Heat and Temperature

Heat and Temperature

The constituent of a body are in continual motion and thus possesses internal energy. When heat is added to the body the internal energy of the body increases and it becomes hotter. So, heat can be defined as a form of energy related to internal energy of a body and creates the sensation of  hotness (or coldness)

The unit of heat is the unit of energy (i.e. work) that is, joule (J). Another widely used unit is calorie (Cal)

1 Calorie = 4.2 joule

Temperature

Temperature of a body is its degree of hotness (or coldness). Thus, temperature is a measure of how hot (or cold) a body is and should not be confused with the amount of heat a body contains.

However from microscopic point of view temperature is a measure of average kinetic energy of particles of an object. Heat energy is transferred from an object at higher temperature to an object at lower temperature until the two has same temperature. So In other words temperature is the thermal state of an object which determines whether a body will gain or lose heat when placed in thermal contact with another body.

Temperature scale

Celsius scale

This scale is based on two points.

Lower fixed point (Ice point): This is the temperature of the pure melting ice , at a pressure of standard atmosphere. It is assigned a value of 0 ⁰C.

Upper fixed point (Steam point): This is the temperature of dry steam from water boiling at a standard atmospheric pressure. It is assigned a value of 100 ⁰C.

 The interval between 0 and 100 ⁰C is divided into 100 equal division and are named degree Celsius  (or ⁰ C).

Fahrenheit

In Fahrenheit scale the ice point is assigned 32 degree and the boiling point of water is assigned 212 degree the interval is divided in 180 equal intervals called degree Fahrenheit. The relation between the Celsius scale ant the Fahrenheit scale is as follows

Kelvin scale

Kelvin scale is based on the triple point of water which is assigned 273 K. The relation between Kelvin scale and the Celsius scale is

 C = K –  273

where C = Temperature in Celsius scale (⁰C) and K = Temperature in Kelvin scale (K).

Thermometers

Different types of thermometers used to measure temperature are

(i) Liquid in glass-bulb thermometer   (Volume of fixed mass of liquid changes with temperature)

            (a) Mercury thermometer

            (b) Alcohol thermometer

(ii)  Constant volume gas thermometer ( Pressure of  fixed mass of a gas changes)

(iii)  Thermocouple thermometer ( Electromotive force (voltage ) produced)

(iv)  Platinum resistance thermometer ( resistance of platinum changes)

Heat capacity

Heat capacity is the amount of heat needed to raise the temperature of a body by 1 K (⁰C). The units of heat capacity are J/ K or J/⁰C.

where C = Heat capacity, Q = Heat required in J, Dq = Change in temperature.

The heat capacity depends on material and mass of a body. 100 g of aluminum needs 900 J of heat to raise its temperature by 10 ⁰C while 100 g of copper needs only 400 J of heat  to raise its temperature by 10 ⁰C.

Specific heat capacity

Specific heat capacity is defined as the amount of heat required to raise the temperature of 1 kg of the substance  by 1 K or 1 ⁰C.

Where   s = specific heat capacity

            C = heat capacity,

            Q = Heat required in J

            Dq = Change in temperature

            m = mass of the substance

The units of specific heat capacity are

J./kg- K or J/kg-⁰C.

We can write C = ms

And                Q = msDq

Therefore

Heat required to raise temperature = mass ´ specific heat ´ change in temperature.

Exercise: In a simple experiment, 100 g of water requires 12600 J of heat to raise it from 30 ⁰C to 60 ⁰C.

(i)            Find the heat capacity of 100 g of water.

(ii)           Find the heat capacity of 1000 g of water.

(iii)          Find the heat needed to raise the 1000 g of water from 30 ⁰C to 40 ⁰C.

Effect of Heat on Matter

When heat is added or removed from a body the following things may occur

(i)            Change of temperature

(ii)          Change of volume

(iii)         Change of magnetization

(iv)         Change of resistance

(v)          Change of state like melting/solidification, boiling/condensation etc.

Expansions of Solids and Liquids

With few exceptions, substances expand when heated. When a body is heated the thermal or kinetic energy of the molecules of the body increases. In solids and liquids the molecules vibrates more vigorously against the intermolecular forces and he intermolecular distances increase resulting in increase in volume. In liquid where the intermolecular forces are comparatively lower then the solids the volume expansion comparatively higher. In gas, where there is negligible interaction between the gas molecules there is an increase in random motion of the molecules. This results in increase in both volume and pressure of the gas.

Example: Gaps between the Rails.

In railway tracks gaps are left between two consecutive rails . This is done to compensate for the expansion that occur due to heating from sunlight in daytime or from friction with the wheels.

Biological importance

Near polar region the water freezes to ice in extreme cold weather. However though the water at the surface cools  down first and frozen to ice water is still available beneath where the temperature at the bottom is at 4 ⁰C. this water remains at this temperature and are not cooled further by convection method. This adobe of water allows aquatic creature to thrive during extreme cold weather.

Melting

When a solid, on heating, changes to a liquid, we call this melting. For a pure substance, melting occurs at a definite or constant temperature. This particular temperature a substance melts is called the melting point. Melting point of ice is 0 ⁰C.

Solidification

The reverse process of melting, i.e. changing a liquid to a solid is called solidification.  A pure substance will freeze at a temperature equal to its melting point.

For example, water freezes to form ice at 0 ⁰C. We call this temperature of 0 ⁰C the freezing point of water.

Boiling

When a pure liquid on heating, changes to a vapor at a fixed of constant temperature, we call this change of state boiling. This particular temperature is known as the boiling point of the substance. The boiling point of water is 100 ⁰C.

Condensation

The reverse of boiling, i.e. the cooling of a vapor to its corresponding liquid state is called condensation.

Evaporation

Evaporation is the change of state of a liquid into its vapor, at any temperature. This process is slower than boiling. It takes place only on the surface of the liquid. Heat is supplied by the surrounding in this process.

Latent heat of fusion

Latent heat of fusion is the amount of heat required to change it from solid to liquid  and vice versa without changing its temperature. Its unit is joule (J)

Specific latent heat of fusion

It is the amount of heat required to change 1 kg of the substance from solid to liquid of vice versa without any change in temperature. Its unit is J/kg.

Latent heat = specific latent heat of fusion  ´ mass

Latent heat of vaporization

It is the amount of heat required to change a substance from liquid state to vapor state or vice versa without changing its temperature.

Its unit is J.

Specific latent heat of vaporization

It is the amount of heat required to change 1 kg of the substance from liquid state to vapor state or vice versa without any change in temperature. Its unit is J/kg.

Latent heat of vaporization = specific latent heat of vaporization ´ mass

Some values for specific latent heats of fusion and vaporization:

Transfer of Heat

There are three basic ways in which heat is transferred:

1. Conduction
2. Convection
3. Radiation

Conduction:

Conduction is the transfer of heat from places of higher temperature to place of lower temperature without movement of matter as a whole. Heat is transferred from hot end to the cold end of an object. This process of heat transfer is predominant in solid material and negligible in gaseous material.

Convection:

Convection is the flow of heat through a fluid from places of higher temperature to places of lower temperature by movement of fluid itself.

Radiation:

Radiation is the process of heat transfer from one place to another in the form of electromagnetic wave. We get heat energy from the sun in the radiation process.

Exercise 1: The body temperature of a baby measures 99.5 ⁰Fby a clinical thermometer measures. What is the temperature in (a) Celsius scale (b) Kelvin scale?

Exercise 2: An iron pipe with a mass of 510 g cools from 35 ⁰C to 20 ⁰C. How much thermal energy is lost from the pipe?

Exercise 3: An electric heating coil supplies 50 W of power to a metal block of mass 0.6 kg and raises the temperature of the block from 20 ⁰C to 45 ⁰C in 90 seconds. Calculate the specific heat capacity of the metal.

Exercise 4: A 250 g metal block is heated to 59 ⁰C. It is put into 200 g of water at 20 ⁰C. the final temperature  of the metal sample and water is 15 ⁰C. Find the thermal energy gained by the water (b) the energy lost by the sample (c) specific heat of the sample.

Exercise 5: A 0.2 kg block of metal is warmed to 100 ⁰C. The block is put into 0.4 kg of water at a temperature of 20 ⁰C. The water warms and the block cools off until both are at same temperature. The new temperature of water is 23 ⁰C. From this data calculate (a) specific heat capacity of the metal (b) heat capacity of the metal block (c) heat lost from the block.