Chapter 17

Heat

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)

Remember

•all matter is made up of molecules which are always in motion

Heat Transfer

•the movement of heat from a warmer object to a cooler one

•three ways = conductionconvectionradiation

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

Conductors

•a substance that conducts heat better & more rapidly than other substances

•Examples = metals (iron, aluminum, silver, copper)

Insulators

•substances that do not conduct heat easily

•Examples = glass, wood, plastic, rubber, styrofoam, air (think layered clothing)

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)

Where are Convection Currents

Active?

Radiation•heat is transferred through empty space

•Examples = sun to Earth, heat from open flame (fire or candle), heat near stove or electric heater

Temperature•a measure of how hot or cold something is

•higher temps = faster moving molecules

•lower temps = slower moving molecules

Thermometers•used to measure temperature

•most contain a liquid that expands & rises when heated, then cools, contracts, & sinks

• Which thermometer shows the freezing point of water?

• Which thermometer shows the boiling point of water?

Types of Thermometers

1.Celcius scale = metric system based on powers of 10

2.Fahrenheit scale3.Kelvin scale = used by

scientists to reach very low temps (absolute zero)

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

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

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

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

Calorimeter

•instrument used to measure the heat given of in a chemical reaction

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

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

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

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

Ice Wedgin

g

Thermostats

•used to control temperature

•electrical appliances (ovens, air conditioners, electric blankets, refrigerators, heating systems) all utilize a switch called a BIMETALLIC STRIP

Thermostat

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

Bimetallic Strip

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

• 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.