Class11 Heat and Temperature Notes

Unit-2

 CHAPTER 9 – Heat and Temperature

Heat

It is a form of energy which flows from body at a higher temperature to a body at lower temperature and it gives the sensation of hotness or coldness. It is also defined as sum of kinetic energy of molecule of given body.

unit:- its unit is joule in SI and C.G.S and practical unit is calorie. 1calorie = 4.18J ≈ 4.2J

Temperature :-  The degree of hotness or coldness of a body is called temperature which measures the average kinetic energy of molecules of body.

Unit:- Its unit is Kelvin in SI and C.G.S and practical unit is Degree celsius.

Note: Heat is extensive property of substance which depends on no of molecules and mass. Temperature is intensive property of substance which does not depends on no of molecules and mass

Thermal equilibrium

When two bodies are in thermal contact, heat flows from higher temperature to lower temperature until both have equal temperature. In thermal equilibrium, heat may or may not be same but temperature must be same.
Note: Thermometer works on the principle of thermal equilibrium an it always records its own liquid temperature then after thermal equilibrium achive by a body and thermometer, than the recorded temperature is temperature of body.

Thermometry

The branch of physics which deals with the study of temperature and its measurement is called thermometry and the device used to measure temperature is called thermometer.

Absolute zero:- It is the lower possible temperature from which no heat transfer arises. Negative temperature is not possible in kelvin scale. Absolute Zero temp=−273.15◦C.

Zeroth law of thermodynamics

Statement : When two bodies are in thermal equilibrium with third body separately then they must be in thermal equilibrium. If A and B are in thermal equilibrium with c separately then, ABC must be in thermal equilibrium.

Different types of temperature scale

1.Celsius
Symbol=◦C
Upper fixed point =100◦C
Lower fixed point =0◦C
Division of scale =100

2. Fahrenheit scale
Symbol=◦F
Upper fixed point =212◦F
Lower fixed point =32◦F
Division of scale =180

3.Kelvin scale
Symbol=K
Upper fixed point =373K
Lower fixed point =273K
Division of scale =100

4.Reaumer scale Symbol=◦R
Upper fixed point =80 ◦ R
Lower fixed point =0◦R
Division of scale =80

C − 0 / 100 − 0 = F − 32 / 212 − 32 = K − 273 / 373 − 273 = R − 0 / 80 − 0 C / 100 = F − 32 / 180 = K − 273 / 100 = R / 80

Properties of thermometric liquid

• It should be shiny and visible.
• It should have high melting and boiling point.
• It should have low specific heat capacty.
• It should be non sticky.

Explain why mercury is used as thermometric substance?

Ans : Mercury is used as thermometric substance because it doesn’t wet the wall of the glass tube.It can be used over the wide range of temperature-39◦ to 357◦. It is opaque and shining liquid. It has lows pecific heat.

Numericals

1.Convert −62.8◦C temperature to Fahernheit:
  solution: 
     C − 0 / 100 − 0 = F − 32 / 212 − 32
 =⇒  −62.8 − 0 / 100 = F − 32/ 180
 =⇒  10F − 320 = −62.8
        : F = −81.04◦F

2.Find the celsius temperature corresponding to 107.0 ◦F
  solution:
    C − 0 / 100 − 0 = F − 32 / 212 − 32
=⇒ C − 0 / 100 = 1070 − 32 /180
              : c = 41.67◦C

3. At what temperature is Fahrenheit equal to Celsius?

C−0 / 100−0 = F−32 / 212−32 ………I Now, we want to know the temperature at which ◦C and ◦F are
equal. So in the above equation, I put both as X
=⇒ X / 100 = X−32
=⇒ x = −40◦C

Solve Yourself with the given hints

4. At what temperature the celsius scale is double the Fahernheit scale.
Hints:
let f=x then c=2x
C – 0 / 100 = F − 32 / 180

5. The distance between upper and lower fixed point is 80 cm. Find the temperature, on Celsius scale if the mercury level rises to height 10.4 cm above the lower fixed point.

Given,
Chance in cm of upper and lower fixed point is 80 cm. Change in temperature between upper fixed point and lower fixed
point =100 – 0 = 100◦C
From this we can write
100 – 0 = 80 cm
1 cm = 100 / 80 = 1.25◦C
mercury level rises to height 10.4 cm from above the lower fixed
point =10.4 × 1.25=13◦C

Shorts Important Question Find and solve It VVI questions

1. A body at higher temperature contains more heat. Comment
2. If your doctor tells you that your body temperature is 310 above absolute zero. Should you worry? Explain
3. At what point of thermometric scale does kelvin scale reading coincide with fahrenheit scale reading?
4. A news paper article about the weather states that ” the temperature of a body measures how much heat the body contains”is this description correct? why or why not?
5. Water can not be used as thermometric substance. Why?

Tricky Question

5. A student claimed that thermometers are useless because a
   thermometer always registers its own temperature. How would you
   respond?
   ans: The thermometer operates under the theory of thermal equilibrium.
   It first achieves thermal equilibrium with our body and records our
   body temperature. Therefore a thermometer is a useful.

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