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9
0.5 × 2 × 9.
Class 11 · English Medium · Cambridge · Physics
Work, energy and thermal physics
Calculate mechanical and thermal energy changes and account for transfers.
Shared cross-level core and stretch unit. Start with the core questions, then use the stretch example for deeper work. The same unit is available in related class and board pathways; it is not a full grade-specific or examination-specification course. A teacher should select the appropriate depth and further topics.
Learn the idea
Work by a constant force is force times displacement in the force direction. Kinetic energy is mv²/2; gravitational potential energy near Earth changes by mgh. Heating without state change can be modelled by Q = mcΔT with approximately constant specific heat capacity c.
Worked example
For a 2 kg object raised 3 m using g = 10 N/kg, ΔE = 60 J. Heating 0.5 kg water by 10°C using c = 4200 J/(kg·°C) requires Q = 21,000 J, neglecting other heating.
- Define the system and transfers.
- Select the relevant equation.
- Account for energy passed to the surroundings.
Stretch example
For an ideal gas expanding at constant pressure, work done by the gas is pΔV. With 100,000 Pa and ΔV = 0.002 m³, W = 200 J. Under ΔU = Q − W, heat in of 500 J gives ΔU = 300 J.
Your written practice
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20
W = Fd.
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60
mgh = 3 × 10 × 2.
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The kettle and surroundings also receive energy.
Identify transfers omitted by the simplified model.
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Energy can change particle arrangement rather than increase average kinetic energy.
Separate latent heating from temperature rise.
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550
ΔU = 800−250.
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Some conventions count work done on the gas as positive; the equation’s signs change accordingly.
Use one convention consistently.
Energy account
Solve a fictional heating calculation and identify two neglected transfers.
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- Show units and assumptions.
- Distinguish useful heating from total input.
- Keep the task on paper; no hot-liquid experiment.
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