• Motor effect: current-carrying conductor in magnetic field experiences a force (F = BIl).
• Electromagnetic induction: moving conductor in magnetic field (or changing flux) induces EMF.
• Step-up transformer: more turns on secondary → higher voltage, lower current.
• Step-down transformer: fewer turns on secondary → lower voltage, higher current.