Direct & Alternating Current (Oxford AQA IGCSE Physics)

Revision Note

Direct Current

  • Direct current (dc) is defined as

A current that always passes in the same direction

  • Electric cells, or batteries, produce direct current (dc)

Examiner Tip

A direct current can have a constant value or a changing value, as long as it is in the same direction

Alternating Current

  • An alternating current (ac) is defined as

A current that is repeatedly changing direction

  • On a graph of current against time:

    • dc has either a positive or negative value

    • ac alternates between positive and negative values

Graphs of ac and dc

The dc graph is a horizontal line with a positive current value, the ac graph is a sinusoidal pattern
Two graphs showing the variation of current with time for alternating current and direct current
  • The frequency of an alternating current is the number of times the current changes direction back and forth each second

  • The period of an alternating current is the time taken for one complete cycle of ac

  • An alternating potential difference produces an alternating current

    • The peak potential difference of this alternating source is the maximum voltage above 0 V that the source reaches as it alternates

Worked Example

A source of potential difference is connected to a circuit with a fixed resistor.

The diagram below shows how the potential difference across a source varies with time. This pattern is known as a square wave-form.

A graph of voltage against time. The graph begins as a horizontal line at minus V0. It then becomes a vertical upwards line up to V0. The line then becomes horizontal for a short time before dropping down vertically to minus V0 before becoming horizontal again. This pattern continues.

Determine whether the current through the resistor is direct or alternating.

Explain your answer.

Answer:

Step 1: Describe the potential difference across the source

  • The potential difference from the source alternates direction repeatedly from positive to negative

Step 2: Relate the potential difference and current through the resistor

  • Potential difference is proportional to current

  • Therefore the current through the resistor is also alternating

Examiner Tip

While alternating current usually has a sinusoidal pattern, it can come in a variety of patterns. Remember the definitions, if it changes from positive to negative repeatedly, it's alternating, if not then it's direct.

Mains Electricity

  • Mains electricity, which is supplied to households, is an alternating source of potential difference which produces an alternating current in circuits

  • It has a defined frequency and voltage

    • In the UK, for example, these are set as 50 Hz and 230 V, respectively

Examiner Tip

Knowledge of specific values for mains electricity is not required for your exam. However, you must understand that the set values for a country or region will be the same for every household in that region.

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