P3: Thermal Physics

Properties of solids, liquids, and gases

Property	Solids	Liquids	Gases
Particle Movement	Can only vibrate in place, cannot freely move	Vibrate, rotate, and translate past each other	Vibrate, rotate, and translate freely, colliding with each other3
Forces in between particles	Strong particles are closely packed	Less than a solid	There is no force between the particles
Shape and Volume	Fixed shape, fixed volume	Changing shape, fixed volume	Changing shape, changing volume
Arrangement	Particles closely packed in a regular pattern	Irregular arrangement, particles close	Irregular arrangement, particles far apart

Pressure effect on gas particle motion

When pressure increases, the particles become closer and is more dense
Therefore, the chances of particles colliding with surfaces is higher
When the temperature of a gas increases at a constant volume, the pressure of a gas increases
When the volume of a gas decreases at a constant temperature, the pressure of a gas increases

Temperature effect on particle motion (in gas, liquids, and solids)

When temperature increases, particle motion is faster
This is because the heat energy increases the kinetic energy of the particles, and makes them move faster
This causes the particles to collide with more force

Brownian motion

The Brownian motion refers to the random motion of particles in a heterogenous mixture of a liquid and solid particles
This random motion happens because the larger molecules are moved by collisions with smaller particles (eg. pollen grains can be moved by water)

Melting and Boiling

The melting point is the point where a solid turns into a liquid
The boiling point is the point where a liquid becomes a gas
The melting point of water is 0°C
The boiling point of water is 100°C
When a substance melts or boils, the particles gain kinetic energy and vibrate and move faster from the heat
The structure and bonds are weakened, and the state changes due to the faster and free movement of particles
During when there is a change in state, there is no change in temperature of the material. However, the substance is still gaining or losing energy.

Condensation and Solidification

Condensation is when a gas turns into a liquid
Solidification is when a liquid turns into a gas
When a substance condenses or solidifies, the particles lose kinetic energy and vibrate and move slower because of the loss of heat
The particles start to move closer together and more slowly, causing a change in state
During when there is a change in state, there is no change in temperature of the material. However, the substance is still gaining or losing energy.

Evaporation

Evaporation is the change from a liquid state to a gas state
Boiling is also a change from liquid to gas
However, boiling is set at fixed temperatures, while evaporation can happen at any temperature and happens on the surface of the liquid
This is because particles at the surface are more energetic and can escape from the surface and turn into a gas
Evaporating cools the original liquid as the escaping of the particles at the surface requires heat energy

Factors affecting evaporation

Higher temperature will cause faster evaporation
The greater the surface area exposed, the faster the rate of evaporation
Wind over the surface of a liquid will cause faster evaporation

Matter and thermal properties

Thermal expansion

When things are heated, the particles begin to move around and vibrate faster, colliding into each other and causing them to be further apart
Therefore, the heated substance expands
Even though the substance is larger, the molecules remain the same size. It is the space between the molecules that changes
Gases expand more than liquids, and liquids expand more than solids (as gases are held by weaker bonds than liquids, and liquids are held by weaker bonds than solids, allowing them to move more)

Applications of thermal expansion

Thermal expansion of a liquid is used to measure heat in a thermometer
A temperature activated switch works by the thermal expansion of two metals that expand at different rates on a bimetallic strip

Consequences of thermal expansion

Thermal expansion of solids can cause them to bend under force under temperature
If this happened to metal railway tracks, road surfaces, or bridges, there could be danger
Therefore, these infrastructures are often built with gaps so the solids can have space around to expand

Measurement of temperature

The physical properties of objects change along with temperature
These physical properties include volume, density, and electrical resistance
If the properties have predictable changes and changes at a constant rate, temperature can be measured through these physical properties
This is done by

Measuring the properties at two fixed points of temperature
Working out the temperature of the other values of that property by following a rule found by the fixed points

Sensitivity, Range, and Linearity

Physical properties are good at measuring temperatures when they have...

Linearity: The physical properties change at a steady rate, which allows us to easily investigate and find the relationships between the properties and temperature
Sensitivity: When the measured property is sensitive and has great change along with temperature, it is going to be able to identify smaller changes of temperature
Range: The range is important when designing a device to measure temperatures, as it is essential to know the limits of the device (the highest and lowest temperature it can measure)

Thermocouple

A thermocouples has two different wires which are twisted together on one end
When the wires are heated, voltage is created between the two wires
A greater voltage reading means a greater temperature
It can measure high temperatures and temperatures that change quickly

Liquid in-glass thermometer

A liquid in-glass thermometer contains a liquid which rises through expansion with temperature
One end has a glass bulb containing large amounts of liquids that expanding and move into the narrow tube
A scale that measures the temperature based on the height of the liquid in the tube
The liquids expands linearly with the temperature
They expand a lot, detecting small changes in temperature, making the device sensitive
There is a wide range of temperatures that can be measured

Thermal processes

Conduction

Conduction is a process where thermal energy is transferred through substances
Metals are good at conducting heat
Non-metals are bad at conducting heat, especially when they are in a liquid or gas state
Substances that are good at conducting are called conductors, and substances that are bad at conducting are called insulators
Materials trapping small amounts of air are good insulators, as air is a gas and is bad a conducting
A substance under heat will have particles that vibrate and move around more
This movement causes the particles to bump into each other and transfer energy across the atoms

Experiments to demonstrate the properties of good and bad conductors

The ability to conduct for different metals can be assessed by connecting them to a conduction ring clamped, and heating them
First, place metal strips on the conduction ring and stick balls on the ends of the metal strips (that are all the same distance away from the center)
Turn on the bunsen burner on low heat so that it aligns with the center of the conduction ring
When the heat is conducted to where the ball is, the wax will melt and the ball will drop
The time it takes for the ball to drop determines the ability of each metal to conduct heat (those that take more time are worse conductors, and those that take less time are better conductors)

Convection

Convection is the main method of energy transfer in liquids and gases (convection does not happen in solids)
When a liquid or gas is heated, the molecules move more and push each other further apart, causing the liquid or gas to expand
Hot liquids and gases are less dense as particles are more spread out
The hot liquid or gas rises because it is less dense, and cold liquid or gas falls to the bottom
When the heat source (like a heater) is on the bottom, the cold liquid or gas is heated again, causing it to rise to the top
The hot liquid or gas on the top will eventually cool and fall back to the bottom, being replaced by newly heated liquid or gas
This causes a motion called the convection current
On the other hand, cooling units are another useful application of convection currents
The cold liquid or gas will sink as it is more dense, and the uncooled or hotter liquid or gas will rise to the top
When the cooling unit is on the top, the uncooled or hotter liquid or gas is cooled, causing it to sink
The cold liquid or gas that has sunk will eventually become warmer, and will rise to the top to be cooled again, while the newly cooled fluid sinks

Experiments showing convection

When potassium permanganate crystals are put in a heated beaker of water, it dissolves and rises as it is heated along with the rest of the heated water
This allows the convection current to be clearly visible because of the purple colour of the dissolved potassium permanganate crystals

Radiation

All objects with heat emit thermal radiation. The more heat they have, the more thermal radiation is emitted
Thermal radiation emits infrared waves
It is the only method of energy transfer that can travel through a vacuum (without a medium)
It is the method in which the sun provides its warmth to the Earth
The colour of an object determines its ability to absorb or emit thermal radiation. Objects that are...

Black can absorb and emit thermal radiation very well
Dark or dull can absorb and emit thermal radiation fairly well
White absorb and emit thermal radiation badly
Shiny absorbs and emit thermal radiation badly (Note: shiny objects absorb thermal radiation badly as they reflect light)

Experiment to demonstrate radiation

To demonstrate the absorbing of thermal radiation, we can carry out the following experiment

Use two conical flasks painted with two different colours and place thermometers in them
Record the initial temperatures of the two flasks
Place the two flasks at the same distance away from a light bulb (the emitter of thermal radiation)
Switch the bulb on for 5 minutes. After 5 minutes, measure the temperature at the end of the reaction and see how much the temperature changed for each one. The flask with the higher temperature should be the better absorber of thermal radiation

To demonstrate the emission of thermal radiation, we can carry out the following experiment

Fill two beakers of different colours with water and place thermometers in them
Once the beakers reach a set temperature of your choice (eg. 90°C), allow them to cool for the same amount of time
Observe the changes in temperature
The amount of lost heat should represent the amount of thermal radiation emitted
The beaker that cooled more should be the better emitter of thermal radiation

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