9th Class Physics Chapter 7 Thermal Properties of Matter Long Questions

Question 1

Describe the main points of the particle theory of matter which differentiate solids, liquids and gases.

Accepted Answer

Kinetic molecular theory Matter is made of very small particles called molecules.; The molecules are always moving: vibrating, rotating or moving in straight lines.; Molecules attract each other with intermolecular forces, which weaken as the distance increases.; The faster the molecules move, the higher the temperature. Diagram Solids The forces are so strong that the molecules are held at fixed positions; they only vibrate about them. So solids have a definite shape and volume. Liquids The forces are weaker and cannot hold the molecules in place; they slide over each other at random. A liquid has a definite volume but no definite shape: it flows and takes the shape of its container. Gases The molecules are far apart, the forces are very weak, and they move freely in all directions. A gas has neither a definite shape nor a definite volume. Plasma At very high temperatures a gas becomes ionised (positive ions and free electrons). This is plasma, the fourth state of matter; it conducts electricity. Forces, strong, weaker, very weak; Motion, vibration, sliding, free random; Shape / volume, fixed / fixed, not fixed / fixed, not fixed / not fixed

Question 2

What is temperature? How is it measured? Describe briefly the construction of a mercury-in-glass thermometer.

Accepted Answer

Temperature Temperature: The degree of hotness or coldness of a body; the quantity that decides the direction of heat flow. In molecular terms, a measure of the average kinetic energy of the molecules. Measurement Temperature is measured with a thermometer, which uses a thermometric property: a property that changes regularly with temperature, such as the expansion of a liquid. Diagram Construction A thin-walled glass bulb at the bottom is filled with mercury.; A narrow capillary tube of uniform bore rises from it; the air above the mercury is removed and the top is sealed.; A thick glass stem around the tube protects it and magnifies the thread.; A scale is marked using the ice point and the steam point. Working When the bulb is heated, the mercury expands and rises up the capillary as a thread. The end of the thread shows the temperature on the scale. Mercury is opaque and silvery, so it is easy to read. Mercury is used because it is easy to see, responds quickly, does not wet glass and stays liquid from −39 °C to 357 °C.

Question 3

Compare the three scales used for measuring temperature.

Accepted Answer

Fixed points Every scale uses two fixed points: the ice point (melting pure ice) and the steam point (steam over pure water boiling at standard atmospheric pressure). Diagram Celsius scale Ice point 0 °C, steam point 100 °C; the gap is divided into 100 equal parts, each 1 °C. Used in daily life. Fahrenheit scale Ice point 32 °F, steam point 212 °F; the gap is divided into 180 equal parts, each 1 °F. Used in daily life, for example for body temperature (98.6 °F). Kelvin scale Ice point 273 K, steam point 373 K, 100 divisions, so 1 K is the same size as 1 °C. Its zero is absolute zero, −273.15 °C, the lowest possible temperature, so it has no negative values. It is the SI scale used in science. Relations T_F = 1.8 T_C + 32;  T_K = T_C + 273 Celsius, 0, 100, 100, ice point; Fahrenheit, 32, 212, 180, arbitrary; Kelvin, 273, 373, 100, absolute zero T_F = 1.8 × 30 + 32 = 86 °F. T_K = 30 + 273 = 303 K. 86 °F, 303 K

Question 4

What is meant by sensitivity, range and linearity of thermometers? Explain with examples.

Accepted Answer

Sensitivity Sensitivity: The ability of a thermometer to detect small changes in temperature. Example: a thermometer marked in 1 °C steps reads to 1 °C; one marked in 0.1 °C steps (a clinical thermometer) is more sensitive and more precise. Range Range: The span of temperature, from low to high, over which a thermometer can measure. Example: a clinical thermometer has a short range, 35–42 °C; a laboratory thermometer measures from −10 °C to 110 °C. The liquid sets the limits: mercury freezes at −39 °C and boils at 357 °C. Linearity Linearity: Equal changes in temperature give equal changes in the reading over the whole range. Example: on a mercury thermometer the marks are evenly spaced, because mercury expands uniformly in a tube of uniform bore. Water could not give a linear scale because it expands unevenly. Sensitivity, How small a change can it show?; Range, How low and how high can it go?; Linearity, Are the marks evenly spaced?

Question 5

Explain how the parameters mentioned in question 7.4 are improved in the structure of a liquid-in-glass thermometer.

Accepted Answer

Diagram Improving sensitivity Thin-walled bulb: heat passes quickly through the glass to the liquid.; Mercury, a metal, is a good conductor and responds quickly.; Small amount of liquid heats up quickly.; Narrow capillary: a small expansion moves the thread a long way.; Alcohol expands about six times as much as mercury, so it can be used for greater sensitivity. Improving linearity The capillary tube has a uniform bore, so equal expansions of the liquid give equal lengths of thread, and mercury expands uniformly. The scale marks are then evenly spaced. Improving range Mercury stays liquid from −39 °C to 357 °C, a long range. For very low temperatures alcohol is used, because it freezes at about −112 °C. A longer tube also gives a longer range. Narrow bore and thin bulb for sensitivity; uniform bore for linearity; the choice of liquid for range.

Question 6

Derive the relations between the Celsius, Fahrenheit and Kelvin scales.

Accepted Answer

Idea A given temperature lies the same fraction of the way from the ice point to the steam point on every scale. Celsius and Fahrenheit Fraction on Celsius: (T_C − 0) ÷ (100 − 0) = T_C ÷ 100.; Fraction on Fahrenheit: (T_F − 32) ÷ (212 − 32) = (T_F − 32) ÷ 180.; Equating: T_C ÷ 100 = (T_F − 32) ÷ 180.; So T_F = (180 ÷ 100) T_C + 32 = 1.8 T_C + 32, and T_C = (T_F − 32) ÷ 1.8. Celsius and Kelvin Fraction on Kelvin: (T_K − 273) ÷ 100.; Equating with T_C ÷ 100: T_K − 273 = T_C.; So T_K = T_C + 273. T_F = 1.8 T_C + 32;  T_K = T_C + 273 T_F = 1.8 × 25 + 32 = 77 °F. T_K = 25 + 273 = 298 K. 77 °F, 298 K

Question 7

Differentiate between heat, temperature and internal energy.

Accepted Answer

Internal energy Internal energy: The sum of the kinetic and potential energies of all the molecules of a body. It depends on the mass, the temperature and the state of the body. Temperature Temperature: The degree of hotness or coldness; a measure of the average kinetic energy of the molecules. It decides the direction of heat flow. Heat Heat: Energy transferred from a hotter body to a colder body because of their temperature difference: energy in transit. Diagram Example A bathtub of warm water has more internal energy than a cup of boiling tea (many more molecules), yet if the cup is put in the bath, heat flows from the tea to the water, because the tea is at the higher temperature. energy in transit, degree of hotness, energy stored; J, K, °C, °F, J; flows because of ΔT, average KE, total KE + PE

Question 8

Describe the construction and use of a clinical thermometer.

Accepted Answer

Use A clinical thermometer measures the temperature of the human body. Normal body temperature is 37 °C (98.6 °F). Diagram Construction A small thin-walled bulb of mercury and a very narrow capillary tube.; Short range, about 35–42 °C, marked in 0.1 °C steps, so it is very sensitive.; A constriction (narrow kink) in the capillary just above the bulb.; A thick glass stem shaped like a prism that magnifies the thread. Working The bulb is held under the tongue or in the armpit for about a minute.; The mercury expands and pushes past the constriction.; When the thermometer is removed, the mercury in the bulb contracts, but the thread breaks at the constriction and stays in place.; The maximum temperature can be read at leisure; then the thermometer is shaken to send the mercury back. Precautions Wash it in cold water, not hot (it would burst: the range ends at 42 °C).; Shake the mercury down before use.

Question 9

What is a thermocouple thermometer? Describe its principle, construction and uses.

Accepted Answer

Principle When two wires of different metals are joined at two junctions and the junctions are at different temperatures, a small electric current flows round the circuit. The bigger the temperature difference, the bigger the current (thermo-electric effect). Diagram Construction Two wires of different metals, such as copper and iron, joined at both ends.; One junction kept at a fixed low temperature (melting ice); the other is the probe.; A galvanometer in the circuit, calibrated to read temperature. Uses and advantages Responds very quickly, because the junction is tiny: good for rapidly changing temperatures.; Wide range, from about −200 °C to over 1000 °C: furnaces, kilns, engines.; The reading can be recorded electrically and at a distance.

Question 10

What are thermometric properties? Why is mercury used in thermometers? Compare mercury and alcohol as thermometric liquids.

Accepted Answer

Thermometric property Thermometric property: A property that changes appreciably and regularly with temperature, such as the volume of a liquid, the pressure of a gas, the resistance of a wire or a thermo-electric current. Qualities of a good thermometric liquid visible; good conductor; uniform expansion; large expansivity; low freezing point and high boiling point; does not wet glass; does not evaporate easily Mercury and alcohol compared Visibility, opaque, shiny, colourless, needs dye; Conduction, good, poorer; Wets glass, no, yes; Freezing / boiling point, −39 °C / 357 °C, about −112 °C / 78 °C; Expansivity, smaller, about six times larger; Best for, ordinary and high temperatures, very low temperatures Mercury is the usual choice; alcohol is used in very cold places.

	Solid	Liquid	Gas
Forces	strong	weaker	very weak
Motion	vibration	sliding	free random
Shape / volume	fixed / fixed	not fixed / fixed	not fixed / not fixed

Scale	Ice point	Steam point	Divisions	Zero
Celsius	0	100	100	ice point
Fahrenheit	32	212	180	arbitrary
Kelvin	273	373	100	absolute zero

Quality	Question it answers
Sensitivity	How small a change can it show?
Range	How low and how high can it go?
Linearity	Are the marks evenly spaced?

Heat	Temperature	Internal energy
energy in transit	degree of hotness	energy stored
J	K, °C, °F	J
flows because of ΔT	average KE	total KE + PE

Property	Mercury	Alcohol
Visibility	opaque, shiny	colourless, needs dye
Conduction	good	poorer
Wets glass	no	yes
Freezing / boiling point	−39 °C / 357 °C	about −112 °C / 78 °C
Expansivity	smaller	about six times larger
Best for	ordinary and high temperatures	very low temperatures

Long Questions with Answers

Set A: Textbook comprehensive questions 5 questions

Describe the main points of the particle theory of matter which differentiate solids, liquids and gases.

Kinetic molecular theory

Diagram

Solids

Liquids

Gases

Plasma

What is temperature? How is it measured? Describe briefly the construction of a mercury-in-glass thermometer.

Temperature

Measurement

Diagram

Construction

Working

Compare the three scales used for measuring temperature.

Fixed points

Diagram

Celsius scale

Fahrenheit scale

Kelvin scale

Relations

What is meant by sensitivity, range and linearity of thermometers? Explain with examples.

Sensitivity

Range

Linearity

Explain how the parameters mentioned in question 7.4 are improved in the structure of a liquid-in-glass thermometer.

Diagram

Improving sensitivity

Improving linearity

Improving range

Set B: More important long questions 5 questions

Derive the relations between the Celsius, Fahrenheit and Kelvin scales.

Idea

Celsius and Fahrenheit

Celsius and Kelvin

Differentiate between heat, temperature and internal energy.

Internal energy

Temperature

Heat

Diagram

Example

Describe the construction and use of a clinical thermometer.

Use

Diagram

Construction

Working

Precautions

What is a thermocouple thermometer? Describe its principle, construction and uses.

Principle

Diagram

Construction

Uses and advantages

What are thermometric properties? Why is mercury used in thermometers? Compare mercury and alcohol as thermometric liquids.

Thermometric property

Qualities of a good thermometric liquid

Mercury and alcohol compared

The rest of Chapter 7

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