rate in the thermal system

Rate in the Thermal System


1. What is the prime mover in the thermal system?- temperature difference

2. What does rate measure in the thermal system?- heat flow

3. How does heat flow?- from areas of high temp to low temp

Rate in the Thermal System4. Why does heat flow?

- molecules with high temperature vibrates faster than molecules at lower temperature – more energy- when the molecules bump into one another, energy is transferred from the high energy molecule to the low energy molecule- this continues until all the molecules have equal energy levels


5. How do we calculate heat flow?

Heat Flow = Heat energy transferred

Elapsed Time

QH = H

t

Rate in the Thermal System6. What are the units for heat flow?

- there are not good, clear cut units for heat flow- any unit of heat / any unit of time- common units – J/s; cal/s; Btu/hr

7. What is a calorie?- amount of heat required to raise the temperature of one gram of water one degree Celsius; kilocalorie = 1000 cal


8. What is a Btu?

- British thermal unit

- amount of heat required to raise the temperature of one pound of water one degree Fahrenheit


9. What is specific heat?- every substance has a specific heat – it is a fixed amount- amount of heat required to raise one gram of a specific substance by one degree Celsius- used to determine the amount of heat required in changing the temperature of a substance


10. What is the equation for calculating heat during a temperature change?H = (m)(c)(T)

H = amount of heatm = mass of samplec = specific heat of substance T = Final temp – initial temp


11. What is the specific heat of water?

- 1 cal/gm-Co or 1 Btu/lb-oF

12. Example – How much heat is required to heat 417 pounds of water from 70oF to 130oF?

- H = (m)(c)(T)

- H = (417 lbs)(1 Btu/lb-oF)(130-70)

- H = (417)(1)(60)

- H = 25,020 Btu


13. Suppose you heat a piece of metal and place it in a cup of cold water. How does the heat flow?

- heat will flow from the ball to the water

- the amount of heat loss by the metal is equal to the amount of heat gained by the water


14. An air conditioner is rated at 10,000 Btu. What does this mean?

- it can remove heat at a rate of 10,000 Btu per hour


15. Example – A 5000 Btu heater runs for 5 hours and 30 minutes. How much heat is moved?

- QH = H

t

- 5000 Btu/hr = H

5.5 hours

- H = 27,500 Btu


16. What is a change of state?

- changing a substance from a solid to a liquid or a gas

17. What is sensible heat?

- heat that is applied that changes the temperature of a substance


18. What is latent heat?

- heat that does not raise the temperature of a substance

- the heat causes the substance to change state (from a solid to liquid)

Rate in the Thermal System19. Example – A sample of ice at -10oC is

heated until it becomes steam at 110oC. Classify the heat changes as latent or sensible. -10oC ice to 0oC ice sensible 0oC ice to 0oC water latent 0oC water to 100oC water sensible 100oC water to 100oC steam latent 100oC steam to 110oC steam sensible

rate in the thermal system

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