Calculating with electric charge: including complex calculations | Combined science

Starter quiz

Match each of the following quantities to their correct definitions.

current

⇔

charge flowing past a point in a circuit per second ✓

electric charge

⇔

a property that can either be positive or negative ✓

potential difference

⇔

energy transferred per unit charge passing between two points ✓
Match each of the following quantities to the correct measurement units.

current

⇔

A ✓

energy

⇔

J ✓

potential difference

⇔

V ✓

power

⇔

W ✓
______ is the amount of energy transferred per second.

'power' ✓
A metal is made of ions and free electrons. What is the charge on the ions?

negative

neutral

positive ✓

positive in some metals and negative in others
In circuits A and B, all of the cells are identical and all of the lamps are identical. Which of the following statements are correct?

The current in B is half the current in A.

The current in B is twice the current in A.

The resistance in B is twice the resistance in A. ✓

The p.d. across the battery in B is twice the p.d. across the cell in A. ✓
A mains appliance has a current of 3.50 A through it. The power of the appliance is ______ W (to 3 sf).

'805' ✓

An ammeter shows a steady reading of 1 A. Which of the following statements describes what this means?

1 ampere flows through per second.

1 coulomb flows through per second. ✓

1 coulomb flows through per ampere.

1 joule is transferred per second.

1 joule is transferred per coulomb.
Match the following symbols for quantities to the symbols used for their units.

$E$

⇔

J ✓

$I$

⇔

A ✓

$P$

⇔

W ✓

$Q$

⇔

C ✓

$t$

⇔

s ✓

$V$

⇔

V ✓
How much charge flows through a motor in 5.0 s if there is a constant current of 2.0 A?

0.40 C

2.5 C

3.0 C

7.0 C

10 C ✓
66 J of energy is transferred to a lamp when 5.5 C of charge flows through it. The potential difference across the lamp is ______ V.

'12' ✓
The potential difference across a television is 230 V and a steady current of 0.39 A flows through it. How much energy does the television transfer every second?

17 J

35 J

90 J ✓

230 J

590 J
An electric fan has resistance 5.2 Ω. When a current of 2.3 A flows through the heater, the energy transferred in one minute is …

28 J.

60 J.

720 J.

1700 J. ✓

3700 J.

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Electric charge, Q, is measured in coulombs (C).
Electric current is a measure of the rate of flow of electric charge, calculated using: I = Q ÷ t.
Potential difference is a measure of the energy transferred by each coulomb of charge, calculated using: V = E ÷ Q.
Combining I = Q ÷ t and V = E ÷ Q gives the equation: E ÷ t = I × V. This is equivalent to P = I × V.
Unknown variables in the power equation can often be calculated using: I = V ÷ R.

Understanding electrical circuits is about memorising equations and procedures.

Give pupils the opportunity to discuss what happens in a circuit before they attempt to answer questions about it.

Electric current - an electrical property; it can either be positive or negative
Coulombs (c) - the unit of electric charge
Electric current - the amount of electric charge flowing past a point in a circuit each second
Potential difference (p.d.) - the amount of energy transferred by each coulomb of charge, passing between two points in a circuit
Power - the amount of energy transferred each second