Heat

HeatHeat Energy:

Heat is thermal energy transferred from one object to another.

Do not confuse heat and temperature!

There are three mechanisms of heat transfer:

1) Conduction – highly energetic atoms collide with less energetic ones giving them energy.

2) Convection - highly energetic molecules move from one place to another, carrying thermal energy with them.

3) Radiation - atoms or molecules emit electromagnetic waves; the energy carried by these waves is converted back to thermal energy when the waves interact with some form of matter.

1.Conduction is the mechanism of heat transfer in which highly energetic atoms or molecules collide with less energetic atoms or molecules, giving them some energy.

Ex: a skillet on a stove

Conduction FURTHER EXPLAINED: If one end of a metal rod is at a higher temperature, then energy will be transferred down the rod toward the colder end because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones.

GoldGold, , silversilver, , coppercopper, and , and aluminumaluminum

are the best are the best conductorsconductors. .

Most other solid substances are Most other solid substances are poorer poorer conductors conductors of heat. of heat.

Liquids (except Liquids (except mercurymercury))

are mostly poor conductors. are mostly poor conductors. Materials which are very poor Materials which are very poor conductors of heat are calledconductors of heat are called

insulators insulators or or nonconductorsnonconductors..

2.2. Convection Convection is the is the mechanism of heat mechanism of heat transfer in which highly transfer in which highly energetic molecules energetic molecules move from one place to move from one place to another.another.Ex: water in a heated potEx: water in a heated pot

Convection: heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it.

Convection above a hot surface occurs because hot air expands, becomes less dense, and rises. Hot water is also less dense than cold water and rises, causing convection currents which transport energy.

Convection

Heated air rises, cools, thenfalls.  Air near heater isreplaced by cooler air, andthe cycle repeats.

Heated water expands and becomes more buoyant. Cooler, more dense water near the surface descends and patterns of circulation can be formed.

 Natural Convection

                          

Air above warmer groundrises.

                           

Inversion layer.  Air near ground is more dense thanair higher up; no convectioncurrents to lift pollutants.

                   

Convection causes ocean water from the tropics to flow in currents towards higher latitudes

3. Radiation is the mechanism of heat transfer in which atoms or molecules emit electromagnetic waves.

ex: Sun

3. Radiation – thermal energy transfer in which hot atoms emit heat in the form of infrared electromagnetic waves.

The energy carried by these waves is converted back to thermal energy only when the waves strike some form of matter.

Radiation is the method by which heat from the sun travels through the vacuum of space to reach earth.

Dark colors absorb heat better than lighter colors. Dark colors absorb heat better than lighter colors. Shiny surfaces reflect heat better that dull Shiny surfaces reflect heat better that dull

surfaces.surfaces.

Heat transfer among objects or substances on earth usually involves convection and/or conduction too.

Specific Heat Capacity

• The amount of heat needed to change the temperature by 1°C for 1g of substance.

• Each substance has a unique specific heat capacity.

Substance s.h.capacity

(J/g °C)

Fresh water

4.18

Ocean water

3.89

Air 1.00

Q=mc∆T

• Q = energy absorbed

• m = mass of substance

• c = specific heat capacity of substance

• ∆T = change in temperature

Sample problem

1. What is the heat absorbed (Q) if 2.0g water is heated from 10°C to 25°C ?

• Q=mc∆T=2.0g x 4.18J/g °C x (25 – 10 °C)=125.4 J

J = joules, the unit for energy

Practice:

1. What is the Q, if 5.2kg sample of limestone has a change in temp. of 16.3 °C? (c = 0.92)

2. What is the change in temp, if a 9.2g sample of mud absorbs 950J of energy? (c = 2.51)

3. An unknown sample absorbs 6000J and has a temp change of 62 °C. It has a mass of 2.6kg, what is the specific heat capacity?

1) What is the Q, if 5.2kg sample of limestone has a change in temp. of 16.3 °C?

Solution Q=5.2 x 0.92 x 16.3Q = 77.98 KJ

2) What is the change in temp, if a 9.2g sample of mud absorbs 950J of energy?

Solution 950 = 9.2 x 2.51 x T950/23.092T= 41.13

3) An unknown sample absorbs 6000J and has a temp change of 62 °C. It has a mass of 2.6kg, what is the specific heat capacity?

Solution 6000J = 2.6 x c x 626000 / 161.2 = c37.2 J/goc = c

Heat of Fusion Q=mHfus

• Amount of heat required to melt 1.0g of a solid at the freezing point into a liquid.

• Amount of heat needed to freeze 1.0 g of a liquid at the freezing point into a solid

• Water has a large heat of fusion.

Heat of vaporization Q=mHvap

• Amount of energy required to change 1.0g of a liquid at the boiling point into a gas at the boiling point.

• If the gas returns back into a liquid, then the same amount of heat must be lost. (condensation)

Latent Heats for a Substance

Latent Heat Sample Question

• Ted is making ice cubes that each contain 50 g of pure water. When the water reaches a temperature of 0 degrees, it begins to freeze. How much heat is lost in the process of the water freezing for each ice cube. (LHF of water is 333 J/g)

Q = mHfus = 50(333) = 16,650 J per ice cube