Chapter 11.1 Temperature, Thermal Energy, and Heat
Moving objects have kinetic energy. Matter is made up of tiny particles that are always moving, so these particles have kinetic energy.
Temperature is a measure of the average kinetic energy of the particles of an object.
The United States uses the Fahrenheit scale to measure temperature.
212* F – water boils
98.6*F – body temperature
32*F – water freezes
Fahrenheit invented his scale in 1724. He used the freezing point of salt water as his starting point of his thermometer at 0*F, then regular water froze at 32*F. He estimated the temperature of the human body to be 96*F (it’s actually 98.6*F), and then boiling water at 212*F.
Most countries use the Celsius scale. 0*C – water freezes
37*C – body temperature
100*C – water boils
Celsius developed his scale in 1742, and decided the freezing point of water should be 100*C and boiling point of water should be 0*C, and he adjusted everything else to fit in between those two numbers. He died 2 years later, and another scientist, Linnaeus, switched the 0 and the 100 around.
And then there is the Kelvin scale, which was invented in 1848. The Kelvin scale is divided into Kelvins, not degrees.
Zero Kelvins is ABSOLUTE ZERO, the lowest temperature possible in the solar system. At absolute zero, particles have no kinetic energy – even the particles in solids have appeared to stop vibrating in place.
0 K = -273*C = -459*F
Different objects at the same temperature can have different amounts of energy. Temperature, thermal energy, and heat are closely related, but they are not the same thing.
Thermal energy – is the total energy of all the particles in an object. It depends on the temperature of an object, the number of particles in it, and how those particles are arranged.
The more particles an object has at a given temperature, the more thermal energy it has.
Example: If all these mugs had coffee in them at the same exact temp of 75*C, which would have the most thermal energy?
70*C 75*C
But if two mugs hold the same amount of coffee, the hotter temperature will have more thermal energy.
Heat – the transfer of thermal energy from a warmer object to a cooler object. An object cannot contain heat, it contains thermal energy.
The warmer object will cool down and the cooler object will warm up until they are the same temperature. When this happens, heat stops transferring. Heat is measured in the units of energy – joules.
11.2 Transfer of Heat
Heat is transferred around you all the time. If it wasn’t, nothing would ever change temperature.
Heat travels only in one direction and by three different methods.
Heat is transferred from warmer areas to cooler areas by conduction, convection, and radiation.
Conduction – transfers heat from one particle of matter to another within an object or between two objects. Must be touching!
The fast moving particles in the hot cookie sheet transfer to the slow-moving particles in the cookie dough, causing them to move faster and make the cookies hotter.
Convection – is a type of heat transfer that occurs only in fluids, such as water and air.
When air is heated, its particles speed up and move farther apart. This makes the heated air less dense, so it rises up. Cooler air flows into its place, then heats up and also rises. Previously heated air cools down, sinks, and the cycle repeats. This flow is called a convection current. Convection currents in air cause wind and weather changes.
Radiation - the transfer of energy by electromagnetic waves. Radiation is the only form of heat transfer that does not require matter.
You can feel radiation from a fire without touching the flames. The suns energy travels to Earth through 150 million kilometers of empty space.
11.3 Thermal Properties
If you walk barefoot from carpet to a tile floor, you will notice that the tile feels colder than the carpet – but they are the same temperature – room temperature!
The difference has to do with how materials conduct heat. Some materials conduct heat well, while other materials do not.
Conductors – a material that conducts heat well (heat transfers in or out)
Metals are good conductors. A metal flagpole in the summer feels hot because the heat easily transfers out of the pole into your hand. The tile floor is a good conductor, too, and feels cold because the heat easily transfers out of your skin and into the tile.
Insulators – materials that do not conduct heat well.
A wooden pole and carpet are good insulators, and a wool blanket slows the transfer of heat out of your body.
Specific Heat – The amount of energy required to raise the temperature of 1 kilogram of a material by 1 kelvin
How can sand on a beach be super hot, and the ocean be super cold? The sun heats them both, but water requires more heat to raise its temperature than sand does.
Temperature does not rise at the same rate for all objects.
To change the temperature of different objects by the same amount, different amounts of thermal energy are required.
To calculate thermal energy changes, use a formula:
Energy change = Mass x Specific Heat x Temperature Change
Energy change = 2 kg x 4,180 Joules/kg x 3 K
Energy change = 25, 080 Joules
Specific Heat of various materials:
Copper = 385 J/kg Silver = 235 J/kg
Water = 4,180 J/kg Iron = 450 J/kg
Glass = 837 J/kg
Thermal Expansion
To loosen a jar lid, hold it under a stream of hot water. The metal lid expands more than the glass as it gets hotter, making it easier to open.
As the thermal energy of matter increases, its particles spread out, causing the substance to expand.
Thermal expansion – the expanding of matter when it is heated.
When matter is cooled, thermal energy is released, so it decreases in volume and contracts.
Exception to the rule - Water expands slightly when it freezes; this is why solid ice floats in a glass of liquid water.