Pearsons sciens year 8 chapter 5.2

5.2.1 Energy Transfer

•    Definition: Energy transfer occurs when energy is passed from one object to another

•    Example: When you kick a ball, the kinetic energy from your foot is transferred to the ball.

•    Key point: Energy moving from object A → object B is transfer. It's different from energy “changing form” (that’s transformation).

•    Revision tip: Think of dominoes – push the first, energy transfers down the line.

•    Practice question: Describe a scenario in daily life where energy is transferred between objects (no change of form necessary).

5.2.2 Energy Transformation

•    Definition: Energy transformation happens when one form of energy changes into a different form.

•    Examples:

◦    Electrical energy → light + sound + heat when using a TV.

◦    A falling object: gravitational potential energy → kinetic energy → heat + sound (when it hits the ground).

•    Flow diagrams: Useful for showing energy changes (arrows from one form → another).

•    Practice question: Write a flow diagram for an electric kettle boiling water: input energy → useful output → wasted output.

5.2.3 Law of Conservation of Energy

•    Statement: Energy cannot be created or destroyed; it can only be converted from one form to another.

•    What it means: If energy seems to “disappear,” it’s been transformed into other forms (often heat or sound).

•    Example: A ball rolling to a stop – its kinetic energy eventually becomes heat (due to friction) and maybe a little sound.

•    Revision tip: Always ask “Where did the energy go?” – helps identify transformations.

•    Practice question: A roller-skater comes to a stop; explain the energy changes using the conservation principle.

5.2.4 Useful Energy, Wasted Energy & Efficiency

•    Useful energy: The portion of energy input that does the intended job (useful output).

•    Wasted energy: Energy that is converted into undesired forms (heat, sound) and is often dissipated into surroundings.

•    Efficiency: A measure of how much input energy becomes useful output.

[

\text{Efficiency} = \frac{\text{useful energy output}}{\text{total energy input}} \times 100%

]

•    Worked example: If a remote-controlled car uses 1800 J of chemical energy and produces 450 J of kinetic energy (useful), then efficiency = (450 ÷ 1800) × 100% = 25%.

•    Practice question: A light bulb receives 1000 J of electrical energy; 800 J becomes light (useful) and 200 J becomes heat (wasted). What’s its efficiency?

5.2.5 Renewable vs Non-renewable Energy Sources & Efficiency in Systems

•    Renewable sources: These are essentially unlimited (in human timeframe), e.g., solar, wind, hydro.

•    Non-renewable sources: Finite supplies, e.g., coal, oil, natural gas — often have high energy losses (low efficiency).

•    System efficiency example: Converting chemical energy of coal → heat → mechanical energy → electrical energy → household usage – much is lost as heat at each stage. Only a fraction becomes useful electrical energy.

•    Practice question: Compare the efficiency of a coal-fired power station (30%) with riding a bicycle (95% conversion) in terms of energy input → useful output.

Key Vocabulary

Term

Definition

Energy transfer

Movement of energy from one object to another.

Energy transformation

Change of energy from one form to another (e.g., kinetic → heat).

Law of conservation of energy

Energy cannot be created/destroyed — only transformed.

Useful energy

The portion of energy input that performs the intended work.

Wasted energy

Energy transformed into less useful forms (often heat or sound).

Energy efficiency

Percentage of input energy that becomes useful output.

Renewable energy source

Energy source that can be replenished and is not depleted by use.

Non-renewable energy source

Energy source with finite supply and often higher losses.

Example Questions You Should Be Able to Answer

1    Explain the difference between energy transfer and energy transformation.

2    Draw a flow diagram showing energy changes in a toaster.

3    State the law of conservation of energy and provide a real-life example.

4    A device uses 2000 J input energy and produces 850 J useful output. Calculate its efficiency.

5    Why are most real-life energy devices less than 100% efficient?

6    Explain why converting coal to electricity involves large energy losses.

7    Compare the benefits of renewable vs non-renewable energy sources in terms of efficiency and sustainability.