Current, Voltage & Resistance — Practice · Grade 12 Science

Chapter: Electricity, Magnetism & Circuits

What this practice covers

By the end of Current, Voltage & Resistance you should be able to:

10 questions, numeric, multiple choice and written answer, each with a worked explanation.

Vocabulary used: electric current, ampere (A), voltage (potential difference), resistance, resistivity.

Problem 1

A charge of 20 C flows past a point in a circuit in 5 seconds. Find the current.
Answer and explanation

4 A

I = Q ÷ t = 20 ÷ 5 = 4 A.

Problem 2

Electric current is measured in:
  1. volts
  2. ohms
  3. amperes
  4. joules
Answer and explanation

amperes

Current is the flow of charge per second, measured in amperes (amps).

Problem 3

A battery transfers 36 J of energy to 3 C of charge. Find the voltage.
Answer and explanation

12 V

V = W ÷ Q = 36 ÷ 3 = 12 V.

Problem 4

Of these four copper wires, which has the LEAST resistance?
  1. a long, thin wire
  2. a long, thick wire
  3. a short, thin wire
  4. a short, thick wire
Answer and explanation

a short, thick wire

Resistance decreases with shorter length and greater thickness, so a short, thick wire resists the least.

Problem 5

A current of 2 A flows through a device for 10 seconds. How much charge passes through it?
Answer and explanation

20 C

Rearranging I = Q ÷ t gives Q = I × t = 2 × 10 = 20 C.

Problem 6

Using the water-in-a-pipe analogy, explain what voltage, current, and resistance each represent.
Answer and explanation

Voltage is like the water pressure from a pump that pushes the water along. Current is like the amount of water actually flowing past a point each second. Resistance is like a narrow section of pipe that restricts the flow. Higher pressure (voltage) drives more flow (current), while a tighter pipe (more resistance) reduces it.

Problem 7

A resistor has 8 V across it and carries 2 A. Find resistance. Show the reasoning, not just the conclusion.
Hints

Amperes are coulombs per second.

Answer and explanation

R=V/I=4 Ω.

Problem 8

Can equal currents in two devices imply equal power?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

No. Power P=VI also depends on voltage.

Problem 9

A constant resistance is a tested operating modelA constant resistance is a tested operating modelCurrent (A)00.20.40.60.8102468Potential difference (V)Slope=1/R=0.10 A/V; R=10 Ω.
Calculated ohmic component relation; temperature and other conditions are assumed fixed.
Interpret the supplied visual. What current does this 10 Ω model predict at 5 V?
Answer and explanation

0.50 A under the same operating conditions.

Resistance relates voltage across a component to current through it; an ohmic model uses constant R so V=IR and the current–voltage graph is linear through the origin. Many real devices are nonohmic or change resistance as they heat. A curved graph does not violate energy conservation; it means a single constant-R approximation is inadequate. Read whether the vertical axis is current or voltage before interpreting the slope as R or 1/R.

0.50 A under the same operating conditions.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 10

An ohmic 12 Ω resistor has 9 V across it for 40 s. Find current, power and transferred energy.
Answer and explanation

I=9/12=0.75 A; P=VI=6.75 W; E=Pt=270 J. The stated constant resistance and voltage must persist over the interval.

I=9/12=0.75 A; P=VI=6.75 W; E=Pt=270 J.

The stated constant resistance and voltage must persist over the interval.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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