At a lecture in Cork in 1843, James Joule, while describing his work on heat and temperature, suggested the principle of conservation of energy - Leaving Cert Physics - Question 7 - 2016
Question 7
At a lecture in Cork in 1843, James Joule, while describing his work on heat and temperature, suggested the principle of conservation of energy. Later in the ninetee... show full transcript
Worked Solution & Example Answer:At a lecture in Cork in 1843, James Joule, while describing his work on heat and temperature, suggested the principle of conservation of energy - Leaving Cert Physics - Question 7 - 2016
Step 1
Distinguish between heat and temperature.
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Answer
Heat is a measure of energy, while temperature is a measure of hotness.
Step 2
State the principle of conservation of energy.
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Answer
Energy cannot be created or destroyed; it can be changed from one form into another.
Step 3
Calculate the height of the Niagara Falls.
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Answer
Using the formula:
E=mgh
g = 9.81 ext{ m/s}^2mgh = mcΔθ$$
The height is calculated as follows:
h = rac{514}{9.81} = 51.4 ext{ m}
Step 4
Suggest a reason for the smaller increase in temperature.
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Answer
The increase in temperature will be much smaller due to energy being converted into other forms, such as energy lost to surroundings.
Step 5
Describe the operation of a heat pump.
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Answer
A heat pump operates by circulating a fluid within a system:
Pipe containing a fluid: The fluid absorbs heat from the cold area.
Valve/compressor: The fluid's state changes, allowing it to release heat to a warmer area.
Latent heat: The fluid undergoes phase changes that involve absorption or release of latent heat.
Heat loss: There may be a loss of temperature/heat to surroundings.
Step 6
State two desirable properties of the fluid used in a heat pump.
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High (specific) latent heat of vaporisation
Low boiling point / volatile / low molecular mass
Step 7
Calculate the mass of fluid that has evaporated.
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Answer
The mass of the fluid vaporised can be calculated using the specific latent heat:
Q=mL
Where:
Q = 12 kJ = 12000 J
L = 4.6 ext{ MJ/kg} = 4600000 ext{ J/kg}
Thus:
m = rac{Q}{L} = rac{12000}{4600000} = 0.0026 ext{ kg}
Step 8
Calculate the fall in temperature of the air in the refrigerator.
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Answer
To find the fall in temperature, we use the formula:
Q=mcΔθ
Where:
Q = 12 kJ = 12000 J
m = 0.74 ext{ kg}
c = 1005 ext{ J/kg K}
Therefore:
Δθ = rac{Q}{mc} = rac{12000}{0.74 imes 1005} = 16.1 °C
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