CBSE Class 7 Science Heat and Temperature, Conduction, Convection and Radiation Worksheet Set 07

Answer: (b) energy. Heat is energy that flows because of a temperature difference.

Teacher's Note:
1. Heat is not a substance.
2. It is energy in transit.

Answer: (b) joule. Heat is energy, so it is measured in joules.

Teacher's Note:
1. Celsius and kelvin measure temperature, not heat.
2. The calorie is an older unit.

Answer: (a) thermometer. A thermometer measures how hot or cold a body is.

Teacher's Note:
1. Barometers measure air pressure.
2. Always match the instrument to the quantity.

Answer: (a) mercury. Mercury expands evenly and is shiny, so it is easy to read.

Teacher's Note:
1. Mercury also does not stick to glass.
2. It does need care because it is poisonous.

Answer: (b) Sun. The Sun supplies almost all the heat that reaches Earth.

Teacher's Note:
1. It reaches us by radiation through empty space.
2. Other sources are far smaller.

Answer: (a) a hotter body to a colder body. Heat moves from higher temperature to lower temperature until both become equal.

Teacher's Note:
1. This is the direction of natural flow.
2. Size of the bodies does not decide it.

Answer: (a) mercury freezes at about −39 °C. Once the mercury freezes, it can no longer show the fall in temperature.

Teacher's Note:
1. Alcohol thermometers are used for very low temperatures.
2. Mercury is fine for ordinary weather.

Answer: (d) conduction, convection and radiation. The vacuum, stopper and shiny walls reduce all three modes of heat transfer.

Teacher's Note:
1. The vacuum between the walls stops conduction and convection.
2. Silvered walls reflect radiation back.

Answer: (b) 37 °C. The average healthy body temperature is 37 °C (98.6 °F).

Teacher's Note:
1. A clinical thermometer is used to measure it.
2. A reading well above 37 °C suggests fever.

Answer: 0 and 100. The interval between them is divided into 100 equal parts.

Teacher's Note:
1. These two points are the reference marks of the scale.
2. Each part is one degree Celsius.

Answer: degree. Temperature tells how hot or cold a body is.

Teacher's Note:
1. Temperature is a measure of hotness, not the quantity of heat.
2. Heat depends on mass and material as well.

Answer: conduction. Heat passes from particle to particle without the particles moving from their positions.

Teacher's Note:
1. Metals conduct best.
2. Solids rarely transfer heat by convection.

Answer: convection. The warm fluid rises and cooler fluid sinks to take its place.

Teacher's Note:
1. This circulation carries heat through the fluid.
2. It does not happen in solids.

Answer: radiation. It needs no medium and travels through space.

Teacher's Note:
1. Radiation is the only mode that works across a vacuum.
2. We feel it as warmth on the skin.

Answer: False. Heat is energy, while temperature is the degree of hotness.

Teacher's Note:
1. A bucket of warm water holds more heat than a hot spoon.
2. Yet the spoon has the higher temperature.

Answer: True. Copper, aluminium and iron pass heat quickly.

Teacher's Note:
1. That is why cooking vessels have metal bases.
2. Their handles are made of poor conductors.

Answer: False. Convection needs a fluid whose particles can move.

Teacher's Note:
1. Only liquids and gases can form convection currents.
2. Solids transfer heat by conduction.

Answer: False. They are poor conductors, called insulators.

Teacher's Note:
1. This is why kettle handles are made of them.
2. They stay cool to touch.

Answer: Heat is the form of energy that flows from a hotter body to a colder body. Temperature is the degree of hotness or coldness of a body.
1. Heat is energy measured in joules; temperature is measured in °C by a thermometer.
2. Heat flows between bodies; temperature decides the direction of flow.

Teacher's Note:
1. Keep the idea of energy for heat and degree for temperature.
2. These two points are enough for full marks.

Answer: A thermometer is an instrument used to measure temperature.
Advantages of mercury: 1. it is a shiny liquid, easy to see; 2. it expands uniformly and does not stick to the glass.

Teacher's Note:
1. Mercury also has a wide range between its freezing and boiling points.
2. It is toxic, so handle a broken thermometer carefully.

Answer: Conduction is the transfer of heat from the hotter part of a solid to the colder part, without movement of the particles of the solid.
Examples: 1. A metal spoon in hot tea becomes hot at the other end. 2. The handle of a pan on a flame warms up.

Teacher's Note:
1. The particles pass vibrations to their neighbours.
2. It is the main way heat travels in solids.

Answer: Convection is the transfer of heat in liquids and gases by the movement of the fluid itself.
Example: when water in a pot is heated, the hot water rises, the cold water sinks, and this circulation heats all the water.

Teacher's Note:
1. Warm fluid is lighter, so it rises.
2. The cooler fluid sinks and is heated in turn.

Answer: It is radiation. There is almost no matter between the Sun and the Earth, and conduction and convection need a material medium.

Teacher's Note:
1. Radiation travels as waves.
2. It needs no medium at all.

Answer: Good conductors: copper, aluminium, iron (silver, gold).
Insulators: wood, plastic, cork (glass, air).

Teacher's Note:
1. Conductors are mostly metals.
2. Insulators are used to prevent heat loss or burns.

Answer: It has a double-walled glass bottle with a vacuum between the walls, which stops conduction and convection. The walls are silvered to reflect heat radiation, and a cork stopper reduces loss from the top.

Teacher's Note:
1. Each part blocks one mode of heat transfer.
2. Together they slow down both heating and cooling.

Answer: Fluffing traps a layer of air between the feathers. Air is a poor conductor of heat, so less body heat is lost.

Teacher's Note:
1. Trapped air acts as insulation.
2. This is also why woollen clothes keep us warm.

Answer: Plastic and wood are poor conductors of heat. They do not become hot, so we can hold the handle safely.

Teacher's Note:
1. Metals would pass heat to our hand.
2. Insulating handles prevent burns.

Answer: (a) Cork, because it is an insulator while the others are good conductors. (b) Expansion, because the others are modes of heat transfer.

Teacher's Note:
1. Look for the property shared by most items.
2. Then name the one that breaks it.

Answer: Light colours reflect more heat radiation and cotton lets sweat evaporate, so we stay cooler. Dark colours absorb more heat and wool traps air, so we stay warmer.

Teacher's Note:
1. Colour affects absorption of radiation.
2. Wool is warm because trapped air is a poor conductor.