PAG5: Measuring Energy Changes

Specific heat capacity relates the energy transferred to a material to its mass and temperature change: E = mcΔT. Measure the mass of the material being investigated and record an initial and final temperature. Use the difference, not the final temperature alone. A temperature rise of 1 °C has the same size as a rise of 1 K; the two temperature scales do not have the same zero.

In an electrical-heating investigation, a suitable joulemeter measures transferred energy. Alternatively, steady measured potential difference and current give an input estimate E = VIt over the measured time. Identify where the energy meter is connected and what its reading includes. A heater’s nameplate power is not automatically its actual operating power, and the energy supplied to the apparatus is not all necessarily stored in the chosen material.

For a suitable metal block, a heater and temperature probe need good contact with the block, using the approved arrangement. Insulation reduces transfer to the surroundings. A poorly coupled probe can lag behind the material temperature. Keep the apparatus stationary and wait for an appropriate consistent reading rather than selecting a convenient peak without explaining the timing.

For water, measure water mass rather than including the vessel by accident. The approved heater must remain suitably immersed, and gentle mixing can reduce temperature differences within the liquid. A lid and insulation reduce unwanted transfers, but do not make the system perfectly isolated. State whether vessel and heater warming are neglected or accounted for in the model. Follow the centre’s electrical and hot-apparatus precautions.

If some measured input heats the surroundings or apparatus, the material receives less than the stated input. Dividing the full input by the material’s measured mΔT can therefore give an apparent heat capacity that is too high. Derive the direction from the equation instead of saying only “heat loss affects accuracy”. A small measured temperature rise also makes a fixed thermometer uncertainty a larger fraction of the result.

A series of energy and temperature readings can reveal an initial thermal lag or a changing heating rate. The gradient of input energy E against ΔT gives an apparent mc under the energy-balance approximation; the axes matter, and continuing losses can bias this slope even if their rate is steady. Compare repeated determinations after an appropriate reset and describe a specific improvement, such as reducing an exposed area or improving thermal contact. Repeats do not remove the same systematic loss in every run.

Determining a metal’s specific heat capacity is the specification’s example for PAG5, which concerns suitable energy-change/transfer measurements and associated quantities such as work done. Use the measured mass, energy and temperature change to explain the chosen example and its limitations.

Other suitable energy activities can contribute across the course, such as measuring electrical energy used to lift a known load through a measured height. For that example, compare measured input with the useful increase mgh; define the system, measurement methods and transfer losses. The group is not restricted to one universally mandated metal-block recipe.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.