Internal energy

OCR

GCSE

Internal energy is defined as the sum of the total kinetic energy and potential energy of all particles that make up a system. Heating a system transfers energy to its particles, resulting in either an increase in temperature (kinetic store) or a change of state (potential store). Candidates must apply the particle model to explain these changes, distinguishing between the breaking of intermolecular bonds during phase changes and the increase in particle speed during heating. Quantitative analysis requires the application of the Specific Heat Capacity and Specific Latent Heat equations.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

What You Need to Demonstrate

Key skills and knowledge for this topic

Award 1 mark for defining internal energy as the total kinetic and potential energy of all the particles (atoms or molecules) that make up a system
Credit responses that explain a change of state by stating energy is supplied to overcome intermolecular forces (increasing potential energy) rather than increasing temperature
Award 1 mark for correct substitution into the specific heat capacity equation: Change in thermal energy = mass × specific heat capacity × temperature change
Candidates must link the flat section of a heating curve to the specific latent heat where energy is transferred without a change in temperature

Marking Points

Key points examiners look for in your answers

Award 1 mark for defining internal energy as the total kinetic and potential energy of all the particles (atoms or molecules) that make up a system
Credit responses that explain a change of state by stating energy is supplied to overcome intermolecular forces (increasing potential energy) rather than increasing temperature
Award 1 mark for correct substitution into the specific heat capacity equation: Change in thermal energy = mass × specific heat capacity × temperature change
Candidates must link the flat section of a heating curve to the specific latent heat where energy is transferred without a change in temperature

Examiner Tips

Expert advice for maximising your marks

💡When analyzing heating/cooling graphs, explicitly annotate horizontal lines as 'change in potential energy/state' and sloped lines as 'change in kinetic energy/temperature' before answering
💡For calculations involving both heating and melting (e.g., ice at -10°C to water at 20°C), calculate the energy for each stage separately and sum them; do not attempt one combined calculation
💡Memorize the definition of specific latent heat precisely: 'energy required to change the state of 1kg of a substance with no change in temperature'

Common Mistakes

Pitfalls to avoid in your exam answers

Stating that the 'kinetic energy of particles increases' during melting or boiling; candidates fail to recognise that energy is increasing the potential store while kinetic energy (temperature) remains constant
Confusing specific heat capacity (energy to raise temperature) with specific latent heat (energy to change state) when selecting formulae
Failing to convert mass from grams to kilograms to match the units of the specific heat capacity constant (J/kg°C), leading to order-of-magnitude errors

Study Guide Available

Comprehensive revision notes & examples

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Key Terminology

Essential terms to know

Sum of kinetic and potential energy stores

Specific Heat Capacity (temperature change)

Specific Latent Heat (state change)

Interpretation of heating and cooling curves

Likely Command Words

How questions on this topic are typically asked

Define

Calculate

Explain

Describe

Plot

Practical Links

Related required practicals

•{"code":"P1","title":"Investigation of Specific Heat Capacity","relevance":"Determination of SHC of materials linking energy input to temperature rise"}

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