Energy Transfers and Particle Motion: CORE Physics Study Notes
Welcome to this important chapter! Don't worry if the physics of heat seems confusing sometimes—we are going to break it down. This section links the tiny world of atoms and molecules to the big changes we see every day, like water boiling or ice melting. Understanding how energy affects particles is the key to mastering the Particle Model of Matter.
Ready to dive in? Let's go!
1. The Particle Model and Internal Energy
Remember from the previous section that matter is made up of tiny particles (atoms or molecules) that are always moving? When we heat something up, we are simply giving those particles more energy.
a. Defining Internal Energy
The particles inside any substance (solid, liquid, or gas) have energy. The Internal Energy of a system is the total energy stored by the particles within that system.
Internal Energy is made up of two parts:
-
Kinetic Energy (KE): Energy due to the movement or vibration of the particles.
(In solids, particles vibrate. In liquids and gases, particles move randomly and quickly.) -
Potential Energy (PE): Stored energy due to the forces between the particles and their positions (bonds).
(This energy increases when particles move further apart, like when a solid melts into a liquid.)
Quick Link: Temperature and Kinetic Energy
When you heat a substance, the particles move faster (or vibrate more intensely). This increase in movement means the average Kinetic Energy of the particles has increased. We measure this increase in average kinetic energy as a rise in Temperature.
🔑 Key Takeaway 1: KE and PE
Temperature only depends on the Kinetic Energy (speed/vibration) of the particles. Internal Energy is the sum of KE + PE.
2. Energy Required for Temperature Change (Heating)
If you put a metal spoon and a wooden spoon into the sun, the metal spoon feels much hotter, much faster. Why? Different materials need different amounts of energy to change their temperature. This leads us to Specific Heat Capacity.
a. Specific Heat Capacity (SHC)
The Specific Heat Capacity (c) of a substance is the amount of energy (in Joules, J) required to change the temperature of 1 kilogram (kg) of that substance by 1 degree Celsius (°C).
Analogy: Think of SHC like a heat "sponge."
- If a material has a high SHC (like water), it is a large sponge. It needs to absorb a lot of energy to feel just a little warmer.
- If a material has a low SHC (like metal), it is a small sponge. It absorbs a little energy and heats up very quickly.
b. The Specific Heat Capacity Equation
We can calculate the energy (Q) needed to change the temperature of a substance using this formula:
\[Q = mc\Delta\theta\]
- \(Q\) = Change in thermal energy (Joules, J)
- \(m\) = Mass of the substance (kilograms, kg)
- \(c\) = Specific Heat Capacity (\(\text{J/kg}^{\circ}\text{C}\))
- \(\Delta\theta\) = Change in temperature (final temp - initial temp) (\(^{\circ}\text{C}\))
Don't worry about the Greek letter \(\Delta\theta\). It just means "the change in temperature."
Example Step-by-Step:
Imagine you have 2 kg of water (\(c = 4200\ \text{J/kg}^{\circ}\text{C}\)) and you want to raise its temperature by 10 °C.
- Identify variables: \(m = 2\ \text{kg}\), \(c = 4200\ \text{J/kg}^{\circ}\text{C}\), \(\Delta\theta = 10^{\circ}\text{C}\).
- Substitute into formula: \(Q = 2 \times 4200 \times 10\).
- Calculate: \(Q = 84,000\ \text{J}\).
🛑 Common Mistake to Avoid
Students sometimes confuse mass (\(m\)) with the Specific Heat Capacity (\(c\)). Remember that mass is how much stuff you have, and SHC is how stubborn that stuff is about changing temperature.
3. Energy Required for Change of State (Phase Changes)
If you boil water, the temperature stops rising when it hits 100 °C, even though you are still supplying energy! Where is that energy going?
a. Heating vs. Changing State
When energy is supplied to a substance, two things can happen:
- If the temperature is rising: The energy is increasing the particle Kinetic Energy (KE). This is where SHC applies.
- If the temperature is constant: The energy is being used to break the bonds holding the particles together. This increases the particle Potential Energy (PE). This is called a change of state (e.g., melting, boiling).
Did you know? During melting or boiling, the potential energy of the substance increases, but the kinetic energy stays the same (because the temperature is constant).
b. Specific Latent Heat (SLH)
The Specific Latent Heat (L) is the amount of energy (in Joules, J) required to change the state of 1 kilogram (kg) of a substance without changing its temperature.
We use different terms depending on the change:
- Specific Latent Heat of Fusion (\(L_f\)): Energy needed for melting (solid to liquid) or freezing (liquid to solid).
- Specific Latent Heat of Vaporisation (\(L_v\)): Energy needed for boiling (liquid to gas) or condensation (gas to liquid).
Analogy: Melting/Boiling is like climbing a hill. You use energy to get to the top (break bonds), but you aren't moving faster sideways (temperature is constant) until you reach the next level.
c. The Specific Latent Heat Equation
The calculation for the energy required during a change of state is simpler, as there is no temperature change involved:
\[Q = mL\]
- \(Q\) = Change in thermal energy (Joules, J)
- \(m\) = Mass of the substance (kilograms, kg)
- \(L\) = Specific Latent Heat (\(\text{J/kg}\)) (use \(L_f\) or \(L_v\) as appropriate)
Example Step-by-Step:
You have 0.5 kg of steam condensing back into water (\(L_v\) for water is approximately \(2,260,000\ \text{J/kg}\)). How much energy is released?
- Identify variables: \(m = 0.5\ \text{kg}\), \(L = 2,260,000\ \text{J/kg}\).
- Substitute into formula: \(Q = 0.5 \times 2,260,000\).
- Calculate: \(Q = 1,130,000\ \text{J}\).
📝 Quick Review: SHC vs. SLH
Always ask yourself: Is the temperature changing?
If YES (heating up): Use Specific Heat Capacity, \(Q = mc\Delta\theta\). (Changing KE)
If NO (changing state): Use Specific Latent Heat, \(Q = mL\). (Changing PE)
4. Summary of Energy Transfers
Understanding these two equations is crucial for the Core Physics exam. Any problem involving heating or cooling a substance will use one or both of these concepts.
a. Applying the Rules (Encouragement!)
When solving problems, read the scenario carefully. If you are heating ice from -10 °C up to steam at 110 °C, you would need to calculate five separate steps:
- Heating the solid (ice) using \(Q = mc\Delta\theta\).
- Melting the ice using \(Q = mL_f\).
- Heating the liquid (water) using \(Q = mc\Delta\theta\).
- Boiling the water using \(Q = mL_v\).
- Heating the gas (steam) using \(Q = mc\Delta\theta\).
Don't worry if doing all five steps seems tricky! CORE physics typically focuses on one or two steps at a time. The most important skill is choosing the correct formula for the process described.
You've made it through the trickiest part of the particle model. Keep practising those equations, and you'll master how energy moves matter!