Notes for Class 10 Physics Heat
1. Internal Energy
Internal Energy of a body is the sum of all kinetic and potential energy of all molecules constituting the body.
It is the amount of heat required to rise the temperature of 1/4200 kg of pure water from 14.5 C to 15.5 C.
It is the amount of heat required to rise the temperature of 1 g of pure water from 14.5C to 15.5C.
4. British Thermal Unit
It is the amount of heat tht is required to rise the temperature of 1 pound of pure water from 63F to 64F.
Difference Between Heat and Temperature
- Heat is the energy in transit from one body to another due to temperature difference.
- It is the total kinetic energy of the body.
- Heat is measured using Joule meter.
- Its unit is Joule.
- Temperature is the degree of hotness or coldness of a body.
- It is the average kinetic energy of the body.
- Temperature is measured using thermometer.
- Its units are F, C and K.
Change in length, breadth and height of a body due to heating is known as Thermal Expansion. It occurs in all the three states, i.e. solids, liquids and gases.
Thermal Expansion of Solids
Solids expand on heating. Their ability to expand depends on their molecular structure. As the temperature is increased, the average kinetic energy of the molecules increases and they vibrate with larger amplitudes. This results in increase in the distance between them. Hence, they expand on heating. Thermal Expansion of solids can be classified into three types.
1. Linear Thermal Expansion
Change in length or any one dimension of a solid on heating is known as LInear Thermal Expansion.
2. Real Expansion
The sum of the observed increase in the volume of a liquid and that of the containing vessel is called real Thermal expansion.
Real Expansion = Apparent Expansion + Expansion of the Vessel
3. Apparent Expansion
Apparent Expansion is the expansion in which only the expansion of liquid is considered and expansion of the vessel is not taken into account. Apparent expansion is less the real expansion.
Anomalous Expansion of Water
The increase in the volume of water as its temperature is lowered from 4 C to 0C is known as anomalous expansion of water.
Effects of Anomalous Expansion of Water
1. In winter, the temperature in the north and south poles of the earth falls. As the temperature fall below 4 C water on the surface expands and stays afloat. Ice continues building up at the surface while the temperature at the bottom remains at 4 C. This helps fish and other forms of marine life to live.
2. During the rainy season a lot of water seeps through the cracks in the rocks. In winter, when the water expands, the rock get broken due to this expansion.
3. In cold climate, water supply pipes burst when the water expands on cooling.
1. Boyle’s Law
The volume of a given mass of a gas is inversely proportional to the pressure, If the temperature is kept constant.
P < 1/V (Here < represents sign of proportionality. Do not write this in your examination paper)
P = C * 1/V
C = PV
The above equation is known as equation of Boyle’s Law.
2. Charle’s Law
The volume of a given mass of a gas is directly proportional to the temperature, if the pressure is kept constant.
V < T (Here < represents sign of proportionality. Do not write this in your examination paper)
V = C * T
C = V/T
The above equation is known as equation of Charle’s Law.
3. Pressure Law
The pressure of a given mass of a gas is directly proportional to the temperature, if the volume is kept constant.
P < T
P = C * T
C = P/T
The above is known as the equation of the Pressure Law.
The instrument that is used to measure temperature is called a thermometer.
Types of Thermometer
1. Ordinary Liquid-in-Glass Thermometer
An ordinary liquid-in-glass thermometer is used in a laboratory to measure temperature within a range of -10C to 110C.
It consists of a glass stem with a capillary tube, having a small bulb at one end. This bulb is filled with a liquid, usually mercury or alcohol coloured with a red dye. The upper end of the capillary tube is sealed so that the liquid will neither spill not evaporate. The air from the capillary tube is also removed.
When the bulb is heated, the liquid in it expands and rises in the tube. A temperature scale is marked on the glass stem to indicate temperatures according to the various levels of liquid in the tube.
2. Clinical Thermometer
A clinical thermometer is a device that is used to find the temperature of the human body. It has a range from 35 C to 43 C (95F to 110F).
It consists of a glass stem with a capillary tube, having a small bulb at one end. This bulb is filled with a liquid usually mercury or alcohol colored with a red dye. The upper end of the capillary tube is sealed so that the liquid will neither spill nor evaporate. The air from the capillary tube is also removed. The glass stem of a clinical thermometer has a construction in its capillary tube near the bulb. This helps to stop the mercury thread from moving back when the thermometer is removed from the patient’s mouth.
In order to find out the temperature, the thermometer is placed in the mouth or in the arm pit of the patient. The liquid in it expands and rises in the tube. A temperature scale is mrked on the glass stem to indicate temperatures according to the various levels of liquid in the tube.
3. Maximum and Minimum Thermometer
This thermometer is used to read the maximum and minimum temperatures reached over a period of time.
This thermometer consists of a fairly large cylindrical bulb with alcohol in it. This bulb is connected through a U-shaped tube filled mercury. At the end of this U-shaped tube another bulb containing alcohol is provided.
When the bulb is heated, alcohol in it expands and drives the mercury round towards the other end of the U-shaped tube. This mercury exerts pressure on the alcohol in the second bulb and its level rises. On each mercury surface, there is a small iron index provides with a light spring to hold it in position in the tube. When the mercury thread is moved, due to expansion or contraction of alcohol in the first bulb, the indices moves and are left in the extreme positions reached over a period of time. The lower end of the index on the left indicates the minimum and that on the right indicates the maximum temperature.
There are three methods of transferring heat from one place into another.
Conduction is a mode of heat transfer by atomic or molecular collisions, without the movement of a bulk of a substance from one position to another, in a body. It mostly occurs in solids.
Convection is a mode of heat transfer by the actual movement of the bulk of the substance from one place to another through large distances. It mostly occurs in liquids and gases.
Radiation is a mode of heat transfer which requires no material medium. Heat energy is carried by infra red electromagnetic waves from one place to another.
A bi-metallic strip is made of pieces of two different metals of different expansion rates, e.g. iron and brass. When it is heated, it bends with the brass on the outside of the curve because brass expands more quickly than iron.
1. Bi-metal Thermometer
A bi-metal thermometer is made of a bi-metallic coil. No liquid is used in such type of thermometer.
It consists of a bi-metallic strip in the form of a long spiral. One end of the spiral is kept fixed, while a light pointer is attached to the other end.
When the temperature rises, the bi-metal strip coil itself into an even tighter spiral due to different expansion rates of the two metals. the pointer moves across the temperature scale and in this way reading is noted.
2. Fire Alarm
A fire alarm is used to warn people when there is a fire.
In a fire alarm, one end of a bi-metal strip is firmly fixed, while the other is free. One terminal of a 6 volt battery is connected to the fixed end of the strip through a 6 volt bulb or bell. The other terminal of the battery is connected with a metallic contact which is just above the free end of the bi-metallic strip.
When a fire starts, heat energy is given off. It raises the temperature of the bi-metallic strip and its free end bends towards the contact. On touching the contact, electric circuit gets completed and the bulb starts to glow or in case of a bell, it rings warning about the fire.
Latent Heat of Fusion
The quantity of heat required to transform 1 kg of ice completely melts into water at 0C is known as Latent Heat of Fusion.
Latent Heat of Vaporization
the quantity of heat required to transform 1 kg of water completely into steam at 100 C is known as Latent Heat of Vaporization.
Effect of Pressure on Melting Point (Regelation)
The melting point of those substances, which expand on freezing, gets lowered when pressure oever one atmosphere is exerted on them.
Take a bare copper wire with weights on its both ends. Place it across a block of ice. The copper wire sinks slowly through the block and weight falls to the floor. Pressure exerted by the copper wire lowers the freezing point of ice and the ice beneath the wire melts. The water flows round the wire and re-freezes on getting above the wire, releasing latent heat energy. This energy is conducted through the copper wire, which helps to melt the ice below the wire. In this way, ice below the wire melts while water above the wire freezes. This process continues until the wire cuts through the ice block.
Effect of Pressure on Boiling Point
If the pressure on the surface of a liquid is increased above the normal atmospheric pressure, its boiling point increases.
Fill a round bottom flask to half its capacity. After boiling the water fro a few minutes, remove the burner and place a cork in the flask. Invert the flask and pour some cold water on the bottom of the flask. After some time, water starts to boil again although no more heat has been provided to it. The reason is that, when the water was boiled, it expelled all the air from the flask. When the flask was corked and allowed to cool the steam condensed into water. Since, no fresh air could enter the flask the pressure inside the flask lowered. This decreased the boiling point of water and water started to boil at normal temperature.
The process of change of a liquid into vapour without boiling is called evaporation.
Factors on which Evaporation Depends
Evaporation depends on the following factors:
1. Nature of Liquid: If the boiling point of a liquid is low, then they evaporate much quickly e.g. Alcohol and Ether.
2. Temperature of Liquid: If the surface temperature of a liquid is increased, then rate of evaporation also increases, e.g. ironing of clothes.
3. Surface Area of Liquid: If the surface area of a liquid is increased, then the rate of evaporation increases, e.g. liquids spread over large areas evaporate more quickly.
4. Dryness of Air: If there is more dryness in the air, then the rate of evaporation increases, e.g. in humid weather, clothes take a longer time to dry.
5. Wind speed: If the wind speed is greater, then evaporation rate increases.
6. Air Pressure on the Surface of The Liquid: If the pressure on the surface of the liquid is increased, the rate of evaporation decreases.
Law of Heat Exchange
For an isolated system comprising mixture of hot and cold substances, the heat lost by hot substances is equal to the heat gained by cold substances.
Heat lost by hot body = Heat gained by cold body
A refrigerator is a device that is used to keep fruits, vegetables and other eatables cool.
A refrigerator consists of a compressor, condenser and evaporator.
Freon is used as the refrigerant in a referigerator. This gas liquifies at normal temperature if the pressure is increased.
1. Compression: Freon gas is first compressed in the compressor of a refrigerator. It is then fed into the condenser.
2. Condensation: In the condenser, the gas is liquified under pressure. It converts into a liquid at normal temperature. This gas is then allowed to pass through a valve into the evaporator.
3. Evaporation: The pressure in the evaporator is comparatively less than in the condenser. Therefore, when liquid Freon enters the evaporator, it evaporates absorbing a large amount of heat. This results in cooling the area around the evaporator. This is where we keep our eatables.
The gas is then again fed into the compressor and the process continues