tp 13 specific heat capacity (shared)
DESCRIPTION
TRANSCRIPT
Thermal physics
A-level Physics
Unit G484: The Newtonian World
Specific heat capacity
Thermal physics
Questions
1. Define internal energy (U).
2. Explain how internal energy can be changed and write an
expression linking U to these mechanisms.
3. Imagine I have two blocks of aluminium and supply 1000 J of
thermal energy to each. The rise in temperature of the two blocks
is different. What is the simplest explanation for this?
4. I have two pieces of metal with the same mass. 1000 J of thermal
energy is supplied to both but, again, the temperature increases
are different. Why?
Internal energy recall LOs
Thermal physics
Learning objectives
At the end of the lesson you will be able to:
• define and apply the concept of specific heat capacity;
• select and use the equation E = mcΔθ ;
• describe an electrical experiment to determine the specific heat capacity of a solid or liquid.
Lesson focus• Specific heat capacity
Thermal physics
Learning outcomes
All of you should be able to• give a full definition of the SHC of a substance;• recall the equation for SHC;• carry out the practical activity efficiently and safely.
Most of you will be able to• identify significant sources of experimental error;• suggest appropriate improvements to the experimental method.
Thermal physics
Definition
The amount of energy needed to raise the temperature of 1 kg of a substance by one degree celsius (or one kelvin).
LO 1: define and apply the concept of specific heat capacity
Specific heat capacity LOs
i.e. E = mc Δθ
hence, c = where, c -specific heat capacity
m - massΔE -
energy suppliedΔθ -
temperature change
Thermal physics
i.e. E = IVt E = mcΔθ
IVt = mcΔθ
c =
electrical energy supplied
= energy gained by water
We need to know
• The energy supplied (E)
• The mass of water (m)
• The temperature change (Δθ)
EmΔθ
LO 3: describe an electrical experiment to determine the specific heat capacity of a solid or liquid.
Measuring the SHC of water LOs
Thermal physics
1. Insert a thermometer and the immersion heater into their respective holes in the block. You may wish to drop a small amount of oil into the thermometer hole to improve the thermal contact between thermometer and block.
2. Allow the thermometer to reach thermal equilibrium and then write down the temperature.
3. Set up a suitable circuit that will enable you to measure the energy input to the heater.
solid block
lagging
immersion heater
thermometer
LO 3: describe an electrical experiment to determine the specific heat capacity of a solid or liquid.
Measuring the SHC of a metal LOs
Thermal physics
4. Turn on the current, noting the time if you are measuring energy using an ammeter and a voltmeter to record power.
5. Energy = current x potential difference x time. Monitor and note the meter readings as the energy is supplied. They may change slightly as things warm up.
6. Allow the block to heat up by about 10 °C, then turn off the current and note the time again.
7. At this point, keep watching the thermometer. The temperature at the turn-off time is not the appropriate final temperature to record. What do you think is the appropriate final temperature?
solid block
lagging
immersion heater
thermometer
LO 3: describe an electrical experiment to determine the specific heat capacity of a solid or liquid.
Measuring the SHC of a metal LOs
Thermal physics
8. Use E = m c Θ to calculate the specific
thermal capacity, c, of your block.
Compare your answer with a data book
value.
9. Was your calculated value of specific
thermal capacity too high or too low?
Which of the measurements you made
is likely to be the one most in error? In
which direction is it in error, and why
might this be?
solid block
lagging
immersion heater
thermometer
LO 3: describe an electrical experiment to determine the specific heat capacity of a solid or liquid.
Measuring the SHC of a metal LOs
Thermal physics
solid block
lagging
immersion heater
thermometer
Analysis
• Plot a graph to find c.
• Plot a second graph of temperature (y-axis) vs time (x-axis). Comment on the shape of the graph. What does the graph tell you?
• Compare your experimental value of c with the published value. Is it too high or too low? Explain the reason for the difference.
Evaluation
• Identify possible sources of error. Which of these errors is the most important?
LO 3: describe an electrical experiment to determine the specific heat capacity of a solid or liquid.
Measuring the SHC of a metal LOs
Thermal physics
Time/s Current/A Voltage/V Temperature/°C Total energy supplied/J
Temperature change () / °C
If E = m c q what graph should you plot to find c?
LO 3: describe an electrical experiment to determine the specific heat capacity of a solid or liquid.
Measuring the SHC of a metal LOs
Thermal physics
solid block
lagging
immersion heater
thermometer
LO 3: describe an electrical experiment to determine the specific heat capacity of a solid or liquid.
Analysis
• Plot a graph to find c.
• Plot a second graph of temperature (y-axis) vs time (x-axis). Comment on the shape of the graph. What does the graph tell you?
• Compare your experimental value of c with the published value. Is it too high or too low? Explain the reason for the difference.
Evaluation
• Identify possible sources of error. Which of these errors is the most important?
Measuring the SHC of a metal LOs
Thermal physics
Using ΔE = mcΔΘ LOs
LO2: select and use the equation E = mcΔθ
G482 Jan., 2010 Q5
Extension
In measurements of the SHC of a metal, energy losses to the surroundings are a source of error. Is this a systematic or random error? Justify your answer.
Thermal physics
LO2: select and use the equation E = mcΔθ
Examiners’ report
Using ΔE = mcΔΘ LOsG482 Jan., 2010 Q5
Thermal physics
LO2: select and use the equation E = mcΔθ
Oxford University entrance exam for physics, 2008
Using ΔE = mcΔΘ LOs
Thermal physics
Oxford University entrance exam for physics, 2008