define the potential difference across a component as the energy transferred per unit charge
Potential Difference and Power
What is Potential Difference?
The potential difference (voltage) across a component is the amount of energy transferred to each unit of charge that passes through it. $$V=\frac{W}{Q}$$ where $W$ is the work (energy) in joules and $Q$ is the charge in coulombs.
Water‑flow analogy: Think of a water pipe. The pressure difference between two points pushes water through. Similarly, a voltage difference pushes electric charge through a circuit.
⚡️ Tip for exams: When you see a question about energy per unit charge, remember the formula $V=W/Q$ and that the units are volts (V).
Energy Transfer in a Circuit
If a charge $Q$ moves through a potential difference $V$, the energy transferred is $$W = Q \times V$$ For example, moving 0.02 C through a 12 V battery transfers $$W = 0.02 \times 12 = 0.24\;\text{J}$$
Practical example: A 9 V battery powering a 3 W LED. The LED draws $$I = \frac{P}{V} = \frac{3}{9} = 0.33\;\text{A}$$ and each coulomb of charge carries 9 J of energy.
Power in Electrical Circuits
Power is the rate at which energy is used or transferred. In terms of voltage and current: $$P = V \times I$$ where $P$ is power in watts, $V$ is voltage, and $I$ is current.
Water‑flow analogy: Power is like the volume of water flowing per second through a pipe. More pressure (voltage) or more flow (current) gives higher power.
📝 Exam tip: If a problem gives power and voltage, find current with $I=P/V$. If it gives current and voltage, find power with $P=VI$.
Units & Quick Reference
| Quantity | Symbol | SI Unit | Example |
|---|---|---|---|
| Energy | $W$ | Joule (J) | 0.24 J (example above) |
| Charge | $Q$ | Coulomb (C) | 0.02 C (example above) |
| Voltage | $V$ | Volt (V) | 12 V (example above) |
| Current | $I$ | Ampere (A) | 0.33 A (example above) |
| Power | $P$ | Watt (W) | 3 W (example above) |
Exam Strategy Checklist
- Identify what is given: voltage, current, power, or energy.
- Choose the correct formula: $V=W/Q$, $P=VI$, or $W=QI$.
- Check units before calculating.
- Show all steps clearly; teachers look for the correct method.
- Use the water‑flow analogy if you need a quick mental model.
Revision
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