# Methods Nitrogen: For this part of the experiment we

Methods

Measuring the Latent
Heat of Vaporisation of Liquid Nitrogen:

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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

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