CBSE Class 7 Science Heat Transfer Assignment Set 01

Answer: (a) conduction. In solids the particles pass heat along from one to the next without moving from their places.

Teacher's Note:
1. This is called conduction.
2. Convection happens only in liquids and gases.

Answer: (a) convection. Hot gases rise up the chimney and cooler air enters from below to take their place.

Teacher's Note:
1. This circulation is a convection current.
2. It carries smoke out of the house.

Answer: (d) 0 °C. Ice melts at 0 °C, which is the lower fixed point of the Celsius scale.

Teacher's Note:
1. The upper fixed point is 100 °C, the boiling point of water.
2. The scale is divided into 100 equal parts between them.

Answer: The colour of the surface and its nature (roughness or smoothness). Dark, rough surfaces absorb more heat; light-coloured, shiny surfaces absorb less.

Teacher's Note:
1. A good absorber of heat is also a good emitter.
2. A good reflector is a poor absorber.

Answer: 35 °C to 42 °C.

Teacher's Note:
1. This is the range of normal and fever temperatures of the human body.
2. Normal body temperature is about 37 °C.

Answer: 1. Sea breeze and land breeze are caused by convection currents in air.
2. A room heater placed near the floor, or a chimney or ventilator, uses convection to circulate warm air.

Teacher's Note:
1. Warm fluids rise and cool fluids sink.
2. This movement carries heat through the fluid.

Answer: Water freezes at 0 °C and boils at 100 °C, so it cannot measure low or high temperatures. It is colourless and wets the glass, and its expansion is not uniform. Mercury is shiny, does not stick to glass, expands evenly and stays liquid over a wide range.

Teacher's Note:
1. Thermometers need a liquid that expands regularly and is easy to see.
2. Mercury meets these needs.

Answer: Most liquids expand when heated. If a flask filled with coloured water is heated, the liquid level in the narrow tube rises. Liquids expand more than solids for the same rise in temperature.

Teacher's Note:
1. This expansion is used in thermometers.
2. Cooling the liquid makes it contract.

Answer: A thermos flask has a double-walled glass vessel with a vacuum between the walls. The walls facing the vacuum are silvered, and the flask is closed by an insulating cork or plastic stopper.
1. The vacuum stops loss of heat by conduction and convection.
2. The silvered surfaces reflect heat radiation back, so little is lost by radiation.
3. The stopper prevents heat loss through the mouth.

Teacher's Note:
1. Heat is blocked in all three ways.
2. So hot things stay hot and cold things stay cold for a long time.

Answer: Conduction is the transfer of heat through a solid from the hotter part to the colder part without movement of the material. Example: the handle of a spoon in hot tea becomes hot.

Teacher's Note:
1. Metals are good conductors.
2. Wood and plastic are poor conductors.

Answer: Radiation is the transfer of heat without any medium, in the form of heat waves. Example: heat from the sun reaching the earth through empty space.

Teacher's Note:
1. It needs no solid, liquid or gas.
2. We feel it when we stand near a fire.

Answer: Light-coloured clothes reflect most of the heat falling on them and keep us cool. Dark clothes absorb more heat and keep us warm.

Teacher's Note:
1. This depends on the absorbing power of the surface.
2. Colour matters most for radiation from the sun.

Answer: During the day land heats up faster than the sea. Air over the land becomes hot and rises, and cooler air from the sea moves in to take its place, forming a sea breeze.

Teacher's Note:
1. The air movement is a convection current.
2. At night the direction reverses to give a land breeze.

Answer: They are poor conductors of heat, so the handles do not become hot enough to burn the hand.

Teacher's Note:
1. Metal pans conduct heat well from the stove to the food.
2. Insulating handles keep our hands safe.

Answer: (c) radiation. Space between the sun and the earth is empty, and only radiation can cross it.

Teacher's Note:
1. Conduction and convection need matter.
2. Radiation does not.

Answer: (d) Wood. Wood does not allow heat to pass through easily.

Teacher's Note:
1. Metals are good conductors.
2. Poor conductors are called insulators.