Question

Describe two sets of conditions of heat and work that cause an increase in internal energy.

Answer 1

this is pretty difficult to conceptualize but you would either have to increase the heat while keeping it isochoric (W=0) or do work on the gas adiabatically (Q=0)

Answer 2

Write one or two sentences describing how a heat engine works

Answer 3

A heat engine converts the flow of heat that results from a difference in temperatures into work..?

Answer 4

Good, to be more specific the difference in temps is the difference between the hot and cold reservoirs, the difference in temps causes heat to flow and part of it can be harnessed to do work

Answer 5

A neatly arranged bookshelf is a very orderly system and has low entropy. Why is it safe to assume that this did not happen naturally? Write one or two sentences explaining your answer

Answer 6

Hm. I'm not 100% sure what they're looking for but I'd say that since the # of disorganized states is much much higher than the # of organized states, the probability of it happening naturally is very very low

Answer 7

During the melting process, how does the energy of the molecules of the substance change as a result of the heat it absorbs?

Answer 8

melting = solid --> liquid, do you think energy goes up or down

Answer 9

down

Answer 10

hm, be careful with this one, going from solid --> liquid means the molecules are getting more spread out/moving faster so the energy has to go up

Answer 11

oh and gas would be the most then right from that logic

Answer 12

yes

Answer 13

How does the heat of fusion of water compare to the heats of fusion of other, similar substances? How does this difference relate to intermolecular forces?

Answer 14

It is higher because of extensive intermolecular hydrogen bonding.

Answer 15

awsome

Answer 16

How much heat is needed to boil 5.0 kg of water at its boiling point? The latent heat of vaporization for water is 2260 kJ/kg. Show your work.

Answer 17

Same formula right the q=

Answer 18

in this case it would just be heat of vaporization * mass

Answer 19

2260*5=11300

Answer 20

good, just stick the joules unit at the end

Answer 21

When the temperature is expected to drop below freezing, a farmer places large tubs of water in an unheated food storage shed. Write two or three sentences explaining why the farmer uses this technique to protect the produce from freezing. Hint: Remember that liquid water has more potential energy than ice. (

Answer 22

well, the water will freeze - is freezing endo or exothermic?

Answer 23

endo

Answer 24

not quite, for endo it would have to increase in energy meaning it would be melting/evaporating

so freezing is exothermic which is why the water will release heat upon freezing thus releasing heat inside the shed and warming it up through convection

Answer 25

A heat engine converts the flow of heat that results from a difference in temperatures into work. Write one or two sentences describing a very efficient heat engine by comparing the temperatures at which it operates.

Answer 26

well if efficiency = 1 - Th/Tc would you want the hot temperature (Th) and the cold temperature (Tc) to be very close or very far apart? as a hint think about what would happen mathematically to the value of 1 - Th/Tc in either case

Answer 27

um close

Answer 28

well, if Th is approximately equal to Tc, then Th/Tc is about 1 so 1 - Th/Tc approaches 0 efficiency

so the heat source and cold reservoir need to be far apart in temperature to be efficient

Answer 29

How could you improve the efficiency of a heat engine by increasing one of the temperatures at which the heat engine operates?

Answer 30

well to make Th and Tc farther apart would you increase Th or Tc

Answer 31

tc

Answer 32

ugh i dont want to guess

Answer 33

well if we assume we have to increase something, then we need to increase Th even higher, if we increase Tc while keeping Th constant they get closer together, so increase Th by adding more fuel/gas/whatever

Answer 34

okay so improve the efficiency increase Th even higher, if we increase Tc while keeping Th constant they get closer together, so increase Th by adding more fuel/gas/whatever

Answer 35

yeah just say increase the hot temperature by adding energy/fuel [I'm not sure about the mechanics of it really]

Answer 36

Calculate the amount of heat needed to warm 24.6 kg of water from a room temperature of 20.0°C to 60.0°C. Show your work.

Answer 37

specific heat 4.186 joule/gram °C

convert kg of water to g then use:

q = mc*deltaT

Answer 38

q = heat = (24.6*1000)g * (4.186 J/gC) * (60-20) = ?

Answer 39

4119024

Answer 40

good

since they gave us kg we could do kJ instead of joules and say 4120kJ as the result

Answer 41

A water heater has an efficiency of 61%. If the water heater takes in 7660 kJ of heat to heat up water for a dishwasher, how much total work does it provide? Show your work.

Answer 42

actually nvm it's just
e = W/Qin solve for W

Answer 43

7660000J

Answer 44

yeah you can just leave it in kJ, plug in 7660 kJ for Qin and 0.61 for e

Answer 45

4672.6

Answer 46

good, units are kJ b/c the original units were also kJ

Answer 47

How much energy is lost to the surroundings while heating the water for one cycle of the dishwasher? Show your work

Answer 48

last one promise

Answer 49

is there anything else given in this problem

Answer 50

no i think it is a continuation of the previous question

Answer 51

oh right

well if it takes in 7660 kJ and only makes 4672.6 kJ of useful work then everything else must be lost as heat

so 7660 - 4672.6 = plug in to a calc and that's your answer

Answer 52

2987.4

Answer 53

J ?

Answer 54

kJ