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Thermal physics

kinetic model

Kinetic particle model of matter

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transfer of heat
Anchor 3

  • Temperature increases the average kinetic energy of particles.

  • As the average kinetic energy of particles increase the particles move faster.

  • The lowest possible temperature is zero Kelvin (-273 degrees Celsius).

  • At this temperature, the kinetic energy of particles is the lowest.

Pressure and temperature at constant volume

  • When the temperature of a gas in a constant volume increases the pressure increases.

  • The average kinetic energy of molecules increases.

  • The molecules vibrate faster.

  • The molecules collide with the internal wall of the container more frequently also with a greater speed.

  • Greater speed gives more momentum and as a result more force.

  • Higher force results in a greater pressure.

Volume and temperature at constant pressure

  • When the temperature of a gas at constant pressure is increased the volume increases.

  • The average kinetic energy of molecules increases.

  • The molecules vibrate faster.

  • The distance between molecules increases.

  • The gas expands.

Pressure and volume at constant temperature

  • Pressure and volume are inversely proportional to each other if the temperature is constant.

  • Increasing pressure decreases the volume.

  • Increasing volume decreases the pressure.

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Thermal properties and temperatuare

  • Thermometers use the physical properties of thermometric substances to measure the temperature change.

thermal properties

Thermometric property​

  • Length of metal - The length of a metal increases as heat is applied to it.

  • The pressure of a fixed volume of gas - The pressure of a fixed volume of gas increases with temperature.

  • The volume of gas or liquid- The volume of a liquid or gas increases with temperature.

  • Resistance- Resistance of metal increases with temperature.

  • When any substance is heated the molecules gain kinetic energy.

  • They vibrate and collide with nearby molecules transferring energy.

  • The space between molecules increases and the material expands.

  • When heat is applied gases expand the most, liquid expands a little, solids expand the least.

°C to kelvin

T (in K) = θ (in °C) + 273

Example

Convert 34°C to kelvin 

Use the formula T (in K)     = θ (in °C) + 273
                                           = 34+273
                                           = 307K

  • An increase in the temperature of an object increases its internal energy

  • An increase in the temperature of an object increases the average kinetic energies of all of the particles in the object.

  • The particles gain more speed.

  • They vibrate faster with higher speed.

Specific heat capacity: It is the energy required per unit mass per unit temperature increase.

Specific heat capacity(c) = 

Change in energy(∆E)

Mass (m) × Change in temperature(∆θ)

Example :​

​

Specific heat capacity of water: 4200 J/(kg K)

Mass of 1ml of water is 1g

Calculate the energy required to increase the temperature of 200ml of water from 10 °C to 100°C.

200ml = 200/1000 l = 0.2l or 0.2 kg
 

∆E = mC∆θ
      = 0.2 × 4200 × (100-10)
       = 75600J

Experiment to find the specific heat capacity of any liquid.​

  • The mass of an empty cup is measured by balance.

  • The liquid is poured into the cup, the mass is measured again.

  • The initial temperature of the liquid is measured using a thermometer.

  • A heater is placed inside the liquid, power supply and a stopwatch are switched ON at the same time.

  • After some time the power supply and switch are turned OFF at the same time.

  • The final temperature is recorded.

  • The reading on the stopwatch is also recorded.

  • The energy supplied is calculated by multiplying the power rating of the heater by time. (E = P ×  t )

  • The change in temperature is the difference in temperatures recorded by the thermometer.

  • The mass of the liquid was measured at the beginning of the experiment.

​

​

Specific heat capacity of liquid is calculated by using the formula below.

Specific heat capacity(c) = 

Change in energy(∆E)

Mass (m) × Change in temperature(∆θ)

Melting point: Temperature at which solid turns into liquid.
Boiling point: Temperature at which liquid turns into gas.
Solidification point: Temperature at which liquid turns into solid.
Condensation point: Temperature at which gas turns into liquid.

Temperature does not change during melting although heat is applied.​

  • The energy given is used to break the intermolecular bonds of solids to turn into liquid.

  • The average kinetic energy of molecules remains the same.

  • Temperature does not change.

  • The melting point of pure water at standard atmospheric pressure is 0 °C or 273K.

  • The boiling point of pure water at standard atmospheric pressure is 100 °C or 373K.

Evaporation

Boiling

  • Boiling occurs at a constant temperature.

  • Boiling occurs throughout the whole liquid.

  • During boiling the temperature of the remaining liquid does not change.

  • This process is faster.

  • Evaporation occurs at any temperature below the boiling point.

  • Evaporation only takes place from the top surface of the liquid.

  • Evaporation causes a cooling effect, so the temperature of the remaining liquid falls.

  • It is a slower process.

Evaporation

  • The process of escaping high energetic molecules from the top surface of any liquid at any temperature other than the boiling point.

Cooling effect

  • During evaporation more energetic liquid molecules escape from the surface, leaving behind the less energetic molecules.

  • The temperature depends on the average kinetic energy of liquid molecuels.

  • As energetic ones leave the average kinetic energy of liquid decreases.

  • Temperature of liquid decreases.

Factors of evaporation

  • The rate of evaporation increases with the increase in the flow of air over it.

  • Higher the temperature the higher the rate of evaporation.

  • The larger the surface area the more the evaporation.

  • Rate of evaporation decreases with the increase of humidity.

  • The rate of evaporation is higher for liquids with lower melting points.

Latent heat

  • It is the  amount of thermal energy required to change the state of a substance.

  • Energy supplied during the latent heat is used to break or weaken the intermolecular bonds, but not to increase the kinetic energy of the molecules.

Transfer of thermal energy

Conduction

transfer of heat

  • Particles at the point being heated gain kinetic energy and vibrate.

  • They collide with the neighbouring molecules, making them vibrate.

  • Heat is transferred without transferring particles.

  • In metals free electron diffusion takes place.

  • The free electrons at the hearing end gain kinetic energy.

  • They move from the hotter to the cooler region of the metal.

  • While moving they collide with the atoms of the cooler region transferring thermal energy.

  • The heat is transferred from the hotter to the cooler region.

Experiment to distinguish between good and bad thermal conductors

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  • Coat two third of the length of each rod with melted wax.

  • When the wax has hardened, insert the rods into the water bath as shown above. Make sure the rods are inserted to the same depth.

  • Pour hot water into the bath.

  • Record the length of wax that remains on each rod after a suitable interval of time.

  • The rod that took the wax longest to melt is the poorest conductor.

  • The rod that took the shortest time to melt is the best conductor.

Convection

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  • When fluid at the bottom is heated, it expands.

  • The expanded fluid is less dense than the surrounding fluid, therefore it rises.

  • As the upper region is cooler it is denser and sinks.

  • The difference in density starts a conventional current.

Radiation

  • Radiation is the transfer of thermal energy in the form of electromagnetic waves.

  • Radiation does not require any medium to take place.

  • Dull and black surfaces emit infra-red radiation at a faster rate than shiny surfaces.

  • Dull and black surfaces also absorb infra-red radiation at a faster rate than shiny silvery surfaces.

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  • A cork is attached to a metal plate with wax.

  • The cork on the dull black plate will fall before the cork on the shiny silvery one.

  • One plate is painted dull black and the other shiny silvery

  • Dull and black surfaces absorb infra-red radiation at a faster rate than shiny silvery surfaces.

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  • Both of the cans are filled with an equal volume of hot water at the same temperature.

  • They are placed at room temperature.

  • As dull and black surfaces emit infra-red radiation at a faster rate than shiny surfaces.

  • It will be observed that the fall in the temperature of water in the dull black box is higher than the shiny surfaced can.

Consequences of thermal energy transfer

(a) heating objects such as kitchen pans

  • Kitchen pans are made up of metal as metals are the best conductor of heat.

  • The handle of a kitchen pan is made up of wood as they are the best insulator of heat.

(b) heating a room

  • A heater heats a room by convection.

  • It heats the air at the bottom of the room.

  • Due to heating the air expands and becomes less dense.

  • Due to lower density, the air rises up and the colder air in the surroundings sinks to the bottom.

  • A conventional current is set up that distributes the heat in the room.

(c) measuring temperature using an infrared thermometer

  • Infra-red thermometers use electromagnetic radiation to detect temperature.

  • Infra-red radiation is focused on a thermocouple.

  • The thermocouple generated a voltage depending on the thermometer.

  • This voltage is converted into a temperature reading.

(d) thermal insulation

  • Air is the best insulator of heat.

  • Buildings use hollow brick that traps air and does not let heat pass through the walls by conduction.

  • Some buildings use a double layer of glass in windows that trap air between them, insulating the room.

  • Thermometric flasks have a double layer of insulation.

  • There is a vacuum between the first and the second layer.

  • Vaccum between the first and second layer prevents heat loss by both conduction and convection.

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