Direct & Alternating Current (Oxford AQA IGCSE Combined Science Double Award)
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 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.
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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