What Does Heat Do?

• Heat Changes the Temperature of Objects

If heat is transferred from an object to the surroundings, then the object can cool down and the surroundings can warm up. When heat is transferred to an object by its surroundings, then the object can warm up and the surroundings can cool down.

Heat, once absorbed as energy, contributes to the overall internal energy of the object. One form of this internal energy is kinetic energy; the particles begin to move faster, resulting in a greater kinetic energy. This more vigorous motion of particles is reflected by a temperature increase. The reverse logic applies as well. Energy, once released as heat, results in a decrease in the overall internal energy of the object. Since kinetic energy is one of the forms of internal energy, the release of heat from an object causes a decrease in the average kinetic energy of its particles.

• Heat Changes the State of Matter

The addition of heat to a sample of matter can cause solids to turn to liquids and liquids to turn to gases. Similarly, the removal of heat from a sample of matter can cause gases to turn to liquids and liquids to turn to solids. Each of these transitions between states occur at specific temperatures - commonly referred to as melting point temperature, freezing point temperature, boiling point temperature and condensation point temperature.

• Heat Does Work

Energy transfer in the form of heat can result in the performance of work upon the system or the surroundings. Devices that utilize heat to do work are often referred to as heat engines. In general, an engine is a device that does work. A heat engine is a device that uses heat transfer as the source of energy for doing work.

The internal combustion engine of an automobile is an example of a heat engine.

Concept of Heat and Temperature

Heat is a form of energy produced by the random motion of its molecules. It remains inside a body in the form of its internal kinetic energy.

Energy transfer that takes place because of a temperature difference is called heat flow or heat transfer, and the energy transferred in this way is called heat.

The measurement of the quantity of heat is called calorimetry.

Temperature is a measure of the average kinetic energy of the particles in an object. When temperature increases, the motion of these particles also increases.

Units of Heat: Joules or calorie (Calorie is mostly used to measure the heat).

Calorie: One calorie is the quantity of heat required to raise the temperature of 1 g of water by 1°C.

1 calorie = 4.2 joules

Calorimetry:

Calorimetry is the field of science that deals with the measurement of the state of a body with respect to the thermal aspects in order to examine its physical and chemical changes. The changes could be physical such as melting, evaporation or could also be chemical such as burning, acid-base neutralization etc.

Principle of calorimetry:

According to the principle of calorimetry, when a body at higher temperature is brought in contact with another body at lower temperature, the heat lost by the hot body is equal to the heat gain by the colder body, provided no heat is allowed to escape to the surroundings.

Heat lost by the hot body = Heat gain by the cold body.

Calorimeter:

The device in which heat measurement can be made by utilising the principle of calorimetry is called a calorimeter

Heat Capacity:

The quantity of heat required to raise the temperature of a body by one degree centigrade in termed as the heat capacity of the body.

Heat capacity = Amount of heat / Rise in temperature

Unit: J/°C or cal/°C

Specific Heat Capacity:

The specific heat capacity of a substance is defined as the heat required to raise the temperature of unit mass of that substance through 1 °C. It denoted by the symbol c.

Unit: J/(g°C) or J/(kg °C)

Enthalpy: the total amount of energy in a system, including both the internal energy and the energy needed to displace its environment

Principle of calorimetry:

Heat Lost = Heat Gained

The heat transfer in a system is calculated using the formula,

q=mcΔt

Where

q is the measure of heat transfer

m is the mass of the body

c is the specific heat of the body

Δt is the change in the temperature

Heat energy gained or lost = Mass of the substance x Specific heat x Change of temperature.

Q = mcΔT

Q = mc(T1-T2)

What is the difference between Latent Heat and Specific Heat?

• Latent heat is the energy absorbed or released when a substance is undergoing a phase change. Specific heat is the quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius (or one Kelvin) at a constant pressure.

• Specific heat doesn’t apply when a substance is undergoing phase change.

• Specific heat causes temperature change where in latent heat there’s no temperature change involved.

Numerical Problems

Examples:

Eg.1. What quantity of heat is required to raise the temperature of 450 grams of water from 15°C to 85°C? The specific heat capacity of water is 4.18 J/g°C.

Solution:

Like any problem in physics, the solution begins by identifying known quantities and relating them to the symbols used in the relevant equation. In this problem, we know the following:

Given:

m = 450 g

C = 4.18 J/(g°C)

T initial = 15°C

T final = 85°C

We wish to determine the value of Q - the quantity of heat. To do so, we would use the equation Q = mcΔT. The m and the C are known; the ΔT can be determined from the initial and final temperature.

T = T final - T initial = 85°C - 15°C = 70.°C

With three of the four quantities of the relevant equation known, we can substitute and solve for Q.

Q = mcΔT = (450 g) x (4.18 J/g/°C) x (70.°C)

Q = 131670 J

Q = 1.3x105 J = 130 kJ (rounded to two significant digits)

Eg.2. Calculate the heat given out when a piece of iron of mass 50 g and specific heat capacity 460 J/kg/°C cools from 80°C to 20°C.

Solution:

Given:

Mass of iron (m) = 50g=0.05kg.

Specific heat capacity of iron, (c)=460 J/(kg°C )

Change in temperature = ΔT = (80 - 20)°C = 60°C

Therefore heat energy Q = mcΔT

0.05 x 460 x 60 = 1380 J

Thus the heat given out by the iron piece is 1380 J

Eg. 3. What is the final temperature of the mixture, if 100 g of water at 70 °C is added to 200 g of cold water at 10°C and well stirred?

Solution:

The specific heat capacity of water is 4200 J/kg°C

Let the final temperature of the mixture be T°C

Then change in temperature of hot water = (70 - T)°C

Change in temperature of cold water = (T-10)°C

According to the principle of calorimetry,

Heat lost by hot water = Heat gained by cold water.

mc(70-T)=mc(T-10)

0.1 x c (70-T) = 0.2 x c (T-10)

Cancelling c on both sides, we get

7 - 0.1T=0.2T-2

9=0.3T

T=30

Hence, the final temperature of the mixture is 30°C

Try Questions:

Q1. What quantity of heat is required to raise the temperature of 500 grams of water from 10°C to 80°C? The specific heat capacity of water is 4.2 J/g°C.

Q2. Calculate the heat given out when a piece of steel of mass 120 g and specific heat capacity 420 J/(kg°C) cools from 120°C to 40°C.

Q3. The temperature of 500 g of a substance is raised to 100 °C and it is then placed in 200 g of water at 15°C. If the final steady temperature rises to 21°C, calculate the specific heat capacity of the substance.

[ take c of water equal to 4200 J/kg°C]

Q4. Karma poured 8kg of cold water at 23 °C to 2kg of hot water in a bucket and the final temperature was 37 °C. What was the initial temperature of the hot water? Neglect the heat lost to the surrounding. (Class XI Physics)

Q5. 45g of water at 50 0C in a beaker is cooled when 50g of copper at 18 0 C is added to it. The contents are stirred till a final constant temperature is reduced. Calculate the final temperature. The specific heat capacity of copper is 0.39 J/gK and that of water is 4.2 J/gK. (From sample question paper)

Multiple Choice Questions:

1. The method of measuring the heat transfer that occurs within a chemical reaction or other physical processes is known as

• thermometry

• calorimetry

• speedometry

• none of the option

2. The heat transfer in a system is calculated using the formula

• q=mc+Δt

• q=mc/Δt

• q=mcΔt

• q=m+c/Δt

3. Calorimetry is performed with the help of a

• thermometer

• galvanometer

• calorimeter

• ammeter

4. This motion of molecules and atoms creates a form of energy known as

• mechanical energy

• heat energy

• potential energy

• kinetic energy

5. The specific heat capacity of iron is

• 900 J/kg°C

• 380 J/kg°C

• 460 J/kg°C

• 4200 J/kg°C