Quantity of Heat and Heat Transfer Chapter 15
What is Heat? Heat is a form of Energy. Usually, if you Add Heat Energy to a substance its Temperature Rises. If you Remove Heat Energy its Temperature Falls. However, if a substance is Changing State as you add (or remove) heat, there may not be any change in temperature.
What is Heat Capacity? The Heat Capacity (C) of an object is the amount of heat energy needed to change its temperature by 1 K (1 oC).
SI Unit of Heat Capacity? What is the SI Unit of Heat Capacity? The joule per kelvin ( J K-1 ).
Q joules of Heat Energy are added to an object. Its Heat Capacity is C joules per kelvin. Its Temperature rises by degrees Celsius. What formula relates Q, C and Δθ? Q = C Δθ Heat needed to change temperature = Heat capacity Change in Temperature
The States of Matter Matter can exist in three different states. the Solid state, the Liquid state, the Gaseous state.
What is Specific Heat Capacity? The Specific Heat Capacity (c) of a substance is the heat energy needed to change the temperature of one kilogram of that substance by one kelvin.
What is the SI Unit of Specific Heat Capacity? The joule per kilogram per kelvin ( J kg-1 K-1 ).
Specific Heat Capacity Formula Provided a substance Does Not Change State when heat energy is added to or taken from it we have: Heat Energy Added = Mass Specific Heat Capacity Rise In Temp Heat Energy Lost = Mass Specific Heat Capacity Fall In Temp In either case: Q = m c
To Measure the Specific Heat Capacity of water by an Electrical Method
How Temperature changes as Heat Energy is Added to a substance
What is Latent Heat? The SI Unit of Latent Heat is the the joule (J). The Latent Heat (L) of an object is the heat energy needed to change its state without a change in temperature. The SI Unit of Latent Heat is the the joule (J).
What is Specific Latent Heat? The Specific Latent Heat ( l ) of a substance is the amount of heat energy needed to change the state of 1 kg of that substance without a change in temperature. The SI Unit of Specific Latent Heat is the joule per kilogram (J kg-1).
Q joules of Heat Energy are needed to Change the State of m kilograms of a substance of Specific Latent Heat l, without a change in temperature. What formula relates Q, m and l? Q = m l Heat energy needed to Change State = Mass × Specific Latent Heat
As the Perspiration Evaporates it takes its Latent Heat from the athlete and his body Temperature Falls.
The Heat Pump
To Measure the Specific Latent Heat of Vaporisation of Water
What is Convection? Convection is the movement of heat through a liquid or gas by means of circulating currents caused by the heat.
Experiment to demonstrate Convection in a Liquid Set up the equipment as shown. Add a small piece of the dye to the bottom of the beaker. Heat the bottom of the beaker. The dye circulates showing convection currents.
Why Convection Occurs When the lower part of the liquid is heated it expands and becomes less dense. Less dense objects float on denser objects. The heated liquid therefore rises above the cooler liquid. This sets up a movement of the liquid called a Convection Current.
Convection in a Domestic Heating System Water in domestic heating systems may circulate by convection. The diagram shows a simple arrangement. Usually however a pump is used to speed up the flow of water through the system.
What is meant by Conduction (of heat)? Conduction is the transfer of heat through a substance by the passing on of vibration from molecule to molecule. There is no overall motion of the substance.
Experiment to show how Different Solids Conduct Heat at Different Rates Use the equipment shown. The rods are all the same size. Stick a coin to the end of each rod with melted candle wax. Pour boiling water in the trough. Wait until the wax melts and the coins fall off. The better the rod conducts, the less time it takes for the coin to fall off.
Experiment to show that Water is a Bad Conductor of Heat
What is Radiation of Heat? Radiation is the transfer of heat energy from one place to another in the form of Electromagnetic Waves.
What is the Solar Constant? The average amount of the Sun’s energy falling per second perpendicularly on 1 metre squared of the Earth’s atmosphere is the solar constant. Its value is about 1.35 kW m-2. (The Solar Constant is also called the Solar Irradiance.)
Solar Heating SOLAR HEATING is using the Sun's energy to heat something. The diagram shows one method. Here water flowing in black pipes under glass absorbs heat and its temperature rises. In another method a solar panel consisting of photocells converts the Sun’s energy into electrical energy which can be used for heating or other purposes.
Dark colours Radiate Heat better The darker the colour and the less shiny an object is, the better it will radiate heat. Hot water at the same temperature is placed in two cans one of which is black and the other is shiny metallic. The black can shows a quicker fall in temperature.
Dark coloured objects absorb heat better than bright or shiny ones Two cans, containing equal masses of cold water are placed in direct sunlight (or radiated heat from a heater) for the same time. The rise in temperature of the water in the black can will be greater. This is because the darker can will absorb heat radiation falling on it better than the shiny one.
What is meant by the U-Value of a structure? The U-value of a structure is the amount of heat energy conducted per second through 1 m2 of that structure when a temperature difference of 1 oC (i.e.1 K) is maintained between its ends. The SI Unit of U-value is the W m-2 K-1
A structure that is a Bad Insulator has a High U-Value. A structure that is a Good Insulator has a Low U-Value.