Chapter 4: Heat

4.1 Understanding thermal equilibrium

A) The difference between Temperature And heat

TemperatureHeat

Is the degree of hotness of a bodyIs a form of energy

Is a base quantityIs a derived quantity

Depends upon the kinetic energy of the moleculesDepend upon temperature, mass and type of material ( spesific heat capacity or specific latent heat) ( Q = mc( or Q = ml) )

The S.I. unit is K

Other unit = 0CThe S.I. unit is Joule(J)

Measured by thermometerMeasured by Joulemeter

Kelvin Scale (K)

The temperature of a substance in Kelvin , also known as absolute temperature ( o C = ( ( + 273 ) KB) Thermal equilibrium (zeroth law of thermodynamics)

a) Initial state b) before thermalc) at thermal

Equilibrium equilibrium

(net transfer of heat)(no net transfer of

heat)1) When hot object comes into thermal contact with a cold object, heat is transferred from the hotter object to the colder object.a) P transfer energy at a higher rate to Q

b) Q transfer energy at a lower rate to P

c) P loses energy and cools down

d) Q gains energy and warms up

e) There is a net transfer of energy from P to Q. The energy transferred is known as heat.

TP decreases whereas TQ increases.f) P and Q are said to be in dynamic equilibrium as energy exchange still goes on between the 2 object.2) After some time,

Rate of transfer of energy from P to Q = rate of transfer of energy from Q to P

a) P and Q are now in thermal equilibriumb) The net transfer of energy or heat transferred = 03) Thermal equilibrium is a state which

a) There is no net flow of heat between two objects

b) Temperature of the two object are equal

4) Two object at different temperatures in thermal contact will eventually come to a state of thermal equilibrium regardless of

a) Massb) Size

c) shape

d) Type of material

5) Good thermal contact ensures that the objects come to thermal equilibrium after a shorter period of time

6) Examples of thermal equilibrium

a) Measuring the temperature of patient

--- thermometer is initially at room temperature

--- while body of patient is at a higher temperature

--- heat energy is transferred from the patient to the

thermometer

--- after sometime, thermometer shows a steady

reading (thermal equilibrium between patient and

thermometer)

--- reading of thermometer equal to temperature of

Patient

b) Cooking using an oven

--- oven has a thermostat to keep its temperature constant

--- when food is placed in an oven, the food absorbs

heat from the oven thereby causing its

temperature to rise--- the process continue until thermal equilibrium is

reached

c) Cooling with a refrigerator

d) Making a cup of warm drink

e) Carrying a bowl with hot soup

--- after some very hot soup is poured into a bowl, a

person may able to carry the bowl with bare

hands

--- but if wait longer, the bowl and hot soup will

reach thermal equilibrium

--- the bowl is at higher temperature (may have to

use a piece of cloth to carry the bowl then)

C) Change in physical properties of materials with temperature1) The physical property of a substance which is sensitive to and varies linearly with changes in temperature of the material is known as thermometric property.2) Important application of thermometric property of a substance is in the construction of thermometer.

3) Example of thermometric properties:

ThermometerThermometric property

Mercury thermometerVolume of mercury varies with temperature

Resistance thermometerElectrical resistance of a wire varies with temperature

Thermocouple thermometerElectromotive force (e.m.f) varies with temperature

Gas thermometerGas pressure varies with temperature

4) Calibration of thermometer

Freeze some pure water.

Crush the ice into small and fill a funnel with them.

When the ice begins to melt inset the bulb of a thermometer so that it is covered with ice.

When the mercury stops shrinking , mark the stem of the thermometer at the mercury level, as 0oC.

Now arrange the thermometer inside a flask so that its bulb is just above the surface of boiling water.

When the mercury stops expanding , mark its level on the thermometer stem ,as 100oC.

Divide the distance between the marks 0oC and 100oC into 100 equal parts, marked as a scale along the stem.

5) The formula is used to calibrate a thermometer

( = temperature of a substance

(o= ice point

(100= steam point

xo = the length of the mercury column at

ice point

x100 = the length of the mercury column at

steam point

x = the length of the mercury column when

the thermometer is placed in a

substance

6) Liquid in-glass thermometer

The specific thermometric property in used in this thermometer the changes of the volumes of mercury with the temperature i.e when the temperature increases ,the volume of the mercury increases.

The sensitivity of the thermometer can be increased by

1 using a thinner-walled glass bulb

2 small bulb

3 reducing the diameter of the capillary tube

Mercury is used in the thermometer because

1 has a higher boiling point 2 does not vaporize readily3 low freezing point

4 does not stick to the glass5 is opaque and therefore it is easier to read.6 expands and contracts uniformly7 good conductor of heatP

Q

Q

P

Q

Cold object

P

Hot object

X0

X100