Methods

Measuring the Latent

Heat of Vaporisation of Liquid Nitrogen:

For this part of the experiment

we only knew the mass of the Nitrogen from equation 1 but if we take the

differential of both sides with respect to time we can get:

(3)

Where

is

the rate at which heat is transferred to the liquid Nitrogen’s surroundings and

is

the rate at which mass of the Nitrogen is lost due to this heat transfer.

Finding the rate of heat transfer would be very difficult and an easier

variable to measure would be to use an electrical heater with a known power to

disperse energy into the nitrogen. This turns equation 3 into:

(4)

Where P is the power of

the electrical heater and

is

the rate of mass lost due to the heat transfer caused by the heater. Now

subtracting equations 4 from 3 and rearranging we come to the final equation

needed for finding the latent heat of vaporisation of nitrogen:

(5)

P can be measured from

the current and voltage of the heater with the simple relation P=VI. So the

latent heat of nitrogen can be found by measuring the rate of mass loss first caused

by the surroundings and then by adding the heater to the liquid nitrogen and

measuring the new rate of mass loss.

Figure 1: Latent heat Calorimeter

Digital Scales

KERN EW4200-2NM

Liquid Nitrogen

Heating Coil

Lascar psu 130

Dewar

flask

Cork

Wires

The

apparatus was set up as shown in figure 1 and the scales were zeroed with the

flask on it so that we would only be measuring the mass of the liquid nitrogen.

The flask was filled to approximately half way with liquid nitrogen and then it

was left for at least three minutes so that the flask and the nitrogen could

reach thermal equilibrium. Then measurements of the decreasing mass on the

scales were taking using a computer program to record the readings from the

scale roughly 4 times a second. Once sufficient data had been taken for the mass

loss due to the surroundings, 5-10 minutes is more than enough data points, it

was time to add the heating coil to the nitrogen, simply inserting the

electrical heater through the cork hole and setting the power on the heater,

again taking measurements for 5-10 minutes. As only the rate of mass loss was

being measured it was not of concern that the coil may add some weight to the

scales. The coil was added during the recording of measurements so we would be

able to see the change in rate of mass loss on one graph. Two electrical power

settings were used in this experiment: 10V and 5V so when the first run of

measurements finished, the flask was replenished with liquid nitrogen to

roughly the same level and the process was repeated