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Current (Edexcel GCSE Physics)
Revision Note
Current & Charge
- Electric current is defined as the rate of flow charge
- In other words, the size of an electric current is the amount of charge passing through a component per second
- The current, charge and time are related by the equation:
- Where:
- I = current, measured in amperes (or amps, A)
- Q = charge, measured in coulombs (C)
- t = time, measured in seconds (s)
- The equation for current can be rearranged using the formula triangle below:
Charge, current, time formula triangle
- The wires in an electric circuit are made of metal, because metal is a good conductor of electric current
- In the wires, the current is a flow of electrons
In metal wires, the current is a flow of electrons. This image shows the electrons flowing through a lattice of metal ions
Worked example
When will 8 mA of current pass through an electrical circuit?
A. When 1 J of energy is used by 1 C of charge
B. When a charge of 4 C passes in 500 s
C. When a charge of 8 C passes in 100 s
D. When a charge of 1 C passes in 8 s
ANSWER: B
Step 1: Write out the equation relating current, charge and time
Step 2: Rule out any obviously incorrect options
-
- Option A does not mention time, so can be ruled out
Step 3: Try the rest of the options by applying the equation to determine the correct answer
-
- Consider option B:
I = 4 ÷ 500 = 8 × 10–3 = 8 mA
-
- Consider option C:
I = 8 ÷ 100 = 80 × 10–3 = 80 mA
-
- Consider option D:
I = 1 ÷ 8 = 125 × 10–3 = 125 mA
-
- Therefore, the correct answer is B
Examiner Tip
Electric currents in everyday circuits tend to be quite small, so it's really common for examiners to throw in a unit prefix like 'm' next to quantities of current, e.g. 10 mA (10 milliamperes). Make sure that you are on the lookout for these prefixes and that you can convert them into standard units, so 10 mA = 10 × 10-3 A.
Ammeters
- The amount of current flowing through a component is measured using an ammeter
- Ammeters should always be connected in series with the part of the circuit you wish to measure the current through
An ammeter can be used to measure the current around a circuit and always connected in series
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