Heat Energy
Theory Questions And Answers
1. What happens when you heat something up?
Answer: Its temperature rises.
2. State whether each of the following statements is true or false.
a) Heat and temperature are the same thing.
Answer: False.
b) Heat and temperature are related to each other, but are different concepts.
Answer: True.
3. What is temperature?
Answer: The temperature of a body is defined as the degree of hotness of the body.
4. Mention three units used to measure temperature.
Answer:
· Degree Celsius (ºC)
· Degree Fahrenheit (ºF)
· Kelvin
5. Heat is a form of energy. What is heat energy?
Answer: Heat energy is the energy that is transferred from one point to another owing to the temperature difference between the two points.
6. Heat is transferred from what point and to what point?
Answer: Heat is transferred from a point of higher temperature to another of lower temperature.
7. What will increase or decrease the temperature of a body?
Answer: Heat energy.
8. a) If we add heat to a body, the temperature will become
Answer: higher.
b) If we remove heat, the temperature will become
Answer: lower.
9. If the temperature of one object is higher than that of the other object, there will be a transfer of energy from the
Answer: hotter object to the colder object, until both objects reach the same temperature.
10. What would a cold object placed in a warm place do?
Answer: It will absorb heat from its surroundings, causing its temperature to rise.
11. What would a warm object placed in a cold place do?
Answer: It will lose heat to its surroundings, causing its temperature to fall.
12. Heat is measure in
Answer: joules (J).
13. State whether each of the following statements is true or false.
a) Temperature is not energy, but a measure of it.
Answer: True.
b) Heat is energy. Heat energy can change the state of a substance. For example, heat causes substances to change from solid to liquid.
Answer: True.
14. What can increase or decrease the temperature of a substance?
Answer: Heat.
15. State four differences between heat and temperature.
Answer:
|
Heat |
Temperature |
|
1. It causes a rise in temperature of bodies |
1. It is the degree of hotness of a body. |
|
2. It is a form of energy. |
2. It is not a form of energy. |
|
3. It is measured in joules. |
3. It is measured in kelvin. |
|
4. It can be transferred from one point to another. |
4. It cannot be transferred from one point to another. |
16. What is a thermometer?
Answer: A thermometer is an instrument used to measure the temperature of a body or place.
17. The most common thermometers have liquid in them and are known as
Answer: liquid-in-glass thermometers.
18. The liquid substance in a thermometer is known as a
Answer: thermometric substance.
19. Name two common thermometric substances.
Answer:
· Mercury
· Alcohol
20. The most preferred thermometric substance is
Answer: mercury.
21. Name the three most common liquid-in-glass thermometers.
Answer:
· The clinical thermometer.
· The laboratory thermometer.
· The maximum and minimum thermometer.
22. The liquid-in-glass thermometer used to measure temperature in the laboratory is the
Answer: laboratory thermometer.
23. The liquid-in-glass thermometer used to measure maximum temperature during the day and minimum temperature during the night is the
Answer: maximum and minimum thermometer.
24. Liquid-in-glass thermometers are constructed based on a fact. What is that fact?
Answer: Liquid-in-glass thermometers are constructed based on the fact that liquids expand when heated and contract when cooled.
25. Mention the three main feature of a liquid-in-glass thermometer.
Answer:
· Bulb
· Stem
· Calibrations
26. a) In a liquid-in-glass thermometer, what serves as a reservoir for thermometric liquid?
Answer: The bulb.
b) Name two features of the stem of liquid-in-glass thermometers.
Answer:
· Uniform capillary glass tube
· Calibrations
c) State the function of each of the following:
i) Stem
Answer: It has a uniform capillary glass tube through which liquid rises when temperature is increased and falls with decrease in temperature.
ii) Calibrations
Answer: Calibrations on the stem help to read the temperature.
27. A liquid that expands or contracts appreciably with a small change in temperature is called a
Answer: thermometric liquid.
28. A thermometer that can record very small changes in temperature is called a
Answer: sensitive thermometer.
29. State four qualities of a thermometric liquid.
Answer:
· It has a large temperature range within which it remains a liquid.
· It expands or cools regularly and uniformly with a rise or fall in temperature.
· It does not wet or cling to the glass tube.
· It does not easily vaporize or condense in the glass tube.
30. State five advantages of mercury over alcohol as a thermometric substance.
Answer:
· Mercury is easily seen but alcohol is colourless and cannot be easily seen in the capillary tube.
· Mercury does not wet glass. Alcohol wets glass and makes accurate reading difficult.
· Mercury is a good conductor of heat. Alcohol is a poor conductor of heat.
· Mercury has a high boiling point (357 ºC) and so can be used to measure very high temperatures. Alcohol has a lower boiling point (78 ºC) and so cannot be used to measure very high temperatures.
· Mercury does not evaporate easily but alcohol evaporates easily.
31. State two advantages of alcohol over mercury as a thermometric substance.
Answer:
· Alcohol has a very low freezing point (-112 ºC) and so can be used to measure very low temperatures without solidifying. Mercury has a higher freezing point (-39 ºC) and so cannot be used to measure very low temperatures.
· Alcohol expands more than mercury (about six times more than mercury).
32. State four reasons why water is not used as a thermometric substance.
Answer:
· It cannot be easily seen in the capillary tube.
· It wets glass.
· Its expansion is not uniform.
· It has a small range of expansion.
33. State five precautions to ensure accurate measurements in the use of liquid-in-glass thermometers.
Answer:
· For even distribution of heat throughout, stir the substance.
· Ensure that the bulb of the thermometer is fully immersed or enclosed in the substance whose temperature is being determined.
· The bulb of the thermometer should not touch the bottom or sides of the container in which we have the substance whose temperature is being determined.
· The thermometer should be held at the stem to prevent the measurement of one’s body temperature.
· Sufficient time should be allowed for thermal equilibrium to be reached to ensure correct temperature reading.
34. The thermometer used for measuring the temperature of the human body is the
Answer: clinical thermometer.
35. Why is the range of the clinical thermometer between 35ºC to 43ºC?
Answer: Because the normal body temperature varies from 35ºC to 43ºC.
36. a) Which part of a clinical thermometer is small and has a thin glass tube which ensures that mercury warms up quickly to make the thermometer a quick acting one?
Answer: The bulb.
b) Which part of a clinical thermometer is a short and thin uniform tube that makes the thermometer very sensitive?
Answer: Capillary tube.
37. State five features of a clinical thermometer and their functions.
Answer:
· Bulb
Function:
ü It serves as a reservoir for mercury.
ü It has a thin glass tube which ensures that mercury warms up quickly to make the thermometer a quick acting one.
· Fine bore
Function: It makes the thermometer sensitive to temperature changes.
· Capillary tube
Function: It makes the thermometer very sensitive.
· Stem
Function: It has calibrations ranging from 35ºC to 43ºC.
· Constriction or kink
Function: It prevents the mercury from falling back into the bulb.
38. Describe how to use the clinical thermometer.
Answer:
· Thermometer bulb is usually placed under the armpit of the patient.
· Mercury expands and forces its way beyond the constriction.
· Thermometer is allowed to be in contact with the patient’s body for about 3 – 5 minutes.
· It is then removed whereby mercury cools and contracts.
· Mercury beyond constriction stays in the tube where body temperature is indicated and can be read.
· Mercury thread breaks at the constriction.
· For the thermometer to be used again, it is shaken vigorously to force the mercury into the bulb.
39. Name the substance used to sterilize clinical thermometers.
Answer: Alcohol antiseptics.
40. Why is boiling water not used to sterilize clinical thermometers?
Answer: Using boiling water for sterilization would cause the glass of the thermometer to crack due to excessive expansion since water boils at 100ºC but clinical thermometers are made to measure body temperature up to 43ºC.
41. What is heat transfer?
Answer: Heat transfer is the movement of heat energy from a region of higher temperature to a region of lower temperature.
42. Name the three processes by which heat is transferred.
Answer:
· Conduction
· Convection
· Radiation
43. a) Can heat be conducted between two bodies which are in contact with each other?
Answer: Yes.
b) What is conduction?
Answer: Conduction is the process by which heat is transferred from one point of a solid to another point without the heated particles moving.
44. a) What are good conductors of heat?
Answer: Good conductors of heat are materials that allow heat to pass through them easily.
b) Mention four examples of good conductors of heat.
Answer:
· Aluminium
· Copper
· Iron
· Zinc
45. a) What are bad conductors of heat?
Answer: Bad conductors of heat are materials that do not allow heat to pass through them easily.
b) Bad conductors of heat are also known as
Answer: Poor conductors of heat.
c) List five examples of bad conductors of heat.
Answer:
· Wood
· Leather
· Cork
· Glass
· Air
· Plastics
· Concrete
46. Why are cooking utensils made of metals such as aluminium which is a good conductor of heat?
Answer: To cause food to cook quickly.
47. Why are the handles of cooking utensils made mostly of wood which is a poor conductor of heat?
Answer: To make it easy to handle utensils when they are hot.
48. Explain the following environmental phenomena.
“A stone floor or cemented floor feels cold to the feet, but a rug or carpet on the same floor feels warm.”
Answer: This is because the stone is a good conductor of heat. Hence, it conducts heat away from the bare feet, making the feet feel cold. The carpet or rug on the other hand, is a poor conductor of heat and so does not conduct heat away from the feet as rapidly as the stone.
49. Describe an experiment to demonstrate conduction in a solid.
Answer:
Method 1
Aim: To demonstrate conduction in a solid.
Materials needed: Candle wax, metal rod, retort stand and source of heat.
Procedure:
· Dip one end of a metal rod in some molten candle wax.
· Allow the candle wax to cool and solidify.
· Clamp the rod in the middle.
· Put the end of the rod which was not covered with candle wax in a hot flame.
Observation: The heat of the flame causes the candle wax along the metal rod to melt.
Conclusion: The heat of the flame was transferred along the metal rod by the process of conduction.
Method 2
Aim: To demonstrate conduction of heat in a solid.
Materials needed: Metal rod, drawing pins, candle wax, stop watch/clock, source of heat (bunsen burner).
Procedure:
· Place or clamp the metal rod in a horizontal manner.
· Attach pins at various intervals on the rod with the aid of the candle wax.
· Heat the rod at one end and start the stop watch at the same time.
· Take note of the time each pin falls when candle wax melts.
Observation: After some time, the candle wax will begin to melt and the pins fall off. Wax closest to source of heat melts in a shorter time, hence pins closest to heat source fall first (pins fall in a specific manner).
Conclusion: Heat moved through the solid rod from the hottest end to coolest end. Conduction has taken place, as heat travelled from region of higher temperature to one of lower temperature.
50. Describe how conduction occurs.
Answer:
· When one end of a solid is heated, its molecules vibrate more strongly.
· The molecules can only vibrate but not move because they are held by strong intermolecular force of attraction.
· The vibrating molecules knock against or collide with other molecules making them also to vibrate.
· In this way, the hot molecules pass on some of their heat energy to the cooler molecules next to them.
· Hence, heat is transferred from one end of a solid to the other by conduction.
51. What is convection?
Answer: Convection is the process by which heat energy is transferred in a liquid or a gas by the movement of molecules of the heated fluid itself.
52. Convection occurs in two mediums. Name them.
Answer:
· Liquids
· Gases
53. The transfer of heat in liquids or gases is accompanied by the
Answer: formation of convection current.
54. What is convection current?
Answer: Convection current occurs when the molecules of a liquid nearest to a source of heat gain heat energy, resulting in an increase in their movement.
55. Complete the following statements that are related to convection currents.
a) The heated molecules of the fluid expand and become less dense than
Answer: the colder molecules around them.
b) As a result, the heated molecules rise up carrying
Answer: the heat energy along with them.
c) The colder and more dense molecules move
Answer: down to take the place of the heated molecules, thus setting up a convection current.
56. Why does convection occur only in liquids or gases?
Answer: Convection occurs only in fluids due to the ability of their molecules to move freely.
57. The occurrence of land breeze and sea breeze is due to
Answer: convection.
58. The ventilation of a room by the opening of windows to allow warm air to escape from a hot room and for cool air to flow in and replace the warm air is an application of
Answer: convection current.
59. The functioning of air conditioners and refrigerators makes use of
Answer: convection.
60. a) State three applications of good and bad conductors of heat.
Answer:
· Cooking utensils are made of metals to cause food to cook quickly.
· The handles of cooking utensils are made mostly of wood to make it easy to handle utensils when they are hot.
· A stone floor or cemented floor feels cold to the feet but a rug or carpet on the same floor feels warm.
b) State three applications or uses of convection currents.
Answer:
· The occurrence of land breeze and sea breeze is due to convection.
· The ventilation of a room by the opening of windows to allow warm air to escape from a hot room and for cool air to flow in and replace the warm air.
· The functioning of air conditioners and refrigerators makes use of convection.
61. Describe an experiment that demonstrates convection in a liquid.
Answer:
Aim: To demonstrate convection in a liquid.
Materials needed: Beaker, water, potassium permanganate crystals, tripod stand and source of heat.
Procedure:
· Fill a flask, beaker or suitable container with water to a suitable level.
· Drop a potassium permanganate (KMnO4) crystal into the water.
· Heat the water gently.
Observation: When the water is heated at the bottom, it expands and becomes less dense. The warm water then rises to the top.
On reaching the top, the water begins to cool. It becomes denser and sinks. It now descends along the cooler side of the beaker. This circulation of water will be repeated as long as there is a temperature difference in the water.
Conclusion: Heat was transferred through the water by convection. The upward movement of the dissolved potassium permanganate shows a convection current.
62. What is the major difference between conduction and convection?
Answer: In conduction, heat energy is passed on from one molecule to another. However, in convection, the molecules move carrying the heat energy along with them.
63. What is radiation?
Answer: Radiation is the process by which heat is transferred from one place to another in the absence of a material medium (through vacuum).
64. Conduction and convection require a material medium while radiation does not require a material medium for the
Answer: transfer of heat.
65. Radiation takes place in a
Answer: vacuum (empty space).
66. The heat from the Sun reaches the Earth through an empty space (vacuum) by the process of
Answer: radiation.
67. a) Materials that emit heat are known as
Answer: good emitters of heat.
b) Name two examples of good emitters of heat.
Answer:
· Smooth surfaces.
· Shiny surfaces.
68. a) Materials that absorb heat are known as
Answer: good absorbers of heat.
b) Name two examples of good absorbers of heat.
Answer:
· Rough surfaces.
· Dark surfaces.
69. a) Good emitters of heat are poor
Answer: absorbers of heat.
b) Poor emitters of heat are good
Answer: absorbers of heat.
70. Describe an experiment that demonstrates radiation.
Answer:
Aim: To demonstrate radiation.
Materials needed: Candle, source of heat.
Procedure:
· Light a candle.
· Place your hands close by the sides of the candle flame.
· What do you feel?
Observation: The heat travelled from the flame to the hand in the absence of a material medium or through the empty space.
Conclusion: Heat travelled from the flame to the hand by radiation.
71. Describe an experiment that shows that dark surfaces are good absorbers of heat and shiny surfaces are poor absorbers of heat.
Answer:
Aim: To show that dark surfaces are good absorbers of heat and shiny surfaces are poor absorbers of heat.
Materials needed: Cork, dull-dark surface, polished surface, bunsen burner, candle wax.
Procedure:
· Place two surfaces, one painted dull black and the other highly polished vertically.
· The two surfaces should be a few centimeters apart.
· Attach a cork by means of candle wax at the back of each side of the two surfaces.
· Light the bunsen burner and place it half way between the two surfaces, so that both surfaces receive equal amount of heat energy.
· Leave the setup to stand for a few minutes.
Observation: The wax behind the dull black surface begins to melt and the cork eventually slides off. The wax behind the polished surface does not melt.
Conclusion: This shows that dull black surfaces absorb more heat than shiny or polished surfaces.
72. Why is it not advisable to wear dark coloured jacket on a sunny day?
Answer: The dark jacket absorbs the heat of the Sun and makes the wearer feel hot and uncomfortable.
73. Why are roofs of factories coated with aluminium paint?
Answer: To emit heat on a sunny day to keep the interior cool.
74. Why are shiny aluminium roofs and light paints used in buildings?
Answer: To reduce the absorption of heat from the Sun.
75. Why are solar panels painted black?
Answer: To absorb the heat energy of the Sun which is transformed into heat energy.
76. Storage tanks containing oil or petrol are usually painted with aluminium paint so that the
Answer: polished surface does not absorb heat.
77. The silvered surfaces of a vacuum flask prevent heat loss by
Answer: radiation.
78. People sell ice water in shiny aluminium containers because shiny aluminium containers do not absorb heat. This keeps the ice water
Answer: cool.
79. Radiators are painted black in order to increase the proportion of heat radiated to make the
Answer: car engine cooler.
80. The Sun gives out invisible hot
Answer: rays (waves).
81. The hot rays of the Sun can pass through vacuum to the
Answer: Earth.
82. What happens when the hot rays of the Sun fall on matter?
Answer: They give out their heat energy to the molecules of the matter, making the matter hot.
83. The vacuum flask is commonly known as
Answer: thermos flask.
84. State three functions of the vacuum flask.
Answer:
· It is used to store both cold and hot items.
· It keeps hot substances hot and it can also keep cold substances cold.
· It prevents heat loss through conduction, convection and radiation.
85. State five features of a vacuum or thermos flask.
Answer:
· It consists of a double-walled glass vessel with a vacuum between the walls.
· Both glass walls facing each other are silvered on the vacuum side.
· It has cork supports at the bottom.
· It has plastic or cork stopper.
· It is enclosed in a metal or plastic container.
86. State the function of the vacuum between the double walls.
Answer: It reduces or stops heat loss or heat gain by conduction and convection.
87. State the function of the silvered and shiny surfaces of the glass wall.
Answer: It reduces radiation.
88. State the function of the cork supports and the plastic stopper.
Answer: They act as insulators and hence prevent heat loss or heat gain by conduction.
89. State three differences between conduction and radiation.
Answer:
|
Conduction |
Radiation |
|
Needs a material medium to travel in. |
Does not need a material medium to travel in. |
|
Does not depend on whether a body is black or white. |
Black bodies absorb heat well. |
|
Heat is passed from one molecule to another. |
Molecules are not involved or it travels in electromagnetic waves. |