Theme B — The Particulate Nature of Matter
B.1.5 Internal Energy — IB Physics HL Revision Notes & Topical Questions
5 min read 9 parts Lesson 7 of 53 1 quiz
Lesson summary
B.1.5 Internal Energy — IB PHYSICS SL notes: Define internal energy as the total random kinetic and intermolecular potential energy and explain how it changes with temperature and state.Internal energy = the sum of the total random kinetic energy and the total intermolecular pote…
- Not included
- Hidden energy
- Gaining thermal energy
- Kinetic part
- Potential part
- State changes
Part 1 of 7Definition
Internal energy
Internal energy the sum of the total random kinetic energy and the total intermolecular potential energy of the particles in a substance.
- U = Ek(random) + Ep(intermolecular), in joules.
Not included
- Not included the kinetic energy of the whole object moving and its gravitational energy as a whole.
Hidden energy
- Hidden energy it is stored in the microscopic motion and arrangement of the particles.
Gaining thermal energy
- Gaining thermal energy the internal energy of a substance increases, and losing it decreases the internal energy.
Part 2 of 7Kinetic and potential contributions
Kinetic and potential contributions
Kinetic part
- Kinetic part it depends on temperature; raising the temperature raises the mean random kinetic energy.
Potential part
- Potential part it depends on the separations and forces between particles.
- It increases when the particles are pulled apart against the attractive forces, as in melting and boiling.
- It also rises slightly when a substance expands on heating.
Temperature
- Temperature only a change in the average kinetic energy changes the temperature.
State changes
- State changes only the potential energy changes, so the temperature stays constant while the internal energy rises.
Part 3 of 7Internal energy of an ideal gas
Internal energy of an ideal gas
No potential part
- No potential part an ideal gas has no intermolecular forces, so its internal energy is all random kinetic energy.
Monatomic gas
- U = (3/2)NkBT = (3/2)nRT, where N is the number of particles.
Depends on T only
- Depends on T only at constant temperature U is constant whatever the pressure or volume.
Use
- Use this result appears in the first law of thermodynamics.
Part 4 of 7Changing the internal energy
Heating
- Heating energy is transferred because of a temperature difference.
- It raises the temperature (Q = mcΔT) or causes a change of state (Q = mL).What the symbols mean: E, Q or q = energy transferred (J) · m = mass (kg; g in chemistry calorimetry) · c = specific heat capacity (J kg−1 °C−1; water 4.18 J g−1 °C−1) · ΔT or Δθ = temperature change (°C or K)
Work
- Work work done on or by the system also changes U.
- Rubbing hands together or pumping a bicycle tyre warms them without a temperature difference supplying the energy.
Heat and internal energy
- Heat and internal energy heat is the energy transferred, while internal energy is the energy stored.
- An object does not contain heat; it contains internal energy.
Part 5 of 7Comparing systems
Comparing systems
Same temperature, different U
- Same temperature, different U a kettle of water at 90 °C has more internal energy than a cup at 90 °C, because there are more particles.
Steam and water
- Steam and water steam at 100 °C has more internal energy than water at 100 °C of the same mass, because the particles are further apart.
Melting ice
- Melting ice it takes in heat, its temperature stays constant and its internal energy increases.
Part 6 of 7Temperature, heat and internal energy
| Quantity | Meaning | Depends on |
|---|---|---|
| Temperature | Mean random kinetic energy per particle; sets the direction of energy flow | Kinetic energy only |
| Internal energy | Total energy stored in random motion and interparticle potential energy | Amount of substance, temperature and state |
| Heat | Energy transferred because of a temperature difference | Process, not stored |
Analysis
- Analysis ask whether the mean kinetic energy changed (temperature) and whether the spacing changed.
- (potential energy)
Ideal gas
- Ideal gas only the first can occur.
Part 7 of 7More from the specification
1
Microscopic thermal energy in solids
- Microscopic thermal energy in solids vibrational kinetic energy of lattice atoms defines the internal thermal energy of crystalline solids across Earth.
2
Internal energy
- Internal energy total random kinetic and potential energy of the particles in a system
3
Specific heat capacity
- Specific heat capacity energy to raise the temperature of one kilogram by one kelvin, as measured for a copper block
4
Latent heat
- Latent heat energy for a change of phase at constant temperature, including latent heat of vaporisation and fusion
5
Thermal transfer
- Thermal transfer conduction, convection and radiation move energy from the surface of a hot object to cooler surroundings
6
Surface temperature
- Effective temperature of the star's radiating surface.
7
Wavelength
- Distance between a point on one wave and the same point on the next wave
8
Solid
- State with a fixed shape and a fixed volume.
9
Fusion
- Joining of light nuclei into a heavier one.
10
Intensity
- Power per unit area perpendicular to the wave direction.
11
Apparent brightness
- The power received per unit area at the observer
12
Thermal equilibrium
- Constant temperature because energy is absorbed and lost at equal rates
13
Phase change
- Change of matter from one state to another
14
Thermal conductivity
- Material constant relating the rate of conduction to the temperature gradient and area
15
Radiation
- Transfer by electromagnetic waves, requiring no medium
16
Liquid
- State with a fixed volume that takes the shape of its container.
17
Copper
- Metal widely used for electrical wires.
18
Convection
- Transfer of thermal energy by the movement of a heated fluid
19
Conduction
- Transfer of thermal energy by lattice vibrations and free electrons
20
Luminosity
- The total power of radiation emitted by a star.
21
Density
- Mass per unit volume.
Key terms
| Term | Meaning |
|---|---|
| Internal energy | Sum of the total random kinetic energy and intermolecular potential energy of the particles |
| Intermolecular potential energy | Energy stored because of the separations and forces between particles |
| Heat | Energy transferred because of a temperature difference |
| Work | Energy transferred by a force moving through a distance |
| Ideal gas | Gas with no intermolecular forces, so internal energy is purely kinetic |
| Monatomic gas | Gas whose particles are single atoms |
Avoid these mistakes
1
Saying a change in internal energy always changes the temperature — it does not during a change of state.
2
Saying an object contains heat — it contains internal energy.
3
Including the motion of the whole object in internal energy — only random microscopic energy counts.
4
Giving an ideal gas potential energy — it has none.
5
Mixing up temperature and internal energy — a bath and a cup at the same temperature differ in U.
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- : AUG UAC .
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