MYP Physics 2.5 Thermal Physics Notes

Heat and Temperature: What Is the Difference?

1. Thermal physics connects what we feel as hot and cold with what is happening to particles (atoms and molecules) at a microscopic scale.
2. Many everyday effects, from a metal spoon feeling colder than a wooden one to water boiling at a fixed temperature, can be explained by how particles move, collide, and transfer energy.

What Is Heat?

1. Heat refers to energy that is transferred from one object to another because of a temperature difference.
2. Heat always flows from a hotter object to a cooler object.
3. Heat is not something an object “contains”; it is a process of energy transfer.
4. It is measured in joules (J).

Water Is Temperature?

1. Temperature describes how hot or cold an object is.
2. Temperature is related to the average kinetic energy of particles in a substance.
3. A higher temperature means particles are moving faster on average.

Heat vs Temperature

1. Two objects can have the same temperature but contain different amounts of energy.
2. A large object at a moderate temperature may contain more thermal energy than a small object at a higher temperature.
3. Temperature does not depend on the amount of substance, but heat transfer does.

Effect of Heating on Particles

1. When a substance is heated, its particles gain kinetic energy.
2. Gaining kinetic energy causes particles to move faster.
3. Faster-moving particles collide more often and with greater force.

Temperature Scales

Two commonly used temperature scales are Celsius and Kelvin.

Celsius Scale

1. The Celsius scale is commonly used in everyday life.
2. It sets 0 °C as the freezing point of water and 100 °C as the boiling point.
3. Celsius is useful for practical temperature measurements.

Kelvin Scale

1. The Kelvin scale is used in scientific contexts.
2. 0 K represents the lowest possible temperature, where particle motion is minimal.
3. Kelvin values are always positive.

Converting Between Scales

1. Kelvin and Celsius values differ by a constant amount.
2. A temperature change of 1 °C is the same size as a change of 1 K.

We Feel Hot Or Cold Mainly By Heat Transfer, Not Temperature Reading

1. Humans can detect small differences in temperature, but our sense is strongly based on the direction and rate of heat transfer.
2. A classic demonstration uses three bowls: cold, warm, and hot water.
   1. Put one hand in cold water and the other in hot water for about a minute.
   2. Then place both hands into the warm water.
3. You will experience the "same" warm water as hot with one hand and cold with the other. This shows your body is responding to whether it is gaining or losing heat energy.

States Of Matter Depend On Particle Arrangement And Forces

1. Particles interact through forces: when they are close they attract, but if pushed too close they repel.
2. The balance between particle energy (motion) and these forces helps determine the state of matter.
   1. In a solid, particles are tightly packed in fixed positions and can only vibrate.
   2. In a liquid, particles remain close together but can move past each other.
   3. In a gas, particles are far apart, forces between them are usually negligible, and they move freely to fill the container.

Brownian Motion Provides Evidence For Constant Particle Motion

1. In 1827, Robert Brown observed pollen grains in water moving randomly.
2. The explanation came later (Einstein and Smoluchowski): the visible particle is constantly bombarded by much smaller molecules.
3. The collisions occur extremely frequently (on the order of $10^{14}$ to $10^{16}$ collisions per second), and at any instant they are slightly unbalanced in direction.
4. That tiny imbalance produces a net force for a short time, so the particle accelerates one way, then another, giving a random path.
5. This is powerful evidence that:
   1. Molecules are in constant motion.
   2. Collisions transfer momentum and energy even when we cannot see the molecules.

Why Some Materials Heat Faster Than Others

1. Different materials require different amounts of energy to change temperature.
2. Some materials warm quickly with little energy input.
3. Other materials need much more energy to produce the same temperature change.

Interpreting Specific Heat Capacity

1. A low specific heat capacity means a substance heats up quickly.
2. A high specific heat capacity means a substance heats up slowly.
3. This explains why water temperature changes slowly compared to many solids.

Factors Affecting Temperature Change

1. The amount of energy transferred
2. The mass of the material
3. The specific heat capacity of the material