Energy Transfer & Transformation — Practice · Grade 8 Science

Chapter: Energy & Heat Transfer

What this practice covers

By the end of Energy Transfer & Transformation you should be able to:

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

Vocabulary used: energy transformation, energy transfer, chemical energy, electrical energy, model, evidence, system boundary, assumption.

Problem 1

A light bulb uses 100 J of electrical energy and produces 20 J of light energy. The rest becomes thermal energy. How many joules become thermal energy?
Answer and explanation

80 J

100 J − 20 J (light) = 80 J of thermal energy.

Problem 2

A car engine converts chemical energy (gasoline) into…
  1. only electrical energy
  2. mechanical energy and thermal energy
  3. only sound energy
  4. matter
Answer and explanation

mechanical energy and thermal energy

Burning gasoline releases chemical energy that becomes mechanical energy (motion) and thermal energy (heat).

Problem 3

Which best describes what happens to energy in any transformation?
  1. Some energy is created
  2. Energy disappears
  3. Energy changes form, but the total amount is conserved
  4. Energy always increases
Answer and explanation

Energy changes form, but the total amount is conserved

Energy transformations change the form of energy, but the total amount stays the same.

Problem 4

Trace the energy transformations that occur when you eat food and then go for a run.
Answer and explanation

Chemical energy stored in food is transformed by your body into mechanical (kinetic) energy for movement and into thermal energy that warms your body, with some chemical energy also stored for later use.

Problem 5

A solar panel receives 500 J of light energy and outputs 90 J of electrical energy. How many joules were NOT converted into electrical energy?
Answer and explanation

410 J

500 J − 90 J = 410 J was not converted into electrical energy (mostly lost as thermal energy).

Problem 6

A motor receives 80 J electrically, produces 50 J of useful motion and 5 J of sound. Account for the rest. Explain the reasoning in your own words.
Answer and explanation

25 J must be accounted for, such as warming of the motor and surroundings.

Add named outputs: 50+5=55 J.

Subtract 80−55=25 J for remaining transfers.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

A motor lifts a weight while warming. Identify the useful energy change and one other output. Show the reasoning, not just the conclusion.
Hints

Include warming even when light is the desired output.

Answer and explanation

The weight gains gravitational potential energy; the motor and surroundings also gain thermal energy.

Problem 8

A solar panel charges a battery. Describe the main sequence of energy transfer and storage.
Hints

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

Answer and explanation

Radiant energy arrives from sunlight, electrical energy is transferred, and chemical energy is stored in the battery, with other outputs such as heating.

Problem 9

Use the illustrative service records to propose a conditional source/storage or demand plan. Explain one benefit, one limitation and two additional evidence needs, tracing rather than creating energy.
Hints

Use the illustrated study titled Energy sources and future choices. Keep the observations, model and conditions separate.

Answer and explanation

Supply X alone does not meet the stated 20-unit evening need, while storage with 15 units charged and 80% useful return delivers 12 evening units. The remaining 8 need another supply or a justified demand reduction. A proposal should compare resource/impact/reliability evidence for that equal service and acknowledge omitted costs and timing. No source is universally best from these few records, and lost useful fraction remains accounted energy elsewhere.

Supply X alone does not meet the stated 20-unit evening need, while storage with 15 units charged and 80% useful return delivers 12 evening units. The remaining 8 need another supply or a justified demand reduction. A proposal should compare resource/impact/reliability evidence for that equal service and acknowledge omitted costs and timing. No source is universally best from these few records, and lost useful fraction remains accounted energy elsewhere.

Response criteria

  • Use equal service and the supplied timing.
  • Calculate/trace storage output and demand gap.
  • Evaluate benefits, limits and missing evidence fairly.

Problem 10

Useful energy can decline while the full account balancesUseful energy can decline while the full account balancesDevice / local objectUseful transfer 70 JSurrounding roomThermal increase 30 JInput account: 100 J = useful 70 J + surroundings 30 J.A smaller boundary can miss a real transfer without energy being destroyed.
The illustrated 100 J input becomes 70 J of the defined useful transfer and 30 J of thermal increase in surrounding objects. The red arrow is energy transfer across the chosen boundary, not a gas or an electric-current arrow. A real account must include the actual stores and partners instead of calling all missing useful energy destroyed.
For this unit’s illustrated case, what is missing if the report counts only the useful 70 J? Would enlarging the boundary change the useful-output definition?
Answer and explanation

The 30 J transferred to surroundings is omitted. Enlarging the boundary includes that change, while useful output still depends on the stated job.

A boundary is an accounting choice, not a wall that energy automatically obeys. Name the objects receiving energy and the pathway by which it reaches them. “Less useful” and “less total energy” are different claims.

The 30 J transferred to surroundings is omitted. Enlarging the boundary includes that change, while useful output still depends on the stated job.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 11

A supplied motor system receives 150 J and gives 90 J to the useful job, 40 J to thermal increase and 20 J to sound/other measured changes. Build a complete account and distinguish a transfer from a store.
Answer and explanation

150 = 90+40+20 J. Transfers name energy moving between systems, while stores/contributions name energy accounted for within chosen objects/arrangements. No supplied energy is missing.

Sum all stated outputs and changes.

Identify actual receiving systems rather than treating arrows as material flowing through every object.

150 = 90+40+20 J. Transfers name energy moving between systems, while stores/contributions name energy accounted for within chosen objects/arrangements. No supplied energy is missing.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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