Physics (AS & A Level) · Feb / March 2017 · Paper 12 · Question 34

A coil contains N turns of insulated copper wire wound on to a cylindrical iron core of diameter D. The copper wire has a diameter d. The resistivity of copper is ρ. Diameter D is much greater than diameter d. What is the total resistance between the two ends of the coil?

1. A\frac{4N\rho D}{d^2}
2. B\frac{4N\rho d}{D^2}
3. C\frac{8N\rho D}{d^2}
4. D\frac{8N\rho d}{D^2}

As printed in the paper