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ELECTRICITY · CHAPTER 5 OF 13

Electric current

Current tells us how quickly electric charge is flowing past a point in a circuit—not simply how much charge is present.

By the end, you’ll be able to…

  • Describe electric current as the rate of flow of charge.
  • Explain what one ampere means.
  • Use and rearrange the equation I = Q ÷ t.
  • Calculate current, charge or time using the correct units.

Current is a rate

Imagine watching one point on a busy walkway. Counting how many people pass that point every second tells you the rate of flow. Electric current works in a similar way: it measures how much charge passes a point each second.

Electric current

The rate at which electric charge flows past a point in a circuit.

Measured in amperes (A)

Amperes are often shortened to amps

Circuit symbol A
INTERACTIVE SIMULATION

Explore electric current

Change the amount of charge passing the measurement point each second. Watch how the movement of charge and the ammeter reading change.

Select a number from 0 to 5. A greater charge flow each second produces a greater current.
Flow of charge
Ammeter 0.0 A 1.0 A 2.0 A 3.0 A 4.0 A 5.0 A
Measurement point
− − − − − − − −
Charge passing each second 0 C/s 1 C/s 2 C/s 3 C/s 4 C/s 5 C/s

Choose a current

No flow Faster flow
No flow means no current No charge passes the marker, so the current is zero.
1 coulomb per second = 1 ampere A small amount of charge passes each second, so the current is small.
2 coulombs per second = 2 amperes More charge passes each second, giving a larger current.
3 coulombs per second = 3 amperes More charge passes each second, giving a larger current.
4 coulombs per second = 4 amperes A large amount of charge passes each second, so the current is large.
5 coulombs per second = 5 amperes A large amount of charge passes each second, so the current is large.

CURRENT AND CHARGE
Current depends on charge flow and time

Electric current tells us how quickly charge flows through a circuit. A larger current means that more charge passes a point in the circuit each second.

The equation

Current = charge ÷ time

I = Q ÷ t

I= current in amperes (A)

Q=charge flow in coloumbs (C)

t=time in seconds (s)

Rearranging the equation

The relationship between current, charge and time can be written in three different ways. Choose the version that has the quantity you need to calculate on its own.

To calculate current:

I = Q ÷ t

A charge of 12 C passes a point in 4 seconds.

I = Q ÷ t

I = 12 ÷ 4

I = 3 A

To calculate charge:

Q = I × t

A current of 3 A flows for 4 seconds.

Q = I × t

Q = 3 × 4

Q = 12 C

To calculate time:

t = Q ÷ I

A charge of 12 C flows with a current of 3A.

t = Q ÷ I

t = 12 ÷ 3

t = 4 s

☆ One ampere has a precise meaning. A current of 1 A means that 1 C of charge passes a point every second.

Interactive practice

Which circuit has the greatest current?

Compare the charge and time for all three circuits. Work out which circuit you think has the greatest current before revealing the calculations.

Select Reveal calculation on each card to check its current.
A

Circuit A

Charge 5 C
Time 2 s
Current = charge ÷ time
5 C ÷ 2 s
= 2.5 A Greatest current
B

Circuit B

Charge 2 C
Time 5 s
Current = charge ÷ time
2 C ÷ 5 s
= 0.4 A
C

Circuit C

Charge 14 C
Time 10 s
Current = charge ÷ time
14 C ÷ 10 s
= 1.4 A
Remember

Circuit C has the greatest amount of charge, but Circuit A has the greatest current. Current depends on both the charge and the time taken.

Common mix-up- Current is not “used up”

A component transfers energy from the electrical system, but it does not consume the charge itself. Charge continues around a complete circuit. This is why we describe current as a flow of charge rather than a supply that disappears inside a bulb.

INTERACTIVE QUICK CHECK

Check your understanding

Answer all three questions about current, charge and time.

Select one answer for each question. You will get feedback straight away.
1 What does a current of 1 A mean?
2 A current of 2 A flows for 5 seconds. How much charge passes?
3 A charge of 18 C passes a point with a current of 3 A. How long does this take?
INTERACTIVE CHALLENGE

If twice as much charge passes a point in the same time, what happens to the current?

Think about the relationship between charge, time and current, then reveal the answer to check your thinking.

The current doubles because current is directly proportional to the amount of charge flowing when the time stays the same.

For example: 6 C ÷ 3 s = 2 A 12 C ÷ 3 s = 4 A

CHAPTER 5

Chapter summary

  • Electric current is the rate of flow of electric charge past a point in a circuit.
  • Current is measured in amperes (A) using an ammeter.
  • A current of 1 A means that 1 C of charge passes a point every second.
  • A greater amount of charge passing each second produces a greater current.
  • Current can be calculated using:

    I = Q ÷ t
  • The equation can be rearranged to calculate charge or time:

    Q = I × t    and    t = Q ÷ I
  • Electric charge is not used up as it moves around a complete circuit. Components transfer energy, but the charge continues through the circuit.

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