Explain that an electric current as the rate of flow of charge and the…

The idea

Electric current is the rate of flow of charge: the charge passing a point each second, measured in coulombs per second, which is the ampere. In a metal conductor the charges that move are electrons, so the current is a flow of electrons through the metal lattice. The electrons carry negative charge and drift in the direction opposite to the conventional current, which is defined as the direction of positive charge flow. To explain a reading, link the size of the current to the quantity of charge passing per unit time: a larger current means more charge passes each second. Use the relationship current equals charge divided by time, I = Q ÷ t, with charge in coulombs and time in seconds to give current in amperes.

What earns the marks

• Defines current as the rate of flow of charge, that is charge passing a point per unit time.
• States that in metals the moving charges are electrons, so the current is a flow of electrons.
• Links a larger current to a greater quantity of charge passing each second.
• Applies the relationship current equals charge divided by time, with charge in coulombs and time in seconds giving current in amperes.

Common mistakes and how to put them right

• Describes current as a flow of energy rather than a rate of flow of charge.

  Put it right: Define it as charge per unit time.

• Claims that positive charges flow through a metal.

  Put it right: State that electrons flow in a metal.

• Treats current as the total charge alone.

  Put it right: Divide the charge by the time to obtain the rate.

Exam tips

• Include the words 'rate of flow of charge' when defining current, since the rate is the key idea.
• Name electrons explicitly when describing conduction in a metal.
• Check units: coulombs divided by seconds gives amperes, so keep charge in C and time in s.