Heat and Energy
ICSE Class 9 · Physics · Chapter 6
What this chapter covers
- Heat as a form of energy
- Heat and temperature
- Anomalous expansion of water
- Global warming and the greenhouse effect
- Energy degradation
Key points
- Heat is energy that flows because of a temperature difference, from higher to lower temperature.
- SI unit of heat: joule. 1 cal = 4.186 J; 1 kcal = 4186 J.
- Temperature is the degree of hotness and measures the average kinetic energy of molecules; heat depends on mass, temperature does not.
- C/100 = (F - 32)/180; T(K) = C + 273.
- Water contracts from 0 °C to 4 °C and expands above 4 °C: minimum volume and maximum density (1000 kg m-3) at 4 °C.
- Water expands by about 9 per cent on freezing, so ice floats.
- Hope's experiment: the lower thermometer settles at 4 °C while the upper one falls to 0 °C.
- Consequences: aquatic life survives under ice, pipes burst, rocks are weathered, icebergs float.
- Greenhouse gases (CO2, CH4, water vapour, N2O, CFCs) trap infrared radiation; extra amounts from human activity cause global warming.
- Global warming melts glaciers, raises sea level, disturbs the monsoon and harms farming, biodiversity and health.
- Energy degradation: in every transformation some energy becomes less useful heat; no device is 100 per cent efficient.
Questions answered
Define heat and state its SI unit.
Heat is the form of energy that flows from one body to another because of a temperature difference. Its SI unit is the joule.
Define the calorie. How many joules make one calorie?
The calorie is the heat needed to raise the temperature of 1 g of water through 1 °C. 1 cal = 4.186 J.
Express 2093 J in calories.
2093 ÷ 4.186 = 500 cal.
Give three differences between heat and temperature.
Heat is energy measured in joules and depends on mass; temperature is a degree of hotness measured in kelvin or °C and does not depend on mass; heat flows from higher to lower temperature, so temperature decides the direction of flow.
A bucket of water at 40 °C and a cup of water at 80 °C are given. Which has more heat? Which has the higher temperature?
The cup has the higher temperature; the bucket, having much more water, usually holds more heat energy.
Convert 25 °C into °F and into K.
F = (9)/(5) × 25 + 32 = 77 °F; T = 25 + 273 = 298 K.
What is meant by the anomalous expansion of water?
Water contracts when heated from 0 °C to 4 °C and expands when heated above 4 °C, so it has its minimum volume and maximum density at 4 °C.
Describe the volume–temperature graph of water from 0 °C to 10 °C.
The volume decreases from 0 °C to a minimum at 4 °C and then increases up to 10 °C.
Past board exam questions
ICSE 2019 sample paper · 2 marks
State the second law of Thermodynamics in energy flow (Law of conservation of energy).
Show answer
In every conversion of energy from one form to another, some energy is always converted into heat that is dissipated to the surroundings and cannot be used; so no energy conversion is 100% efficient and the useful energy keeps decreasing at each step of energy flow (e.g. along a food chain). Heat flows on its own only from a hotter body to a colder body.
(The total energy, however, remains conserved — it is neither created nor destroyed.)
ICSE 2019 sample paper · 2 marks
State two main human activities which are responsible for the increase of carbon dioxide gas in the atmosphere.
Show answer
1. Burning of fossil fuels (coal, petrol, diesel) in industries, power plants and vehicles.
2. Deforestation (cutting down of forests), which reduces absorption of CO₂ by plants.
ICSE 2019 sample paper · 3 marks
(i) Define global warming. (ii) State two ways by which global warming impacts on life on earth.
Show answer
(i) Global warming is the gradual rise in the average temperature of the earth's atmosphere and surface due to the enhanced greenhouse effect caused by increasing amounts of greenhouse gases (CO₂, methane etc.).
(ii) 1. Melting of glaciers and polar ice raises the sea level, flooding low-lying coastal areas. 2. Changes in climate and rainfall patterns affect agriculture (lower crop yields) and cause droughts, floods and the extinction of some plant and animal species. (Also spread of diseases, heat waves.)
ICSE 2019 sample paper · 3 marks
A frozen pond in a cold region: the air above the pond is below 0°C and the surface of the pond is covered with a layer of ice. A is a point in the water just below the ice and B is a point at the bottom of the pond. (i) State the expected temperatures at A and B. (ii) Name the phenomenon responsible for these temperatures mentioned in part (i)
Show answer
(i) Temperature at A (just below the ice) = 0°C; temperature at B (at the bottom) = 4°C.
(ii) Anomalous expansion of water — water has its maximum density at 4°C, so water at 4°C sinks to the bottom while colder water stays at the top and freezes.