LIWA

Heat

ICSE Class 7 · Physics · Chapter 5

What this chapter covers

Key definitions

Melting is the change of a solid into a liquid when it is heated.

Boiling is the rapid change of a liquid into a gas at a fixed temperature, with bubbles forming all through the liquid.

Condensation is the change of a gas into a liquid when it is cooled.

Heat transfer is the movement of heat energy from a hotter object or region to a colder one.

Conduction is the transfer of heat through a material from its hotter part to its colder part, from one particle to the next, without the particles moving from their places.

Convection is the transfer of heat in a liquid or a gas by the actual movement of its heated particles from one place to another.

Radiation is the transfer of heat from a hot object in the form of invisible rays, without needing any material in between.

Key points

Questions answered

What is heat? How is it different from temperature?
Heat is a form of energy that flows from a hotter object to a colder one because of a difference in temperature. Temperature only tells how hot or cold an object is. A bucket of warm water and a cup of water at the same temperature have the same temperature, but the bucket holds more heat.
Name the SI unit of heat and one other unit. How are they related?
The SI unit is the joule (J). Another unit is the calorie (cal). One calorie is about 4.2 joules.
Change 50 cal into joules.
50 × 4.2 = 210 J.
Write the lower and upper fixed points on the Celsius, Fahrenheit and Kelvin scales.
Celsius: 0 °C and 100 °C. Fahrenheit: 32 °F and 212 °F. Kelvin: 273 K and 373 K.
Change 25 °C into kelvin and into degrees Fahrenheit.
Kelvin: 25 + 273 = 298 K. Fahrenheit: 25 × 9 ÷ 5 = 45, and 45 + 32 = 77 °F.
Why are small gaps left between lengths of railway line?
On hot days the steel rails expand. The gaps give them room to expand; without them the rails would push against each other and bend.
Why does the temperature of boiling water stay at 100 °C even though the stove is still on?
While water is boiling, the heat supplied is used to change the water into steam, so it changes the state of the water instead of raising its temperature.
Why do cooking pans have plastic or wooden handles but metal bottoms?
The metal bottom is a good conductor, so heat from the flame reaches the food quickly. The plastic or wooden handle is an insulator, so the heat does not reach your hand.