Thermal Energy

Heat Energy and Temperature

 When you hop into your car in the afternoon, touch the different surfaces. 

As long as they have all been in the same level of sunlight, they should all be the same temperature - right? but they are not

How about the beach, you walk bare feed on the grass and you watch out for prickles, but it is fine. walk on the sand and your feet start burning so you run to the water. The water is cooler. but why? why is the grass cool, the sand scorching and the water cold?

They have all been exposed to the same level of heat energy from the sun - so they should all be the same temperature...right? 

But they are not the same temperature, so something is going on here...

The image above shows that for the same amount of heat energy (Joules), the sand is hotter. But why? 

It is because it takes different amounts of energy to heat 1 kg of different substances by 10C. 

So when different substances are exposed to the same amount of energy, they will change temperature by different amounts.

When the sun shines, it gives the same amount of energy to both the sand and the water.

They will absorb the same amount of energy—but for this amount of energy, the sand will heat more. 

This is because it has a lower "heat capacity" than water. 

This is called the substance's Specific Heat Capacity.

Heat Energy vs Temperature

Heat energy and temperature are separate things. Temperature requires particles. Heat energy can move from the sun to the earth with no particles. 

The sun will give the earth a set amount of energy per hour. But different substances will get to different temperatures while sitting in the direct sunlight. Imagine a playground next to the beach. The metal slide will be hot, but the wood not. 

The things that get the hottest the fastest will also get the coldest the fastest. The hot metal slide will be a cold metal slide at night time. 

Things that get the hottest the fastest will also get the coldest the fastest. Conversely the things that take the longest to heat up will also take the longest to cool down. Think of your dinner, you are busy, so it starts to go cold - but different parts of your dinner will go colder faster than others. If you are having toast and baked beans for dinner, the toast will co colder faster than the baked beans. But likewise, it is quicker to turn bread into toast than to heat your baked beans. 

So: 

Heat energy - it is energy e.g. Infrared Heat Waves from the Sun

Temperature - what happens to the substance when exposed to that Energy

Well, what happens to the substance when it is exposed to Heat Energy? Its molecules start moving Faster!

The faster molecules move, the more Kinetic Energy the molecules have. 

So, for different substances, it takes different amounts of Heat Energy to force their molecules to move a certain amount (Kinetic Energy)

The more the molecules move, the more kinetic energy they have; the hotter the substance, the higher its Temperature.

So, the temperature results from the Kinetic Energy of the molecules in the substance.

Therefore, space has no temperature. But if you were in space in a "Temo" spacesuit, your molecules would absorb the heat energy and move faster. Thus, your temperature will increase - to about 2000C. Have a watch of the video below.

Specific Heat Capacity

You put two things into the oven for 3 minutes on high:



You then get them out

Which one is hotter?

Which one cools down the fastest?

Why?


Things that heat up quickly will cool down quickly/ Think of the metal dish. This is because they have a low heat capacity

So only small amounts of energy are needed to change their temperature

Give them small amounts of energy and their temperature increases quickly

They release small amounts of energy and their temperature drops quickly


Things that have a high heat capacity take ages to heat up. But also take a long time to cool down. Like the stone plate

It takes a lot of energy for it to increase its temperature, so it does so slowly

It will then release this energy slowly, so its temperature will reduce slowly

This is why metal is used for your stove top, but stone is used for stone grills and Hangi

Metal

Heats fast = low thermal capacity = cools fast 


Stone

Heats slowly = high thermal capacity = cools slowly (eg stone)


 The above video is actually REALLY good!

They actually explain the difference between stone and steel in a geeky Science way. You should watch it

The Thermal Capacity is how much energy a substance is capable of absorbing before its temperature increases by 10C 

To put it another way. It is the amount of energy (J) that 1Kg of a substance must absorb or release to change its temperature by 10C

To put it yet another way, the heat capacity is unique to a particular substance. Different substances have different heat capacities. To calculate the heat capacity of a substance, you measure how much energy you need to use to change 1kg of the mass by 10C in temperature:


Heat capacity (c) = Energy (J) / (1kg x 10C)


Heat capacity (c) = Energy [J] / (Mass [kg] x temperature change [0C])


The bigger the heat capacity number, the more energy it will take to heat up that substance. However, it will also take longer for that substance to release all that heat energy. 

If we heat a block of metal with the same mass as a block of water, you will see that the metal will get hotter quicker for the same amount of energy and cool down quicker — this is because it has a lower heat capacity.

The interactive below allows you to compare the Specific Heat Capacity of different substances. 

Have a play and see what you discover

Latent Heat Capacity

 Imagine that you are ..


This is as far as I've gotten to

Sorry

Please refer to your SciPads and the Past Papers on the OLE

Also, there is a good heat ppt on the OLE


Heat Energy Transfer

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