Revision: Heat Science Secondary School (English Medium) (5 to 8) Class 7 CBSE

One kilo-calorie of heat is the heat energy required to raise the temperature of 1 kg of water from 14.5°C to 15.5°C.

The kinetic energy due to random motion of the molecules of a substance is known as its heat energy.

The sum of the potential energy and kinetic energy of a molecule is called its internal energy.

The total internal energy of all the molecules of a substance is called its thermal energy.

Heat is that form of energy which flows from a hot body to a cold body when they are kept in contact.

The measurement of the quantity of heat is called calorimetry.

Heat energy lost by the hot object = Heat energy gained by the cold object. This is called as ‘Principle of heat exchange’

The temperature at which the pressure and volume of a gas theoretically reach zero is called absolute zero.

Coefficient of Linear expansion is equal to the change in length of a rod of length 1m when its temperature rises by 1°C.

Temperature is a parameter which tells the thermal state of a body (i.e., the degree of hotness or coldness of the body). It determines the direction of flow of heat when two bodies at different temperatures are placed in contact.

Solid substances that conduct heat easily are called good conductors of heat.

Conduction is the process by which heat flows from the hot end to the cold end of a solid body without any net bodily movement of the particles of the body.

OR

The process by which heat flows from the hot end to the cold end of a solid body without any net bodily movement of the particles of the body is called conduction.

Substances that do not conduct heat easily are called bad conductors of heat.

Convection is the process by which heat is transmitted through a substance from one point to another due to the actual bodily movement of the heated particles of the substance.

OR

The process by which heat is transmitted through a substance from one point to another due to actual bodily movement of the heated particles of the substance is called convection.

OR

The mode of heat transfer by actual motion of matter (bulk transport of fluid) from the source of heat, which occurs only in fluids, is called convection.

The transfer of heat energy from one place to another via emission of EM energy (in a straight line with the speed of light) without heating the intervening medium is called radiation.

OR

The transfer of heat energy from one place to another via emission of EM energy (in a straight line with the speed of light) without heating the intervening medium is called radiation.

Statement: In steady-state heat flow by conduction in a bar with ends maintained at different temperatures T_C and T_D, the heat flow is proportional to the temperature difference and the area of cross-section A, and inversely proportional to the length L.

Also written as:

Where K is the thermal conductivity of the material.

Key Points:

• Gases are poor conductors; liquids have intermediate conductivities; solids are generally good conductors.
• The greater the value of K, the more rapidly the material conducts heat.

Statement: All bodies emit radiant energy depending on their temperature. The heat emitted (H) by a body is given by:

Where:

• σ = Stefan-Boltzmann constant
• e = Emissivity (for perfect radiator, e = 1)
• A = Area of the body
• T = Temperature (in Kelvin)

Key Points:

• Black bodies absorb and emit more radiant energy than bodies of lighter colors.
• Thermal radiation is partially reflected and partially absorbed when it falls on other bodies.

• Heat is the energy that flows from a hot body to a cold body when they are kept in contact.
• The S.I. unit of heat is joule (J), while calorie and kilocalorie are commonly used units.
• One calorie is approximately equal to 4.2 joule, and kilocalorie is used to measure the energy value of foods.

• If no heat flows between two bodies in contact, they are said to be at the same temperature, even though their thermal energies may be different.
• The S.I. unit of temperature is kelvin (K), and absolute zero (0 K) is the temperature at which molecular motion ceases.

• The transfer of heat from the hot part to the cold part of an object is called conduction of heat.
• Conduction takes place through solid substances only — it requires a medium.
• Heat travels by molecular collisions: fast-vibrating molecules pass energy to slower neighbours.
• Copper conducts heat faster than aluminium, which conducts faster than steel.
• Conduction of heat through a substance depends on the property of that substance.
• Good conductors: silver, copper, aluminium, brass — all metals.
• Bad conductors: wood, cloth, air, paper — most non-metals.
• Good conductors of heat are also good conductors of electricity, and bad conductors of heat are also bad conductors of electricity.

• Convection occurs only in fluids (liquids and gases) — not in solids.
• In conduction, molecules vibrate but stay in place.
• In convection, molecules physically move from one place to another.
• Heating reduces density → hot fluid rises; cool fluid sinks → a convection current is set up.
• Convection currents transfer heat to the entire mass of the fluid.
• Potassium permanganate makes convection currents visible as magenta-coloured streams.

• When water is heated from the top, its density decreases, and it stays at the top. Since hot water cannot sink, convection does not occur and the bottom remains cool.
• Radiation is the transfer of heat without a medium — through electromagnetic waves.
• Heat from the Sun reaches us through radiation across the vacuum of space.
• All objects above 0 K emit thermal radiation (electromagnetic waves).
• Radiation is a two-step process: thermal energy → EM waves → thermal energy.
• Black or dark surfaces absorb more heat radiation; absorption also depends on the intrinsic properties of the substance.
• An infrared camera uses the radiation emitted by objects to see at night — useful for military surveillance.
• Copper is an excellent conductor; plastic is a bad conductor (insulator).
• Heat readily conducts through metals (copper and steel) but not through non-metals (wood and plastic).