Physics: Thermal Energy

HEAT

Heat is a type of energy that flows between objects because of their difference in temperature.

• it is thermal physics on the move.

Heat energy is measured in joules (J).

Two objects are said to be in thermal equilibrium when they have the same temperature.

• they cease to exchange heat

The zeroth law of thermodynamics states that if object A and B are in  equilibrium with a third object C (thermometer) then objects A and B are in thermal equilibrium.

The temperature of an object depicts what direction the energy flows in.

• temperature flows from hot to cold

-> no flow -> thermal equilibrium -> same temperature

• temperature flows one way, but microscopically, it flows both ways 

THERMAL EXPANSION

When something is heated, the vibration of the molecules increases and their displacement or amplitude increases.

As the amplitude of vibration increases, the average distance between the molecules becomes larger which accounts for the increase in length.

Expansion and contraction are very large forces. 

What happens when liquid in a flask is heated?

The flask expands, the volume of water decreases initially as it is heated first and thermally expands. 

There is also a convection current occurring: liquid in the bottom of the flask is heated and rises to the top, then cool water goes to the bottom. In this picture, the flask is heated, then the water gets heated and the molecules expand, causing a rise in temperature.

Gases expand when heated, and contract when cooled.

To fasten two metals together, we use thermal expansion.

BEHAVIOR OF WATER BETWEEN 0*C AND 40*C

Most liquids expand when heated, and contract when cooled. However, water is an exception.

Between 0*C and 40*C, water contracts when the temperature increases and expands when temperature decreases. (Opposite of other liquids)

• water has the highest density and smallest volume at 40*C

Heat is defined as the transfer of energy from a body at a higher temperature to one that is at a lower temperature by conduction, convection, and radiation.

When the atoms of a solid are heated, they vibrate to and fro equilibrium positions alternately attracting and repelling each other. As a result, the particles contain energy that is partly due to their kinetic and potential energies, constituting what is known as the internal energy of a system.

It is essentially the total energy of the particles of a substance in a system.

However, in the case of a gas, the intermolecular forces are weak resulting in the internal energy to be almost entirely kinetic.

kinetic: due to the vibratory motion of the atoms and according to the kinetic theory depends on the temperature

potential energy: stored in the interatomic bonds that are continuously stretched and compressed as the atoms vibrate and it depends on the size of the bonding forces as well as the particle separation.

There are two ways in which the internal energy of a body can be changed:

1. By doing work - the more work done on a system, the greater the internal energy of the system - e.g. rubbing hands together and the gas compression in a bicycle pump

2. Heat transfer - causes a rise in the internal energy of the body receiving the heat in which turn increases the temperature of the body

HEAT CAPACITY

The heat energy required to raise the temperature of a body/substance by 1 degree of temperature (one kelvin/celsius)

• Symbol: C (uppercase)

• S.I unit: joule per kelvin (or degrees Celsius) J/K or J/C

C = change in Q/Change in T (DON’T USE Q USE Eh)

Q = amount of heat energy absorbed (gained) or removed (lost)

• the change in internal energy

T = change in temperature (final - initial)

Change in temperature (T) = final temperature - initial temperature

You don’t have to convert from Celsius to kelvin because the raise in temperature is the same.

The amount of heat a body receives is dependent on two properties of the body

1. The mass: the smaller the mass, the greater the temperature change

2. The material from which it is made: objects of the same mass but different materials will undergo different temperature rises when applied with the same amount of heat

SPECIFIC HEAT CAPACITY

Defined as the amount of heat required to raise the temperature of 1kg of a substance by one degree of temperature (celsius/kelvin).

Symbol: c (lower case C)

S.I: joule per kilogram kelvin (or degree celsius) (J kg^-1 K^-1)

Remember Q is represented by Eh

Q is the amount of energy regained or lost (Eh)

M is mass

T is temperature

To derive a formula containing heat capacity and specific heat capacity:

To find Q (Eh) which is the specific amount of energy absorbed or lost by a system, we multiply the heat capacity x mass x change in temperature.

So the resulting formula is:

Eh = mcΔt

Relationship Between Specific Heat Capacity and Heat Capacity

Remember that both formulas can be written in terms of Eh

• Heat capacity (Eh) = CΔT

• Specific heat capacity (Eh) = mcΔT

Therefore, CΔT = MCΔT

• divide both sides by Δt

C = mc (heat capacity = specific heat capacity x mass)

-> Measured in J/K (joules per kelvin)

• kg x J/kgK = J/kg

• m = mass of substance

• c = specific heat capacity

• C = heat capacity

The greater the specific heat capacity of a substance, the more energy needs to be transferred to the substance if the other conditions (ΔT and the mass, m) are the same.

The specific heat capacities of some metals. We don’t need to know them but know that the specific heat capacity of water is 4200 J/kgK.

Deriving another formula:

We know power = work/time or energy/time

But we also know that power = heat energy transferred/time (Eh/T)

So, power is equal to volts (V) x amperes (I)

• P = IV

Using the past formula P = Eh/t,

IV = Eh/t

Therefore Eh = IVt

Eh = IVt = mcΔt = cΔt = Pt

The temperature change will always be negative if the object is being cooled, and positive if it is being heated.

Make sure every value is in the right units! Just because we don’t pay attention to the kelvin and Celsius, doesn’t mean we ignore the rest.

• Eh should be in Joules

• Mass should be in kilograms

• C/c should be in J/kgC or J/kgK

Also keep in mind that energy is lost to the surroundings as light and sound too, so in reality we’d probably need to transfer more joules to get it to raise to that temperature.

Latent heat

Latent heat is defined as the heat that is absorbed or released during a change of state. The latent heat is the amount of energy that is required to change the state of a body without changing the temperature.

When going from solid to liquid to gas, there is latent energy being absorbed, as this energy is the one responsible for creating a wider distance between the particles so that they may move apart.

• this is why an increase latent heat results in an increase in potential energy

When going from a gas, to liquid to solid, the latent energy is being released. This is because the energy between the particles decreases and the particles move closer together.

The latent heat of vaporisation is the amount of heat energy required to change state from liquid to gas at a constant temperature.

The latent heat of fusion is the amount of heat energy required to change state from solid to liquid at a constant temperature.

Melting point: the single temperature at which that substance changes from solid to liquid

Boiling point: the single temperature at which that substance changes from liquid to gas

specific latent heat of vaporization

Latent heat of vaporisation is defined as the amount of heat energy required to change 1kg of liquid to gas (and vice versa) at constant temperature. The symbol is

• specific latent heat of vaporization is 226000J/kg

Formula: L = Eh/m (SI unit: J/kg)

Eh = mlv

When a solid moves to a liquid, or a liquid to a vapour, heat energy is being gained. This means that the particles are absorbing latent heat energy.

Specific latent heat of fusion

Defined as the amount of heat energy required required to change 1kg of a substance from a solid to a liquid (and vice versa) at constant temperature. Symbol is

• specific latent heat of fusion of water = 33400J/kg

Formula: Lf = Eh/m (SI unit: J/kg)

Eh = mlf

Specific latent heat is given by the formula L (specific latent heat) = energy transferred (Eh) / mass, and given in the units joules per kilogram (J/kg)

When a liquid moves to a solid, or a gas to a liquid, latent heat energy is being released as the substance loses energy.

Evaporation vs boiling

Differences

Evaporation

• no bubbles

• Takes place only on the surface

• Slower

• Temperature fluctuates

• Temperature comes from surroundings

N.B dependent on the surface area of the water, e.g. water in a bottle and water in a wide basin, water in the wide basin will evaporate faster because the evaporation happens layer by layer

Boiling

• has bubbles

• Takes place throughout the liquid

• Faster process

• Boils at a specific temperature

• Temperature remains constant

• Temperature comes from an external force

Similarities

• both involve changing from liquid to gas

• Both need energy to take place

• Both are physical changes

Heat transfer

Convection, conduction and radiation

Conduction

Thermal conduction is the process in which heat energy flows from the hotter to colder regions of the material without the particles moving from their position.

• Heat energy is only passed on by neighbouring particles

• For conduction to occur, heat has to be in contact with the solid

• If something is not conducting, it is insulating (e.g. plastic, wood)

There are two types of thermal conduction:

1. Free electron diffusion

2. Lattice vibration

All metals are good conductors of heat because they have free electrons that move around and vibrate along the metal.

In contrast, insulators like wood don’t have these free electrons.

Liquids and gases are poor conductors of heat.

Convection

Convection is the process whereby heat flows through a substance by the particles moving and changing position. Think of when water boils.

As the fluid is heated, the particles gain energy, become less dense (as they spread out from each other) and rise to the top of the fluid. At the top of the fluid, they give their extra energy to the particles at the top. This causes them to become more dense and sink back down to the bottom.

The process is repeated.

convection current: a flow of liquid or gas caused by a change in density, in which the whole medium moves and carries energy with it

• An example of this is water boiling.

• The movement of hotter areas in a liquid can be seen using a dye, potassium permanganate.

• Convention currents were used in mining to ventilate the cave - an underground fire was used.

Convection causes air to move between cave passages, rooms, and in and out of the cave itself. Convective air movements are driven by differences in air temperature, warmer air is less dense than cooler air (which has more oxygen for workers to breathe) causing it to be more buoyant.

There are two types of convection:

1. Natural convection

Natural cooling and heating of mass of molecules.

Examples: land and sea breeze

• the breeze heats up on land, rises, where it cools and comes down as rain

Hang gliding

• uses rising convection currents of warm air (thermals) which make them stay in air longer (since the convection currents are moving up)

Ocean and wind currents

• the hot water rises to the top, where it cools and sinks back down, creates whirls in the pool that are known as sea currents

2. Forced convection:

An outside object causes a substance to undergo convection.

This can most simply be explained by a fan and a heating source. Essentially, a substance is placed over an energy source (candle, fire, etc) and a fan is put next to it. The fireplace heats up the substance, and then fan blows away the hot air released. The cool air replaces the hot air and is heated up again. This process repeats.

Thermal Radiation

Thermal radiation comes from electromagnetic waves that are given off by objects as a result of their temperature.

• does not require the presence of matter to transport heat from hotter to colder (unlike conduction and convection)

It has its own spectrum: infrared radiation to visible light to the ultraviolet region

Properties of Electromagnetic Waves

• waves travel in straight lines

• Travel at the speed of light

• Unaffected by electric or magnetic feels

Black Body/Temperature Radiation

Black bodies absorb all the radiation that fall on it, and reflects/transmits none. They are good absorbers and emitters of radiation.

• a black container containing hot water would gain heat faster than a silver container with hot water because it is a good absorber.

• Matte black is the best radiator and emitter

Also called temperature radiation because the relative intensities of different wavelengths present depend only on the temperature of the body

• e.g. of black bodies: pupil of eye, hole

• A good absorber is a good emitter

Shiny surfaces, on the other hand, absorb very little radiation (most is reflected). They are bad emitters and absorbers of radiation.

• a bad absorber is a bad emitter

Greenhouse effect

This is a process in which the earth, which receives short-wavelength radiation from the sun, emits long wavelength radiation and is trapped or absorbed by greenhouse gases such as carbon dioxide.

• this trapping of radiation increases the temperature of the atmosphere, giving name to global warming.

• A greenhouse also operates this way

Vacuum

A vacuum flask, also known as a thermos, is an insulating double walled container that allows its contents to stay hot or stay cold for long periods of time.

• To do this, it essentially blocks convection, conduction and radiation

It blocks conduction by: it has double glass walls in the sides of the flask to prevent this. It also has a cork or plastic stopper (because plastic doesn’t transmit heat, it traps the air inside)

It blocks convection by: the stopper cork, but once the stopper cork is removed convection is inevitable

It blocks radiation by: two silver (silver because it’s shiny and shiny things are bad radiators) coatings are used on the walls of the bottle.

Solar water heater

Solar water heaters heat water stored in the hot water tank using solar collectors. Mainly used for industrial, agricultural production and daily life. A solar water heater has 3 parts:

1. Solar collectors

2. Water tank

3. Circulation lines

How to reduce heat loss from a house

1. Install double glazing in the windows (reduces heat transfer by conduction)

2. Install fibre glass loft insulation in the roof (contains air pockets, a poor conductor, which reduces heat transfer by conduction)

3. Install cavity wall insulation (creates air pockets, reduces heat transfer by convection, stops heat from rising up through the house)

4. Install draught excluders around the gaps in doors and windows (stops heat transfer by convection)

5. Install aluminium foil behind radiators (reflects radiation back into the house to keep it warm (stops heat transfer by radiation).

Factors affecting emission/absorption rates of infrared radiation

1. Colour & Texture of surface

• dull/black: better absorbers and emitters

• Shiny/silver: worse absorbers and emitters

2. Surface temperature

• higher temperature: faster emission

• Lower temperature: slower emission

3. Surface area

• large area: faster emission

• Smaller area: slower emission

The temperature of an object affects the wavelength and intensity.

Very hot objects have shorter wavelengths than older objects.

• they produce a very visible light

• The sun emits short wavelength radiation

No radiation is reflected in a very black object. A perfect black body is also the best possible emitter.

A body at a constant temperature absorbs and emits radiation at the same rate.