*Show all work -- you may receive partial credit!*

Liquid nitrogen: boils at T(boil) = -196 Celsius latent heat of vaporization = 2.00 x 10^5 J/kg latent heat of fusion = 2.57 x 10^4 J/kg Copper: thermal conductivity = 390 J/s*m*C

One must keep an electronic camera must be very cold to avoid noisy images. A camera is connected to a reservoir of liquid nitrogen to chill it. The liquid nitrogen is at its boiling temperature. A copper wire of diameter d = 10 millimeters runs from the reservoir to the camera. The camera is kept at temperature T(camera) = -100 Celsius.

**Question 1:**
Heat flows from the camera at a rate Z = 2.9 Joules/sec.
What is the length of the wire?

Answer: The cross-section area of the wire is A = pi*r^2 = pi*(0.005 m)^2 = 7.85 x 10^(-5) m^2 The rate of heat transfer through the wire is Q A*(delta T) --- = k ----------- t L Solve for length of wire, L: t L = --- * k*A*(delta T) Q 1 = ---------- *(390 J/s*m*C)*(7.85 x 10^(-5) m^2)*(96 C) 2.9 J/s = 1.0 m

**Question 2:**
How much energy flows into the nitrogen reservoir
every day?

Answer: J s hour 2.9 --- * 3600 --- * 24 ---- = 2.5 x 10^5 J/day s hour day

**Question 3:**
What mass of liquid nitrogen boils off per day?

Answer: Heat required to boil m kilograms of liquid is Q = m*(latent heat of vaporization) = m*Lv Solve for mass m: Q 2.5 x 10^5 J/day m = --- = ------------------ = 1.25 kg/day Lv 2.0 x 10^5 J/kg

*
This page maintained by Michael Richmond.
Last modified Jan 13, 1998.
*

Copyright © Michael Richmond. This work is licensed under a Creative Commons License.