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Charge flow is the total amount of electrical charge that passes through a cross-section of a conductor, measured in coulombs (C). Free GCSE Physics notes.
Electrical Work and Energy Calculations (Part 2/2) is part of Energy transfers in GCSE AQA Higher Physics.
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Charge flow is the total amount of electrical charge that passes through a cross-section of a conductor, measured in coulombs (C). Free GCSE Physics notes.
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Use this lesson to stay inside Energy transfers while connecting the detail back to the wider Electricity topic.
Read the core explanations from Electrical Work and Energy Calculations (Part 2/2) before testing yourself from memory.
When you switch on a kettle, electrical energy flows from the mains supply and transforms into heat energy to boil water. But how much energy actually transfers? This is where electrical work calculations become crucial - they tell us exactly how much energy moves through a circuit and gets converted to other forms.
Understanding energy transfers in circuits helps you predict heating effects, calculate electricity bills, and design efficient electrical systems. The key insight is that electrical work depends on three factors working together: voltage (the push), current (the flow), and time (how long it flows).
These equations are connected through Ohm's Law (V = IR). The choice depends on which quantities you're given in the question. Both formulas give the same answer - they're just different routes to the same destination.
Power connects directly to work done through the relationship P = W/t. This means work done equals power multiplied by time: W = Pt. This is why your electricity metre measures energy in kilowatt-hours (kWh) - it's literally power × time.
Worked example: Calculating Work Done in a Heating Element
A 3kW electric heater operates at 230V for 2 hours. Calculate the work done and the current flowing. [6 marks]
Answer: Work done = 21.6 MJ, Current = 13.0 A
Notice how we used two different approaches to reach the same answer. This cross-checking technique is valuable in exams - if your answers don't match, you know to look for an error.
Worked example: Energy Transfer in a Car Starter Motor
A car starter motor draws 80A from a 12V battery for 3 seconds. Calculate the energy transferred by the battery. [3 marks]
Answer: Work done = 2880 J or 2.88 kJ
Power tells us how quickly energy transfers in a circuit. A 100W bulb transfers 100J of energy every second, while a 2000W kettle transfers 2000J every second. The relationship P = W/t connects power to work done over time.
This explains why high-power devices cost more to run. Your electricity bill charges for energy used (measured in kWh), which equals power × time. A 2kW heater running for 5 hours uses 10kWh of energy, costing about £3 at typical UK rates.
Circuit devices convert electrical energy into other forms with different efficiencies. LED bulbs convert about 80% of electrical energy to light, while old filament bulbs only convert 5% to light (the rest becomes waste heat).
Energy calculation questions are mark-rich in GCSE physics, but students often lose marks through careless errors rather than conceptual misunderstanding. The most common mistakes involve unit conversions and formula selection.
Examiners frequently test whether you can distinguish between energy (measured in joules) and power (measured in watts). Remember: energy is the total amount transferred, power is the rate of transfer. A 60W bulb uses 60J every second.
In 6-mark calculation questions, show every step clearly. Mark schemes typically award: 1 mark for correct formula, 1 mark for substitution with units, 1 mark for numerical answer, 1 mark for correct units in final answer, plus additional marks for method in multi-step problems.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Charge flow is the total amount of electrical charge that passes through a cross-section of a conductor, measured in coulombs (C). It is related to current by Q = It.
Electrical work done is the energy transferred by an electrical current flowing through a component, measured in joules (J). It represents the conversion of electrical energy into other forms such as heat, light or kinetic energy.
Energy transferred can be calculated with W = VIt, using voltage × current × time. A car starter motor drawing 80A from a 12V battery for 3 seconds transfers 12 × 80 × 3 = 2880 J, or 2.88 kJ.
Work done by an electric heater is found with W = Pt after converting to SI units. A 3kW heater operating for 2 hours (7200s) does 3000 × 7200 = 21,600,000 J, or 21.6 MJ; at 230V its current is 3000 ÷ 230 = 13.0A.
Energy is the total amount transferred, measured in joules, while power is the rate of transfer, measured in watts. A 60W bulb uses 60J every second, and a 2000W kettle transfers 2000J every second.
Electricity bills charge for energy used, measured in kilowatt-hours (kWh), which equals power × time. A 2kW heater running for 5 hours uses 10kWh of energy, costing about £3 at typical UK rates.
A 3 kW heater at 230 V draws a current of about 13.0 A. Rearranging P = VI gives I = P ÷ V = 3000 W ÷ 230 V = 13.0 A.
A car starter motor that draws 80 A from a 12 V battery for 3 seconds transfers 2880 J, or 2.88 kJ. Using W = VIt, W = 12 × 80 × 3 = 2880 J.
W = VIt gives the electrical work done, in joules, from potential difference V in volts, current I in amps and time t in seconds. It comes from E = QV with Q = It, and gives the same answer as W = Pt.
An electrical energy calculation can be checked by using a second method. For a 3 kW heater at 230 V for 7200 s, W = Pt and W = VIt both give 21,600,000 J; if the two answers do not match, there is an error.
A 2 kW heater running for 5 hours uses 2 × 5 = 10 kWh of energy, which costs about £3 at typical UK rates. Electricity bills charge for energy used in kWh, which equals power × time.
P = W/t means power equals work done ÷ time, so power is how quickly energy transfers. Rearranged, work done equals power multiplied by time, W = Pt, which is why electricity meters measure energy in kilowatt-hours.
A 60 W bulb uses 60 J of energy every second, because power is the rate of energy transfer. In the same way, a 100 W bulb transfers 100 J each second and a 2000 W kettle 2000 J each second.
A 3 kW heater used for 2 hours transfers 21,600,000 J, or 21.6 MJ. Converting to SI units gives 3000 W and 7200 s, so W = Pt = 3000 × 7200 = 21,600,000 J.