Heat and Temperature
Section: Physics | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
Hot and Cold
Hot soup feels hot on your tongue, ice cream feels cold in your hand, a warm bath feels comfortable. But why do things feel hot or cold? It's all about particle movement!
- Everything is made of tiny particles that are constantly moving and vibrating
- The faster they move, the hotter something feels
What is Temperature?
- Temperature tells us how hot or how cold something is – it measures how fast particles are moving (vibrating) on average
- What temperature tells us:
- hot soup (80°C) = particles moving fast
- warm water (40°C) = particles moving medium speed
- cold water (10°C) = particles moving slowly
- ice cream (-20°C) = particles moving very slowly
- Temperature also tells us which way energy will flow:
- energy always flows from hot to cold, never from cold to hot
- think of it like water flowing downhill
Worked Example: Ice Cream Melting
Museli leaves ice cream at -20°C (very cold) in a room at 24°C (warmer) and wants to explain what happens.
- Step 1: find the temperature difference. 24°C − (−20°C) = 44°C
- Step 2: apply the rule. Thermal energy flows from the warm air into the cold ice cream
- Step 3: state the key rule. The bigger the temperature difference, the faster energy transfers
Temperature and Particle Speed
| Temperature | Particle Speed | Example |
|---|---|---|
| Very High (100°C) | Vibrating very fast | Boiling water |
| High (80°C) | Vibrating fast | Hot soup |
| Medium (40°C) | Vibrating moderately | Warm bath |
| Low (10°C) | Vibrating slowly | Cold tap water |
| Very Low (-20°C) | Vibrating very slowly | Freezer |
- Temperature measures the average speed/energy of particles – think of it like the average test score in your class
What is Heat?
- Heat is the total amount of thermal energy in an object – it's all the energy from all the particles added together
- Heat can be transferred from one object to another - one of the ways this happens is by conduction (thermal energy passing between particles that are touching); see the Transfer of Thermal Energy topic for the other methods
- The key difference:
- Temperature = average energy (like average test score)
- Heat = total energy (like total points from all students)
Common Mistakes
- Temperature and heat are NOT the same thing! This is a very common mistake
Heat vs Temperature: The Key Difference
Scenario 1: Same Volume, Different Temperatures
Two glasses with the SAME amount of water: Glass A is 100 ml at 20°C (cold), Glass B is 100 ml at 60°C (hot).
- Glass B has a higher temperature (particles moving faster)
- Glass B has more heat (more total energy)
- Same number of particles, but they have more energy each
Scenario 2: Different Volumes, Same Temperature
This is the tricky one that confuses many students! Two glasses at the SAME temperature: Glass A is 100 ml at 50°C (small), Glass B is 200 ml at 50°C (large).
- Same temperature (particles moving at the same average speed)
- Glass B has more heat (twice as much!) because more particles means more total energy, even though the average energy per particle is the same
- A bathtub full of warm water (40°C) has more heat than a cup of boiling water (100°C) – the cup is hotter (higher temperature), but the bathtub has more heat (more total energy because it has millions more water particles)
Visual Comparison
| Property | Small Glass (100ml at 50°C) | Large Glass (200ml at 50°C) |
|---|---|---|
| Temperature | 50°C | 50°C (same) |
| Particle speed | Fast (average) | Fast (average) – same |
| Number of particles | Fewer particles | More particles (twice as many) |
| Total heat energy | Lower | Higher (twice as much) |
The Sparkler Mystery
A sparkler can reach temperatures of about 1000°C – that's incredibly hot! So why doesn't it burn you badly when a spark lands on your hand?
- Very small mass = very few particles: one spark contains only a tiny amount of material compared to your hand
- Few particles = small total energy (low heat): even though each particle has lots of energy (high temperature), there aren't many particles, so the total energy (heat) is very small
- Large temperature difference = energy transfers quickly: spark at 1000°C, air at 24°C is a huge difference, so energy flows rapidly from spark to air. By the time the spark touches your skin, its temperature and heat have both decreased a lot
Comparing Different Materials
Temperature is really useful for comparing different materials and amounts.
Worked Example: Hot Soup vs Cold Water
Muchindu compares a bowl of hot soup at 80°C with a glass of cold water at 10°C.
- Step 1: compare temperatures. The soup has a higher temperature (its particles are moving faster on average)
- Step 2: note that temperature lets us compare easily, even though the soup and water have different amounts and types of particles – the soup is simply "hotter"
- Step 3: predict energy flow. We know energy will flow from the soup to the surroundings, not the other way
Absolute Zero: The Coldest Possible
- Absolute zero is the coldest possible temperature that can exist, at -273°C (also called 0 Kelvin), where particles would theoretically stop moving completely
- The discovery:
- a scientist named Lord Kelvin predicted this in the 1800s
- he noticed that as temperature drops, particles slow down
- he predicted that at some point, particles would stop moving completely – the lowest possible temperature, "absolute cold"
- What happens at absolute zero:
- at -273°C, particles would theoretically stop moving completely
- no particle movement means no kinetic energy
- this is as cold as anything can possibly get
- Can we reach it? No, not completely. Scientists have gotten incredibly close (within billionths of a degree), but it's impossible to reach exactly 0 Kelvin
- there are fundamental laws of physics that prevent it
- this shows there's a limit to how cold things can get (but no limit to how hot!)
- it helps scientists understand how particles behave, and is used in advanced scientific research and technology
Temperature Scales
| Event | Celsius (°C) | Kelvin (K) |
|---|---|---|
| Absolute zero | -273°C | 0 K |
| Water freezes | 0°C | 273 K |
| Room temperature | 20°C | 293 K |
| Water boils | 100°C | 373 K |
Temperature Sign and the State of Water
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