Electrical Power, Energy & Circuit Design — Practice · Grade 12 Science

Chapter: Electricity, Magnetism & Circuits

What this practice covers

By the end of Electrical Power, Energy & Circuit Design you should be able to:

8 questions, numeric and written answer, each with a worked explanation.

Vocabulary used: kilowatt-hour, internal resistance, emf.

Problem 1

A9 V component draws.20 A. Find power.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

1.8 W

P=VI.

Problem 2

Is a kWh power or energy?
Answer and explanation

Energy; it is power multiplied by time.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 3

A 4 W device operates for 2 hours. Find energy in kWh.
Hints

List the given quantities, select a relationship, and check units before substituting.

Answer and explanation

0.008 kWh

.004 kW×2h=.008 kWh.

Problem 4

Why can a shorted battery overheat?
Answer and explanation

Very low external resistance permits large current; internal resistance then dissipates substantial I²r heat.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 5

A 2 kW heater operates 0.5 h. Find energy. Show the reasoning, not just the conclusion.
Hints

Kilowatt-hour is energy, not power per hour.

Answer and explanation

1 kWh, equivalent to 3.6×10⁶ J.

Problem 6

Two lamps give equivalent lighting at 10 W and 50 W. What comparison does power support?
Hints

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

Answer and explanation

The 10 W lamp uses less energy per equal operating time; broader impact also requires lifetime and other factors.

Problem 7

A design adds branch demand within a stated source modelA design adds branch demand within a stated source modelCurrent (A)Branch A2Branch B3Branch C1Source total6Ideal parallel 12 V loads: total power=72 W.
Calculated ideal branch demand; no household electrical work is part of this exercise.
Interpret the supplied visual. If one 3 A branch is switched off, what ideal total remains?
Answer and explanation

3 A and 36 W at unchanged 12 V, assuming the source maintains that voltage.

For ideal parallel loads sharing 12 V, source current is the sum of branch currents. Here 2+3+1=6 A and source output power is 72 W. A real source and conductors have operating limits and losses, so an ideal current sum is only part of a design comparison. Consider voltage regulation, continuous versus peak demand and the stated component ratings. Use supplied-data design here rather than modifying household wiring.

3 A and 36 W at unchanged 12 V, assuming the source maintains that voltage.

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 8

A 15 W device is replaced by a 9 W device providing the same service for 2500 h/year. Find annual energy saved in kWh and joules, and name one uncounted life-cycle term.
Answer and explanation

Saving=(0.015−0.009)×2500=15 kWh=5.40×10⁷ J/year. Production, transport, replacement lifespan or disposal is outside this operational-only result.

Saving=(0.015−0.009)×2500=15 kWh=5.40×10⁷ J/year.

Production, transport, replacement lifespan or disposal is outside this operational-only result.

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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