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Current, Charge and Potential Difference

Potential difference, power and current

  • 3 things to know
  • Unit AS 1

Overview

Equation relating potential difference to power and current and volt in terms of power and current.

What you need to know

Answer each one before you reveal it.

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Key term

State the equation relating potential difference to power and current, and define every symbol with its unit.

Numbers

A motor operating at 12 V draws a current of 2.5 A for 5.0 minutes. Calculate the electrical energy supplied.

Key term

Show that the watt is equivalent to the volt ampere.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    State the equation relating potential difference to power and current, and define every symbol with its unit.

    Model answer

    The equation is V = P/I, where V is the potential difference in volts, P the power in watts and I the current in amperes; rearranged it gives P = VI.

  2. 2

    Show how V = P/I follows from V = W/Q and I = Q/t.

    Model answer

    Power is energy per unit time, P = W/t, and substituting W = QV gives P = QV/t; since Q/t is the current I this becomes P = VI, so V = P/I.

  3. 3

    Define the volt in terms of power and current.

    Model answer

    One volt is one watt of power dissipated per ampere of current, so 1 V = 1 W A⁻¹.

  4. 4

    Calculate the power dissipated when a current of 3.0 A flows through a component with 230 V across it.

    Model answer

    The power is P = VI = 230 × 3.0 = 690 W, and 690 W ÷ 3.0 A returns the 230 V as a check.

  5. 5

    A lamp is rated 60 W at 230 V. Calculate the current it draws at its rated voltage.

    Model answer

    The current is I = P/V = 60 ÷ 230 = 0.26 A, and 0.261 A × 230 V gives back 60 W as a check.

  6. 6

    A motor operating at 12 V draws a current of 2.5 A for 5.0 minutes. Calculate the electrical energy supplied.

    Model answer

    The power is 12 × 2.5 = 30 W, so the energy is 30 × 300 = 9.0 × 10³ J, and 9000 J ÷ 300 s returns 30 W as a check.

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