Electric current

AS Level Physics · Electricity. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

An electric current is a flow of charge carriers. In a metal the carriers are free electrons. In a salt solution they are positive and negative ions. Current is the charge passing a point each second.

Charge is quantised: it comes only in whole-number multiples of the elementary charge e = 1.60 × 10−19 C. A charge of half an electron charge on an object is not possible.

The carriers in a wire move slowly. Their average speed along the wire is the drift speed, usually less than a millimetre per second. The current is still large because the number of carriers in each cubic metre of metal is huge.

Rules to remember

Common mistake

Students leave the time in minutes or hours in Q = It. Change the time to seconds first, and in I = Anvq make sure the area is in m2.

Worked example

A wire of cross-sectional area 2.5 × 10−6 m2 carries a current of 2.0 A. The number density of free electrons is 1.0 × 1029 m−3 and e = 1.6 × 10−19 C. What is the drift speed?

  1. Rearrange I = Anvq to give v = I ÷ (Anq).
  2. Anq = 2.5 × 10−6 × 1.0 × 1029 × 1.6 × 10−19 = 4.0 × 104 C m−1.
  3. v = 2.0 ÷ (4.0 × 104) = 5.0 × 10−5 m s−1.

Answer: The drift speed is 5.0 × 10−5 m s−1 (0.050 mm s−1).

Practice questions

Try each one, then open the answer.

1. How many electrons pass a point in a wire when a charge of 2.0 C flows? (e = 1.60 × 10−19 C)

  1. A
    3.2 × 10−19
  2. B
    1.25 × 1018
  3. C
    6.25 × 1018
  4. D
    1.25 × 1019
Show answer

Answer: D. N = Q/e = 2.0 ÷ 1.60 × 10−19 = 1.25 × 1019. 6.25 × 1018 is the number in one coulomb.

2. A charge of 240 C passes through a lamp in 2.0 minutes. What is the current?

  1. A
    0.5 A
  2. B
    2.0 A
  3. C
    120 A
  4. D
    480 A
Show answer

Answer: B. I = Q/t = 240 ÷ 120 s = 2.0 A.

3. A phone battery is rated at 3000 mA h. How much charge can it deliver when fully charged?

  1. A
    3.0 C
  2. B
    180 C
  3. C
    3000 C
  4. D
    10 800 C
Show answer

Answer: D. Q = It = 3.0 A × 3600 s = 10 800 C. 1 mA h is 1 mA flowing for one hour.

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