Calculating Electric Charge

What is the unit of charge?

• Amperes (A)

• Coulombs (Q)

• Colombs (C)

• Watts (W)

How many electrons carry one coulomb of charge?

• 6 x 10¹⁸

• 6 x 10¹⁵

• 6 x 10²¹

• 6.02 x 10²³

What formula could be used for calculating charge?

• Q = I/t

• Q = It

• Q = I²t

• t = I/Q

Calculate the charge flowing through a wire in 5 s if the current is 3 A.

Answer explanation

Q = It

Q = 3 A x 5 s

Q = 15 C

A wire has 3 A flowing through it for 5 seconds. How many electrons will flow through the wire in this time?

Give your answer in scientific notation, do not attempt to write out the full number.

Answer explanation

Q = It

Q = 3 A x 5 s

Q = 15 C

1 C of charge is carried by 6 x 10¹⁸ electrons.

15 C of charge requires 15 x 6 x 10¹⁸ electrons =

90 x 10¹⁸ electrons

A charge of 20 C passes through a wire over the span of 4 seconds. What was the current flowing through the wire?

Answer explanation

Q = It; so I = Q/t

I = 20 C / 4 s

I = 5 A

A circuit with an ammeter reading of 12 A was conducting electricity for one minute. How much charge passed through the wire during this time?

Answer explanation

Q = It

Q = 12 A x 60 s

Q = 720 C

A current of 4 A produced 100 C of charge. How long was the current switched on?

Answer explanation

Q = It; so t = Q/I

t = 100 C / 4 A

t = 25 s

The voltage across a light bulb is 12 V. Calculate the electrical energy transferred when 50 C of charge passes through it.

Answer explanation

E = QV

E = 50 C x 12 V

E = 600 J

The voltage across a battery is 6 V. Calculate the charge if the battery transfers 2 kJ of energy. Give your answer to 3 s.f.

Answer explanation

E = QV; so Q = E/V

Q = 2000 (convert kJ to J) / 6 V

Q = 333.3333... C

Q = 333 C (3 s.f.)