Direct Current and Transient Analysis
151
Observe from Figure 2.49 that when R 3 = 250 Ω, I 2 = I 3 . Also observe that the current I
is the least affected, whereas the currents I 3 and I 2 are equally affected by the changes
in R 3 , over the range 0–500 Ω.
Example 2.10
Write a MATLAB program that returns in a tablelike format the resistance values (R)
versus length L of a copper wire with a diameter of √
____
300 mills (the cross-sectional area:
300 CM [circular mills]), over the range 50 ft ≤ L ≤ 500 ft in linear increments of 50 ft,
assuming that the temperature remains constant at 20°C.
ANALYTICAL Solution
Recall that the resistance R is given by
R
L
A
ϭ
rho *
(at a room temperature of 20°C)
where rho = ρ denotes resistivity in CM Ω/ft, and the rho of copper is 10.37 CM Ω/ft,
at 20°C; L defi nes its length in feet; A defi nes the cross-sectional area in CM (circular
mills); A (area of a circle) = pi * r
2
= π * d
2
/4; 1000 mills = 1 in. and π/4 sq. mills = 1 CM,
then if d is given in mills, A = d
2 (in CM).
MATLAB Solution
>> L = 50:50:500;
>> Rho =10.37;
>> A = 300;
>> R=Rho.*L./A;
>> disp(‘*************************’)
>> disp(‘Length (ft) Resit (Ohms)’); results = [L’ R’];
>> disp(‘*************************’)
>> disp(results)
>> disp(‘*************************’)
****************************
Length (ft)
Resit (Ohms)
****************************
50.0000
1.7283
100.0000
3.4567
150.0000
5.1850
200.0000
6.9133
250.0000
8.6417
300.0000
10.3700
350.0000
12.0983
400.0000
13.8267
450.0000
15.5550
500.0000
17.2833
****************************
Example 2.11
Write a program that returns in a tablelike format the resistance of a copper wire of
100 ft long with a fi xed cross-sectional area A = 1 CM as a function of temperature
variations (∆T), over the range 0–100°C, in linear increments of ∆T = 10°C.
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7/23/2008 1:38:48 PM