The heat capacity of a substance (sometimes called the specific heat ) is the amount of energy required to heat a unit mass of the substance through a unit change in temperature. It is common to cite the heat capacity of a substance in units of
energy/mass-temperature change
commonly expressed as
joules/kilogram-degree
or some similar system of units. The heat capacities of substances are important to know when examining the effects of gravitational collapse and radioactive decay on planetary melting and differentiation. In this experiment you will measure the heat capacity of water.
The heat capacity will be measured by adding a known amount of energy to the water electrically and measuring the rise in temperature. You will immerse an electrical resistor in water and apply a voltage across it, measuring both this voltage and the current through the resistor. The product of the voltage and the current is the power heating the resistor, and the power is the energy per unit time. Hence the energy put into heating the water is
energy = voltage * current * time
or
joules = volts * amps * seconds
Wire the circuit up according to the diagram on the blackboard. Adjust the rehostat so that the maximum resistance is in the circuit. Weigh the inner can of the calorimeter, then fill the can with cold water (10-15 degrees below room temperature) and weigh it again. There must be enough water to cover the heater resistor when the calorimeter is assembled.
Connect to the power supply briefly and adjust the rehostat so that the power, as indicated by the current and voltage readings, is between 8 and 12 watts. If this is not possible, consult the lab instructor. NEVER connect the power unless the heater resistor is under water!
With the power off, stir the water and record its temperature. Turn the power on and read the clock. As the temperature of the water increases (stir it occasionally), observe and record the voltage and current readings periodically- they might fluctuate as other equipment in the building is turned on and off. If you wish, adjust the rehostat as necessary to keep the readings constant.
When the water temperature is about 10 degrees above room temperature, turn off the power and note the time. Record the total time power was applied. Stir the water until the temperature reaches a maximum, and record the temperature.
From the average voltage and current readings and the time interval, compute the total energy delivered to the system. Knowing the temperature rise, use the heat capacity of the inner calorimeter can (stamped on it) to compute the amount of energy that went into heating the can. The rest of the energy went into heating the water. From it you can compute the heat capacity of the water.
Disconnect all the wires and pour out the water before you leave. Be sure to leave the thermometer in a safe place where it cannot roll onto the floor!