Current, Potential Difference, Resistance and Electrical Power
Electric current is the rate of flow of electric charge, measured in amperes (A), and is driven around a circuit by a potential difference (voltage), measured in volts (V), while resistance, measured in ohms, opposes the flow of current. These three quantities are linked by potential difference = current x resistance (V = I x R). Series and parallel circuit rules, current-potential difference graphs for resistors, filament lamps and diodes, the required practical investigating how resistance depends on the length of a wire, and calculating charge and electrical power go beyond the core GCSE course by requiring quantitative use of the fact that resistance is directly proportional to wire length at constant cross-sectional area and temperature.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Identify whether the circuit is series or parallel: in series, the current is the same at every point and potential differences across components add up to the supply potential difference; in parallel, the potential difference is the same across every branch and the currents in the branches add up to the total current.
- Choose the correct equation for the quantity required: charge = current x time (Q = I x t), potential difference = current x resistance (V = I x R), or power = current x potential difference (P = I x V), noting that power can also be written as P = I^2 x R.
- Substitute the given values, keeping current in amps, potential difference in volts, resistance in ohms, charge in coulombs, time in seconds and power in watts.
- For a current-potential difference graph, read the gradient at a point to compare resistance: a resistor at constant temperature gives a straight line through the origin (constant resistance, obeys Ohm's law); a filament lamp curves as it heats up and its resistance increases; a diode allows current in only one direction and has very high resistance in the reverse direction.
- For the required practical on resistance and wire length, keep the potential difference low and switch the circuit off between readings to avoid heating the wire, since a change in temperature would change the resistance and make the results unreliable.
- Plot resistance (y-axis) against length (x-axis) for the practical: a straight line through the origin shows resistance is directly proportional to length, and the gradient gives the resistance per unit length of the wire.
Worked example
In the required practical, a student measures the resistance of a length of resistance wire connected in a circuit with an ammeter and a voltmeter. At a wire length of 0.80 m, the ammeter reads 0.40 A and the voltmeter reads 2.0 V. (a) Calculate the resistance of this length of wire. (b) Using the fact that resistance is directly proportional to length, predict the resistance of a 1.2 m length of the same wire.
- Write down the equation linking potential difference, current and resistance: resistance = potential difference / current.
- Substitute the values for the 0.80 m length: resistance = 2.0 / 0.40.
- Calculate: 2.0 / 0.40 = 5.0 ohm.
- Since resistance is directly proportional to length, resistance per metre = 5.0 / 0.80 = 6.25 ohm per metre.
- Predict the resistance of the 1.2 m length: resistance = 6.25 x 1.2.
- State the final answer with its unit: resistance = 7.5 ohm.
Practice questions
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Q1State the equation linking potential difference, current and resistance.Show answer
Answer: Potential difference = current x resistance (V = I x R).
Q2In a series circuit, the potential difference across the supply is 12 V and across one resistor is 5.0 V. Calculate the potential difference across the second resistor in the same series circuit.Show answer
Answer: 7.0 V (12 - 5.0), since potential differences in series add up to the supply potential difference.
Q3A current of 3.0 A flows through a resistor for 20 s. Calculate the charge that flows. Use Q = I x t.Show answer
Answer: 60 C (3.0 x 20).
Q4State how the resistance of a filament lamp changes as it gets hotter.Show answer
Answer: The resistance increases as the lamp gets hotter.
Q5State the resistance of a diode when it is connected in reverse bias.Show answer
Answer: Very high (almost infinite), so almost no current flows.
Q6A resistor of resistance 12 ohm has a current of 0.50 A flowing through it. Calculate the power dissipated. Use P = I^2 x R.Show answer
Answer: 3.0 W (0.50^2 x 12 = 0.25 x 12).
Q7In the resistance-length practical, state why the circuit should be switched off between readings.Show answer
Answer: To prevent the wire heating up, since a change in temperature would change its resistance and make the results unreliable.
Exam-style questions
Written in the style of a IGCSE Science exam paper, with a full mark scheme.
A wire of length 0.60 m has a resistance of 4.5 ohm. Using the fact that resistance is directly proportional to length at constant temperature, calculate the resistance of a 1.5 m length of the same wire.
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A student investigates how the resistance of a wire depends on its length, using a circuit with a variable-length wire, an ammeter, a voltmeter, a low-voltage power supply and a switch. (a) Describe how the student should carry out the investigation to obtain a set of results relating resistance to length. (4 marks) (b) State the shape of the graph of resistance (y-axis) against length (x-axis) that the student should expect, and explain what the gradient of this graph represents. (2 marks)
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Free printable worksheet
Want more practice on paper? Download the current, potential difference, resistance and electrical power worksheet pack - 6 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 26 of IGCSE Science Workbook, the whole course as one free printable PDF.
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