Electricity is the flow of electric charge. Three linked quantities describe every circuit: current, voltage and resistance.
Basic Quantities
| Quantity | Symbol | Unit | Meaning |
|---|---|---|---|
| Current | I | ampere (A) | Rate of flow of charge |
| Potential difference (voltage) | V | volt (V) | Energy per unit charge |
| Resistance | R | ohm (Ω) | Opposition to current flow |
Ohm’s Law
Ohm’s law states that the current through a conductor is directly proportional to the potential difference across it, provided temperature remains constant.
Formula: V = IR (and therefore I = V/R, R = V/I).
Series and Parallel Circuits
- Series: Rtotal = R₁ + R₂ + R₃ … (current is the same through all components; voltages add up).
- Parallel: 1/Rtotal = 1/R₁ + 1/R₂ + 1/R₃ … (voltage is the same across each branch; currents add up).
Electrical Energy and Power
- Power: P = VI (also P = I²R and P = V²/R). Unit: watt (W).
- Energy: E = P × t = VIt. Unit: joule (J); kilowatt-hour (kWh) for domestic use.
Worked Example
A resistor of 5 Ω carries a current of 2 A. Find (a) the potential difference and (b) the power dissipated.
- V = IR = 2 × 5 = 10 V
- P = VI = 10 × 2 = 20 W
Summary
Ohm’s law (V = IR) links voltage, current and resistance. Resistors add directly in series but add reciprocally in parallel. Electrical power is P = VI, and energy is E = Pt.
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