Deck 13: Magnetic Fields Due to Currents

Question 1

In an overhead straight wire, the current is north. The magnetic field due to this current, at our point of observation, is:

A) east
B) up
C) north
D) down
E) west

Answer 1

west

Question 2

The magnetic field outside a long straight current-carrying wire depends on the distance R from the wire axis according to:

A) R
B) 1/R
C) 1/R²
D) 1/R³
E) 1/R^3/2

Answer 2

1/R

Question 3

Two parallel wires carrying equal currents of 10 A attract each other with a force of 1 mN. If both currents are doubled, the force of attraction will be:

A) 1 mN
B) 4 mN
C) 0.5 mN
D) 0.25 mN
E) 2 mN

Answer 3

4 mN

Question 4

Two parallel long wires carry the same current and repel each other with a force F per unit length. If both these currents are doubled and the wire separation tripled, the force per unit length becomes:

A) 2F/9
B) 4F/9
C) 2F/3
D) 4F/3
E) 6F

Question 5

A constant current is sent through a helical coil. The coil:

A) tends to get shorter
B) tends to get longer
C) tends to rotate about its axis
D) produces zero magnetic field at its center
E) none of the above

Question 6

In Ampere's law, the symbol is:

A) an infinitesimal piece of the wire that carries current i
B) in the direction of
C) perpendicular to
D) a vector whose magnitude is the length of the wire that carries current i
E) none of the above

Question 7

A long straight wire carrying a 3.0 A current enters a room through a window 1.5 m high and 1.0 m wide. The path integral around the window frame has the value (in T.m

A) 0.20
B) 2.5x10^-7
C) 3.0x 10^-7
D) 3.8 x10^-6
E) none of these

Question 8

The magnetic field at any point is given by where is the position vector of the point and A is a constant. The net current through a circle of radius R, in the xy plane and centered at the origin is given by:

A) $\pi$ AR²/ $\mu$ ₀
B) 2 $\pi$ AR/ $\mu$ ₀
C) 4 $\pi$ AR³/3 $\mu$ ₀
D) 2 $\pi$ AR²/ $\mu$ ₀
E) $\pi$ AR²/2 $\mu$ ₀

Question 9

The magnetic field B inside a long ideal solenoid is independent of:

A) the current
B) the core material
C) the spacing of the windings
D) the cross-sectional area
E) the direction of the current

Question 10

Solenoid 2 has twice the radius and six times the number of turns per unit length as solenoid 1. The ratio of the magnetic field in the interior of 2 to that in the interior of 1 is:

A) 2
B) 4
C) 6
D) 1
E) 1/3

Question 11

If the radius of a pair of Helmholtz coils is R then the distance between the coils is:

A) R/4
B) R/2
C) R
D) 2R
E) 4R

Question 12

If R is the distance from a magnetic dipole, then the magnetic field it produces is proportional to:

A) R
B) 1/R
C) R²
D) 1/R²
E) 1/R³