A 3.00 m length of copper wire at 20 degrees Celsius has a 1.20 m long section w

Question

A 3.00 m length of copper wire at 20 degrees Celsius has a 1.20 m long section with diameter 1.4 mm and a 1.80 m long section with diameter 0.70 mm . There is a current of 2.1 mA in the 1.4 mm diameter section. What is the potential difference between the ends of the 3 m length of wire? (express in 2 significant figures) What I have already solved for: (a) Current in 0.7 mm section = 2.1 mA (b) Electric field in 1.4 mm diameter section = 2.3*10^-5 V/m (c) Electric field in 0.7 mm diameter section = 9.4*10^-5 V/m I have already tried to solve this using the formula: Delta V = (E1 + E2)*d When I did so, I got an answer of 3.5*10^-4 V and the system counted it wrong. Please help me figure out what I am doing wrong!

Explanation / Answer

E=V/d ==IR/L =I(pL/A)/L =Ip/A

b)

E1=(2.1*10^-3)*(1.67*10^-4)/pi*(1.4*10^-3/2)^2

E1=0.2278V

c)

E2=(2.1*10^-3)*(1.67*10^-4)/pi*(0.7*10^-3/2)^2

E2=0.9113 V

c)

delta(V)=(E1+E2)d =(0.2278+0.9113)*3*10^-3

delta(V)=3.4*10^-3 V or 3.4mV

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