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High-frequency signals are often transmitted along a coaxial cable, such as the one shown in FIGURE P26.66. For example, the cable TV hookup coming into your home is a coaxial cable. The signal is carried on a wire of radius R1while the outer conductor of radius R2is grounded (i.e., at V=0V). An insulating material fills the space between them, and an insulating plastic coating goes around the outside.

a. Find an expression for the capacitance per meter of a coaxial cable. Assume that the insulating material between the cylinders is air.

b. Evaluate the capacitance per meter of a cable having R1=0.50mmand R2=3.0mm.

Short Answer

Expert verified

a. The expression for the capacitance per meter of a coaxial cable is 20lnR2R1.

b. The capacitance of a cable is 31pF/m.

Step by step solution

01

Given Information (Part a)

The signal is carried on a wire of radius R1andR2

Voltage=0V

02

Explanation (Part a)

a. Applying Gauss's law for the cylinder with linear charge density , to get electric field outside of a cylinder as

E2rL=Q0E(r)=Q20Lr

Potential difference for two cylinders will be

V=R1R2E(r)dr=Q20LR1R2drr=Q20LlnR2R1

Find the capacitance per unit length as

CL=QLV=20lnR2R1

Hence, the expression for the capacitance per meter of a coaxial cable is=20lnR2R1

03

Given Information (Part a)

The signal is carried on a wire of radius R1and R2

Voltage=0V

04

Explanation (Part b)

In the given numerical case, we will have

CL=20lnR2R1

Substitute the values,

localid="1648629781694" CL=28.8510-12ln3mm0.5mm

Multiply the expression,

=3.110-11F/m

Hence, the capacitance of a cable is31pF/m

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