1. Chapter 17
Heat

2. Heat
a form of energy caused by the internal motion of molecules of matter
18th century scientists believed that heat was an invisible, weightless fluid capable of flowing from hotter objects to colder ones (called caloric)

3. Remember
all matter is made up of molecules which are always in motion

4. Heat Transfer
the movement of heat from a warmer object to a cooler one
three ways = conduction
convection
radiation

6. Conduction
heat is transferred through a substance or from one substance to another by direct contact of one molecule to the next
faster moving molecules collide with slower moving molecules
works best in solids

7. Conductors
a substance that conducts heat better & more rapidly than other substances
Examples = metals (iron, aluminum, silver, copper)

8. Insulators substances that do not conduct heat easily
Examples = glass, wood, plastic, rubber, styrofoam, air (think layered clothing)

9. Convection transfer of heat through liquids & gases
liquid or gas heats up, molecules move faster, substance expands, becomes less dense & rises
process reverses on the way down (cools, molecules slow, substance contracts, becomes more dense & sinks)

10. Where are Convection Currents Active?

11. Radiation heat is transferred through empty space
Examples = sun to Earth, heat from open flame (fire or candle), heat near stove or electric heater

13. Temperature a measure of how hot or cold something is
higher temps = faster moving molecules
lower temps = slower moving molecules

15. Thermometers used to measure temperature
most contain a liquid that expands & rises when heated, then cools, contracts, & sinks

16. Which thermometer shows the freezing point of water?
Which thermometer shows the boiling point of water?

17. Types of Thermometers
Celcius scale = metric system based on powers of 10
Fahrenheit scale
Kelvin scale = used by scientists to reach very low temps (absolute zero)

19. Measuring Heat heat CANNOT be measured directly
an increase in temp indicates that heat is being added
a decrease in temp indicates that heat is being removed

20. Measuring Heat
measured in units called calories (cal) – the amount of heat needed to raise the temp of 1 gram of water 1°C
1 cal = 4.19 Joules

21. Specific Heat Capacity
the ability of a substance to absorb heat energy
the # of calories needed to raise the temp of 1 gram of substance 1°C

22. Specific Heat of Water water has a high specific heat
it takes a lot of energy to heat a body of water (like a lake)
water heats slowly & loses heat slowly

23. Calorimeter
instrument used to measure the heat given of in a chemical reaction

24. Thermal Expansion
most substances (solids, liquids, & gases) expand when their temperature increases
think about how bridges, railroad tracks, & sidewalks all have spaces between each section

25. Thermal Expansion
solids – molecules vibrate faster causing the distance between molecules to increase
liquids – molecules move faster & farther apart (one big exception is water)
gases – molecules already far apart, additional heating & expansion may lead to an explosion if contained

26. Describe what happens when you leave the lid on a container of food you’re heating up in the microwave.

27. Water Expansion
unlike most, water expands when it cools (4°C to 0°C), not when it heats
this is why ice floats – as it cools & expands, it becomes less dense than the liquid water
also causes major damage to roads & sidewalks

28. Ice Wedging

29. Thermostats used to control temperature
electrical appliances (ovens, air conditioners, electric blankets, refrigerators, heating systems) all utilize a switch called a BIMETALLIC STRIP

30. Thermostat

31. Bimetallic Strip
the two faces of the strip are made of two different metals joined together
different metals expand at different rates as they warm up
strip is straight at room temperature, and bends in different directions when heated or cooled

32. Bimetallic Strip

33. Bimetallic Strip
you can coil a long strip, making it more sensitive to small changes in temp

34. In this diagram, the green metal would be chosen to expand faster than the blue metal if the device were being used in an oven. In a refrigerator you would use the opposite. In a refrigerator, as the temperature rises the blue metal expands faster than the green metal. This causes the strip to bend upwards, making contact so that current can flow. By adjusting the size of the gap between the strip and the contact you control the temperature.