2.1.1 Charge & Current
Current
- 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
- Current flows from the positive terminal to the negative terminal of a cell
Charge flows from the positive terminal to the negative terminal
Charge
- 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 negatively charged electrons
In metal wires, the current is a flow of negatively charged electrons. This image shows the electrons flowing through a lattice of metal ions
Calculating Electric Charge
- The charge, current and time are related by the equation:
- Where:
- Q = charge measured in Coulombs (C)
- I = current measure in amps (A)
- t = time measured in seconds (s)
- This equation can be rearranged with the help of the following formula triangle:
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 sC When a charge of 8 C passes in 100 s
D When a charge of 1 C passes in 8 sANSWER: B
Step 1: Write out the equation relating current, charge and time
Q = It
- This can be rearranged to make current I the subject of the equation:
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
Exam 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
Measuring Current
- Current 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
- Ammeters measure the amount of charge passing through them per unit time, so the ammeter has to be in series so that all the charge flows through it
An ammeter can be used to measure the current around a circuit
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