Ampere

The idea

Current is measured in amperes (A): one ampere is a flow of one coulomb of charge per second, 1 A = 1 C/1 s; small currents are expressed in milliampere (10⁻³ A) or microampere (10⁻⁶ A).

A big unit and a small unit

A bulb in a torch draws a small current, a heater draws a big one. We need a unit that works for both.

If 1 C of charge crosses a wire every second, what is the current?

• 1 ampere
• 1 volt
• 1 coulomb per ampere

Coulombs each second

Slide the charge each second. The boxes are seconds and the squares are coulombs.

The ampere and its small parts

When 1 C passed every second the current was 1 A. For very small currents, the milli- and micro- units are handy.

Current is measured in amperes (A). One ampere is a flow of one coulomb of charge each second. Small currents are expressed in milliampere (1 mA = 10⁻³ A) or microampere (1 µA = 10⁻⁶ A).

Notes

Current is measured in amperes: 1 A is 1 C of charge flowing each second. Small currents use mA (10⁻³ A) and µA (10⁻⁶ A).

Check yourself

A current of 2 A flows for 5 s. How many coulombs of charge cross a cross-section in that time?

Answer: 10 C

Each second 2 C cross, so in 5 s the charge is 2 C × 5 = 10 C.

Which current is the smallest?

A current is 0.005 A. How many milliamperes (mA) is that?

Answer: 5 mA

1 mA = 10⁻³ A = 0.001 A, so 0.005 A is 5 mA.

What does a current of 1 A mean?

• 1 mA. 1 mA is 10⁻³ A, which is a thousand times bigger than 1 µA = 10⁻⁶ A.
• 1 A. 1 A is a million times bigger than 1 µA.
• 1 µA — correct. Yes!
• 1 C of charge flows each second — correct. Yes!
• 1 C of charge flows in a whole minute. One ampere is 1 coulomb each second, which is much faster than one coulomb a minute.
• 1 J of work is done on each coulomb. That describes the potential difference of 1 volt. A current of 1 A means 1 C of charge flowing each second.