1The push
Potential difference (p.d.), or voltage, is the push from the cell that drives the current. It is measured in volts (V).
potential differencevolt
Lesson 6.3 notes
Potential difference and resistance
Potential difference measured in volts, voltmeters, cell and lamp ratings, and resistance in ohms.
2Measuring p.d.
A voltmeter measures p.d. It is connected across a component, in parallel, so it can measure the p.d. between the component's two ends.
voltmeter
3Cells add up
Cells joined in series make a battery. Their p.d.s add up: three 1.5 V cells give 4.5 V, so the lamp glows more brightly.
battery
4Ratings
Cells and lamps are labelled with a voltage rating. A lamp given too little p.d. glows dimly; too much, and it glows brightly, then blows.
rating
5Resistance
Resistance is how hard it is for current to flow through something. It is measured in ohms (Ω). For the same p.d., more resistance means less current.
resistanceohm
6Working out resistance
resistance (Ω) = p.d. (V) ÷ current (A). A resistor with 6 V across it and 0.3 A through it has a resistance of 6 ÷ 0.3 = 20 Ω.
7Conductors and insulators
Conductors have a low resistance. Insulators have such a huge resistance that almost no current flows, even with a big p.d.
Explore: Resistance lab
Choose a component and how many cells to use. Read the voltmeter and the ammeter, and see the resistance worked out.
Common mistake: A voltmeter goes across a component, not in the loop
An ammeter goes in the loop (in series) because the current flows through it. A voltmeter goes across a component (in parallel) because it measures the p.d. between the two ends of the component.
Remember
- Potential difference (p.d.) is the push from the cell, measured in volts (V) with a voltmeter connected across a component.
- In series, the p.d.s of the cells add up, and the p.d.s across the components add up to the battery's p.d.
- Resistance, in ohms (Ω), is how hard it is for current to flow: resistance = p.d. ÷ current.
Going further at GCSE
GCSE: Voltage and meters → GCSE: Resistance → GCSE: Investigate resistance →