Thermal Physics Flashcards
Properties of a solid
- The particles are close together.
- They are tightly bonded to their neighbors.
- They vibrate about fixed positions.
Properties of liquids
- The particles are held together by relativity weak forces.
* They slide over each other.
Properties of gas
- The particles have broken free from their neighbors.
- They are widely separated.
- They are free to move around.
What causes kinetic energy and potential energy to increase when heating?
Kinetic energy increases if the particles are able to move around freely and faster.
Electric potential energy increases if the particles break free of their neighbors and become more disordered.
For electrical potential energy:
•The electrical potential energy is large and negative when two atoms are very close together.
•As the separation increases, their potential energy also increases.
•The electrical potential energy is maximum and zero when the atoms are completely separated.
U = -k q₁q₂ / r
What does a change of state involve?
- An input of energy
- No change in temperature
- Molecules breaking free of one another
- An increase in potential energy
What happens in between a change of state?
- An input of energy raises the temperature
- Molecules move faster
- Kinetic energy increases
Why can energy be put into a system without the temperature rising?
This occurs during a change of state; the energy goes to breaking the bonds between neighboring molecules.
NB: The energy which must be supplied to cause a change of state is called latent heat.
What is temperature?
Temperature is a measure of the average kinetic energy of the molecules in a system.
Why is it that melting ice takes much less energy than boiling the same amount of water?
OR
Why is the specific latent heat of vaporization higher than the specific latent heat of fusion?
When a solid melts, only very few bonds are broken as the molecules are still bonded to most of their immediate neighbors. When a liquid boils, more bonds are broken as each molecule breaks free of all its neighbors.
More energy is required to break the bonds in a liquid to form a gas than in a solid to form a liquid per unit mass.
What is evaporation?
This is when a liquid changes into a gas without boiling.
Evaporation occurs at all temperatures. The most energetic molecules at the surface gains energy from the surroundings and eventually possess enough kinetic energy to break free from its bonds and escape.
How does evaporation cause cooling?
The most energetic molecules at the surface gains energy from the surroundings and eventually have enough kinetic energy to break free from its bonds and escape. This causes the average kinetic energy of the liquid to lower as the most energetic molecules have left so the temperature is reduced. So the liquid is cooled.
What is internal energy?
The internal energy of a system is the sum of the random distribution of kinetic and potential energies of its atoms or molecules.
How can the internal energy of a gas increase?
- Heating the gas
* Doing work on the gas (compressing it)
FIRST LAW OF THERMODYNAMICS
Increase in internal energy = energy supplied by heating + energy supplied by doing work
ΔU = q + w
