Heat and Thermodynamics
- Heat is a form of energy, which measures the sensation or perception of warmness or coldness of a body or environment
- Its unit are calorie, kilocalorie or joule.
- 1 calorie = 4.18 joule.
- Temperature is the measurement of hotness or coldness of a body.
- When two bodies are placed in contact, het always flows from a body at higher temperature to the body at lower temperature.
- An instrument used to measure the temperature of a body is called a thermometer.
- The normal temperature of a human body is 370C or 98.40 F
- - 400 is the temperature at which Celsius and Fahrenheit thermometers read same.
- The clinical thermometer reads from 960F to 1100
- White roof keeps the house cooler in summer than black roof because white roof reflects more and absorbs less heat rays whereas black roof absorbs more and reflects less heat rays.
- Ice wrapped in a blanket does not melt away quick because woolen blanket is a bad conductor of heat.
- Silver is the best conductor of heat.
- Cooking utensils are made of aluminum, brass and steel because these substances have low specific heat and high conductivity.
- Thermal expansion is the increase in size on heating.
- A solid can undergo three types of expansions
(ii) Superficial expansion
(iii) Cubical expansion
- Telephone wires are kept loose to allow the wires for contraction in winter.
- A gap is provided between two iron tracks of the railway track, so that rails can easily expand during summer and do not bend.
- The amount of heat required to raise the temperature of unit mass of a substance through 10C, is called its specific heat.
- When temperature of water is increased from 00C, then its volume decreases upto 40C, becomes minimum at 40C and then increases.
- This behaviour of water around 40C is called anomalous expansion of water.
- The heat energy absorbed or released at constant temperature per unit mass for change of state is called the latent heat.
- Latent heat of fusion of ice is 80 cal/g.
- Latent heat of vaporization of steam is 536 cal/g.
- Hot water burns are less severe than that of steam burns because steam has high latent heat.
- It is the slow process of a conversion of liquid into its vapour even below its boiling temperature.
- The amount of water vapour in air is called humidity.
- Relative humidity is measured by hygrometer.
- Relative humidity increase with the increase of temperature.
Transmission of Heat
- Transfer of heat from one place to other place is called transmission of heat.
- In solids, transmission of heat takes place by conduction process.
- In liquids and gases, transmission of heat takes place by convection process. In room, ventilators are provided to escape the hot air by convection.
- Heat from the Sun reaches the Earth by radiation.
- Simple pendulum is a heavy point mass suspended from a rigid support by means of an elastic and inextensible string.
- The maximum time period of a simple pendulum is 84.6 min.
- The time period of a simple pendulum does not depend upon the mass, shape and size of the bob and its amplitude of oscillation. A pendulum clock goes slow in summer and fast in winter.
- If a simple pendulum is suspended in a lift descending down with acceleration, then time period of pendulum will increase. If lift is ascending, then time period of pendulum will decrease.
- If a lift falling freely under gravity, then the time period of the pendulum is infinite.