Experiment 1: Charging objects and the electroscope
Some objects get electric charge when you rub them. A plastic pen rubbed on dry hair can pick up small pieces of paper.
How do we know an object is charged? We use an electroscope. It has a metal knob on top, a metal rod, and two very thin metal-foil leaves. The leaves hang inside a glass jar so that wind does not move them.
Bring a charged rod near the knob. Charge moves along the metal. Both leaves get the same kind of charge, so they push each other apart. The leaves spread. The more charge, the wider they spread. If the rod goes away, the charge goes back and the leaves fall.
An electroscope tells you if something is charged and roughly how much. It does not tell you which kind (+ or −) without extra steps.
Experiment 2: Forces between charged bodies
Hang two light balls from threads. Charge them by touching them with charged rods.
- Same charges (both + or both −): the balls swing apart. This is repulsion.
- Opposite charges (+ and −): the balls swing together. This is attraction.
The closer the balls are, the stronger the push or pull. This simple test is how scientists first showed that there are two kinds of charge.
Experiment 3: Does a material conduct?
Make a circuit with a cell, a bulb and a small gap. Put the material in the gap so it touches both ends.
- Bulb glows: current flows, so the material is a conductor (copper, iron, pencil lead).
- Bulb stays off: no current, so the material is an insulator (rubber, plastic, wood).
Pencil lead is graphite. It surprises many students, because it is not a metal, but it still conducts.
If the bulb is very dim, the material conducts a little. Use a bright bulb and a fresh cell for a fair test.
Experiment 4: Build a circuit from a diagram and read the meters
In a circuit diagram, symbols show the parts and lines show the wires. Build it in the same order as the diagram: cell, then switch, then the resistor, and back to the cell.
- Ammeter (measures current in amperes, A): join it in series, in the same line as the resistor, so all the current passes through it.
- Voltmeter (measures voltage in volts, V): join it in parallel, across the two ends of the resistor.
Connect the positive (+) terminal of each meter towards the positive end of the cell. Before you switch on, check the scale. Read the pointer straight from the front, not from the side. Open the switch between readings so the wire does not heat up.
Experiment 5: Find resistance from voltage and current
Resistance tells how hard it is for current to flow. Use Ohm's law, R = V ÷ I.
- Join one cell. Read V and I.
- Add a second cell, then a third. Read V and I each time.
- For each row, work out V ÷ I.
- Take the average. That is the resistance in ohms (Ω).
Example table for one resistor:
| Cells | V (V) | I (A) | V ÷ I (Ω) |
|---|---|---|---|
| 1 | 1.5 | 0.30 | 5 |
| 2 | 3.0 | 0.60 | 5 |
| 3 | 4.5 | 0.90 | 5 |
V and I both double together, but V ÷ I does not change. This is why it is the resistance of the part. Draw a graph of V (up) against I (along): you get a straight line through the origin, and its slope is R.
Key formulas and definitions
- R = V ÷ I (ohm = volt ÷ ampere)
- V = I × R, I = V ÷ R
- Ammeter: in series. Voltmeter: in parallel.
- Same charges repel. Opposite charges attract.
- Conductor: bulb glows. Insulator: bulb stays off.
Worked examples
1. A voltmeter across a resistor reads 6 V and the ammeter reads 2 A. Find the resistance.
R = V ÷ I = 6 ÷ 2 = 3 Ω.
2. An experiment gives these readings: (1.5 V, 0.25 A), (3.0 V, 0.50 A), (4.5 V, 0.75 A). Find the resistance.
V ÷ I = 1.5 ÷ 0.25 = 6, 3.0 ÷ 0.50 = 6, 4.5 ÷ 0.75 = 6. Every row gives 6, so R = 6 Ω.
3. The ammeter shows 150 mA when the voltmeter shows 3 V. What is R?
Change mA to A first: 150 mA = 0.15 A. R = 3 ÷ 0.15 = 20 Ω.
4. Two charged balls move toward each other. What can you say about their charges?
They attract, so they have opposite charges (one + and one −).
5. A student connects the voltmeter in series with the resistor. What will she see?
The voltmeter has a very high resistance, so almost no current flows. The ammeter reads nearly 0 and the bulb does not glow. A voltmeter must be in parallel.
Common mistakes
- Putting the voltmeter in series. It must be across (in parallel with) the part you measure.
- Reading milliamps as amps. Change 250 mA to 0.25 A before you divide.
- Thinking spread leaves tell you the sign of the charge. They only show that the leaves have the same charge.
- Using only one reading. Take at least three and average V ÷ I, so one bad reading does not spoil the answer.