INTERNAL ENERGY is the total kinetic energy and potential energy of all the particles in a system.
INTERNAL ENERGY = sum of:
KINETIC ENERGY of particles — from their random motion (vibration, translation, rotation).
POTENTIAL ENERGY of particles — stored in the bonds and intermolecular forces between them.
When a substance is heated, the energy supplied goes into the internal energy of the system.
This increased internal energy can produce TWO EFFECTS:
1. TEMPERATURE RISE — particles move faster (higher average KE).
2. CHANGE OF STATE — particles gain enough PE to overcome intermolecular forces (bonds break/form).
NOTE: Both effects cannot happen simultaneously for a pure substance at its melting or boiling point — during a change of state, temperature stays constant even as energy is supplied.
Temperature and Kinetic Energy
TEMPERATURE is a measure of the AVERAGE KINETIC ENERGY of the particles.
Higher temperature = faster-moving particles = higher average KE.
Lower temperature = slower-moving particles = lower average KE.
At the same temperature:
Different materials have different internal energies because they have different numbers of particles and different potential energy arrangements.
IMPORTANT DISTINCTION:
TEMPERATURE (°C or K) — measures average KE per particle.
THERMAL ENERGY (J) — total energy transferred — depends on temperature difference AND mass AND specific heat capacity.
Example:
A large cool lake and a small hot cup of tea:
The tea is hotter (higher temperature = higher average KE per particle).
But the lake has far greater total internal energy (more particles, more total KE + PE).
Heating and Cooling — Two Effects
When energy is SUPPLIED to a substance:
EFFECT 1 — TEMPERATURE RISES:
Particles move faster (KE increases).
This happens when no change of state is occurring.
Calculated using ΔE = mcΔθ.
EFFECT 2 — CHANGE OF STATE:
Temperature stays CONSTANT while state changes.
Energy goes into increasing POTENTIAL ENERGY — breaking intermolecular bonds.
Calculated using E = mL (latent heat equation).
On a HEATING CURVE (temperature vs time for constant energy input):
Sloping sections: temperature rising (KE increasing).
FLAT sections: change of state occurring (temperature constant, PE increasing).
Flat section during melting = melting point.
Flat section during boiling = boiling point.
On a COOLING CURVE: the reverse — flat sections at condensation and freezing points.
⚠️ Common Mistake
During a CHANGE OF STATE, temperature stays CONSTANT — energy goes into potential energy (breaking bonds), not kinetic energy. Students often think heating always raises temperature — but at melting/boiling point, temperature is flat on the heating curve until the change is complete.
📌 Key Note
Internal energy = KE + PE of all particles. Heating → increases internal energy → either raises temperature (KE) or causes change of state (PE). Temperature = average KE per particle. During change of state: temperature constant, PE increasing. Heating curve: slopes = temperature rise; flat sections = changes of state.
🎯 Matching Activity — Internal Energy Concepts
Match each statement to the correct concept. — drag the symbols on the right to match the component names on the left.
Internal energy
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Temperature
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Flat section on heating curve
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Sloping section on heating curve
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Total kinetic + potential energy of ALL particles in the system
Change of state — temperature constant, energy increasing PE (breaking bonds)
Temperature rising — energy increasing KE of particles
Measure of AVERAGE kinetic energy per particle
🎯 Test Yourself
Question 1 of 2
1. A substance is heated at constant power. Its temperature stays constant for several minutes. What is happening?
2. A large cool swimming pool and a small hot cup of tea — which has greater internal energy?
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