Current, voltage and resistance
Connect the movement of charge with the energy supplied by a circuit.
What you will learn
- Explain current and potential difference.
- Use V=IR with consistent units.
Different quantities describe a circuit
Current is the rate at which charge passes a point. Potential difference is energy transferred per unit charge between two points. Resistance describes the ratio of potential difference across a component to the current through it.
An ammeter measures current in series. A voltmeter measures potential difference in parallel across the component. These arrangements reflect what each instrument is measuring.
Calculate and interpret resistance
Rearrange V=IR to isolate the required quantity before substituting. For an ohmic conductor at constant temperature, V is proportional to I and resistance remains constant. A filament lamp may heat up and change resistance, so proportionality is not universal.
Worked example
A resistor carries 0.30 A when the potential difference is 9.0 V. Find its resistance.
Show the worked solution
- Rearrange to R=V/I.
- Substitute 9.0/0.30.
- The result is measured in ohms.
Answer 30 Ω
Common mistakes
- Convert milliamps to amps before calculating.
- Current is not used up as it passes through a resistor.
What can you explain now?
What current flows through a 20 Ω resistor connected across 6 V?
Compare with the explanation
0.30 A
I=V/R=6/20=0.30.
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