Concepts of heat and temperature | ICSE Class 9 Physics Notes

This note covers heat as energy, temperature and hotness, the direction of heat flow, thermal equilibrium, the distinction between heat and temperature, joules and calories, thermometers, temperature scales and simple conversions.

What does temperature tell us about a body?

Definition: Temperature is a measure of the hotness or coldness of a body. A body at a higher temperature is hotter than a body at a lower temperature.

The word body here means the object being considered. Hot and cold are relative descriptions: they compare one object with another. A kettle containing boiling water is hotter than a box containing ice. Temperature gives this comparison a measurable meaning.

A thermometer is an instrument used to measure temperature. It provides a numerical reading on a temperature scale, which is a system for assigning numbers to temperatures. The number and its unit must be stated together.

Why is a number alone incomplete?

The SI unit of temperature is the kelvin, written K. SI means the International System of Units. The degree Celsius, written °C, is another commonly used temperature unit. A temperature reading is incomplete if the scale or unit is missing.

A temperature does not tell us how much energy has passed into an object. It describes how hot or cold the object is. Energy transfer and temperature are connected, but they answer different questions about a physical situation.

For example, the temperature of hot tea can decrease as it stands in cooler surroundings. The thermometer reading describes the tea's changing hotness. To explain why it changes, we must also consider the energy transferred between the tea and its surroundings.

How should temperatures be compared?

Use readings expressed on the same scale when deciding which body is hotter. Comparing bare numbers from different scales is misleading because the scales assign different numbers to the same temperature. A change of scale does not itself heat or cool the body.

Measurement replaces a vague description such as hot or cold with a value that can be recorded and compared. This is especially useful when touch gives an uncertain impression or when an object cannot safely be touched.

Why can our sense of touch give a misleading impression?

We often judge whether something is hot or cold by touching it. However, the temperature sense provided by touch is somewhat unreliable, and its range is too limited for scientific measurement. It can compare sensations without giving a dependable temperature reading.

What does the three-container activity show?

Consider three containers labelled A, B and C. These letters identify the containers, not physical quantities. Container A holds cold water, B holds hot water, and C holds a mixture of cold and hot water. The hot water must not be hot enough to burn a hand.

1. Place the left hand in the cold water in container A.
2. Place the right hand in the hot water in container B.
3. Keep the hands in their separate containers for 2 to 3 minutes.
4. Move both hands into container C at the same time and compare the sensations.

The left hand can report that the water in C feels hot, while the right hand reports that the same water feels cold. These different impressions do not establish two different temperatures for that water. They show the limitation of using touch as a measuring method.

The drawing shows three water containers labelled A, B and C. One hand is immersed in each of the first two containers; both hands are immersed together in the third.

See Fig. 3.1 in your NCERT textbook

What conclusion follows from the observation?

A temperature measurement should come from a suitable thermometer rather than from the sensation in a hand. The activity does not mean that touch is useless in everyday life. It means that touch cannot always be relied upon to decide how hot an object is.

Separate the observation from the conclusion. The observation is the different feeling in the two hands. The conclusion is that the sense of touch may deceive us. Do not replace that conclusion with an invented temperature for any container.

What is heat, and why is it a form of energy?

Definition: Heat is energy transferred between bodies, or between a body and its surroundings, because of a temperature difference.

The surroundings are the environment outside the body being considered. A temperature difference exists when two bodies have unequal temperatures. This difference is the reason for the transfer described as heat.

The SI unit of heat is the joule, written J. Heat has an energy unit because it is energy being transferred. It is not a substance that fills an object, and it is not another name for the temperature of that object.

How is heat related to other forms of energy?

Rubbing the palms together makes them feel warmer. In this example, work done in rubbing produces warming. Work is a way of transferring energy through the action of a force as something moves. A force is a push or pull.

The reverse connection is illustrated by a steam engine: energy supplied through hot steam can be used to move its pistons, the moving parts that drive the mechanism. These examples connect heating with energy transformations rather than with the movement of a material called heat.

What can happen when a body is heated?

Its temperature may rise, it may expand, or it may change state. Expansion means an increase in dimensions. A change of state means a change between forms of matter such as solid, liquid and gas.

The word “may” matters. Heating does not guarantee that temperature rises throughout every process. During boiling or freezing, water can exchange energy while its temperature remains unchanged. Temperature change is therefore one possible effect of energy transfer, not its definition.

Note: Use “heat supplied” or “heat transferred” when describing energy passing because of a temperature difference. A thermometer measures temperature; it does not directly measure the amount of heat transferred.

In which direction does heat flow, and when does the flow stop?

When bodies at different temperatures are in thermal contact, meaning that heat can pass between them, heat flows from the hotter body to the colder body. Their temperature difference determines the direction of this flow.

The transfer continues until their temperatures become equal. Thermal equilibrium is the condition in which bodies have the same temperature and there is no net heat transfer between them. “Net” refers to the overall transfer considered in one direction.

Why does ice-cold water warm up?

A glass of ice-cold water left on a table on a hot summer day becomes warmer. The surroundings are initially hotter than the water, so energy passes from the surroundings towards the colder water. The water's temperature approaches that of its surroundings.

The explanation must name both sides of the transfer. Saying that the water “gets heat” is incomplete unless the source of that heat is identified. Here, the source is the warmer environment around the glass.

Why does hot tea cool down?

A cup of hot tea on the same table loses heat to the cooler surroundings. The direction is reversed because the tea, unlike the ice-cold water, is initially at a higher temperature than the environment. The tea cools towards the surrounding temperature.

The two examples use the same principle. First compare temperatures, then identify the hotter side, and finally state the direction of transfer. Describing “cold” as flowing out of the water obscures the energy transfer that actually explains the change.

Equality of temperatures is the stopping condition. It is not necessary to claim that the bodies have become identical in every other respect. Thermal equilibrium concerns temperature; the bodies can still be different objects.

How do heat, temperature and internal energy differ?

Heat describes energy transferred because of a temperature difference. Temperature describes hotness or coldness and determines the direction of heat flow. These quantities are related, but neither their meanings nor their units are interchangeable.

What does internal energy mean?

Internal energy is the energy associated with the particles within a body. It includes their kinetic energy, meaning energy of motion, and their potential energy, meaning energy associated with their interactions and relative positions.

For this description, consider the body as a whole at rest. The energy of the entire body moving from one place to another is distinct from the energy of the motion and interactions of its constituent particles.

This distinction helps make the language of heat precise. A body possesses internal energy. Heat refers to a transfer of energy because the body and something else have different temperatures. Internal energy and heat both have energy units, but they describe different things.

Why must the wording be precise?

“The body has a high temperature” describes its hotness. “Heat passes into the body” describes an energy transfer. “The body has internal energy” describes energy associated with its particles. Each statement has a different purpose.

Heating water during boiling provides a useful warning against merging these ideas. Energy can continue to enter while the temperature stays unchanged. Therefore, a fixed temperature reading does not by itself prove that no energy has been transferred.

When comparing heat and temperature, pair each definition with its unit. When explaining a process, also identify the warmer and cooler bodies. This keeps the measurement of hotness separate from the explanation of energy passing between objects.

How are joules and calories related?

The calorie, written cal, is another unit used for heat energy. Use the exact relation 1 cal = 4.186 J. The calorie and joule express the same kind of quantity, so this is an energy conversion, not a temperature conversion.

A conversion changes the numerical expression of the energy. It does not change the amount of energy itself. The same transfer can be described in calories or joules, provided that the conversion factor and unit are stated correctly.

Which way should the conversion go?

To express calories as joules, multiply the numerical value by 4.186. To express joules as calories, divide the numerical value by 4.186. Keep the exact factor during the calculation and distinguish an exact result from a rounded result.

Let QJ mean the numerical value of a heat transfer expressed in joules, and QC its numerical value expressed in calories. These symbols represent numerical values in specified units, not two separate amounts of energy.

QJ = 4.186 × QC

QC = QJ / 4.186

Worked example 1. Express 1 cal in joules, using 1 cal = 4.186 J exactly.

Formula: QJ = 4.186 × QC. Substitute: QJ = 4.186 × 1. Answer: 4.186 J exactly. Both expressions describe the same energy.

Worked example 2. Express 4.186 J in calories, using 1 cal = 4.186 J exactly.

Formula: QC = QJ / 4.186. Substitute: QC = 4.186 / 4.186. Answer: 1 cal exactly. The inverse conversion recovers the starting value.

What should a unit check establish?

The final unit must answer the question asked. A calculation converting heat cannot end in °C or K, because those are temperature units. Conversely, a thermometer reading cannot be converted into joules merely by using the calorie-to-joule relation.

Note: The conversion factor is exactly 4.186 here. Replacing it with a shorter rounded factor changes the stated relation. Keep the specified value in both the working and the answer.

How does a thermometer turn temperature into a readable value?

A thermometer uses a physical property that changes sufficiently with temperature. In a liquid-in-glass thermometer, the useful property is the volume of the liquid. Volume means the space occupied by a substance.

Mercury and alcohol are examples of liquids used in such thermometers. Their volumes vary linearly with temperature over a wide range. Here, linearly means that equal temperature changes correspond to equal changes in the relevant liquid volume over that range.

What do the bulb and scale do?

The bulb is the reservoir containing the thermometric liquid, the liquid whose changing volume indicates temperature. The liquid extends into a narrow tube. Its level can be read against the marked temperature scale.

As the thermometer reaches the temperature of the object being measured, its indication settles. The scale links the observed liquid level to a temperature value. Calibration means assigning temperature values to the instrument's indications using reference temperatures.

The drawing shows a long, narrow thermometer with numbered scale markings along its length and a bulb at the lower end. The scale and liquid column run beside one another.

See Fig. 3.4 in your NCERT textbook

How is the value of a small division found?

Find the temperature difference between two neighbouring numbered marks, then count the equal small intervals between them. Dividing that temperature difference by the number of intervals gives the temperature represented by one small interval.

Worked example 3. Two larger marks on a thermometer differ by 1 °C, and the space between them contains five equal divisions. What does one small division represent?

Answer: 1 °C divided by 5 gives 0.2 °C per division. Count the spaces between the marks, rather than treating both end marks as additional divisions.

The range of a thermometer is the interval between the lowest and highest temperatures it can measure. The value of one small division describes the scale's detail. Range and division value are therefore different features, and both should be checked before reading the instrument.

How should a laboratory thermometer be used correctly?

A laboratory thermometer measures temperatures during laboratory work, including the temperature of water. Its range is generally from −10 °C to 110 °C. “Generally” matters: the actual instrument's scale must be inspected before it is used.

What is the correct method for measuring water temperature?

1. Inspect the thermometer's range and the value represented by one small division.
2. Place its bulb in the water, with the bulb surrounded by water on all sides.
3. Hold the thermometer upright. Keep the bulb away from the bottom and sides of the container.
4. Wait until the liquid thread becomes steady, then read the scale while the bulb remains in the water.

Read with the liquid level along the line of sight. Handle the glass carefully and avoid holding the thermometer by its bulb while reading. The aim is to obtain the temperature of the water under observation using the correct position and reading method.

Photograph: Measuring temperature of water with a laboratory thermometer (NCERT Class 7 Figure 3.5). The photograph shows a hand holding a laboratory thermometer upright in a beaker of water. The lower end is immersed in the water. The thermometer extends vertically above the beaker.

Why must the thermometer remain in the water?

When a laboratory thermometer is taken out of hot water, its mercury level begins to fall. A reading taken after removal no longer reliably records the earlier water temperature. Wait for a steady level, then read before removing it.

This sequence has two separate requirements: allow the indication to settle, and keep the bulb in the substance during the reading. A reading taken too soon and a reading taken after removal are different mistakes.

A clinical thermometer is designed to measure human body temperature. The glass clinical thermometer described here reads from 35 °C to 42 °C. It should not be used to measure hot water or another object's temperature.

Choosing an instrument therefore comes before taking a reading. Check what it is designed to measure, its range, and its scale divisions. A visible scale alone does not establish that a thermometer is suitable for every temperature measurement.

How do the Celsius, Fahrenheit and Kelvin scales compare?

The ice point and steam point are the temperatures at which pure water freezes and boils under standard pressure. They provide reference points for temperature scales. Pressure means force per unit area; standard pressure is the reference pressure specified for these fixed points. The pressure condition and the purity of the water are part of the description.

On the Celsius scale, these points are 0 °C and 100 °C. On the Fahrenheit scale, whose unit is the degree Fahrenheit, written °F, they are 32 °F and 212 °F. The reference interval contains 100 Celsius degrees or 180 Fahrenheit degrees.

How are Celsius and Fahrenheit readings related?

Let tC denote the numerical temperature reading in degrees Celsius and tF the reading in degrees Fahrenheit. The same fraction of the reference interval gives the same temperature on either scale.

Derivation: How are Celsius and Fahrenheit readings converted?

Use the ice and steam points to compare equal fractions of the two linear scales.

1. The Celsius interval between the fixed points is $100 - 0 = 100$ degrees. The Fahrenheit interval is $212 - 32 = 180$ degrees.
2. For the same temperature, the fractional distances above the ice point are equal: $\frac{t_F - 32}{180} = \frac{t_C}{100}$.
3. Multiply by 180 and simplify: $t_F - 32 = \frac{9}{5}t_C$. Adding 32 gives the Fahrenheit reading.

Conversion results: $t_F = \frac{9}{5}t_C + 32$. Reversing the steps gives $t_C = \frac{5}{9}(t_F - 32)$.

The subtraction of 32 accounts for the Fahrenheit reading at the ice point. Omitting it would wrongly identify the zero points of the two scales. The denominators represent their different numbers of equal intervals between the reference temperatures.

The horizontal axis shows Celsius temperature and the vertical axis Fahrenheit temperature. A rising straight line joins the ice-point readings, 0 °C and 32 °F, to the steam-point readings, 100 °C and 212 °F. Dashed guides mark the intervals.

See Fig. 10.1 in your NCERT textbook

How does kelvin relate to Celsius?

Let T denote the numerical temperature reading in kelvins. The relation is $T = t_C + 273.15$. A kelvin and a Celsius degree have the same size, but the two scales have different zero points.

Derivation: Why is 273.15 added to a Celsius reading?

Equal unit sizes mean that temperature intervals have equal numerical values on these two scales.

1. At absolute zero, the Kelvin reading is 0 and the Celsius reading is −273.15.
2. At any other temperature, measure the interval above this common reference: $T - 0 = t_C - (-273.15)$.
3. Subtracting a negative number adds its magnitude, giving $T = t_C + 273.15$. Subtract 273.15 from both sides to reverse the conversion.

Inverse conversion: $t_C = T - 273.15$. The offset changes the numerical reading while preserving the physical temperature.

Absolute zero is the zero point of the Kelvin scale, corresponding to −273.15 °C. Write kelvin temperatures with K, without a degree sign. A Celsius reading needs the degree sign as part of °C.

Worked example 4. Express the Celsius ice-point temperature, 0 °C, in kelvins. Use T = tC + 273.15, where T is the numerical kelvin reading and tC the numerical Celsius reading.

Answer: T = 0 + 273.15, so the temperature is 273.15 K. This is the same temperature expressed on another scale.

Worked example 5. The triple-point temperature of neon is 24.57 K. Express it in degrees Celsius and degrees Fahrenheit.

Formula: $C = T - 273.15$ and $F = \frac{9}{5}C + 32$, where C and F are the numerical Celsius and Fahrenheit readings, and T is the numerical kelvin reading.

Substitute: $C = 24.57 - 273.15 = -248.58$. Then $F = \frac{9}{5}(-248.58) + 32 = -447.444 + 32 = -415.444$.

Answer: −248.58 °C and approximately −415.44 °F. Retain the unrounded intermediate value before rounding the final Fahrenheit reading.

Worked example 6. The triple-point temperature of carbon dioxide is 216.55 K. Express it in degrees Celsius and degrees Fahrenheit.

Formula: $C = T - 273.15$ and $F = \frac{9}{5}C + 32$, where C and F are the numerical Celsius and Fahrenheit readings, and T is the numerical kelvin reading.

Substitute: $C = 216.55 - 273.15 = -56.60$. Then $F = \frac{9}{5}(-56.60) + 32 = -101.88 + 32 = -69.88$.

Answer: −56.60 °C and −69.88 °F. Both readings describe the same temperature as 216.55 K.

Worked example 7. The triple-point temperature of water is 273.16 K. Find its Celsius reading and explain why the conversion subtracts 273.15 rather than 273.16.

Formula: $t_C = T - 273.15$. Substitute: $t_C = 273.16 - 273.15 = 0.01$.

Answer: 0.01 °C. The triple point is the condition in which ice, liquid water and water vapour coexist in equilibrium. It is not the zero point of the Celsius scale; subtracting 273.16 would incorrectly assign it a Celsius reading of zero.

Temperature-scale conversions concern a body's hotness. Calorie-to-joule conversions concern energy. Keep these two kinds of conversion separate even though both involve numbers and units.

Glossary

• Heat — Energy transferred between bodies or between a body and its surroundings because their temperatures differ.
• Temperature — A measure of hotness or coldness that determines the direction of heat flow between bodies.
• Thermal contact — A condition in which bodies can transfer heat between one another.
• Thermal equilibrium — The condition of equal temperatures with no net heat transfer between the bodies concerned.
• Internal energy — Energy associated with the motion and interactions of the particles within a body.
• Joule — The SI unit of energy, also used to express the amount of heat transferred.
• Calorie — A unit of heat energy related to the joule by the exact conversion 1 cal = 4.186 J.
• Kelvin — The SI unit of temperature, written K without a degree sign.
• Thermometer — An instrument that assigns a temperature reading using a property that changes with temperature.
• Calibration — Assigning temperature values to an instrument's indications using established reference temperatures.
• Thermometer range — The interval between the lowest and highest temperatures an instrument can measure.
• Ice point — The temperature at which pure water freezes under standard pressure.
• Steam point — The temperature at which pure water boils under standard pressure.
• Absolute zero — The zero point of the Kelvin scale, corresponding to −273.15 °C.

Common errors and misconceptions

• Misconception: Heat and temperature are different names for one quantity. Correct: Heat is transferred energy; temperature measures hotness or coldness. Their SI units are J and K respectively.
• Misconception: The hands give dependable temperature readings. Correct: Touch can be misleading; the two hands may report different sensations in the same water after different prior exposure.
• Misconception: Cold flows from ice-cold water into warmer surroundings. Correct: Heat passes from the warmer surroundings towards the colder water.
• Misconception: Supplying heat always raises temperature. Correct: Temperature may rise, but water can exchange energy during a change of state without its temperature changing.
• Misconception: A laboratory thermometer should be removed before reading hot water's temperature. Correct: Read it while its bulb remains immersed, after the liquid thread becomes steady.
• Misconception: The calorie-to-joule relation converts temperature. Correct: It converts energy units. Use 1 cal = 4.186 J exactly; Celsius and kelvin require a separate relation.
• Misconception: Zero degrees Celsius means zero kelvin. Correct: The scales have different zero points. Zero degrees Celsius corresponds to 273.15 K.

Exam-style questions with model answers

Q1. Define heat and state its SI unit. [2 marks]

1. Heat is energy transferred between bodies, or between a body and its surroundings, because of a temperature difference.
2. The SI unit of heat is the joule, represented by the symbol J.

Q2. Distinguish heat from temperature using their meanings, SI units and roles when two bodies at unequal temperatures are in thermal contact. [3 marks]

1. Heat is energy transferred because of a temperature difference, whereas temperature is a measure of the hotness or coldness of a body.
2. Heat is expressed in joules, symbol J. Temperature has the SI unit kelvin, symbol K, so the two quantities do not share a unit.
3. Temperature determines which body is hotter and hence the direction of heat flow. Heat is the energy that passes from the hotter body towards the colder body.

Q3. Ice-cold water and hot tea are placed in surroundings warmer than the water but cooler than the tea. State the heat-flow direction for each and the eventual thermal condition. [3 marks]

1. Heat passes from the warmer surroundings towards the ice-cold water because the water initially has the lower temperature. The water therefore becomes warmer.
2. Heat passes from the hotter tea towards the cooler surroundings. The tea loses energy through this transfer and its temperature decreases.
3. Each reaches thermal equilibrium with its surroundings when their temperatures become equal. At that stage there is no net heat transfer between that body and its surroundings.

Q4. Express 1 cal in joules, using 1 cal = 4.186 J exactly, and verify the result by converting it back to calories. [2 marks]

1. Multiplying the calorie value by 4.186 gives 1 × 4.186 = 4.186 J exactly.
2. Converting back gives 4.186 / 4.186 = 1 cal, which recovers the original energy value.

Q5. A liquid-in-glass laboratory thermometer is used to measure water temperature. Explain five precautions concerning the scale, position, bulb, settling time and reading. [5 marks]

1. Inspect the instrument's temperature range and the value of one small division before measuring. This establishes whether the scale is suitable and how to interpret its markings.
2. Hold the laboratory thermometer upright rather than tilted while it is in the water. Maintain this position while observing the liquid column and taking the reading.
3. Keep the bulb surrounded by water on all sides, without touching the sides or bottom of the container. Do not hold the thermometer by its bulb.
4. Wait until the liquid thread becomes steady before recording the temperature. A moving level indicates that the thermometer has not yet reached a settled reading.
5. Read with the liquid level along the line of sight while the bulb remains immersed. Removing it from hot water causes the mercury level to begin falling.

Q6. The left hand is held in cold water and the right hand in hot water for 2 to 3 minutes. Both are then placed together in mixed water. Explain the observation and conclusion. [3 marks]

1. The left hand, previously in cold water, can report that the mixed water feels hot. This is a sensation rather than a numerical temperature measurement.
2. The right hand, previously in hot water, can report that the same mixed water feels cold. Thus, the hands can give conflicting impressions of the same water.
3. The conclusion is that touch cannot always be relied upon to judge temperature. A suitable thermometer provides a temperature reading instead of relying on these sensations.

Q7. Convert 0 °C to kelvins using T = tC + 273.15, where T is the numerical kelvin reading and tC the numerical Celsius reading. Explain whether this conversion changes the water's temperature. [2 marks]

1. Substitute tC = 0: T = 0 + 273.15 = 273.15, so the temperature is 273.15 K.
2. The conversion changes the scale used to express the temperature. It does not physically heat or cool the water.

Q8. A thermometer has five equal divisions between marks differing by 1 °C. Calculate the value of one division and explain how it differs from the thermometer's range. [2 marks]

1. One small division represents 1 °C divided by 5, giving 0.2 °C per division.
2. This is the scale interval, whereas the range extends from the lowest to the highest temperature the thermometer can measure.

Key takeaways

• Heat is energy transferred because of a temperature difference; temperature describes how hot or cold a body is.
• Heat flows from higher to lower temperature during thermal contact, until the bodies reach thermal equilibrium.
• The SI unit of heat is the joule, while the SI unit of temperature is the kelvin.
• Use the exact relation 1 cal = 4.186 J, multiplying to obtain joules and dividing to obtain calories.
• Touch can give misleading impressions, so use a suitable thermometer to obtain a dependable temperature reading.
• Read a laboratory thermometer while its bulb remains immersed, after its liquid column has become steady.
• Heating may raise temperature, cause expansion or change state; energy transfer does not invariably produce a temperature rise.
• Celsius and Kelvin scales have equal-sized units but different zero points; changing the scale does not change the physical temperature.

Test yourself

What condition causes energy transfer to be described as heat?

A temperature difference causes energy to pass between bodies or between a body and its surroundings.

What happens to net heat transfer at thermal equilibrium?

There is no net heat transfer between the bodies because their temperatures are equal.

Why does ice-cold water warm in hotter surroundings?

Heat passes from the warmer surroundings towards the colder water because of their temperature difference.

Does a thermometer directly measure heat transferred?

No. It measures temperature, while heat is energy transferred because of a temperature difference.

What exact relation connects calories and joules?

One calorie equals 4.186 joules exactly; both units express energy rather than temperature.

Why is a laboratory thermometer read before removal from hot water?

Its mercury level begins to fall after removal, so the reading must be taken while immersed.

What conditions accompany the ice-point and steam-point definitions?

They refer to pure water freezing and boiling under standard pressure.

Does supplying heat guarantee a temperature rise?

No. During a change of state, water can exchange energy without its temperature changing.