Heating up the classroom with

Thermal Energy

• Measure of kinetic energy of molecules

• Thermometer• Fahrenheit• Celsius• Kelvin

On the Kelvin scale, there is a reference point called absolute zero, which is defined as a temperature of 0 Kelvins. Absolute zero is impossible to reach.

• Measurement related to the quantity of molecules.

• Thermal energy depends on the mass, temperature, and phase(solid, liquid, or gas) of an object.

• As the temperature of an object rises so does its thermal energy.

• Thermal energy always moves from warmer to cooler objects.

• Heat is the result of molecules vibrating quickly.

• Heat is moving energy.• Transfer is the result of a difference in

temperature.

Calorimeter

•A calorimeter is an instrument used to measure changes in thermal energy.•Uses the principle that heat flows from a hotter object to a colder object until both reach the same temperature.•According to the law of conservation of energy the thermal energy released by a test sample is equal to the thermal energy absorbed by its surroundings. Calorimeter is sealed to prevent thermal energy from escaping.

•A heated piece of aluminum is placed in water. The change in temperature of the water is measured. Thermal energy absorbed by the water is calculated using the specific heat equation.

Calorimeter

Count Rumford (Benjamin Thompson)

• Count Rumford used a drill to make a cannon in 1798.

• He discovered that heat cannot be a form of matter.

Thermal contraction and expansion

•Thermal expansion is an increase in the volume of a material due to a temperature increase.•Thermal expansion occurs when particles of matter move farther apart as temperature increases.•Thermal expansion is used in glass thermometers. As temperature increases, the alcohol in the tube expands and its height increases. The increase in height is proportional to the increase in temperature.

Specific Heat– The ability of a substance to absorb

heat energy (specific heat) Different substances absorb heat at different rates-amount of energy required to raise the temp. of 1 kg of material by 1 degree Kelvin– units: J/(kg·K) or J/(g·°C)

Specific Heat Some things heat up or cool down faster than others.

Land heats up and cools down faster than water

Why does water have such a high specific heat?

Water molecules form strong bonds with each other; therefore it takes more heat energy to break them. Metals have weak bonds

and do not need as much energy to break them.

water metal

Specific Heat Cont….

-The lower a material’s specific heat, the more it’s temperature rises when a given amount of energy is absorbed by a given mass-The greater the mass of the object the more heat is absorbed

Temperature Conversions

• Formula (273+ºC)= Kelvin

How to calculate changes in thermal energy

Q = m x T x Cp

Q = change in thermal energy

m = mass of substance

T = change in temperature (Tf – Ti)

Cp = specific heat of substance

1. A piece of iron at a temperature of 145°C cools off to 45°C. If the iron has a mass of 10g and a specific heat of 0.449 J/g·oC, how much heat is given up?

2. How much heat is required to warm 230 g ofwater from 12°C to 90°C?

3. A 32-g silver spoon cools from 60°C to 20°C.How much heat is lost by the spoon?

Practice Problems

• The transfer of heat by direct contact between objects or particles.

• Heat transferred through space.

• EXAMPLE: The sun’s rays causing a sunburn.

• Heat transferred by the movement of molecules within a substance.

• Movement occurs from warmer areas to cooler areas.

• The movement of a substance that is caused by differences in temperature and density.

• EXAMPLES: wind boiling water

• NATURE WANTS EVERYTHING TO BALANCE ! !

• Conductors are materials that transfer heat easily.

• Examples: metals like copper and gold

• An insulator is a material that does not transfer heat easily.

• Examples: • liquids and gases• A jacket

ReviewWhat type of heat transfer is involved?1. Heating a room with a fireplace2. Egg cooking in a frying pan3. Roof of a house becoming hot

What is the formula for converting a Celsius temperature to a Kelvin temperature?

What is the boiling point of water on the Kelvin scale?

What is the freezing point of water on the Kelvin scale?

Questions

• What are the three types of heat transfer?

• How is conduction different from radiation?

References

Wilmot High School

Lexington