quiz 5

# quiz 5 - Physics 2Ba quiz 5 solutions(Dated 13:13 Problem 1...

This preview shows pages 1–2. Sign up to view the full content.

Physics 2Ba quiz 5 solutions (Dated: February 14, 2009 13:13) Problem 1 A copper wire of length L and radius b is attached to another copper wire of length L and radius 2 b , forming one long wire of length 2 L . This long wire is attached to a battery, and a current is ﬂowing through it. Relative to the electric ﬁeld within the wire of radius b , the magnitude of the electric ﬁeld within the wire of radius 2 b is A) two times stronger. B) four times stronger. C) two times weaker. D) four times weaker. E) equal. The correct answer is D . The current through each wire is the same and so we have I 1 = J 1 A 1 = J 2 A 2 = I 2 ± ± σ 1 E 1 A 1 = ± ± σ 2 E 2 A 2 . In the last line, we used the fact that material in each wire is the same, therefore they have the same conductivity. The ratio of the electric ﬁelds is E 2 = E 1 A 1 A 2 = E 1 πb 2 4 πb 2 = 1 4 E 1 Problem 2 The ﬁgure below shows a steady electric current passing through a wire with a narrow region. What happens to the drift velocity of the moving charges as they go from region A to region B and then to region C? A) The velocity increases all the time. B) The velocity decreases all the time. C) The velocity decreases from A to B and increases from B to C. D) The velocity increases from A to B and decreases from B to C. E) The velocity remains constant. The correct answer is D . The current through any part of the wire must be the same and since each region is made up of the same material, the free electron density, n e , is the same. We know that I = JA , where J is the current density and A is the cross-sectional area. As we move from region

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

## This note was uploaded on 03/28/2009 for the course PHYS 2b taught by Professor Schuller during the Winter '08 term at UCSD.

### Page1 / 3

quiz 5 - Physics 2Ba quiz 5 solutions(Dated 13:13 Problem 1...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online