Explain that potential difference (voltage) is the energy transferred per…

The idea

Potential difference, often called voltage, measures how much energy each coulomb of charge transfers as it moves between two points in a circuit. Divide the energy transferred in joules by the charge that moved in coulombs and you obtain the potential difference in volts. A 12 V battery therefore transfers 12 J for every coulomb that passes through it, so 5 C moved transfers 60 J. Because the volt is defined as one joule per coulomb, the unit itself states the relationship: 1 V = 1 J/C. A larger potential difference means more energy is transferred per unit charge, which is why a 12 V supply drives more energy into each coulomb than a 1.5 V cell does.

What earns the marks

• States that potential difference is the energy transferred per unit charge passed.
• Identifies the volt as equivalent to one joule per coulomb.
• Uses the relationship that energy transferred equals charge multiplied by potential difference to compare two supplies.
• Explains that a larger potential difference transfers more energy for each coulomb moved.

Common mistakes and how to put them right

• Confuses potential difference with current.

  Put it right: Keep potential difference as energy per unit charge and current as charge per second.

• Treats the volt as a unit of energy.

  Put it right: State that the volt equals one joule per coulomb.

• Reverses the ratio and divides charge by energy.

  Put it right: Divide energy transferred by charge moved to obtain potential difference.

Exam tips

• Read the unit given in the question and convert to joules and coulombs before substituting.
• Check the answer by confirming that volts multiplied by coulombs gives joules.
• Eliminate options that describe current or power rather than energy per unit charge.