PROJECT HEAT PART 2

HEAT ENERGY

When caloric theory failed to explain the production of heat due to friction, as observed in paddle –wheel experiment, scientists put forward the dynamic theory of heat according to which:

1. Heat energy is the total KE of all the molecules due to transitional, vibrational and rotational motions.
2. Heat lost or gained by the system depends on initial state final state and path taken up by the process.
3. Work is the transfer of mechanical energy irrespective of temperature difference while heat is the transfer of thermal energy due to molecular collisions from the body of higher temperature to lower temperature.
4. generally temperature of body rises due to supplied heat but following two situations also found to exist:

·         When liquid in the thermos flask is vigorously shaken or when gas in cylinder is suddenly compressed then temperature of liquid or gas gets raised even without supplying heat. In these situations, work done on the system becomes a source of heat energy.

·         When heat is supplied to a body either at its MP or BP, then supplied heat is used up in changing the state of body and temperature of body does not change.

CONVERSIONS OF HEAT ENERGY INTO OTHER FORMS

When we rub our palms, they gets heated but only up to a maximum temperature because work done changes into heat energy. When temperature of palm become above then atmospheric temperature, then heat is lost in the environment.

JOULE’S MECANICAL EQUIVALENET OF HEAT/ WORK-HEAT EQUIVALENECE/ GEENRALIAITONOF 1^ST LAW OF THERMODYNAMICS

Whole work (i.e. mechanical energy) can be converted into heat but whole heat can never be converted into work.

When work (i.e. mechanical energy) is converted into heat then ratio of work done to heat produced is a conversion factor:-

J is a conversion factor, called as the mechanical equivalent of heat.

VALUE OF J

If there is change of state then but if there is change in temperature only then

CONVERSION OF PE into THERMAL ENERGY

1.      FREELY FALL BODY: - when water falls from the height h, then loss of PE is converted into heat or thermal energy and temperature of water sises slightly i.e. why temperature of dam water at ground level is more w.r.t. to its higher level.

So, expression of rise in temperature of a body on converting its total PE into heat is given by

2.      MELTING OF ICE;- if ice block falls down throh some height melts partially then its PE gets converted into heat of melting.

3.      Thus,

4.      if ice-block completely melts down then height required for compree melting is

CONVERSION OF KE INTO THERMAL ENERGY

1. STOPPING OF MOVING MASS: - on striking with a target, the KE of bullet fired from a gun gets converted into heat. By, this heat, temperature of bullet increases by ∆θ

2. MELTING OF CULLET (v=):- If temperature of bullet rises upto MP of bullet then bullet melts down:

For melting of bullet without increase in temperature, required velocity will be (∆θ=0)

Note: -

·         During melting of ice at 273 K, volume decreases. Therefore, positive work is done on ice-water system by the atmosphere.

·         During driving i.e. when a car is in the motion, the temperature of air inside the tyre increases due to motion of air particles and according to charles P T, so air pressure inside the tyre increases.

·         In Harbour town, relative humidity is ore than in desert ton. Hence climate of Harbour town is without extremes of hot and cold.

SPECIFIC HEAT / SPECIFIC HEAT CAPACITY

(A)  GRAM SPECIFIC HEAT: - if mass of a body is ‘m’ and by giving Q heat to the body, change in temperature is ∆θ then:

              S is the specific heat of the substance which depends upon the nature of substance, temperature and state.

If m=1 and ∆θ= 1C then S=Q, thus specific heat of a substance is the amount of heat raise the temperature of unit mass of substance by 1c.

Notes:-

·               Acc. to delong and petit’s law for alelement except carbon:-

Atomic wtx specific heat=constant

So, more is the atomic wt., less is the specific heat and viea –versa.

Ex. Specific heat of H₂ is max (3.5 cgs unit) b/c its atomic wt. is minimum.

        SUBSTANCES

          H₂ (max0

          H₂O

If value of S is more, ten by giving certain amount of heat at constant mass, change in temperature is less.

Since, specific heat of H2O is very large, thus, by absorbing or releasing large amount of heat, its heat changes by small amount. Due to this reason, H₂O is used as a coolant in radiators and also in hot water bottles.

MOLAR SPECIFIC HEAT

By definition, one mole of any substance is the quantity of the substance , whose mass M gram is numerically equal to molecular mass M.

Thus, amount of heat required to raise the temperature of one mole gas by 1C is called as molar specific heat. Thus for µ moles:

(a)    molar specific heat at constant pressure

(b)   moalr specific heat at constant temperature

specific heat of an ideal gas is independent of T.

a vessel moving with velocity v, contained an ideal gas, ten its KE (closed vessel)

when this closed vessel is suddenly stopped then total KE will increase the temperature of gas because process will be adiabatic.

Thus, rise in temperature of agas when vessel is suddenly stopped is given by:

CALORIFIC VALUE OF FUEL

Calorific value is the toal quantity of heat liberated when unit mass of fuel is burnt completely in presence of sufficiently quantityof air/oxygen.

GROSSS CALORIFIC VALUE / HIGHER

Gram calrotifi cv lae is the toal quantity of heat liverated when one unitof the fuel has been burnt completely and the products of combustion have been cooled to room temperaute.

NET CALORIFIC VALUE/LOWER

Net calorific value is the toal quantity of heat liberated when one unit of the ful has been burnt completely and the products of combustion are allowed to escape as such.

VARIATION OF SPECIFIC HEAT/ MOLAR SPECIFIC HEAT

Variation of molar specific heat of solid with temperature

Debye explaied the varaitonof Cv with temperature successfully onteh bassi of plank’s quantum theory.

When a solid is heated through a small range of temperature , its volume nearly remains contant. Therefore , specific heat of a solid may be called as its specific heat at contast volume.

From graph, it is cleat that at T= tend s tat