Welcome to the World of Thermal Energy and Gases!
Ever wondered why the beach sand gets burning hot while the sea stays cool? Or why a bicycle pump feels warm after you've used it? This chapter is all about how energy moves through substances and how gases behave when we squash them or heat them up. We will look at Specific Heat Capacity, Latent Heat, and the "rules" that Gases follow.
1. Specific Heat Capacity (SHC)
If you heat up 1 kg of iron and 1 kg of water for the same amount of time, the iron will get much hotter than the water. This is because every material needs a different amount of energy to increase its temperature. We call this Specific Heat Capacity.
Definition: Specific heat capacity is the amount of energy required to raise the temperature of 1 kg of a substance by \(1^\circ\text{C}\).
The Formula
To calculate the energy involved, we use this formula (don't worry, this is on your formula sheet!):
\(\Delta Q = m \times c \times \Delta \theta\)
- \(\Delta Q\) = Change in thermal energy (Joules, \(J\))
- \(m\) = Mass (kilograms, \(kg\))
- \(c\) = Specific heat capacity (Joules per kilogram degree Celsius, \(J/kg^\circ\text{C}\))
- \(\Delta \theta\) = Change in temperature (degrees Celsius, \(^\circ\text{C}\))
Analogy: Think of SHC like a sponge. Some materials are like big sponges (water)—they can "soak up" a lot of heat energy before they show a change in temperature. Others are like tiny sponges (metals)—they "overflow" (get hot) very quickly.
Core Practical 14.11 (Part 1): Finding SHC
You need to know how to investigate this. Typically, you use an immersion heater to heat a known mass of water. You measure the energy supplied using a joulemeter and the temperature rise using a thermometer. By rearranging the formula to \(c = \frac{\Delta Q}{m \times \Delta \theta}\), you can find the SHC.
Quick Review: High SHC means the material is good at storing heat without getting too hot (like the water in your central heating radiators!).
2. Specific Latent Heat (SLH)
When you boil a kettle, the water stays at \(100^\circ\text{C}\) even though the heater is still on. Where is that energy going? It's being used to break the bonds between particles to turn the liquid into a gas. This "hidden" energy is called Latent Heat.
Definition: Specific latent heat is the energy needed to change the state of 1 kg of a substance without changing its temperature.
The Formula
This one is also on your formula sheet:
\(Q = m \times L\)
- \(Q\) = Thermal energy for a change of state (Joules, \(J\))
- \(m\) = Mass (kilograms, \(kg\))
- \(L\) = Specific latent heat (Joules per kilogram, \(J/kg\))
Two Types of Latent Heat:
- Specific Latent Heat of Fusion: The energy to change between solid and liquid (melting or freezing).
- Specific Latent Heat of Vaporisation: The energy to change between liquid and gas (boiling or condensing).
Top Tip: In exam questions, look at the graph! If the temperature line is flat (horizontal), the substance is changing state and you use \(Q = m \times L\). If the line is sloping, the temperature is changing and you use \(\Delta Q = m \times c \times \Delta \theta\).
3. Gas Behaviour and Temperature
Gases are made of particles moving randomly at high speeds. When they collide with the walls of a container, they exert a force. This force over an area creates pressure.
Temperature and Pressure
If you heat a gas (in a sealed container):
- The particles gain kinetic energy and move faster.
- They hit the walls more often and with more force.
- Therefore, the pressure increases.
Absolute Zero and the Kelvin Scale
As you cool a gas, the particles move slower. Scientists figured out that at \(-273^\circ\text{C}\), particles would stop moving entirely! This is called Absolute Zero.
The Kelvin scale starts at absolute zero:
- \(0\text{ K} = -273^\circ\text{C}\)
- To turn Celsius into Kelvin: Add 273
- To turn Kelvin into Celsius: Subtract 273
Example: Room temperature (\(20^\circ\text{C}\)) is \(20 + 273 = 293\text{ K}\).
Key Takeaway: The temperature of a gas (in Kelvin) is directly proportional to the average kinetic energy of its particles.
4. Gas Pressure and Volume (Physics Only - 14.16P-14.20P)
If you have a fixed mass of gas at a constant temperature, there is a special relationship between its pressure and its volume.
Squashing Gases
If you decrease the volume (squash the gas), the particles are more crowded. They hit the walls more often, so the pressure increases.
The formula (on the formula sheet) is:
\(P_1 \times V_1 = P_2 \times V_2\)
- \(P_1\) and \(V_1\) are the starting pressure and volume.
- \(P_2\) and \(V_2\) are the final pressure and volume.
Work Done and Temperature (Higher Tier Only)
When you do work on a gas (by compressing it quickly), you transfer energy to it. This increases the internal energy of the gas and can cause its temperature to rise. This is why the nozzle of a bike pump gets hot when you pump up a tyre—you are doing work on the air inside!
Summary Checklist
- Can you define Specific Heat Capacity and use the \(\Delta Q = m \times c \times \Delta \theta\) formula?
- Do you know that Specific Latent Heat happens at a constant temperature?
- Can you convert between Celsius and Kelvin? (Add/Subtract 273).
- (Physics Only) Can you use \(P_1 \times V_1 = P_2 \times V_2\)?
- (Higher Tier) Do you understand that doing work on a gas increases its temperature?
Don't worry if the formulas look scary at first! Just remember: they are tools to help you find the answer. Always list what you know (\(m = \dots, c = \dots\)) before you start calculating.