P1a(i) Heating Houses You will learn about: The differences between heat and temperature How energy flows How to read a thermogram

Temperature Temperature is a measure of how hot something is. It uses a man-made scale or unit: ° C which means degrees Celsius. You may find thermometers that use Fahrenheit or Kelvin too. The particles in a substance are constantly vibrating. When the temperature increases, the particles vibrate faster. Therefore the kinetic energy increases. So, temperature is the measure of the average kinetic energy the particles have. This thermometer reads a temperature of -33°C. It could also be -26°F. Remember we use °C as the standard unit

Heat Heat is a measurement of the internal energy an object has. The unit of energy is a Joule. Hotter objects emit more thermal energy Colder objects do not emit as much thermal energy Remember cold objects emit less thermal energy… they do not emit cold energy Careful… Blue flames are actually hotter than orange flames. Therefore they emit more thermal energy. Use your knowledge of the Bunsen burner… the roaring flame is blue Remember… work and energy are precious like a precious jewel… so the unit for work and energy is the Joule!

What is the difference between temperature and heat? A swimming pool at 25°C… …is at a lower temperature than a cup of coffee at 60°C… …but the swimming pool contains more water, so it stores more thermal energy than the cup of tea. Here we have a cup of coffee with a higher temperature but a lower amount of thermal energy. So, temperature is just a scale of how hot something is. Heat is the amount of thermal energy stored in it

Cooling down Hot objects cool down at a faster rate than warmer objects. Freezing cold objects warm up at a faster rate than cool objects. Objects want to be at the same temperature as the room: room temperature. The further away from room temperature the object is then the faster the rate at which it transfers energy to get to room temperature Room temperature, 20°C Warm up at a faster rate Warm up at a slower rate Cool down at a faster rate Cool down at a slower rate 0°C 40°C Temperature (°C) Time (seconds) Steep gradient so object cools at a faster rate Shallow gradient so object cools at a slower rate Room temperature

Why is this? How does energy flow? Energy, in the form of heat, flows from where it is warm to where it is cold. If you leave your front door open then the warm thermal energy in your house will go outside to where it is cold. As the heat has now left, the house feels colder. Heat moves from hot to cold… straight out your front door! Here, your hands are warmer than the ice cubes. So heat moves from hot to cold. So from your hand and into the ice. So the ice warms up and starts to melt. And as your hands have transferred (or ‘lost’) some heat they now feel colder! That’s why ice feels cold! Never cold to hot…

Thermogram Thermograms are made using a special camera that identifies heat. Whitered yellow White, red and yellow represent the hot areas. blackdark blue purple Whilst black, dark blue and purple represent the cold areas. Red so hot area Dark blue so cold area Be Careful: Look back at slide 3… Bunsen burners are hottest when blue and coldest when red… Hot Cold

Quick Questions… 1.What is the unit of temperature? 2.What is the unit to measure heat? 3.A loaf of bread is taken out of the freezer. It does not defrost immediately. Why does it take a couple of hours to defrost? 4.What does this picture show? 5.Nitrogen freezes at -210°C and boils at -196°C. When a lump of ice at -5°C is dropped into liquid nitrogen, the nitrogen boils. Why?

Quick Questions… 1.What is the unit of temperature? °C or Degrees Celsius 2.What is the unit to measure heat? J or Joules 3.A loaf of bread is taken out of the freezer. It does not defrost immediately. Why does it take a couple of hours to defrost? The bread is about -5°C and the room temperature is about 20°C. The difference is small. So the room will transfer thermal energy into the bread at a slow rate (remember heat moves from hot to cold, so in this case from the room into the bread) so it will take a long time to defrost. However, if the room was much hotter and the bread much colder then the temperature difference would be greater so the bread would defrost at a faster rate. 4.What does this picture show? Thermogram of an overheated engine. Red and white represents hot. Tyres are also hot so the driver must have recently stopped the car to inspect the engine problem. 5.Nitrogen freezes at -210°C and boils at -196°C. When a lump of ice at -5°C is dropped into liquid nitrogen, the nitrogen bois. Why? The ice is very, very hot compared to the liquid nitrogen. The ice transfers its heat (remember from hot to cold) to the nitrogen. Nitrogen boils at a really cold temperature so the heat from the ice is sufficient to boil it.