1: Temperature and Heat Transfer

Question 1

What is temperature?

Answer 1

Macroscopic: Temperature is a measure of the degree of hotness of an object.
Microscopic: Temperature is a measure of the average kinetic energy per molecule in a body

Question 2

Which properties of substances allow us to indicate temperature?

Answer 2

thermometric properties: temperature-dependent, physical properties. some examples include colour, volume etc.

Question 3

What are two features of reference points to define a temperature scale?

Answer 3

They are fixed and easily reproducible.

Question 4

What are the fixed points for the celcius temperature scale?

Answer 4

lower fixed point: ice point (the temperature of melting ice under normal atmospheric pressure)
upper fixed point: steam point (the temperature of steam above boiling water under normal atmospheric pressure)

Question 5

How is the range between the fixed points divided in the celcius temperature scale?

Answer 5

It is divided into 100 equal divisions with each division being called 1°C.

Question 6

Express a temperature T (ºC) in Fahrenheit and Kelvin.

Answer 6

T (ºF) = 9/5 T(ºC) + 32
T (K) = (ºC) + 273

Question 7

How does a liquid-in-glass thermometer work?

Answer 7

The liquid inside expand and contract uniformly when temperature changes. The level of liquid risen up from its bulb indicates the temperature measured by the thermometer.

Question 8

What is meant by calibrating an unmarked thermometer?

Answer 8

Marking a scale on a thermometer

Question 9

How do you calibrate a thermometer by using a practical method?

Answer 9

1. Put the thermometer into a beaker of melting ice (0°C) and mark the liquid level.
2. Put the thermometer into a beaker of boiling water (100°C) and mark the liquid level.
3. Divide the length between the two marked points to 10 portions. Each portion stands for 10°C.

Question 10

How do you calibrate a thermometer by a graphical method?

Answer 10

1. Measure the liquid level when the thermometer is put in melting ice and boiling water.
2. Draw a graph of length of liquid against the temperature.
3. Read from the graph or calculate the calibration from a uniform proportion.

Question 11

(S4 Test 1) A faulty mercury-in-glass thermometer reads 1.2C at ice point and 99.6C at steam point. What is the actual temperature when the faulty thermometer reads 25C?

Answer 11

Assumption: the length of the liquid column varies linearly with temperature, because mercury’s volume varies linearly with temperature.
Approach: using proportion, uniform ratio, or equation of straight line
(99.6-1.2)/100 = 0.984: for every 1ºC increase shown on the faulty thermometer, the real temperature increase is 0.984ºC.
Starting from 0ºC, where the faulty thermometer reads 1.2ºC, the faulty reading is increased by (25-1.2)ºC. The real temperature increases by (23.8/0.984)ºC, which means the real temperature is 24.2ºC.

Question 12

What affects the accuracy of a thermometer?

Answer 12

1. The length of the liquid column should vary linearly with temperature for a thermometer to be accurate.
   The accuracy is guaranteed by uniform cross-sectional area of the capillary tube, and the uniform expansion of the liquid per unit temperature.

Question 13

What is sensitivity and how can a thermometer become more sensitive?

Answer 13

Sensitivity refers to the ability to record a small change in temperature. For liquid-in-glass thermometer, sensitivity can be increased by using a narrower tube (more variation in column length), increasing the amount of liquid, and using a liquid that expands more when heated.

Question 14

Can an accurate thermometer be less sensitive, and can a sensitive thermometer be inaccurate?

Answer 14

Yes: an accurate thermometer is less sensitive if the capillary tube is thicker, or there is less liquid.
Yes: a sensitive thermometer can have an inaccurate scale after marking the scale randomly.

Question 15

What is response time and how can it be shortened?

Answer 15

Response time is the time needed for the reading of the thermometer to become stable. It can be faster if the thermometer has thin walls, or if highly conducing liquid is used.

Question 16

What are two advantages and a disadvantage of using a mercury thermometer?

Answer 16

advantages: can measure high temperatures up to 357°C, has a quick response to temperature changes
disadvantage: mercury is poisonous.

Question 17

Give two reasons why water is not used in thermometers.

Answer 17

1. It has a non-uniform expansion, in which it contracts when heated from 0ºC to 4ºC, but expands when heated to 4ºC or beyond.
2. It has a high specific heat capacity, meaning that it has to absorb a large amount of energy with only a small change in temperature.

Question 18

What are two advantages and a disadvantage of using an alcohol thermometer?

Answer 18

advantages: can measure low temperatures down to -115°C, is not poisonous
disadvantage: slow response to temperature changes.

Question 19

How does a rotary thermometer with copper and iron bimetallic strip work? (given: the copper strip is put on the outside)

Answer 19

When heated, copper expands more than iron, which makes the strip in the thermometer bend, displaying different temperatures.

Question 20

What are the properties, molecular arrangement, and molecular movement in a solid?

Answer 20

properties: fixed volume and shape
arrangement: closely packed fixed in positions
movement: vibrate about a fixed position

Question 21

What are the properties, molecular arrangement, and molecular movement in a liquid?

Answer 21

properties: fixed volume but unfixed shape
arrangement: closely packed but not fixed in positions
movement: move freely

Question 22

What are the properties, molecular arrangement, and molecular movement in a gas?

Answer 22

properties: no fixed volume or shape
arrangement: far apart, not fixed in positions
movement: move freely at high speeds

Question 23

How can we infer how much kinetic energy a body has?

Answer 23

The faster the molecules move in a body, the more kinetic energy it possesses. The molecules move faster at a higher temperature and move slower at a lower temperature. Therefore, the temperature of the body is a measure of the average kinetic energy due to the motion of the molecules in the body.

Question 24

What determines the direction of heat transfer?

Answer 24

The temperature difference (instead of the internal energy difference)

Question 25

What is internal energy and what is its unit?

Answer 25

Internal energy measures the amount of total energy stored in a body in joules(J) or kilojoules(kJ)/megajoules(MJ). It is given by no. of molecules x (average kinetic energy + average potential energy).

Question 26

What are 3 factors that can change the amount of stored internal energy inside a body?

Answer 26

Mass, temperature, state of matter

Question 27

When the internal energy of a body is increased, when is the internal energy account for an increase in kinetic energy and potential energy respectively?

Answer 27

increase in kinetic energy: temperature rise
increase in potential energy: change of state

Question 28

What is the minimum temperature of a body?

Answer 28

Absolute zero (-273°C)

Question 29

What is the conversion between joules, kilojoules, and megajoules?

Answer 29

1MJ=1000kJ=1,000,000J
1kJ=1000J

Question 30

What is thermal equilibrium?

Answer 30

Thermal equilibrium occurs when two bodies of different temperature are put in contact, and when the bodies reach the same temperature (heat transfer stops)

Question 31

What is meant by the conduction of heat?

Answer 31

Conduction is the process where heat is transferred from an object with higher temperature to another through direct contact.

Question 32

List 3 good heat conductors and 3 good heat insulators.

Answer 32

Conductors: copper, silver, aluminum
Insulators: water, wood, air

Question 33

How do cooking utensils and food containers utilise the principle of conduction and insulation?

Answer 33

Cooking utensils are made of good hesat conductors to cook effectively, and their handles are made of heat insulators like wood or plastic to allow users to grasp the utensils easily.
Foam food containers are used because of their insulating property.

Question 34

How do animals and humans utilise the principle of conduction and insulation to keep warm?

Answer 34

Maintaining temperature: feathers, fur, and fat are good insulators of heat. They help to keep animals warm in winter.
A jacket filled with goose-down is a good insulator of heat because there are a lot of air trapped inside the jacket.

Question 35

How can conduction of heat in non-metallic solids be explained microscopically?

Answer 35

In solids, the particles are packed together closely. When a part of a solid is heated, the particles near the heat source gain more kinetic energy and vibrate vigorously, and they collide with neighbouring particles to make them vibrate faster as well. The process continues and the enery is transferred along to object.

Question 36

Why are non-metallic liquids and gases worse conductors when compared to solids?

Answer 36

The particles of liquids are packed irregularly, and the particles of gases are far apart, which makes the conduction process inefficient as the vibration of hotter particles cannot induce the vibration of neighbouring particles effectively.

Question 37

How can the conduction of heat in metals be explained microscopically?

Answer 37

In metals, the outermost electrons in atoms are loosely held, which makes them free elctrons that can move at high speeds when heated. When a part of the metal is heated, both the particles and free electrons move faster. In addition, unlike ions, free electrons can move quickly to the cold region and transfer their energy to the ions there by collision, making it more efficient to conduct the heat to other parts of the metal.

Question 38

What is convection?

Answer 38

Convection is the process of heat transfer through the movement of the fluid (liquid or gas) itself.

Question 39

What is meant by a convection current?

Answer 39

The flow of fluid due to temperature difference.

Question 40

What is the convection current of water when it is heated?

Answer 40

The water at the bottom is heated first. The heated water expands and becomes less dense. Then the cold water sinks because of its higher density, and it replaces the hot water. Therefore, the cold water can also be heated, and the water is heated.

Question 41

What are some examples of convection in daily life?

Answer 41

The air conditioners are put at the top of the room. Cold air produced by the air conditioner sinks and the warmer air near the floor rises. The air in the room is cooled down rapidly.

Question 42

How do radiators (heaters) utilise the principle of convection?

Answer 42

Heaters are placed near the floor. The air is warmed up and rises, while cold air near the ceiling sinks, making it heat the whole room effectively.

Question 43

How can sea breezes and land breezes be explained by convection?

Answer 43

Land heats up and cools down more quickly than the sea due to a lower specific heat capacity.
In the day, the warmer air above the land rises, and the colder air over the sea flows in to take its place. There is onshore wind near the ground, or sea breeze.
At night, the warmer air over the sea rises, and the cooler air over the land flows in to take its place. There is an offshore wind near the ground, or land breeze.

Question 44

What is radiation?

Answer 44

Radiation is a process in which heat is transferred from one place to another by means of electromagnetic waves. It does not require any medium or particles. (it can take place in a vacuum.) It takes place in all directions.

Question 45

Why must the heat transfer from the sun to the Earth be done by radiation?

Answer 45

There is a vacuum between the Earth and the sun, meaning that there are no particles and therefore neither conduction or convection can transfer the heat.

Question 46

What is the common type of electromagnetic wave objects at everyday temperature emit?

Answer 46

Infrared rays (IR)

Question 47

Does everything emit or receive radiation?

Answer 47

Yes, everything above absolute zero (-273ºC) is both an absorber and a radiator or radiation.

Question 48

Does a colder object have a net gain or loss of radiation?

Answer 48

It has a net gain of radiation, and therefore its temperature increases. (vice versa for an object hotter than the surroundings)

Question 49

Does a hotter object have a net gain or loss of radiation?

Answer 49

It has a net loss of radiation, and therefore its temperature decreases.

Question 50

Under what condition does an object have no net gain or loss of radiation?

Answer 50

When its temperature is the same as that of the surroundings.

Question 51

Which types of surfaces are poor absorbers of radiation? What other properties do they have regarding emission and reflection of radiation?

Answer 51

Shiny, light-coloured surfaces. They are also poor emitters of radiation and good reflectors of radiation.

Question 52

Which types of surfaces are good absorbers of radiation? What other properties do they have regarding emission and reflection of radiation?

Answer 52

Dull, black surfaces. They are also good emitters of radiation and poor reflectors of radiation.

Question 53

How do greenhouses utilise the principle of radiation?

Answer 53

The other radiation waves (eg. visible light) from the sun are converted to infra-red radiation given out by the warm objects inside. Since glass can trap infra-red radiation, it makes the greenhouse warm.

Question 54

(S4 Test 1) A Bain-marie is used to keep food warm. Water is put in at the lower tray while food is put in the upper tray. There is a heater under the water tray. List three ways how the bain-marie can keep the food warm for a long time.

Answer 54

1. cover: The cover reduces heat loss from the hot food to the surroundings through convection.
2. shiny, silvery surface: The shiny and silvery surface is a poor emitter of radiation. It reduces heat loss from the hot food to the surroundings through radiation.
3. water as a temperature regulator: Water has a high specific heat capacity, so hot water can keep the food warm for longer.

X water reduces heat loss by convection because water is not a part of the food.