THERMAL ENERGY AND HEATSPH4C/SPH3U

THERMAL ENERGY• James Prescott Joule (1818-1889) spent much of his

honeymoon studying waterfalls.• He noticed that the water at the bottom

of a waterfall had a higher temperature than at the top.

• How might this happen?

THERMAL ENERGY AND HEAT• Thermal energy and heat play significant roles in

our lives from the furnaces that heat our homes to winds generated by the uneven heating of the Earth’s surface. Even most of the food that we consume is converted into thermal energy.

THERMAL ENERGY• The total kinetic energy and potential energy of the

atoms or molecules of a substance. • Depends on: mass, temperature, nature and state

of matter

HEAT• A measure of the energy transferred from a warm

body to a cooler body because of a difference in temperature.

CALCULATING HEAT• The amount of heat released or gained during a

temperature change can be found from,

• Where,– is the heat in Joules

– is the mass of substance

– is the specific heat capacity J/kgoC

– is the change in temperature

SPECIFIC HEAT CAPACITY• Scientists define specific heat capacity of a

substance as the amount of heat needed to raise the temperature of 1 kg of that substance by 1 0C.

• These are generally known values for most substances.

EXAMPLEA beaker of 250 g of water is heated over a bunsen burner from room temperature (20 0C) to boiling point (100 0C). The heat capacity of water is 4180 J/kg0C. How much heat does the water gain?

TEMPERATURE• A measure of the average kinetic energy of the

atoms or molecules of a substance.

HOW HEAT SPREADS FROM ONE REGION TO ANOTHER• All things are made up of molecules• When objects are heated, they absorb thermal

energy.• This means that the molecules are absorbing the

thermal energy.• With more energy, the molecules are able to move

faster.• When the molecules move faster, the temperature

of the object increases.• Temperature increase means the object gets hotter.

EXAMPLE 1 • Consider two samples of water: 100 g at 50 °C and

500 g at 50° C

50° C

50° C

50° C

EXAMPLE 1 • The two samples have the same temperature, but

the bigger sample contains more thermal energy because there’s more of it. If the samples were mixed, no heat transfer would occur because they are the same temperature

50° C

50° C

50° C

EXAMPLE 2 • Consider two other samples of water: 500 g at 50°C

and 500 g at 90° C

50° C 90° C 70° C

EXAMPLE 2 • The masses are the same but the warmer sample

has more thermal energy because the water particles have more motion, that is, the average kinetic energy of the molecules is greater at a higher temperature. When the two samples are mixed, heat would transfer from the 90 °C sample to the 50° C.

50° C 90° C70° C

EXAMPLE (GRADE 11 PHYSICS)A beaker containing 250 g of water at 250C is poured into another beaker that initially contains 350 g of water at 850C. What is the final temperature of the mixed water?

METHODS OF HEAT TRANSFER

THREE METHODS OF HEAT TRANSFER• Conduction– Process by which the collision of atoms and electrons

transfers heat through a material or between two materials in contact.

• Convection– Process of transferring heat by a circulating path of fluid

particles.

• Radiation– Process in which energy is transferred by means of

electromagnetic waves.

CONDUCTION• Collision of atoms and electrons transfers heat• Particles with more kinetic energy transfer some of

their energy to neighbouring particles with lower kinetic energy increases the kinetic energy of the neighbouring particles.

• Occurs mainly in solids• Two types of conduction– Molecular vibration

– Free electron diffusion

Note: Conduction is not the main form of heat transfer in liquids and gases because their molecules are spaced further apart.

MOLECULAR VIBRATION

• When heat is supplied to one end, the molecules at the hot end start to vibrate more vigorously.

• In the process, they ‘bump’ into their neighboring molecules. In doing so, some energy is transferred to the neighbour.

• The neighbour molecule gains energy and starts to vibrate more vigorously. The cycle continues.

FREE ELECTRON DIFFUSION

• This form of conduction takes place only in metals. As only metals have free electrons.

• The electrons are freed from the molecule when heated and they travel towards the cold end.

• At the cold end they collide into a molecule therefore passing all their energy to the molecule.

METHODS OF CONDUCTIONMolecular vibration Free electron diffusion

Occurs in all solids

Slow process

Occurs in metals only

Fast process

This explains why metals heat up faster:

1. Metals have 2 mechanisms of conduction occuring at the same time.

2. In metals, free electron diffusion is the main mechanism, which is faster.

CONDUCTORS AND INSULATORS• Materials that can conduct heat easily

and readily (eg. Metals) are known as conductors.

• Materials that do not conduct heat easily (eg. Water, air, plastic) are known as insulators.

CONVECTION• Transferring heat by a circulating path of fluid

particles.• Occurs in liquids and gases• Does not occur in solids because the molecules are

not free to move around

EXAMPLETaking the example of heating water• Water at the bottom is heated first• Heated water expands• When water expands density decreases• Heated water of lower density starts to rise• Cooler water of higher density rushes in from sides

to take its place• The cooler water gets heated and the cycle repeats.• Convection currents are set up.

CONVECTION• Since Earth’s surface is over 70 percent water,

water has a large effect on Earth’s climate. Therefore, regions closer to large bodies of water tend to experience more moderate weather conditions than regions farther from them.

RADIATION• Energy is transferred by means of electromagnetic

waves.• Radiation does not require a medium to transfer

heat. (can occur in a vacuum)• Sun releases electromagnetic waves (heat is

contained in the waves as infra-red)• Hotter objects radiates more heat.

RADIATION• Any substance at a higher temperature than its

surroundings will emit radiant energy, usually as infrared radiation. The warmed matter then transfers some of its thermal energy to substances at lower temperatures or re-emits it as IR.

EMITTERS AND ABSORBERS• The Sun gives out the heat.–It is known as an emitter / radiator

• The Earth takes in the heat.–It is known as an absorber.

CONSERVATION OF ENERGY

Law of Conservation of Energy

When energy changes from one form to another, no energy is created or destroyed.

In ideal situations, no energy is lost to

friction.

In real situations, some energy is needed to overcome friction.

This results in the production of waste thermal energy and, sometimes, sound

energy.

ENERGY TRANSFORMATION

CONSERVATION OF ENERGY (GR. 11)• The amount of heat gained by a cold substance is

equal to the amount of heat lost by a hot substance.

EXAMPLE (GRADE 11 PHYSICS)A beaker containing 250 g of water at 250C is poured into another beaker that initially contains 350 g of water at 850C. What is the final temperature of the mixed water?

LATENT HEAT AND CHANGES OF STATE

HEATING/COOLING CURVE• We learned that when you add heat to a substance, the

temperature increases. When you remove heat, the temperature decreases.

• BUT, an interesting thing occurs when an object is undergoing a change of state. During a change of state, the temperature remains constant. The heat being added or removed is going into breaking or creating the bonds between the particles in the different states. If you measured the temperature of a solid substance to the point where it melts to a liquid, then continued heating the liquid until it boiled and turned entirely to a gas, you would get the following graph of temperature versus time.

HEATING/COOLING CURVE

LATENT HEAT• Latent Heat of Fusion

– The amount of thermal energy absorbed when a substance melts or released when it freezes.

– Where is the specific latent heat of fusion.

• Latent Heat of Vaporization– The amount of thermal energy absorbed when a substance

evaporates or released when it condenses.

– Where is the specific latent heat of fusion.

EXAMPLEA 300 g block of ice at -25 0C is heated until it eventually becomes 300 g of water vapour at 110 0C. How much total heat does this take?