thermal physics. matter is most commonly found in solid, liquid or gas form. we will discuss the...
TRANSCRIPT
THERMAL PHYSICS
Matter is most commonly found in solid, liquid or gas form. We will discuss the properties of these diff erent states of matter.
STATES OF MATTER
Solids have a fixed shape and a fixed volume.The molecules in a solid have a rigid
structure.The force of attraction between molecules is
strong.
SOLIDS
Liquids have a fixed volume but change shape to fi t the container.
The molecules in a liquid stay in contact but move around freely.
The force of attraction is strong enough to keep the molecules from completely moving away.
LIQUIDS
Gases do not have a fixed volume or shape.The molecules in a gas are far apart and
move quickly.The forces of attraction between molecules
is negligible.
GASES
The temperature of a gas is the average kinetic energy of its molecules.
Objects with a high temperature have molecules moving at a high speed. If the temperature is decreased the speed of the molecules decreases.
TEMPERATURE IN GASES
The pressure of a gas on a surface is due to the impacts of gas molecules with the surface.
When a molecule impacts the surface it exerts a tiny force. Billions of these impacts occur every second creating a steady pressure on the surface.
PRESSURE IN A GAS
The following diagram shows the names of the different changes of state.
CHANGES OF STATE
The changes of state can be described by the movement of molecules.
Melting – when a solid is heated the molecules begin vibrating to the point at which they break free of the rigid structure.
Freezing – When a liquid is cooled the molecules slow down and form a rigid structure.
CHANGES OF STATE
The changes of state can be described by the movement of molecules.
Boiling – When a liquid is heated the molecules move quickly and break free from each other.
Condensing – When a gas is cooled the molecules move more slowly and the force of attraction increases.
CHANGES OF STATE
Molecules in a solid are in a fixed structure.Molecules in a liquid move in contact with
each other.Molecules in a gas are far apart and move at
high speed.Increasing the temperature of a gas increases
the average speed of its molecules.The pressure of the gas on a surface is
caused by its molecules repeatedly hitting the surface.
Practice: pg 71 #1,2 and pg 73 #1,2
SUMMARY
PRESSURE, TEMPERATURE AND VOLUME IN GASES
In 1785, Robert Brown observed pollen grains floating on water. He observed that the pollen grains moved randomly.
Using molecular theory, it has been explained that the small water molecules were constantly colliding with the large pollen grain. This caused it to move randomly.
This motion is called Brownian Motion.
RANDOM MOTION OF PARTICLES
If we had a container of sealed gas, what would happen to the pressure if we increase the temperature? (Note: we are keeping the same volume.)
The pressure would increase because the molecules would be moving faster and there would be more collision with the walls of the container.
PRESSURE AND TEMPERATURE
Imagine a piston filled with a gas.
The temperature and mass of the gas are constant.
If we force the piston down, what will happen to the pressure?
GAS PRESSURE AND VOLUME
If the piston is forced down, the volume decreases.
The pressure in the tube will increase because the molecules will impact the surface more often.
GAS PRESSURE AND VOLUME
What would happen to the piston if we lift the piston upwards?
GAS PRESSURE AND VOLUME
If the piston is forced up, the volume increases.
The pressure in the tube will decrease because the molecules will impact the surface less often.
GAS PRESSURE AND VOLUME
We see that when volume decreases the pressure increases. When the volume increases the pressure decreases.
This means that volume and pressure are “inversely proportional”.
GAS PRESSURE AND VOLUME
PressureSymbol – PUnit – PascalUnit Symbol – Pa
VolumeSymbol – VUnit – meter cubed or centimeter cubedUnit Symbol – m3 or cm3
VOLUME AND PRESSURE
For a fixed mass of gas at a constant temperature:
pressure x volume = constant
When you are comparing two situations you can use the following equation:
P1V1 = P2V2
BOYLE’S LAW
A fixed mass of gas has an initial volume of 15cm3. When the volume is increased to 45cm3 the pressure is measured at 60kPa. What was the original pressure?
BOYLE’S LAW EXAMPLE
Brownian motion is the random motion of small particles due to the impacts of gas molecules on each particle.
The pressure of a gas in a sealed container increases if the gas temperature is increased.
For a fixed mass of gas at constant temperature, the pressure x volume = constant.
SUMMARY OF PRESSURE, VOLUME AND TEMPERATURE IN GASES
EVAPORATION
Evaporation is when high energy molecules in a liquid leave the surface of the liquid.
EVAPORATION
Why do we sweat when we’re hot?The water is on the surface of our skin.
The high energy water molecules will leave the surface. The average kinetic energy of the molecules on the skin will decrease. That means the temperature will decrease.
EVAPORATION
1. Increase the temperature because that increases the kinetic energy of the molecules.
2. Create a draught across it because the moving air molecules will collide with the water molecules.
3. Increase the surface area because there will be more molecules to escape from the surface.
HOW CAN WE INCREASE EVAPORATION?
Evaporation in a liquid occurs when high energy molecules leave the surface.
The average kinetic energy of the remaining liquid decreases which cools the liquid.
We can increase evaporation by increasing the temperature, passing a draught over the surface, or increasing the surface area.
SUMMARY OF EVAPORATION
THERMAL EXPANSION
Most substances expand when heated.Some substances expand more than
others.Gases expand much more than liquids
and solids.
THERMAL EXPANSION
EXPANSION OF GASES
EXPANSION OF LIQUIDS - THERMOMETERS
EXPANSION OF SOLIDS – EXPANSION GAPS
EXPANSION OF SOLIDS – STEEL TIRES
EXPANSION OF SOLIDS – BIMETALLIC STRIPS
When the temperature of an object increases the object expands.
Gases expand much more than liquids or solids.
Some applications of expansion include liquid thermometers, bimetallic strips, expansion gaps on bridges, and fitting steel tires.
SUMMARY – THERMAL EXPANSION
THERMOMETERS
Every thermometer depends on a physical property that changes with temperature.
For liquid-in-glass thermometers the thermometric property is the thermal expansion of liquids.
Amos Dolbear used the number of cricket chirps per minute to estimate the temperature. The thermometric property in that cases is the rate of cricket chirps.
THERMOMETRIC PROPERTY
To make a thermometer we need fixed points to define the scale.
The Celsius scale uses two fixed points:0oC, the temperature at which pure ice melts100oC the temperature at which pure water boils
Once these two points are marked on the thermometer we can divide the length into 100 equal intervals.
HOW DO WE MAKE THERMOMETERS?
When the bulb becomes warmer the liquid expands and moves up the capillary tube.
The liquid is usually mercury or coloured alcohol.
HOW DOES A LIQUID-IN-GLASS THERMOMETER WORK?
Thin glass bulb
Narrow glass capillary tube
Thermocouples are electrical thermometers. The thermometric property is the voltage created when two diff erent metals are in contact. This voltage varies with the temperature.
THERMOCOUPLES
Thermocouples are better for measuring temperatures that are changing rapidly.
They can also measure much higher temperatures than liquid-in-glass thermometers.
ADVANTAGES OF THERMOCOUPLES
For liquid-in-glass thermometers alcohol is better for lower temperatures because it freezes at -114oC. Mercury freezes at -38oC so would not be suitable for very low temperatures.
The range is the lowest to the highest temperature a thermometer can measure.
RANGE OF THERMOMETERS
The sensitivity of a thermometer is the extent to how much the thermometric property changes in a 1oC temperature rise.
Alcohol expands about 5 times as much as mercury in a 1oC temperature change. That means that alcohol has a higher sensitivity.
SENSITIVITY OF THERMOMETERS
The expansion of a liquid in a thermometer is not constant. That means that the temperatures between 0oC and 100oC and not exactly accurate.
A perfectly linear thermometer will make a straight line between 0oC and 100oC . The average thermometer will make a slightly curved line.
THE LINEARITY OF A THERMOMETER
The linearity of a thermometer tells how straight the line is on the graph. The greater the linearity the greater the accuracy of the thermometer.
THE LINEARITY OF A THERMOMETER
Each type of thermometer depends on a physical property that depends of the temperature.
The fixed points of the Celsius scale are 0oC and 100oC.
The range of a thermometer is from the lowest to highest temperature it can measure.
The sensitivity of a thermometer is the change in its thermometric property for a change of 1oC.
The greater the linearity of a thermometer, the closer the readings are to a standard thermometer.
SUMMARY OF THERMOMETERS
THERMAL CAPACITY
The thermal capacity of an object is the energy that must be supplied to raise its temperature by 1oC.
For example, if it takes 2000J to raise the temperature 1oC, the thermal capacity is 2000J/oC.
THERMAL CAPACITY
Thermal capacity is measured in J/oCE is the energy supplied in Joules2 is the final temperature in oC
1 is the initial temperature in oC
THERMAL CAPACITY
thermal capacity =
E
2 - 1
The thermal capacity for a beaker with 1L of water will be different for a beaker with 2L of water.
This is a problem when comparing the thermal capacity of different substances.
The specific heat capacity will give a value for any mass of a substance for easy comparison.
SPECIFIC HEAT CAPACITY
The temperature rise depends on: The amount of energy supplied The mass of the substance The nature of the substance
The specific heat capacity, c, is defined as the energy needed to raise the temperature of 1kg of the substance by 1oC.
SPECIFIC HEAT THERMAL
c is the specific heat capacity J/kgoCE is the energy in Joulesm is the mass in kilograms2 is the final temperature in oC
1 is the initial temperature in oC
SPECIFIC HEAT CAPACITY
c =
Em(2 - 1)
2000J of heat energy is applied to 4kg of a substance. The temperature went from 20oC to 23oC. What is the specific heat capacity?
EXAMPLE PROBLEM
The thermal capacity of an object is the energy that must be supplied to raise its temperature by 1oC.
The specific heat capacity, c, is defined as the energy needed to raise the temperature of 1kg of the substance by 1oC.
THERMAL CAPACITY SUMMARY
thermal capacity =
E
2 - 1
c =
Em(2 - 1)
CHANGE OF STATE
When ice is heated and it melts, its temperature remains at 0oC until all the ice has melted.
This temperature is called the melting point.
When water is heated and boils, its temperature remains at 100oC (the boiling point). Boiling occurs throughout the liquid at its boiling point but evaporation occurs at the surface when it is below the boiling point.
CHANGING STATE
Energy is supplied to an object to raise its temperature.
When an object is changing state (ie. melting) the temperature remains constant.
The energy supplied when it changes state is called latent heat.
Latent mean hidden. It is called latent because energy is supplied to the object but the temperature does not change.
CHANGING STATE
LATENT HEAT GRAPH
The specific latent heat of fusion is the energy needed to melt 1kg of a substance at its melting point. The unit is joule per kilogram (J/kg).
The specific latent heat of vaporisation is the energy needed to change 1kg of a substance at its boiling point from liquid to vapour.
SPECIFIC LATENT HEAT
l =
Em
• l is the specific latent heat of fusion in J/kg
• E is the energy supplied in J
• m is the mass of substance melted in kg
Energy is needed to melt a solid or boil a liquid.
Latent heat is energy used or released when a substance changes its state without changing its temperature.
Specific latent heat of fusion (or vaporisation) is the energy needed to melt (or boil) 1kg of the substance without change of temperature.
CHANGE OF STATE SUMMARY
HEAT TRANSFER
We will look into the three types of heat transfer; conduction, convection and radiation.
THREE WAYS TO TRANSFER HEAT
When you place hot water in a mug the fast moving water molecules bump into the molecules of the mug, causing the mug to heat up.
When heat is transferred through contact we call it thermal conduction.
CONDUCTION
Materials that transfer heat easily are called conductors. Materials that do not conduct heat easily are insulators.
Metals are good conductors. Wool is a good insulator.
CONDUCTORS AND INSULATORS
Metals contain electrons that move around freely. Why does this make them better conductors?
When metals are heated the free electrons transfer kinetic energy to the cooler parts.
In non-metallic solids energy is only transferred by atoms shaking each other. It is much less effective than free moving electrons in metals.
CONDUCTORS AND INSULATORS
Convection happens whenever a fluid (ie. gas or liquid) is heated.
The fluid expands where it is heated and becomes less dense. This causes it to rise.
As the hot fluid rises the cool fluid moves in to take its place, creating a convection current.
CONVECTION
Convection happens whenever a fluid (ie. gas or liquid) is heated.
CONVECTION
Convection happens whenever a fluid (ie. gas or liquid) is heated.
CONVECTION
Every object around us emits infra-red radiation. The hotter an object is, the more radiation it emits.
Different surfaces absorb and emit infra-red radiation differently.
RADIATION
When you wear a dark shirt in the sun you feel warmer than in a light shirt.
Dark surfaces absorb infra-red radiation better than light surfaces.
RADIATION - COLOUR
Shiny surfaces do not absorb infra-red radiation as well as matt surfaces.
Infra-red radiation reflects off of the shiny surface because it is smoother than a matt surface.
RADIATION - SHAPE
Thermal conduction is heat transfer through contact. Metals are better conductors due to the free electrons.
Convection takes place because when fluids are heated they become less dense and begin to rise.
All bodies emit infra-red radiation. Dark, matt surfaces are better emitters and absorbers of infra-red radiation than light, shiny surfaces.
SUMMARY – HEAT TRANSFER