Circuits, Resistance and Components
Current is the rate of flow of electric charge, measured in amperes, and is the same everywhere in a series circuit but splits between the branches of a parallel circuit. Potential difference is the energy transferred per unit charge, measured in volts, and resistance, which opposes the flow of charge, is linked to the other two by potential difference = current x resistance.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Identify the circuit type first: in series there is one loop and one path for the current; in parallel the current has more than one path.
- Apply the series rules: current is the same at every point, potential difference is shared between the components in proportion to their resistance, and total resistance is the sum of the individual resistances.
- Apply the parallel rules: the potential difference across each branch equals the supply potential difference, the currents in the branches add up to the total current from the supply, and the total resistance is less than the smallest single resistance.
- Use potential difference = current x resistance, rearranging to current = potential difference / resistance or resistance = potential difference / current, and always work in volts, amperes and ohms.
- Interpret current-potential difference graphs by shape: a straight line through the origin means resistance is constant (an ohmic conductor); a curve that flattens as current rises means resistance is increasing, as in a filament lamp whose temperature rises so the ions vibrate more and impede the electrons; a graph with current in one direction only is a diode.
- Learn the two sensors as resistance changes: the resistance of a thermistor decreases as temperature increases, and the resistance of a light-dependent resistor decreases as light intensity increases, which is why they are used in thermostats and in automatic lighting circuits.
Worked example
A 12 V battery is connected in series with a 3 ohm resistor and a 5 ohm resistor. Calculate the total resistance, the current in the circuit, and the potential difference across the 5 ohm resistor.
- Total resistance in series is the sum: 3 + 5 = 8 ohms.
- Rearrange potential difference = current x resistance to give current = potential difference / resistance.
- Substitute for the whole circuit: current = 12 / 8 = 1.5 A.
- The current is the same everywhere in a series circuit, so 1.5 A flows through the 5 ohm resistor.
- Find the potential difference across it: potential difference = current x resistance = 1.5 x 5 = 7.5 V.
- Check: the potential difference across the 3 ohm resistor is 1.5 x 3 = 4.5 V, and 7.5 + 4.5 = 12 V, which matches the supply.
Practice questions
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Q1Define electric current.Show answer
Answer: The rate of flow of electric charge, measured in amperes.
Q2State the equation linking potential difference, current and resistance.Show answer
Answer: Potential difference = current x resistance.
Q3Two 4 ohm resistors are connected in series. Calculate the total resistance.Show answer
Answer: 4 + 4 = 8 ohms.
Q4What happens to the total resistance when a second resistor is added in parallel, and why?Show answer
Answer: It decreases, because there are more paths for the charge to flow through, so the total current for a given potential difference increases.
Q5Describe the current-potential difference graph for a filament lamp.Show answer
Answer: A curve that becomes less steep as the potential difference increases, because the filament gets hotter and its resistance increases.
Q6How does the resistance of a thermistor change as it gets hotter?Show answer
Answer: It decreases.
Q7In a parallel circuit, how do the branch currents relate to the total current?Show answer
Answer: The currents in the branches add up to the total current drawn from the supply.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Explain why the resistance of a filament lamp increases as the current through it increases.
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A light-dependent resistor is connected in series with a fixed resistor across a 6 V supply, and the output is taken across the fixed resistor. Explain how the output potential difference changes as night falls.
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Free printable worksheet
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This topic is chapter 11 of GCSE Physics Workbook, the whole course as one free printable PDF.
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