Physics: Heat & Temperature (Prt2)

Quantity of Heat & Heat Transfer

Heat

is a form of energy

Conduction

is the movement of heat energy through a substance by the passing on of molecular vibration from molecule to molecule, without any overall movement of the substance

Demo of heat transfer by conduction

• Set up equipment as shown

• Heat will travel from the source to the other end causing the wax to melt and the pin to fall

• Whichever falls first is at the end of the best conductor

Demo of the a poor conductor of heat

• Set the Bunsen burner under the top of the water

• The ice at the bottom doesn’t melt even though the water at the top is boiling

• Therefore, water is a poor conductor 

Convection

is the transfer of heat through a fluid by means of circulating currents of fluid caused by the heat

Demo of convection in a liquid

• Use the apparatus shown and drop in some copper sulphate or potassium permanganate to act as a dye

• Place the heat source under one of the corners and note the movement of the water around the apparatus

Radiation

is the transfer of heat energy from one place to another in the form of electromagnetic waves

U-value

is a measure of the heat flow through 1m² of material each second when a temperature difference of 1K exists between the two sides

Solar constant

represents the average amount of energy falling normally on each square meter of the earth’s atmosphere in one second

Specific heat capacity

is the change in the heat energy associated with one kilogram of a substance heating up or cooling down by one kelvin

Heat capacity

a substance that heat energy needed to change the temperature of the substance by one kelvin

Give the equation that define the heat capacity?

ΔE=C Δθ

Give the equation that define the specific heat capacity?

ΔE=mcΔθ

Give the equation that define the specific latent heat?

ΔE=ml

Define specific latent heat?

is the amount of heat energy taken in or released when 1 kg of the substance changes from one state to another without a change in temperature

Define latent heat of fusion?

is the amount of heat energy taken in or released when 1 kg of the substance change from a solid to a liquid (vice versa) without a change in the temperature.

Define latent heat of vaporisation?

(l_v) is the amount of heat energy taken in or released when 1 kg of the substance changes from liquid to a gas (vice versa) without a change in temperature

Heat pump

is a device that extracts available heat from a cold area and transfers it to a warmer area

Operation of a heat pump?

• The selection on the left is include the fridge; the section on the right is at the back of the fridge.

• A special liquid is pumped around the pipe as shown. Note this liquid should have a high specific latent heat of vaporirsation and low boiling point.

• At the bottom of the diagram just above the dotted line is an expansion valve. This allows the liquid to expand quickly and in doing so it goes from a liquid to a gas and as it does it takes in energy from the environment outside the pipe. This pipe is inside the fridge so the air in that section cools down.

• At the top of the circuit outside the dotted line a pump is used to compress the gas which causes it to go back into a liquid state, and in the process, it gives heat energy back out to the surroundings.