Thermal Physics State the distinguishing properties of solids, liquids and gases

Kinetic Theory The three phases of matter are …solid, liquid and gas Matter is made up of tiny particles called atoms and molecules These particles are in constant motion. Solid molecules vibrate Liquid molecules slide over each other Gases are in constant random motion

Example 1Match the following statements with a phase of matter.

1. Particles vibrate in fixed positions solid2. Particles vibrate but changes position liquid3. Particles move around at high speeds gas4. No fixed shape or volume gas5. Fixed volume but no fixed shape liquid6. Forces of attraction is weak gas

Extension questionWhen a perfume is sprayed, why can a person standing some distance away get the fragrance?

Small fast moving air molecules collide with the large slow moving perfume particles.

Temperature is the measure of the average kinetic energy of molecules.

When gas molecules are heated they gain kinetic energy and move faster.

Example 2

1.a. What do think would happen if a spray can is thrown into a fire? It may explode

1.b. Use the kinetic molecular theory to explain why this happens? The gas molecule in the can may get hot and move faster. This will cause them to hit the sides of the container harder and faster. The pressure of the can will increase and explode.

1.c. From the above example what do you think is the relationship between pressure and temperature? The higher the temperature the greater the gas pressure

2. How is pressure in a gas created?

Gas pressure is created when gas molecules hit the walls of the container it is stored in.

3. What is the effect of pressure in a fixed container if:

a. The gas is cooled decreaseb. The gas is heated increasesc. The gas expands increased. The gas is compressed increase

Brownian MotionSmoke cell experiment-

When observed from a microscope, brightly coloured specks are seen dancing randomly. This is caused by small, fast moving air molecules colliding with large, slow moving smoke particles.

The lamp and the lens allows us to see the brightly coloured specks dancing in random motion.

Dust particles- When observed dust particles are seen moving in random motion. This is caused by small, fast moving air molecules colliding with large, slow moving dust particles.

Evaporation occurs when high energy surface molecules escape from the liquid. This causes the liquid to lose

energy and cool down. It occurs at any temperature and is not boiling. It leave the liquid feeling cooler.

Apply perfume/alcohol to your body...You will feel COOL !Evaporation is a cooling effect

Wind or draught causes air molecules to collide with the liquids surface molecules and transfers energy for the liquid molecule to escape. The higher the temperature the more energy obtained by the surface molecules. The higher the evaporation.Try spreading a liquid spill. This increases the surface area of the liquid and evaporation increases.The higher the humidity the lower the evaporation

Evaporation occurs at all temperatures. It occurs when 1) the temperature is high- more evaporation2) The surface area is large- more evaporation3) The wind or draught is blowing over the surface of the water- more evaporation

Example 3

Why do people wrap a bottle containing milk with a damp cloth?

The more energetic surface molecules of the liquid escape from the damp cloth taken away some energy with it. This causes the cloth to cool thereby cooling the milk.

Expansion

Solids

Example 4

Why are there expansion gaps in roads and bridges ? Use the concept of contraction, expansion and the kinetic molecular theory in your answer.

During hot days the bridge expands due to the heat energy given to its molecules. The expansion gaps actually prevents the bridge from collapsing due to the expansion.

Explain how axles can fit into gear wheels but it is so difficult to move it apart.Liquid nitrogen cools the big axle causing the molecules to loose heat energy. This causes the axle to shrink and it fits into the smaller wheel. The axle than is allowed to heat up and expands and the axle and wheel locks.

Bimetallic strips

Example 5

How many metals does a bimetallic strip have? 2 Which metal expands first, copper or iron? copper When expansion occurs in this case what happens? Bends away from copper

Example 6How are bimetallic strips used in the following instances?

a) fire alarm

b) thermostat

a. fire alarms The circuit of the alarm is broken. In an event of a fire the bimetallic strip gets hot and completes the circuit. When this happens the strip bends away from the copper and switches the siren on

b. thermostat The circuit of the thermostat is complete and when a certain temperature is reached the strip bends away from the copper strip and breaks the cct. allowing the device to cool.

copper

iron

Alarm or heater circuit

Expansion of liquidsAt 0C water freezes and expand. This is due to the spaces between it hydrogen bonded molecules

Example 7

Explain why fish can survive in a frozen lake?

Due to the molecular arrangements of the hydrogen bonds of water it actually expands when it freezes. Water reaches its maximum density when it reaches 4C. In the lake the dense water that is 4C sinks to the bottom and the water on the top freezes and drop to low temperatures. The fishes survive in the bottom where the water is warmer.

Thermometers

boiling point - Temperature at which a liquid turns into gas without any increase or decrease in temperature.

melting point- Temperature at which a solid turns into liquid without any increase or decrease in temperature.

What is temperature? Measure of the average kinetic energy of the molecule

Liquid-in-glass thermometer

What happens to a liquid when it is heated? It expands

This

principle is used in a liquid-in-glass thermometer

1) The liquid expands rapidly by the change in temperature over a wide range. 2) Mercury or coloured alcohol is used as the liquid.

MERCURY ALCOHOL

BOILING POINT 357C 78C

MELTING POINT -39C -115C

Which liquid would you use to measure low temperatures? ExplainAlcohol, takes a very low temperature to freeze.

Which liquid would you use to measure high temperatures? ExplainMercury, it has a very high boiling point

Define lower fixed point and upper fixed pointlower fixed- freezing point of pure waterupper fixed- boiling point of water

How are the points found?

lower fixed- freezing point of pure water is reached and the thermometer is placed on the water and the mercury rise is marked.upper fixed- boiling point of pure water is reached and the thermometer is placed on the water and the mercury rise is marked.

How would you calibrate a thermometer?Dividing the upper and lower fixed point into 100

Sensitivity, Range and LinearitySensitivity- When a thermometer gives a large response to a small change in temperature. A Thermometer can be made sensitive by having a large bulb and a very narrow tube.

capillary

Lower fixed point

Range- The values within the highest and lowest temperature that a thermometer can measure.

Linearity- The markings between the upper fixed point and the lower fixed point is increased by the same amount.

Accuracy – An accurate thermometer gives the correct values of temperature. This is different form sensitivity.

Thermocouple thermometer

You are required to draw a labelled diagram of a thermocouple thermometer.

A difference in temperature between the hot junction and cold junction causes a voltage to be produced. The sensitive voltmeter gives a reading. The voltmeter is calibrated to read temperature.

Used to measure temperature in hard to reach places such as cracks in walls and volcanoes.

Large range of temperature to be measured When there is a small temperature change To measure very high temperature

Thermal Capacity

When a match burns its internal energy increases

Specific heat capacity

If 1kg of water and 1 kg of aluminium was heated for the same time which would have a higher temperature rise?

Water is greedy. It needs 5 times as much temperature rise to produce the same energy. So aluminium will have the higher temperature rise.

4200J of energy is required to raise the temperature of 1kg water to 1C. 4200 J is called the specific heat capacity.

Specific heat capacity of a substance is the energy required to raise the temperature of 1kg of substance by 1C

To calculate the amount of energy needed we use the following equation

Example SHC

Why does the inside of a pie get hotter than the outside after it has been heated in a microwave oven?

The specific heat capacity of the outside of the pie is greater than the specific heat capacity of the inside of the pie. The higher the specific heat capacity the longer it takes to heat up.

What does 8 kW means?This is the power rating. I means that 8000J of energy is given out every second.

A tank holding 60kg of water is heated by a 3kW heater. If the specific heat capacity of water is 4200J/kgC, Estimate the time taken for the temperature to rise from 10C to 60C.

First we need to find the energy needed:

Q = m c ∆T = 60 kg x 4200 x (60-10) = 12600000J To find the time we use the power equation: P = W/t t = W/P t= 12600000/3000 = 42000 s

Specific Latent HeatWhat happens when you melt 1kg of ice completely?

To melt 1kg of ice at we must provide 340 000 J of energy.

During the process of melting the temperature of the water stays the same, it does not get warmer. The energy provided is called hidden heat or latent heat. The number 340000J is called the specific latent heat of fusion for water. Fusion means melting.

The specific latent heat of fusion is the energy required to melt 1kg of substance without changing its temperature (lf).

Example Melting

Why are ice cubes used to cool down drinks?During the meting process of ice, the ice removes heat energy from the cool drink. The ice melts without its temperature changing at 0 C. This causes the drink to remain cool.

The equation used to find the energy for melting is:

Q = mlf

Vaporisation What happens when water is heated to 100C?

The latent heat is used to overcome the forces between the molecule and water is changed into steam. To boil 1 kg of water into steam at 100C we must provide a latent heat of 2300000J. This number is called the Latent heat of vaporisation. Notice that the temperature does not change.The latent heat of vaporisation Is the energy required to boil 1 kg of a substance into gas without changing its temperature. The equation used to find the energy for boiling/vaporisation is:

Q = mlv

Example BoilingExplain why the latent heat of vaporisation of water is greater than latent heat of fusion of ice.The liquid molecules need a lot of energy to overcome the forces holding them together to become a gas. Latent heat of vaporisation is required increase kinetic energy and expansion of the gas molecules. This requires a lot of energy.

Example

Calculate the quantity of heat required to convert 5 kg of ice at -7C to Water vapour at 100C.

cice ( SHC of ice)= 2100J/Kg.C

cwater (SHC of water )= 4200J/Kg.C

lf ice ( laten heat of fusion)= 336J/kg

Lv water(Latent heat of vaporisation)2273J/kg

What changes must ice go through to become water vapour?

T (C)

Raise temp from -7C to 0C

Melting a 0C

Raise temp from 0C to 100C

Vaporisation at 100C

time(s)

You are required to use the above graph to show when boiling, melting and temperature change takes place. Notice that during melting and boiling the temperature stays constant.

Q = mc∆T ( energy that doesn’t require change in phase)

Q = ml ( energy that require change in phase)

step1 Q = mc∆T (raising heat from 7C t0 0C)

= 5 x 2100x 7

= 73500J

step2 Q = ml f (melting phase)

= 5 x 336

= 1680J

step3 Q = mc∆T (raising heat from 0C t0 100C)

= 5 x 4200x 100

= 2100000J

step4 Q = mlv (boiling or vaporisation phase)

= 5 x 2273

= 11365J

Total energy required is equal to = 73500J + 1680J +2100000J + 11365J = 2186545J

Extended students are required to study the experiments on page 68,73 and 74

You must go over the details of the experiment and the worked examples (Describe an experiment to measure specific latent heats for steam and for ice and specific heat capacity of a solid block)

Example 1

Q = mlv

VIt = (m1 – m2) lf

Lf= VIt/(m1 – m2)

Lf= VIt/(m1 – m2) = 12V x 2Ax 3750/40g= 2250J/kg

The liquid molecules need a lot of energy to overcome the forces holding them together so that they can become a gas. Latent heat of vaporisation is required increase kinetic energy and expansion of the gas molecules. This requires a lot of energy.

Example 2

Refer to graph

Boiling point has been reached. During boiling the temperature stays the same 100°C

The energy is used to break the bonds that are holding the water in liquid form so that it can become a gas and the energy is required to push the gas molecules apart.

Example 3

Internal energy

Aluminium has a higher Specific capacity than copper so aluminium’s temperature rise is less

Transfer of thermal energy

All students must study the experiment to show conduction on page 77 of the textbook

Heat is transferred by Conduction , Convection and Radiation

Condution Copper is the best

conductor of heat Heat is transferred

through the rod and melts the Vaseline. The nails will fall

During heat conduction of a metal the heat travels fom hot to cold.

WHEN A METAL IS HEATED THE MOLECULES VIBRATE FASTER.

THEY COLLIDE WITH THE NEIGHBOURING

MOLECULES TRANSFERING HEAT

Convection

Convection takes place in gas and liquids. The hot fluid gets less dense and rises. When it is cooled it gets more dense and falls. The cycle repeats itself.

RadiationRadiation from the

electromagnetic spectrum such as infrared, radio waves etc travels as fast as the speed of light. It does not need a medium to travel. An example is the way we get heat from the sun.

WHICH MATERIALS ARE GOOD EMITTERS?

Dark matted surfaces are good emitter whilst light shiny surfaces are poor emitters.

Extended students are required to read page 79 and describe an experiment that shows which materials are good emitters and bad emitters

Leslie's cube is a metal box with four differently painted sides.If it is filled with boiling water from a

kettle you can use either your hands or a radiometer to try and decide which surfaces give out the most heat radiation.

Haa Ha

Leslie cuberadiometer