Current (Oxford AQA IGCSE Combined Science Double Award)
Revision Note
Current
Electric current is defined as the rate of flow of electric charge
In other words, the size of an electric current is the amount of charge passing through a component per second
Current is measured in amperes, which has the symbol, A
A current of 2 A means that 2 coulombs of charge passes through a point in a circuit every second
Current flows from the positive terminal to the negative terminal of a cell
Note that this is the opposite direction to electron flow
Flow of charge
The following equation links charge flow, current and time:
Where the symbols:
Q stands for charge flow (measured in coulombs, C)
I stands for current (measured in amperes, A)
t stands for time (measured in seconds, s)
You can rearrange this equation with the help of the formula triangle shown below
Cover up the quantity you need to find
The position of the other quantities tells you the operation you need to perform to find them
Charge flow formula triangle
Worked Example
Calculate the current through a lamp when a charge of 4 C passes through it in 500 s.
Answer:
Step 1: Write down the known quantities
Charge, Q = 4 C
Time, t = 500 s
Step 2: Write down the relevant equation
Step 3: Rearrange for current, I
Step 4: Substitute the known quantities into the equation
Examiner Tip
Electric currents in everyday circuits tend to be quite small, so it's common for examiners to throw in a unit prefix like 'm' next to quantities of current, e.g. 10 mA (10 milliamperes). Be on the lookout for these prefixes and know you can convert them into standard units, so 10 mA = 10 × 10-3 A.
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