Know the difference between direct current (d.c.) and alternating current (a.c.)
4.2.2 Electric Current – Direct vs Alternating
What is Electric Current?
Electric current is the flow of electric charge. Think of it like a stream of tiny water droplets (electrons) moving through a pipe (wire). The amount of charge that passes a point in one second is called the current, measured in amperes (A). Mathematically, $$I = \frac{Q}{t}$$ where I is current, Q is charge, and t is time.
Direct Current (d.c.) ⚡️
In d.c. the electrons move in one direction only, just like a one‑way street. - The voltage stays constant or changes very slowly. - Used in batteries, flashlights, and most small electronic devices. - Example: A 9 V battery powers a toy car. The electrons flow from the negative to the positive terminal, always in the same direction.
Alternating Current (a.c.) 🔄
In a.c. the electrons change direction periodically, like a pendulum swinging back and forth. - The voltage oscillates sinusoidally: $$V(t) = V_0 \sin(\omega t)$$ where V₀ is the peak voltage and ω is the angular frequency. - Used in household mains supply (230 V in most countries) and in power plants. - Example: The electricity that powers your TV and lights flows back and forth many times per second (50 or 60 Hz).
Key Differences
| Feature | Direct Current (d.c.) | Alternating Current (a.c.) |
|---|---|---|
| Direction of flow | One way | Reverses periodically |
| Typical source | Batteries, solar cells | Power plants, mains supply |
| Voltage behaviour | Constant or slowly varying | Oscillates sinusoidally |
| Common use | Portable electronics | Household appliances, long‑distance transmission |
Exam Tips 📚
- Remember the definition: Current is charge per unit time, $I = Q/t$.
- Use the analogy: d.c. = one‑way street, a.c. = traffic lights turning red/green.
- Key formulae: Ohm’s law $V = IR$ works for both, but note that $V$ in a.c. is often given as RMS value.
- Multiple choice: Look for words like “constant” (d.c.) or “oscillates” (a.c.).
- Diagram questions: Label the direction of electron flow for d.c. and the alternating arrows for a.c.
Revision
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