Download 4.5 THERMAL ENERGY AND HEAT . PRACTICE

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4.5 THERMAL ENERGY AND HEAT
PRACTICE
(Pages 149—152)
Understanding Concepts
•
Thermal energy is the total kinetic energy and potential energy of all the particles of a substance. Temperature is the
average kinetic energy of the particles of a substance.
2. (a) The average kinetic energy of the milk molecules is less than the average kinetic energy of the water molecules. The
milk is at a lower temperature.
(b) The water has far greater thermal energy. Not only is it at a greater temperature; there is also far more of it.
(c) Heat will eventually stop transferring from the water to the milk when both are at the same temperature. At this point,
all the molecules will have the same average kinetic energy.
4. (a) Curling irons and clothes irons have plastic handles because plastic is not a good conductor I heat.
(b) High-quality cooking pots have copper bottoms because copper is a very goo con uc or [heat. This will ensure an
even temperature across the bottom of the p1ulting in a uniform transfer of heat into the food.
(e) A metal skewer inserted into a potato willdtcltjeat into the interior of the potato faster than the potato itself will.
Metals tend to be excellent conductors of heat.
(d) Smoke rises up a chimney due to the upwanetion f the hot air.
5. This statement is true. The particles themselves are not transferred during conduction. Instead, heat energy moves from
particle to particle as a result of collisions between adjacent particles.
6. Since normal body temperature exceeds 25°C, heat would be conducted away from your body if air were a good
conductor of heat. This would have a cooling effect.
7. An electric room heater is best placed near the floor. In this way, the warm air rising from the heater by convection has an
opportunity to be distributed throughout the room. If placed near the ceiling, the warm air would simply stay near the
ceiling.
8. In most eases, the density of a substance decreases as it is heated. Heating generally results in expansion, resulting in the
same mass taking up a greater volume. This corresponds to a smaller density.
9. Heat radiation is vastly different from conduction and convection because radiant heat does not require any matter to be
present to be transferred, For example, heat can only move through a vacuum by radialion.
10. (a) in 8.4 kg
c = 4.18 x I0 J/kg°C
Q = rncAt
At 6.0°C
= 8.4 kg(4, 18 x to J/kg°C6.0°C)
Q=2.l x I05J
The amount of heat needed is 2. I x I o5 J.
(b) ni2.I kg
c = 2.5 x l0 J/kg°C
Q = incA!
At = 32°C
= 2.1 kg(2.5 x io3 J/kg°C)(32°C)
Q= I.7x I0,I
The amount of heat needed is 1,7 x l0 5.
11. (a) in 3.7 kg
c4.l8x l&J/kg0C
QmcAt
At = 31°C 24°C = 7°C
= 3.7 kg(4.18 x l0 J/kg°C)(7°C)
Q=lxlO5J
The amount of heat lost is I x i05 J.
(b) in = 540 g = 0.540 kg
c=2.4x 10J/kg°C
QmcAt
At = 78°C 14°C = 64°C
= 0.540 kg(24 x 102 J/kg°C)(64°C)
Q=8.3x lO3J
The amount of heat lost is 8.3 x I0 3.
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100 UnIt 2 Energy, Woth, and Power
Copyright © 2002 Nelson Thomson LearnIng
12. (a) c
Q
mAt
Q
(b) zn=—
cAt
(c)
me
13. Q=0.050 Mi 5.Ox 103J
In = 5.0 kg
&=42°C -32°C=10°C
—
5.0x104 i
5.0 kg(l0°C)
= l.0xl03 J/kg°C
The specific heal capacity of the liquid is 1.0 x I o J/kg°C.
14. Ql.OMJ=l.0xlO6J
Q
c3.8x 102 J!kgDC
At=32°C20T= 12°C
i OxlO6 J
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3.8x10 i!kg°C(12°C)
m=2.2x102 kg
The mass of brass that could be heated is 2.2 x 102 kg.
IS. Q=4.2x104J
Q
m2.Skg
c8.4x 102J/kg°C
4’x101 J
Ar=?
2.5 kg(8.4x 102 J/kg°C)
& = 20°C
Since the initial temperature of the glass was 41°C, the final temperature
will be 2 1°C. heat is lost.
16. ‘n 120g0.l2Okg
in2l0g0.210kg
= 7.8 x 10J/kg°C
4. IS x 10’ J/kg°C (assuming water)
Let the final lemperature be Ic.
At = t— 21°C
&—t—9lC
IIl.C..Atm + IIlC_At = 0
0.120 kg(7.8 x 102 J/kg°C)(tç— 21°C) + 0.210 kg(4.18 x io J!kg°C)(11— 91°C)
=0
93.6i 1965.6°C + 877.8t 79879.8°C = 0
97l.41r= 81845.4°C
îç= 84°C
The final temperature of the coffee will be 84°C.
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Section 4.5 Questions
(Page 153)
Understanding Concepts
I. Thermal energy is the total kinetic and potential energy in the atoms
and molecules ofa substance. Heat is the movement
of thermal energy by one of the methods of heat transfer.
2. The material that the rug is made of is a poor conductor of heat. whereas
the tiled floor is a good conductor. Since your
feet are warmer than the tiled floor and rug, heat will move out of your
feet but at a faster rate when in contact with the
tiled floor than with the rug. As a result, the tiled floor will feel cold, whereas
the rug will not.
3. (a) Conduction is the most likely method of heat transfer through a
metal.
(b) Radiation is the only method of heat transfer through a vacuum.
(c) Convection is the most likely method of heat transfer through a liquid.
Copyright © 2002.Nelsan Thomson Learning
Chapter 4 Energy, Work, Heat, and Power 101