Unit notes and practice questions

Grade 9 Physics unit 7: Temperature and Thermometry

Everything here is for unit 7 of the Grade 9 Physics textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.

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Cards: 10
Practice questions on unit 7

What unit 7 covers

1.Temperature and Our Life
2.Extreme Temperature Safety
3.Temperature Change and its Effects
4.Measuring Temperature with Different Thermometric Scales
5.Types of Thermometers and Their Use
6.Conversion between Temperature Scales
7.Thermal Expansion of Materials

Short notes for unit 7

The cards filed under this chapter on the Grade 9 Physics page. Open one at a time.

Fixed Points of Temperature ScalesReference tableCelsius · Fahrenheit · Kelvin

Scale	Ice Point	Steam Point	Divisions
Celsius	$0^\circ\text{C}$	$100^\circ\text{C}$	$100$
Fahrenheit	$32^\circ\text{F}$	$212^\circ\text{F}$	$180$
Kelvin	$273.15\text{ K}$	$373.15\text{ K}$	$100$

When you use it

Reference the standard calibration fixed points and division counts across standard scales.

Watch out

A temperature difference of 1 degree Celsius is equal to 1 Kelvin, but it is not equal to 1 degree Fahrenheit.

Drafted from Grade 9 Physics, page 140 onwards, then checked twice before it went up

Linear Thermal ExpansionFormula

\Delta L = \alpha L_0 \Delta T

\Delta L = \alpha L_0 \Delta T

$\Delta L$: change in length $\text{m}$
$\alpha$: coefficient of linear expansion $(^\circ\text{C})^{-1}$
$L_0$: initial length $\text{m}$
$\Delta T$: change in temperature $^\circ\text{C}$

When you use it

Calculate the expansion or contraction in length of a solid rod when its temperature changes.

Watch out

Do not confuse the change in length with the final length. The final length is the initial length plus the change in length.

Drafted from Grade 9 Physics, page 140 onwards, then checked twice before it went up

Temperature Scale ConversionFormula

T_C = \frac{5}{9}(T_F - 32) = T_K - 273.15

T_C = \frac{5}{9}(T_F - 32) = T_K - 273.15

$T_C$: temperature in Celsius $^\circ\text{C}$
$T_F$: temperature in Fahrenheit $^\circ\text{F}$
$T_K$: temperature in Kelvin $\text{K}$

When you use it

Convert temperature readings between Celsius, Fahrenheit, and Kelvin scales.

Watch out

Subtract 32 before multiplying by 5/9 when converting Fahrenheit to Celsius. Do not use degree signs with Kelvin.

Drafted from Grade 9 Physics, page 140 onwards, then checked twice before it went up

Thermal Expansion of Holes and CavitiesCommon mistake

When you use it

Use when determining whether a hole or inner diameter expands or contracts during heating.

Watch out

A hole in an object does not shrink when heated. It expands at the exact same rate as if it were filled with solid material.

Drafted from Grade 9 Physics, page 140 onwards, then checked twice before it went up

Thermometers and Thermometric PropertiesReference tableMercury thermometer · Alcohol thermometer · Resistance thermometer · +1

Thermometer	Thermometric Property	Measuring Range
Mercury thermometer	Length of mercury column	$-30^\circ\text{C}\text{ to }300^\circ\text{C}$
Alcohol thermometer	Expansion of alcohol volume	$-115^\circ\text{C}\text{ to }78.15^\circ\text{C}$
Resistance thermometer	Electrical resistance of platinum	$-270^\circ\text{C}\text{ to }700^\circ\text{C}$
Thermocouple	Voltage between two junctions	$-270^\circ\text{C}\text{ to }2300^\circ\text{C}$

When you use it

Select appropriate thermometer types and identify their physical thermometric properties.

Watch out

Alcohol thermometers are chosen for very cold environments because alcohol freezes at a much lower temperature than mercury.

Drafted from Grade 9 Physics, page 140 onwards, then checked twice before it went up

Specific heat capacity and latent heatFormula

Q = mc\,\Delta T \text{ and } Q = mL

Q = mc\,\Delta T \text{ and } Q = mL

$Q$: the heat energy transferred because of a temperature difference. A body does not contain heat; it contains internal energy, and heat is that energy while it is moving $\mathrm{J}$
$m$: the mass of the substance being heated, cooled or changed $\mathrm{kg}$
$c$: the specific heat capacity of the material: the energy that raises 1 kg of it by 1 K. For water it is about 4200 J per kilogram per kelvin $\mathrm{J\,kg^{-1}\,K^{-1}}$
$\Delta T$: the change in temperature. A Celsius interval and a kelvin interval are the same size, so the number is the same either way $\mathrm{K}$
$L$: the specific latent heat: fusion for melting and freezing, vaporisation for boiling and condensing. For water these are about 334 000 and about 2 260 000 joules per kilogram $\mathrm{J\,kg^{-1}}$
$C$: the heat capacity of one whole body, which is its mass times c. A big block has a larger C than a small block of the same material $\mathrm{J\,K^{-1}}$

When you use it

Use whenever a question heats, cools, melts, freezes or boils something and asks for the energy. While the thermometer is still moving, the energy is mass times specific heat capacity times temperature change; while it holds steady at a melting or boiling point, it is mass times specific latent heat.

Watch out

Students reach for the temperature form on a melting or boiling step. Asked for the energy that turns 0.5 kg of ice at 0 °C into water at 0 °C, they look for a temperature change, find none, and either write zero or invent one. Melting has no temperature change at all: it is mass times the latent heat of fusion, which comes to 167 000 J. On a heating curve, use the temperature form on the sloping parts and the latent heat form on the flat parts, then add the steps. The other slip is swapping C for c. Capital C belongs to one particular object and is measured in J/K; small c belongs to the material and is measured in J/(kg K), and small c is the one printed in the data table.

Practice questions on unit 7

10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.

Question 1

Which of the following is the correct scientific definition of temperature?

A.The total kinetic energy of all particles in a body
B.The measure of the average kinetic energy of the particles in a sample of matter
C.The amount of heat expanding inside a thermometer
D.The volume of mercury contained inside a thin glass tube

Show the answer and why

Answer: B The measure of the average kinetic energy of the particles in a sample of matter

Why

Temperature is defined as the degree of hotness or coldness of a body, or the measure of the average kinetic energy of the particles in a sample of matter.

Question 2

What temperature range is generally referred to as room temperature, in which human beings feel comfortable?

A.10 °C to 14 °C
B.18 °C to 22 °C
C.28 °C to 32 °C
D.36.5 °C to 37.2 °C

Show the answer and why

Answer: B 18 °C to 22 °C

Why

According to the textbook, human beings are comfortable with temperatures between 18 °C and 22 °C, which is referred to as room temperature.

Question 3

What is the SI unit of temperature?

A.Degree Celsius (°C)
B.Degree Fahrenheit (°F)
C.Kelvin (K)
D.Degree Centigrade (°C)

Show the answer and why

Answer: C Kelvin (K)

Why

The SI unit for temperature is the kelvin (symbol: K). Note that no degree sign is used with kelvin.

Question 4

How many equal divisions are between the ice point and the steam point on the Fahrenheit scale?

A.100
B.180
C.212
D.273

Show the answer and why

Answer: B 180

Why

On the Fahrenheit scale, the ice point is 32 °F and the steam point is 212 °F. The difference is 212 - 32 = 180 equal parts.

Question 5

Which type of thermometer uses the voltage generated between two welded junctions of different metals as its thermometric property?

A.Thermocouple
B.Thermistor
C.Resistance thermometer
D.Alcohol thermometer

Show the answer and why

Answer: A Thermocouple

Why

A thermocouple consists of two different metal wires welded to form two junctions. It generates a voltage that depends on the temperature difference between the junctions.

Question 6

Why is an alcohol thermometer used instead of a mercury thermometer to measure very low temperatures such as -50 °C?

A.Alcohol expands less than mercury
B.Mercury freezes at a higher temperature, while alcohol measures down to -115 °C
C.Alcohol is a toxic liquid and mercury is non-toxic
D.Mercury only functions above 100 °C

Show the answer and why

Answer: B Mercury freezes at a higher temperature, while alcohol measures down to -115 °C

Why

A mercury thermometer is suitable between -30 °C and 300 °C, while an alcohol thermometer can measure temperatures from -115 °C to 78.15 °C, making it suitable for very cold conditions.

Question 7

A student in Addis Ababa reads a room temperature of 20 °C. What is this value in degrees Fahrenheit (°F)?

A.43 °F
B.52 °F
C.68 °F
D.72 °F

Show the answer and why

Answer: C 68 °F

Why

Using the conversion formula: T_F = 1.8 × T_C + 32 = 1.8 × 20 + 32 = 36 + 32 = 68 °F.

Question 8

In a laboratory experiment, hot water has a temperature of 345.15 K. What is this temperature in degrees Celsius?

A.72 °C
B.100 °C
C.618.3 °C
D.273.15 °C

Show the answer and why

Answer: A 72 °C

Why

Using the formula T_C = T_K - 273.15: T_C = 345.15 - 273.15 = 72 °C.

Question 9

At what temperature do the Celsius and Fahrenheit scales read the exact same number?

A.0°
B.40°
C.-40°
D.-273.15°

Show the answer and why

Answer: C -40°

Why

Setting T_C = T_F = x in the formula x = 1.8x + 32 gives -0.8x = 32, which leads to x = -40. Thus, -40 °C equals -40 °F.

Question 10

A 2 m long copper rod (α = 1.7 × 10^-5 (°C)^-1) is heated so that its temperature rises from 0 °C to 150 °C. What is the increase in length (ΔL) of the rod?