GRADE XI PHYSICS: CHAPTER 3 (HEAT)

Dr Vince (YU)

ရူပေဗဒဘာသာရပ္  Chapter 3 မွာ Heat, Heat Transfer, Heat Emission  တို႕နဲ႕ဆိုင္တာ ေတြကို ေလ့လာရမွာျဖစ္ပါတယ္။ Grade 11 Physics Chapter 3 အေျဖလႊာေတြ စစ္ေဆးရာမွာ ေက်ာင္းသားအခ်ဳိ႕က ျပဌာန္းစာအုပ္မွာ မရွိတဲ့ Definition ေတြကို ေရးခ်တာ ေတြ႕ရပါတယ္။ ေမးခြန္းတစ္ခုကို ကိုယ္နားလည္သလို ေျဖလို႕ရပါေသာ္လည္း ဘာသာရပ္ စကားလံုးေတြပါမလာရင္ အမွတ္ေပးလို႕မရသလို တစ္ခ်ဳိ႕အရပ္သံုး စကားလံုးေတြနဲ႕ ဘာသာရပ္စကားလံုး မတူတာေတြရွိတာေၾကာင့္ ျပဌာန္းစာအုပ္ပါ Definitions ေတြကိုသာ အမိအရ က်က္ထားေစခ်င္ပါတယ္။ ျပဌာန္းစာအုပ္မွာ မရွိတဲ့ Definition ေတြကို အမွတ္ေပး/မေပး အာမ မခံႏိုင္ေပမယ့္ ျပဌာန္းစာအုပ္ပါ အတိုင္း ေရးတာကိုေတာ့ အမွတ္အျပည့္ ေပးမွာျဖစ္လို႕ ေသခ်ာတာကိုသာ အပိုင္ရေအာင္ လုပ္ထားေစခ်င္ပါတယ္။ မေသခ်ာတာကို ဘဝနဲ႕မရင္းလိုက္ပါနဲ႕။

(၁) က်က္မွတ္ရန္ Definitions/laws မ်ား

(1) Heat Conduction (2) Thermal conductivity (3) Temperature Gradient (4) Heat Convection (5) Heat Radiation (6) The best absorber (7) Black Body (8) Total Emissive Power (9) Stephan-Boltzmann’s Law (10) Emissivity (e) (11) Kinetic Theory of Gas (12) Brownian motion (13) Temperature (14) Heat

(2) Fill in the blanks.

  1.    ___________ help keep unwanted heat away.

  2.    An ordinary sized particle is called a ________________ particle.

  3.    Body tissue is a good ________ of heat.

  4.    Every object including the sun emits energy in the form of ________ radiation.

  5.    For a person wearing warm clothes, the ______ makes him warm by reducing heat losses.

  6.    Heat conduction constant is also known as _________________.

  7.    If thermal conductivity of the body is large, it is a good thermal _____________.

  8.    In cold regions where rooms are heated by fire, heating is done by ________ process.

  9.    One of the most important thermal insulators is the _________.

10.  Some of the weather conditions are created by heat _________ process.

11.  The _______ warms the earth and is the major source of heat for the earth.

12.  The black body is taken as a reference body to study the ____________ of bodies.

13.  The rate of heat conduction is ______ proportional to the length of the conducting media.

14.  Thermal radiation is also known as ____________ radiation.

Ans: (1) Insulators (2) macroscopic (3) insulator (4) electromagnetic (5) air (6) thermal conductivity  (7) conductor (8) convection (9) air (10) convection (11) sun (12) emissivity (13) inversely                 (14) infrared

(3) Say True or False

  1.    Body tissues are poor conductors of heat.

  2.    Electromagnetic waves cannot travel through vacuum.

  3.    Good conductors are used where heat has to be readily transmitted.

  4.    Heat cannot be removed from the body by means of conduction.

  5.    Hot liquid is lighter than cold liquid of the same kind.

  6.    If a liquid is heated from the top, convection would not occurred.

  7.    Metals are good thermal conductors.

  8.    Radiation cannot pass through materials.

  9.    Radiation does not require a material medium.

10.  Some heat may be transferred by conduction in liquids and gases.

11.  The emissivity of the object other than a black body is greater than 1.

12.  The flow of warm blood is the major factor in body heat transport.

13.  The temperature difference in human body is only a few degrees.

14.  The values of thermal conductivity is constant for all types of material.

15.  Thermal conductivities of conductors are greater than those of thermal insulators.

16.  Thermal conductivities of metals are greater than those of thermal insulators.

Ans: (1) True (2) False (3) True (4) True (5) True (6) True (7) True (8) False (9) True        

        (10) True (11) False (12) True (13) True (14) False (15) True (16) True

(4) Quiz:

  1.       Why does a person wearing woollen sweater feels warm?

Ans: For a person wearing wool and down clothes, it is the air that makes him warm by reducing heat losses. When wearing wool and down clothes, the wool and the down trap air in the woollen fibres and down feathers, and this air acts as an insulator. Thus a person wearing wool and down clothes feels warm.

  2.       One end of a poker is placed in fire. After some time the other end becomes hot. Explain how heat is transferred along the poker. Name the method of heat transfer in this case.

Ans: At first the end of the poker placed in the fire gains heat energy. Then the other end becomes hot by successive distribution of heat energy among the adjacent parts. This method of heat transfer is heat conduction.

  3.       A silver spoon and a wooden spoon are at room temperature. Why does the silver spoon feel cold when it is touched?

Ans: Silver is the better conductor of heat than wood. When we touch the silver spoon, heat is taken from our body more quickly than in touching the wood. Thus, we feel cold.

  4.       If a person wearing ordinary clothes travels out into space, the liquid in the body will boil. Why? Explain how a space suit can prevent this effect.

Ans: The boiling point of water at normal pressure is 100 ºC. But the boiling point decreases as the pressure decreases. At the space, the pressure is very low and so is the boiling point. Thus, if a person wearing ordinary clothes travels out into space, the liquid in the body will boil when heat enters to his body. To prevent this, a space suit is constructed to make sure there is no heat transfer between the man and his environment. The suit consists of two layers. Between the layers is vacuum so that no conduction and convection take place. The outside is painted silvery to reduce heat radiation. 

  5.       In cold regions it is seen that birds on the branches of trees often ruffle their feathers. Why do the birds feel warm by ruffling their feathers?

Ans: By ruffling, air can enter and be trapped among feathers. This trapped air serves as an insulator to prevent heat conduction and convection or heat transfer to the surrounding. Thus the birds feel warm.

  6.       How does a blanket wrapped round our body keep us warm on a cold day?

Ans: When we wrap a blanket round our body, it is the air that makes us warm by reducing heat losses. When wrapping the blanket, the blanket traps air in the blanket’s fibres, and this air acts as an insulator. Thus, a blanket wrapped round our body keep us warm on a cold day.

  7.       How is heat transmitted from the sun to the earth?

Ans: Heat transfer from the sun to the earth is neither by conduction nor convection because in the space between the sun and the earth, there are hardly any molecules and this space is vacuum. Heat is transmitted from the sun to the earth by means of (electromagnetic) radiation. Electromagnetic waves can travel through vacuum.

  8.       Is heat radiation a form of electromagnetic waves? Does it need a medium for propagation? What is its speed? Can heat radiation be explained by Maxwell's theory of electricity and magnetism?

Ans: Yes, No, 3 ´ 10⁸ m s^-1, Yes

(5) Calculation အတြက္ Formula မ်ား

1. The rate of heat conduction

H = kA (T2-T1)/l

H = the rate of heat flow, k = thermal conductivity, A = cross-sectional area of conducting medium,      l  = length of the conducting medium, T₂ – T₁ = temperature difference

2. The rate of heat convection

H = qA(T₂ – T₁)

H = the rate of heat convection, q = Heat convection constant

3. The rate of heat radiation

H = e s A T⁴

H = the rate of heat radiation, T = absolute temperature of the body, e = the emissivity of the body,

s = Stephan’s constant

4. Total emissive power

e₀ = s T⁴

where   e₀ = total emissive power of a black body, T = absolute temperature of the body,

s  = Stephan’s constant (5.685 × 10^–8 W m^–2 K^–4)

5.The Total Emissive Power of Objects Other Than a Black Body

e = ee₀ = e s T⁴

e = emissive power of the object, e = emissivity, e₀ = total emissive power of a black body

6. The kinetic energy of the molecule

½ mv² = 3/2 KT

k = Boltzmann’s constant, m = mass of molecule, v = speed of molecule

T = absolute temperature of the gas