Measuring specific heat capacity
I'm doing an experiment on the thermal properties of metals. It involves a metal rod of copper that is pre-wrapped in insulation, and an electrical power source to supply a power transistor used to heat the rod. I measure how the temperature increases with time, and calculate the power supplied, to determine the specific heat capacity.
However, all of my measurements so far have been roughly twice as large as reference values. I asked the demonstrator assigned to my experiment and he said that a factor of 2 in this case "isn't too bad". I'm not sure how limited I am with the rods I've been given but how can I get more accurate results? I've wrapped the rod in more insulation and seen a slight improvement in values for specific heat capacity but not much. What else can I change about my experiment?
However, all of my measurements so far have been roughly twice as large as reference values. I asked the demonstrator assigned to my experiment and he said that a factor of 2 in this case "isn't too bad". I'm not sure how limited I am with the rods I've been given but how can I get more accurate results? I've wrapped the rod in more insulation and seen a slight improvement in values for specific heat capacity but not much. What else can I change about my experiment?
How can we answer that?
You haven't told us
-how you measured temperature
-how you measured the power supplied
-how you did the calculation
A factor of 2 (a 100% error) in an experiment has never, in all my experience, been ok.
What in heaven's name does the demonstrator mean?
-it's a crap experiment and everyone gets the same error?
You haven't told us
-how you measured temperature
-how you measured the power supplied
-how you did the calculation
A factor of 2 (a 100% error) in an experiment has never, in all my experience, been ok.
What in heaven's name does the demonstrator mean?
-it's a crap experiment and everyone gets the same error?
Original post by Stonebridge
How can we answer that?
You haven't told us
-how you measured temperature
-how you measured the power supplied
-how you did the calculation
A factor of 2 (a 100% error) in an experiment has never, in all my experience, been ok.
What in heaven's name does the demonstrator mean?
-it's a crap experiment and everyone gets the same error?
You haven't told us
-how you measured temperature
-how you measured the power supplied
-how you did the calculation
A factor of 2 (a 100% error) in an experiment has never, in all my experience, been ok.
What in heaven's name does the demonstrator mean?
-it's a crap experiment and everyone gets the same error?
Thanks for the reply.
Temperature was measured using a thermocouple that was already attached to the metal rod. Made a measurement of the temperature from the digital meter connected to the thermocouple every 30 seconds for around 6 or 7 minutes to get the rate of change of temperature.
Power supplied was from P = VI for the power source as I made a note of V and I before starting measurements and kept these constant. Then I calculated the energy supplied over the measurement period from Q = Pt. Using this, I calculated the specific heat capacity, c, from Q = mcdeltaT.
And yeah, I thought that too about the values. I'm not sure if he just meant that we're limited by recording techniques and equipment quality. The equipment seems decent though (it's a 2nd year experiment that's being carried out over a few weeks). As recommended background, we were asked to read up about some simple types of dislocations/defects in crystalline structures so I thought it might be related to that but he implied that the values I obtained for specific heat capacity weren't.
Quite simply, it is a duff experiment, but that's part of the reason for doing it. Your analysis of where the errors come in and analysing why you get a result which is different to the text book value is the real skill here,
Without seeing an image of the exact equipment you are using it will be hard to say where your errors are but for starters:
The heat loss through the insulation.
The parts of the metal block that aren't insulated (there will be some)
Energy used to heat the insulation.
Not to mention the inherent error in all pieces of equipment that you use and the outside chance that your thermocouple might not be correctly calibrated all need to be considered.
In some experiments an answer within an order of magnitude is acceptable.
Without seeing an image of the exact equipment you are using it will be hard to say where your errors are but for starters:
The heat loss through the insulation.
The parts of the metal block that aren't insulated (there will be some)
Energy used to heat the insulation.
Not to mention the inherent error in all pieces of equipment that you use and the outside chance that your thermocouple might not be correctly calibrated all need to be considered.
In some experiments an answer within an order of magnitude is acceptable.
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