# 8.1–8.2 Batteries, Bulbs and Circuits

F.2 Integrated Science · Chapter 8 · Lesson 6

Trace the path, distinguish current from voltage, and connect meters correctly.

## Figure

The ammeter is in series with the bulb; the voltmeter is connected across the bulb.

## Key points

- **Complete circuit**: a source of electrical energy and a closed conducting path are needed for sustained current.
- **Conductors** allow electricity to pass through; metals and graphite are examples. **Insulators (絕緣體)** do not allow current to pass easily.
- **Dry cell**: converts chemical energy into electrical energy. A battery supplies energy; charges already exist in the circuit.
- **Series (串聯)**: one path; current is the same at every point. Removing a bulb opens the whole path.
- **Parallel (並聯)**: separate branches; removing one branch leaves the others complete.
- **Current**: the amount of electric charge passing a point each second; measured in amperes (A).
- **Voltage (電壓)**: the electrical energy transferred per unit charge between two points; measured in volts (V).
- **Directions**: in the external metal circuit, conventional current flows from + to −; electron drift is from − to +.
- **Cells in series**: voltages add when cells face the same direction. Cell size alone does not tell you its voltage.

## Must know

| Check | Correct connection or rule |
|---|---|
| Ammeter: current | In series; current enters the + terminal |
| Voltmeter: voltage | In parallel across the component; + to the higher-potential side |
| Current units | $1\,\mathrm{A}=1000\,\mathrm{mA}$ |
| Voltage units | $1\,\mathrm{kV}=1000\,\mathrm{V}$; $1\,\mathrm{V}=1000\,\mathrm{mV}$ |
| Meter range | Smallest range that safely includes the expected reading |
| Practical safety | Use low-voltage cells; open the switch before changing connections |

Never connect the cell terminals directly together or connect an ammeter directly across a cell.

## Common mistakes

- ✗ “The first series bulb uses up current.” ✓ Current is the same through both bulbs; bulbs transfer energy, without using up charge.
- ✗ “An open circuit has no battery voltage.” ✓ No sustained current flows, but a working battery still has voltage across its terminals.
- ✗ “Electrons flow from + to −.” ✓ Electron drift is from − to + in the external metal circuit; conventional current is opposite.
- ✗ “Put the ammeter across the cell.” ✓ Put it in series with the component; connecting it across the cell risks a short circuit.
- ✗ “A bigger cell must have higher voltage.” ✓ Check the label; cell size alone does not determine voltage.

## Quick check

1. What two conditions are needed for sustained current in a simple circuit?
2. One bulb is removed. What happens to the other bulbs in a series circuit and in separate parallel branches?
3. How do you connect an ammeter and a voltmeter to measure a bulb's current and voltage?
4. Convert $0.25\,\mathrm{A}$ to milliamperes and $1500\,\mathrm{mV}$ to volts.
5. Two $1.5\,\mathrm{V}$ cells are connected in series in the same direction. What is their total voltage?

## Answers

1. A source of electrical energy and a closed conducting path.
2. Series: all go out. Parallel: bulbs in the remaining complete branches stay lit.
3. Ammeter in series; voltmeter in parallel across the bulb. Observe +/− polarity.
4. $250\,\mathrm{mA}$; $1.5\,\mathrm{V}$.
5. $3.0\,\mathrm{V}$.

## Did you know?

Pencil 'lead' contains graphite, a form of carbon that can conduct electricity.