lesson
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If you supply the exact same 1,000 joules of electrical energy to a 1 kg block of aluminum and a 1 kg block of lead, the lead heats up nearly seven times hotter. Different materials store thermal energy differently, and measuring that difference accurately in a lab is one of the most tested skills in physics.
The specific heat capacity (c) is the amount of energy needed to raise the temperature of 1ย kg of a substance by 1โC (or 1ย K). We calculate it using ฮE=mcฮฮธ, where ฮE is thermal energy in joules (J), m is mass in kilograms (kg), and ฮฮธ is temperature change in degrees Celsius (โC).
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How do we wire an experiment to measure each of these variables without letting energy escape undetected?
The Required Apparatus
A standard setup uses a 1ย kg metal cylinder with two drilled boreholes: one for an immersion heater and one for a temperature probe or thermometer. We measure electrical energy input directly using a digital joulemeter, or calculate it from a voltmeter, ammeter, and stopwatch using E=VIt.
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Air is a poor thermal conductor, so adding a few drops of oil or water into the thermometer borehole eliminates insulating air gaps and ensures good thermal contact. Wrapping the block in thick foam and resting it on an insulating mat prevents thermal energy from escaping into the bench and air.
Taking a single start and end reading can hide heating delays, so how do we extract the most reliable value of c from continuous data?
Graphical Analysis
By recording temperature every 60ย s as energy increases, we can plot a graph of temperature (ฮธ) on the y-axis against work done (E) on the x-axis. Rearranging ฮฮธ=mc1โฮE shows that the linear portion of this graph has a gradient of mc1โ.
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Even with insulation, what happens when the block gets significantly hotter than room temperature?