Electricity & Magnetism
Electric circuits, Ohm's Law, electromagnetism and motors
Electric current (I) is the flow of charge. Measured in amperes (A). Voltage (V) = energy per unit charge, measured in volts (V). Resistance (R) = opposition to current, measured in ohms (Ω).
Ohm's Law: V = IR. If you know any two, you can find the third.
Series circuits: Same current everywhere. Voltages add up. Total resistance = R1 + R2 + ...
Parallel circuits: Voltage same across branches. Currents add up. 1/Rtotal = 1/R1 + 1/R2 + ...
Power: P = IV = I²R = V²/R. Measured in watts (W). **Energy** = P × t (joules).
Magnetism: Like poles repel, unlike attract. Magnetic field lines go from N to S.
Electromagnets: Coil of wire + current → magnetic field. Stronger with: more turns, more current, iron core.
Motor effect: Current in magnetic field → force (Fleming's Left-Hand Rule: thuMb = Motion, First = Field, seCond = Current).
Electromagnetic induction: Moving wire in field → induced voltage (generator effect). Basis of all power generation.
Transformers: Vp/Vs = Np/Ns. Step-up: more secondary turns → higher voltage.
Key Points to Remember
- 1Ohm's Law: V = IR
- 2Series: current same; Parallel: voltage same
- 3Power P = IV; Energy = Pt
- 4Transformer: Vp/Vs = Np/Ns
Pakistan Example
Load Shedding and WAPDA — Electricity in Pakistan
Pakistan's National Grid uses step-up transformers at Tarbela and Mangla dams to transmit at 500 kV (reduces energy lost as heat — since P_loss = I²R, higher voltage means lower current). Near homes, step-down transformers bring it to 220V. Every ceiling fan uses the motor effect — AKU Physics examiners love transformer and motor questions.