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Heat and Temperature

If you hold an ice cube in your hand, you will feel it very cold. If you dip your finger in hot water, you will feel warm. An object appears cold because the heat from our body flows into it and an object appears warm because heat from the object flows into our body. Thus, we can say that it is the flow of thermal energy into our body which makes us feel hot. On the other hand, the flow of energy out of our body makes us feel cold (Figure 8.1).
Description: Description: 72530.png
Figure 8.1 Feeling Hot or Cold is all about Gaining or Loosing Heat

The part of the thermal energy that flows from hot body to cold body is called heat.
Heat is a kind of energy that gives us a sensation of hotness or coldness. A quantitative measure of the sensation of hotness or coldness is given by temperature. Therefore, temperature is the degree of hotness or coldness of a body. It is related to the average kinetic energy of the molecules of a material. In other words, temperature is the measure of the average kinetic energy of the molecule of a substance. Temperature determines the natural direction of flow of heat. Heat always flows from a hot body to a cold body.
Difference Between Heat and Temperature

1. Heat is a form of energy.
1. Temperature is the measure of the degree of hotness or coldness of a body.
2. Heat is responsible for the change in temperature in a body.
2. Temperature is one of the effects of heat.
3. SI unit of heat is joule.
3. SI unit of temperature is kelvin.
4. The direction of transfer of heat does not depend on the quantity of heat.
4. The direction of transfer of heat depends on the temperature.

Measurement of Temperature

A thermometer is a device for the measurement of temperature. (Thermo is a Latin word which means heat and ‘meter’ means measuring device.)
Liquid Thermometers Thermometers which use a liquid as the thermometric fluid are called liquid thermometers. The two most commonly used thermometric liquids are mercury and alcohol.
Use of Mercury in Thermometers Mercury is used as a thermometric liquid because of the following advantages.
  1. It is an opaque and shiny liquid and hence can be easily seen through the glass of the thermometric tube.
  2. It does not stick to glass.
  3. It has a fairly large expansion for a small change in temperature.
  4. It has a low freezing point (39 °C) and a high boiling point (357 °C). Hence, it can be used over a wide range of temperature.
  5. It can be easily obtained in pure state.

Temperature Scales

The temperature scales commonly used are Celsius scale, Fahrenheit scale and Kelvin scale.
To construct these scales, two fixed temperatures, which are called fixed points are selected.
The lower fixed point of a thermometer is the melting point of ice under 1 atmospheric pressure and the upper fixed point is the boiling point of pure water under 1 atmospheric pressure (Figure 8.2). The interval between upper fixed point and lower fixed point is divided into 100 equal parts in case of Celsius scale and Kelvin scale, whereas in case of Fahrenheit scale, it is divided into 180 parts.


Celsius Scale
Fahrenheit Scale
Kelvin Scale
Freezing point of pure water
273 K
Boiling point of pure water
373 K


Note: The symbol ‘°’ is not used in Kelvin scale unlike °C and °F.


Description: Description: FIG-8.2.tif

Figure 8.2

Relationship Between Fahrenheit and Celsius Scales of Temperature The number of divisions between 0 and 100 of Celsius scale is 100. Let C be any temperature on this scale and F be the corresponding temperature on the Fahrenheit scale. In the Fahrenheit scale, the number of divisions between 32 and 212 is 180. Therefore,
Description: 71304.png 

or Description: 71313.png
or Description: 71321.png
and Description: 71331.png.

Using the above formulae, temperature measured in one scale can be converted into the other scale.

Relationship Between Celsius and Kelvin Scales of Temperatures
Temperature of 273°C in Celsius scale corresponds to zero kelvin in the Kelvin scale. Therefore, by subtracting 273 from the temperature in the Kelvin scale, we get the corresponding temperature in the Celsius scale. Similarly, by adding 273 to the Celsius temperature, we get the corresponding temperature on the Kelvin scale. This is represented in the following relation:
°C = K – 273
K = °C + 273

For example, 35°C can be expressed on the Kelvin scale as
35° + 273 = 308 K.
Similarly, 373 K can be written on the Celsius scale as 373 – 273 = 100°C.
  • Sometimes doctors also use an instrument called a strip thermometer to measure our body temperature.
  • Digital thermometers are also available.

Effects of Heat

Some of the important effects of heat on a body are as follows:
  1. thermal expansion
  2. increase in temperature
  3. change in physical state
Thermal Expansion Objects generally expand when heated, i.e., when their temperature is increased. This effect is called thermal expansion. Except for water in the temperature range of 0°C to 4°C, all substances namely, solids, liquids and gases expand on heating, but the extent of expansion is different for different substances.
Generally, gases expand more than liquids and liquids show greater expansion than solids.
The phenomenon of increase in the size of a substance due to heating is known as thermal expansion.
Solids have a definite shape and size. In solids we observe three types of thermal expansion.
  1. Linear expansion
  2. Superficial expansion (area expansion)
  3. Volume expansion
Let ‘1’ denote the initial dimensions, ‘2’ the final dimensions and ‘’ the change in the dimension or temperature; the co-efficient of thermal expansion can be in general defined as the fractional change in the dimension per unit change in temperature. When the substance is heated or cooled:
Description: 71406.png
 Description: 71415.png
Description: 71424.png 
 Description: 71434.png
Description: 71461.png

 Description: 71470.png

αβγ are the co-efficients of linear, superficial and volume expansions, respectively. The unit of
αβ and γ is Description: 71478.png
  • It is found that the ratio between the coefficients of thermal expansion in solids is α : β : γ :: 1: 2: 3.
  • In solids three types of thermal expansion are possible. These are linear expansion, superficial expansion and volume expansion. But in liquids and gases only volume expansion is possible (since they do not possess definite shape).
  • Expansion on heating has some exceptions. Some examples are:
    1. Water contracts when the temperature is increased from 0° C to 4° C. On heating further, it expands. This is called anomalous expansion of water.
    2. Stretched rubber band contracts on heating.


Applications of Thermal Expansions of Solids

  1. Metallic rims are fitted on wooden wheels by heating: When heated, the rims expand. The hot rim can be easily placed around the wooden wheels. When water is poured over it, the metal rim contracts and fits tightly to the wheel.
  2. Thermostat switches and bimetallic strips: Bimetallic strips have two metallic strips (say brass and iron), which are riveted together. On heating, the bimetallic strip bends so that brass is on the outside of the curve. This is because brass expands more than iron for the same temperature change.
    Bimetallic strips are used to make or break electric circuits. These are used in the manufacture of thermostat switches. Thermostat switches are devices that automatically break an electric circuit when the temperature increases beyond a certain point and thus help the maintenance of a stable temperature. Thermostat switches are used in electric irons, electric stoves, refrigerators, air conditioning equipments and other such devices related to heating or cooling.
Increase in Temperature due to Heating  Increase in temperature is one of the effects of heat. The temperature of a body increases when it is heated and decreases when it is cooled. This is because, when heat is given to a body, the average kinetic energy per molecule of the given material increases and the average kinetic energy per molecule decreases when heat is removed from the body.
The increase in the temperature of a body depends on:
  1. the quantity of heat energy supplied to the body
  2. the mass of the body
  3. the material of the body
More massive the body, more is the heat required to raise its temperature by T. Thus, we write the amount of heat required as
Q  m · T (where m is the mass of the body and T is the change in temperature)
or Q = Cm · T
where the constant of proportionality, C, is called the specific heat capacity of the material. Therefore,
Description: 73224.png with SI unit Jkg1 K1.
The specific heat capacity of the material is the quantity of heat required to increase the temperature of 1 kg of a substance by 1 kelvin.
The CGS unit of specific heat is cal g1 °C1.
For example, specific heat of iron is 460 J kg1 K1 and specific heat of water is 4180 J kg1 °C1.

Thermal Capacity of a Substance

Thermal capacity is the amount of heat required to raise the temperature of a body through 1°C. Its SI unit is JK1.
The thermal capacity or heat capacity of a substance of mass m and specific heat C is equal to mC joule. If H is the thermal capacity, then H = mC.
The unit of thermal capacity is joule/degree celsius (J°C1).


Principle of Calorimetry

Calorimetry refers to the measurement of heat. Whenever hot and cold substances are mixed to find the resulting temperature, we assume that there is no loss of heat. Thus,
Heat lost by the hot body = Heat gained by the cold body.

Change of State
The physical state of a substance can be changed by heating or cooling. For example, water can exist in solid (ice), liquid (water) and gaseous state (steam). Ice can be converted into water and water can be converted into steam by heating. Conversely, steam when cooled becomes water, which on further cooling becomes ice.

Change of a substance from one physical state to another is called change of state.

The temperature at which a solid melts is called the melting point. On cooling, the molten solid again freezes at the same temperature. Hence, the melting point is the same as the freezing point for a given material.

The temperature at which a liquid boils to form vapour is called the boiling point. The vapour condenses into liquid at the same temperature (Figure 8.3).

Description: Description: FIG-8.4.tif

Figure 8.3 Change of State

In addition to this, there is also a process called evaporation (different from boiling and vaporisation) associated with liquids. In this process, a liquid enters into vapour state at all temperatures.
A common example of evaporation is observed when some water is spilled on the floor. Water evaporates as vapour even though the temperature is less than the boiling point (100°C).


Boiling (Vapourisation)


It takes place at all temperatures.


It occurs at a definite temperature.


The temperature may change during evaporation.


The temperature remains constant during boiling.


It takes place only on the surface of the liquid.


Boiling takes place all over the volume of the liquid.


The rate of evaporation depends on the area of the free surface of the liquid.


The rate of boiling does not depend on the area of free surface of the liquid.

Latent Heat
During change of state (melting or boiling), there is no change in temperature, even if heat energy is added. This is because heat added is being utilized only for the change of state and hence there is no increase in temperature. Thus, the heat required is called the ‘latent heat’, meaning ‘hidden’ heat.
Therefore, latent heat is the amount of energy in the form of heat released or absorbed by a substance during change of state, without change in temperature. It is the amount of heat energy required to completely change the state of 1 kg of solid to liquid (or liquid to vapour) at its melting point (or boiling point).
So, the quantity of heat required to change the state of ‘m’ kg of a substance is Q = mL or Description: 71777.png.
Its unit is JKg1.
Consequences of Large Lf and Lv
  1. Ice has large Lf (3,36,000 J). Therefore, the ice cubes floating on water melt slowly. This is used as an advantage in keeping drinks cool.
  2. Large Lf of ice prevents snow in mountains from melting quickly. It changes into water slowly due to heat of the sun and keeps rivers flowing without floods.
  3. Steam has large Lv. Hence a lot of heat is released when steam condenses to water. So, steam causes more severe burns than boiling water, even though both are at 100°C.
Change in Volume during Change of State The change in volume during melting is quite small compared to that during boiling. Usually, volume increases when a solid becomes a liquid, for example, wax. There are some exceptions to this. The volume decreases when ice becomes water. However, volume increases enormously when a liquid becomes vapour and there are no exceptions to this.

Transfer of Heat

When you pour hot tea into a cup you will observe that the cup gets warmer and the tea a little cooler. This is because the hot tea transfers some of its heat energy to the cup and cools down, while the cup gains the heat energy and gets warmer.

The flow of heat energy from a body at higher temperature to another part of the same body or another body at lower temperature is called transfer of heat. The transfer of heat stops as soon as the temperature of the two bodies becomes equal. The two bodies are then said to be in a state of thermal equilibrium.

In nature, there are three distinct ways in which heat is transferred from a hot body to a cold body.

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