Short on time? This page is the recap. For explanations, Figures 1–3 and the worked examples, use the full study guide.
Recap
- A circuit is made of electrical loops containing elements: wires, batteries, resistors, bulbs, capacitors, switches, ammeters and voltmeters.
- A closed loop is a complete path through which charges could flow.
- One element can belong to several loops. Count loops without the battery too.
- Schematics show connections only. Wires are ideal, so length and shape do not matter. Crossing wires without a dot are not connected.
- Schematics use conventional current: out of the long (+) line of the battery, back into the short (−) line.
- Arrangement matters. The same parts wired in one loop or in separate branches behave differently.
Key definitions
| Term | Meaning | What happens |
|---|---|---|
| Closed circuit | complete loop with a source | charges could flow |
| Open circuit | a break anywhere in the loop | no current anywhere in that loop |
| Short circuit | path with no change in potential, connected across an element | a bypassed bulb or resistor has zero potential difference and carries no current |
| Variable element | normal symbol with a diagonal arrow through it | its value can be adjusted (for example a dimmer) |
| Meter | How it is connected | Ideal behaviour |
|---|---|---|
| Ammeter (circle with A) | in the loop: the current passes through it | acts like a plain wire |
| Voltmeter (circle with V) | across the element: one lead on each end | lets no charge through |
Assumptions behind the answers
- Wires and closed switches are ideal: no change in potential along them.
- Meters are ideal unless the question says otherwise.
- Schematics show conventional current unless told otherwise.
Mistakes to avoid
- Saying current reaches the gap and stops. An open loop has no current anywhere.
- Letting a shorted bulb share the current. It has no potential difference, so no current.
- Forgetting loops without the battery when counting loops.
- Putting an ammeter across an element. That shorts it.
- Putting a voltmeter in the loop. That opens the circuit.
- Reading meaning into wire length or position on a schematic.
Quick self-check
- A switch in a single loop with a battery and two bulbs is opened. Which bulbs carry current? (Neither)
- A battery is connected to two bulbs, each in its own branch. How many closed loops are there? (3: battery with each bulb, and the two bulbs together)
- A plain wire is connected across one of two bulbs in a single loop. What happens to that bulb? (It goes out: short circuit, no potential difference across it)
Next: practice questions.