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Current and amps

Electricity and magnetism · level 4, lesson 2 of 6

A question to start

A phone charger might say "2 A". A kettle draws about 12 A. What is an amp counting?

The idea

Current is how much charge flows past a point each second:

\[I = \frac{Q}{t},\]

where \(Q\) is the charge in coulombs and \(t\) the time in seconds. Current is measured in amps (A): 1 A is 1 coulomb per second. That is about \(6.25 \times 10^{18}\) electrons every second.

Device Current, about
A small LED 0.02 A (20 milliamps)
A phone charging 2 A
A toaster 7 A
A kettle 12 A

In a single loop, the current is the same everywhere. Charge is not used up; it goes all the way round.

count the charge passing here each secondflow

Worked examples

1. From charge. 30 C flow past a point in 15 s: \(I = 30/15 = 2\) A.

2. Charge from current. A 0.5 A current runs for 2 minutes: \(Q = It = 0.5 \times 120 = 60\) C.

3. Electrons. 1 A is \(6.25 \times 10^{18}\) electrons a second, so 2 A is \(1.25 \times 10^{19}\) a second.

Try it

  1. 120 C flow in a minute. What is the current?
  2. How much charge flows when 3 A runs for 10 s?
  3. In a single loop, the current near the battery is 0.4 A. What is it on the far side of the bulb?
  4. How long does it take 1 A to move 3,600 C?
Answers
  1. \(120/60 = 2\) A.
  2. \(3 \times 10 = 30\) C.
  3. 0.4 A: the current is the same all the way round a single loop.
  4. 3,600 s, one hour.

Common mistake

"The bulb uses up some of the current." The current coming out of the bulb is the same as the current going in. The bulb takes energy from the charge, not the charge itself.

Where it leads