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Charge flow and electrical power Charge and current4.1 Energy, power and resistance4.2 Power in circuits4.2.5
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Power in circuits

OCR A AS Physics · 4.2.5

Explore power in circuits within energy, power and resistance. Potential difference measures energy transferred per unit charge. Combine it with current to calculate power, and investigate how components respond to changing voltage.

Specification outcomes (3)
  • 4.2.5(a) Use and apply power equations P = IV, P = I²R, and P = V² / R.
  • 4.2.5(b) Use energy transfer equation W = VIt.
  • 4.2.5(c) Recognise the kilowatt-hour (kWh) as a unit of energy and calculate cost of electrical energy.
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H156 v2.0 · May 2026

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A wearable warning indicator is connected to a supply that maintains 3.0 V across the indicator whenever it is on. During each 0.20 s pulse, a charge counter measures 0.012 C passing through it. Current is constant during a pulse and zero between pulses. The indicator produces one identical pulse every 1.0 s; it produces 600 pulses in 10 minutes. Ignore any current used by other electronics. (a) Calculate the current while the indicator is on. [2] (b) Calculate its electrical power while it is on. [2] (c) Calculate the electrical energy transferred during one pulse using charge and potential difference. [2] (d) Calculate the total electrical energy transferred in 10 minutes and the average power over that time. [3] (e) A pupil says the current leaving the indicator must be smaller because the indicator has used up some electrons. Explain why this statement is wrong, distinguishing charge transfer from energy transfer. [2]

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