Heat — Exercise Solutions Guide
Use this page alongside the official Samacheer Kalvi textbook. Solve every exercise question on your own first, then check the methods and answers below.
I. Multiple Choice Questions (MCQs)
Q1. When an object is heated, the molecules that make up the object
(a) begin to move faster ✓ (b) lose energy (c) become heavier (d) become lighter
Q2. The unit of heat is
(a) newton (b) joule ✓ (c) volt (d) Celsius
Q3. One litre of water at 30 °C is mixed with one litre of water at 50 °C. The temperature of the mixture will be
(a) 80 °C (b) more than 50 °C but less than 80 °C (c) 20 °C (d) around 40 °C ✓
Q4. An iron ball at 50 °C is dropped in a mug containing water at 50 °C. The heat will
(a) flow from the iron ball to the water (b) not flow from the iron ball to the water or from the water to the iron ball ✓ (c) flow from the water to the iron ball (d) increase the temperature of both
Q5. We reduce the heat by adding \_\_\_\_\_ while preparing fruit juice
(a) sugar (b) lime (c) ice cubes ✓ (d) salt
Q6. The SI unit of temperature is
(a) Calorie (b) Joule (c) Celsius (d) Kelvin ✓
Q7. Our normal body temperature is
(a) 34 °C (b) 36 °C (c) 35 °C (d) 37 °C ✓
Q8. Two tumblers A and B have water at 50 °C each. If water from both is poured into tumbler C, the temperature of water in tumbler C will be
(a) 100 °C (b) 0 °C (c) 50 °C ✓ (d) between 0 °C and 100 °C
Q9. \_\_\_\_\_ exists when two objects in thermal contact no longer affect each other's temperature
(a) Thermal expansion (b) Thermal equilibrium ✓ (c) Average temperature (d) Coolness
II. Fill in the Blanks
Q1. Heat flows from a higher temperature body to a lower temperature body.
Q2. The hotness of an object is determined by its temperature.
Q3. The SI unit of temperature is kelvin.
Q4. Solids expand on heating and contract on cooling.
Q5. Two bodies are said to be in the state of thermal equilibrium if there is no transfer of heat taking place.
Q6. We feel the heat on our body when the Sun shines.
Q7. Heat energy can be generated by burning fuels like coal, wood, charcoal, gasoline etc.
Q8. When electric current flows through a conductor, heat energy is produced.
Q9. Heat is a form of energy.
Q10. Temperature determines the direction of the flow of heat.
Q11. The coldest temperature in the world was measured in the Antarctic continent.
Q12. Temperature measures the average kinetic energy of molecules.
III. True or False (with corrections)
IV. Match the Following
Match Set 1 — Textbook Exercise
| Column A | Column B |
|---|---|
| 1. Heat | (d) joule |
| 2. Temperature | (e) kelvin |
| 3. Thermal Equilibrium | (c) No heat flow |
| 4. Ice cube | (a) 0 °C |
| 5. Boiling water | (b) 100 °C |
Match Set 2 — Additional
| Column A | Column B |
|---|---|
| 1. Source of heat | (e) Sun |
| 2. Electric current | (d) Electric kettle |
| 3. Gasoline | (a) Heat energy |
| 4. Unit of heat | (b) Calorie |
| 5. Human body temperature | (c) 37 °C |
V. Analogy
Q1. Heat : Joule :: Temperature : Kelvin
Just as the SI unit of heat is the joule, the SI unit of temperature is the kelvin.
Q2. Ice cube : 0 °C :: Boiling water : 100 °C
Ice melts at 0 °C; water boils at 100 °C at standard pressure.
Q3. Total kinetic energy of molecules : Heat :: Average kinetic energy : Temperature
Heat is total kinetic energy; temperature is average kinetic energy per molecule.
Q4. Movement of molecules : Heat :: Heat energy : Calorie
Heat is caused by molecular motion; calorie is a unit of heat energy.
Q5. Expansion in length : Linear expansion :: Expansion in volume : Cubical expansion
Linear expansion is change in length; cubical expansion is change in volume.
Q6. Ordinary glass : Glass tumbler :: Pyrex glass : Laboratory glassware
Ordinary glass cracks on heating; borosilicate (Pyrex) glass resists thermal shock and is used in labs.
VI. Give Reasons
Q1. Why does an ordinary glass bottle crack when boiling water is poured into it, but borosilicate glass does not?
Ordinary glass expands unevenly when suddenly heated — the inner surface heats and expands while the outer surface is still cold. This uneven expansion causes cracking. Borosilicate (Pyrex) glass has a very low coefficient of thermal expansion, so it expands very little and evenly, preventing cracks.
Q2. Why do electric wires sag in summer but become tight in winter?
In summer, higher temperatures cause the metal wires to expand (thermal expansion), making them longer and causing them to sag. In winter, lower temperatures cause the wires to contract (shrink), making them taut and straight again.
Q3. Why is a rivet heated before fixing it in a hole to join two metal plates?
Heating the rivet makes it malleable (soft and red-hot), so it can be easily hammered into a new rivet-head shape. When it cools, it contracts and grips the plates tightly, creating a very strong joint.
VII. Very Short Answers
Q1. List any three electrical appliances at home from which we get heat.
Water heater, iron box, and electric kettle.
Q2. What is temperature?
Temperature is the measure of the degree of hotness or coldness of a substance. Its SI unit is kelvin (K); other common units are degree Celsius (°C) and degree Fahrenheit (°F).
Q3. What is thermal expansion?
The expansion of a substance (solid, liquid, or gas) on being heated is called thermal expansion.
Q4. What is thermal equilibrium?
Thermal equilibrium is the state reached when two objects in thermal contact are at the same temperature and there is no net transfer of heat between them.
VIII. Short Answers
Q1. What difference does heating a solid make in its molecules?
When a solid is heated, its molecules gain kinetic energy and begin to vibrate faster. They spread slightly apart, causing the solid to expand. The higher the temperature, the greater the molecular motion and the more the solid expands.
Q2. Distinguish between heat and temperature.
| S.No. | Heat | Temperature |
|---|---|---|
| 1 | Depends on both temperature and number of molecules | Related to how fast molecules vibrate |
| 2 | Measures total kinetic energy of molecules | Measures average kinetic energy of molecules |
| 3 | SI unit: Joule (J) | SI unit: Kelvin (K) |
| 4 | Other units: Calorie | Other units: °C, °F |
| 5 | Has the ability to do work | Indicates degree of hotness or coldness |
Q3. How can heat energy be generated?
Heat energy can be generated by: (1) Burning fuels like wood, kerosene, coal, charcoal, petrol, or oil (combustion). (2) Friction — rubbing two surfaces together. (3) Passing electric current through a conductor. (4) Solar radiation from the Sun.
Q4. What is heat? Give its units.
Heat is a form of energy that flows from a body at higher temperature to a body at lower temperature. It causes molecules to move faster and raises temperature. Units: Joule (SI unit) and Calorie (1 calorie = 4.184 joules).
Q5. Define one calorie.
One calorie is the amount of heat energy required to raise the temperature of one gram of water by one degree Celsius (from 14.5 °C to 15.5 °C).
Q6. Differentiate between linear expansion and cubical expansion.
| Feature | Linear Expansion | Cubical Expansion |
|---|---|---|
| Definition | Increase in length on heating | Increase in volume on heating |
| Example | Railway track rods expanding | A metal ball expanding when heated |
| Dimension | One-dimensional (length) | Three-dimensional (volume) |
Q7. How do solids expand on heating?
When a solid is heated, its molecules gain kinetic energy and vibrate with greater amplitude. This increased vibration pushes molecules slightly further apart, causing the solid to occupy more space — i.e., it expands in length, area, and volume.
Q8. How is heat energy transferred?
Heat energy always flows from a region of higher temperature to a region of lower temperature. The three modes of transfer are: conduction (through solids), convection (through liquids and gases), and radiation (through empty space/vacuum, no medium required).
Q9. What type of glassware is used in laboratories? Why?
Borosilicate (Pyrex) glass is used in laboratories because it has a very low coefficient of thermal expansion. It does not expand much on heating, so it can withstand sudden temperature changes without cracking.
IX. Detailed Answers
Q1. Explain thermal expansion with suitable examples.
Thermal expansion is the increase in the size (length, area, or volume) of a substance when it is heated. As temperature rises, molecules vibrate more vigorously and move further apart, causing the substance to expand.
Examples of thermal expansion:
1. Railway tracks — Small gaps are left between rail sections so that they can expand in summer without bending. 2. Concrete bridges — Expansion joints are provided so that the bridge can expand and contract without cracking. 3. Electric wires — Wires sag in summer due to expansion and become tight in winter due to contraction. 4. Iron rims on wooden wheels — The iron rim is heated, expands, and is slipped onto the wheel. On cooling, it contracts and grips the wheel tightly. 5. Riveting — Hot rivets are driven through holes in metal plates. One end is hammered into a new head. On cooling, the rivet contracts and holds the plates tightly together. 6. Thermometers — Mercury or alcohol expands when heated and rises in the capillary tube, indicating temperature.
Q2. List and describe the sources of heat.
1. The Sun — The primary natural source of heat and light. Solar radiation reaches Earth through the vacuum of space by radiation. 2. Combustion (burning fuels) — Burning wood, kerosene, coal, charcoal, petrol, or diesel releases heat energy through chemical reactions. 3. Friction — Rubbing two surfaces together generates heat. Ancient people used this method by rubbing two stones to create fire. 4. Electricity — When an electric current passes through a conductor, heat is produced (Joule heating). Examples: electric iron, water heater, electric kettle. 5. Nuclear reactions — Nuclear fission and fusion release enormous amounts of heat energy (advanced topic).
Q3. Explain the working principle of a thermometer and describe different types.
A thermometer measures temperature using the thermal expansion of a liquid (mercury or alcohol) inside a glass tube with a bulb at one end. When the bulb is placed in contact with a hot object, the liquid expands and rises in the narrow capillary tube. The height of the liquid column corresponds to the temperature on a calibrated scale.
Types of thermometers:
1. Clinical thermometer — Measures human body temperature (35 °C to 42 °C). Uses mercury or digital sensors. 2. Laboratory thermometer — Measures a wider range (-10 °C to 110 °C) for experiments. 3. Digital thermometer — Uses electronic sensors; gives a numeric readout; safer than mercury types. 4. Six's maximum-minimum thermometer — Records the maximum and minimum temperatures over a period (used in weather stations).
X. Higher Order Thinking Skills (HOTS)
Q1. When a window is left open on a winter night, do you feel uncomfortable because "cold is getting in" or because "heat is escaping"?
Heat always flows from higher temperature to lower temperature. On a winter night, the room temperature (around 36 °C) is much higher than the outside temperature (around 22 °C). The discomfort is caused by heat escaping from the warm room to the cold outside. As warm air leaves, cold air enters through gaps. So it is the loss of heat that makes us feel cold, not "cold entering."
Q2. Suppose your normal body temperature were lower than it is. How would the sensation of hot and cold change?
A lower-than-normal body temperature is called hypothermia (below 37 °C). The body would feel a persistent cold sensation. Blood vessels near the skin would constrict to reduce heat loss, and shivering would occur as muscles try to generate heat. The person would perceive normal room temperature as cold and would need more warmth to feel comfortable.
Q3. If you heat a circular disk with a hole, what change do you expect in the diameter of the hole?
The diameter of the hole increases on heating. Think of the hole as part of the material — when the entire disk expands, every dimension increases, including the hole. A useful way to visualise: imagine the hole is very large, nearly the disk's radius, making it a ring. As atoms gain energy, the ring's radius grows, so the hole diameter also increases. This principle is used in engineering — to insert one hollow pipe into another of the same diameter, heat one pipe so its hole expands and the other pipe fits inside.
XI. Numerical Problems
Q1. A kettle with 1 litre of cold water on a gas stove takes 5 minutes to boil. A small electric kettle with 0.5 litre takes the same 5 minutes. Which gives more heat in 5 minutes? How many times as much?
The gas stove heats 1 litre of water from room temperature to boiling in 5 minutes. The electric kettle heats 0.5 litre in the same time. Since the gas stove heats twice the amount of water to the same final temperature in the same time, it supplies twice as much heat.
Q2. One calorie of heat is needed to raise the temperature of 1 gram of water by 1 °C (from 30 °C to 31 °C). How much heat is needed to raise 1 gram of water from 30 °C to 35 °C?
Temperature rise = 35 °C − 30 °C = 5 °C Heat needed = 1 calorie/°C × 5 °C = 5 calories
Common Mistakes
1. Confusing heat and temperature — Heat is total kinetic energy (measured in joules); temperature is average kinetic energy (measured in kelvin or °C). They are not the same.
2. Saying "cold flows into a room" — Cold does not flow. Heat always flows from hot to cold. What you feel is heat leaving your body.
3. Writing "steam forms when heat is released from water" — Steam forms when water absorbs heat. Ice forms when water releases heat. Mix these up and you lose marks.
4. Saying "all glass cracks with hot water" — Ordinary glass cracks because of uneven expansion. Borosilicate (Pyrex) glass has low thermal expansion and does NOT crack easily.
5. Confusing linear and cubical expansion — Linear expansion = increase in length. Cubical expansion = increase in volume. Do not write "expansion in volume is linear expansion."
6. Writing "unit of heat and temperature are the same" — They are not. Heat is measured in joules (or calories); temperature is measured in kelvin (or °C, °F).
7. Thinking mixing equal volumes at different temperatures gives the sum — 1 L at 30 °C + 1 L at 50 °C = approximately 40 °C (the average), NOT 80 °C. You add equal quantities; the temperature averages out.
Exam Checklist
Before your exam, make sure you can answer yes to every item below:
- [ ] I can define heat and state its SI unit (joule) and common unit (calorie).
- [ ] I can define temperature and state its SI unit (kelvin) and common units (°C, °F).
- [ ] I can distinguish heat from temperature (at least 4 differences in a table).
- [ ] I can name the three modes of heat transfer and give an example of each.
- [ ] I can define thermal expansion and give at least 3 real-life examples.
- [ ] I can explain why gaps are left in railway tracks and bridges.
- [ ] I can explain why borosilicate glass is used in labs and does not crack.
- [ ] I can state why electric wires sag in summer and tighten in winter.
- [ ] I can explain the riveting process using thermal expansion.
- [ ] I can define thermal equilibrium and give the condition (no heat flow, same temperature).
- [ ] I can list at least 4 sources of heat (Sun, combustion, friction, electricity).
- [ ] I can define 1 calorie correctly (heat to raise 1 g of water by 1 °C).
- [ ] I can distinguish linear expansion from cubical expansion.
- [ ] I can solve simple numerical problems involving calories and temperature changes.
- [ ] I can answer HOTS questions about heat flow direction and thermal expansion of a hole in a disk.