An 8-g iron ball bearing initally at a temperature T = 50o C is dropped into a l

Question

An 8-g iron ball bearing initally at a temperature T = 50o C is dropped into a large (assume infinite) bucket of water at T = 10o C.

1)
Approximately how much energy flows into the water from the bearing before an equilibrium temperature is reached? (Iron has an atomic weight = 56 g/mole.)

18 J
60 J
140 J
820 J
2700 J




2)
Compare the time t50->40 needed to cool the bearing from 50o C to 40o C, with time t20->10 needed to cool from 20o C to 10o C.

t50->40 < t20->10
t50->40 = t20->10
t50->40 > t20->10




3)Say that a solid glass window conducted heat away from your house at a rate of 1 W on a cold day. If you used a window twice as thick, twice as wide, and twice as high, what would be the heat loss through the window?
1/2 W
1 W
2 W




4)
Two perfume bottles are opened in a very still room (neglect all air turbulence). Assume the molecules from perfume A have molecular weight 130 g/mole; the perfume B molecules have 200 g/mole. Under the assumption that the molecules have approximately the same mean-free path (e.g., they are approximately the same size), compare the times tA and tB for the two perfumes to diffuse to the edges of the room.

tA = 0.65 tB
tA = 0.81 tB
tA = tB
tA = 1.24 tB
tA = 1.54 tB




5)
What is the chance that for 30 random spins, 29 will be up?

1/30
30 / 230
29 / 600
(29/900) / 230
none of the above

Explanation / Answer

1)
Approximately how much energy flows into the water from the bearing before an equilibrium temperature is reached? (Iron has an atomic weight = 56 g/mole.)

18 J
60 J
140 J
820 J
2700 J

To calculate the amount of energy uses the following expression:

Q = mcdT

= 8 g* 0.450 J/ g-K (10-50)

=-144 J

= -140 J

Negative sign indicates release of energy

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