Physics Notes: Important Points on Heat and Thermodynamics

Heat and Thermodynamics

Heat

Heat is a form of energy, which measures the sensation or perception of warmness or coldness of a body or environment
Its unit is a calorie, kilocalorie or joule.
1 calorie = 4.18 joule.

Temperature

Temperature is the measurement of the hotness or coldness of a body.
When two bodies are placed in contact, heat always flows from a body at a higher temperature to the body at a 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 98.4^{0 F}
- 40⁰ is the temperature at which Celsius and Fahrenheit's thermometers read the same.
The clinical thermometer reads from 96⁰F to 110⁰
The white roof keeps the house cooler in summer than the black roof because a white roof reflects more and absorbs fewer heat rays whereas the black roof absorbs more and reflects fewer heat rays.
Ice wrapped in a blanket does not melt away quickly because the woollen blanket is a bad conductor of heat.
Silver is the best conductor of heat.
Cooking utensils are made of aluminium, brass, and steel because these substances have low specific heat and high conductivity.

Thermal Expansion

Thermal expansion is the increase in the size of heating.
A solid can undergo three types of expansions-
(i) Linear expansion
(ii) Superficial expansion
(iii) Cubical expansion
The relation between the coefficient of linear expansion (α), the coefficient of superficial expansion (β), the coefficient of cubical expansion (γ)
α: β:γ = 1: 2: 3
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.

Specific Heat

The amount of heat required to raise the temperature of a unit mass of a substance through 1⁰C, is called its specific heat.
When the temperature of the water is increased from 0⁰C, then its volume decreases up to 4⁰C, becomes a minimum at 4⁰C and then increases.
This behavior of water around 4⁰C is called the anomalous expansion of water.

Latent Heat

The heat energy absorbed or released at constant temperature per unit mass for a change of state is called latent heat.
The 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.

Evaporation

It is the slow process of a conversion of a liquid into its vapour even below its boiling temperature.
The amount of water vapour in the air is called humidity.
The relative humidity is measured by the hygrometer.
Relative humidity increase with the increase of temperature.

Transmission of Heat

Transfer of heat from one place to another place is called the transmission of heat.
In solids, the transmission of heat takes place by the conduction process.
In liquids and gases, the transmission of heat takes place by the convection process. In the room, ventilators are provided to escape the hot air by convection.
The heat from the Sun reaches the Earth by radiation.

Simple Pendulum

A simple pendulum is a heavy point mass suspended from 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 the time period of the pendulum will increase. If the lift is ascending, then the time period of the pendulum will decrease.
If a lift falling freely under gravity, then the time period of the pendulum is infinite.