Page 345 - Schaum's Outline of Theory and Problems of Applied Physics

330 MAGNETISM [CHAP. 27

SOLVED PROBLEM 27.19
A sample of cast iron exhibits a magnetic field of B = 0.50 T when the magnetic intensity is H = 10 A/m.
(a) Find the permeability of cast iron at this value of H. (b) What would the field be in air at this value
of H?
B 0.50 T
(a) µ = = = 0.05 T·m/A
H 10 A/m

T·m A
(b) B = µ 0 H = 4π × 10 −7 10 = 1.257 × 10 −5 T
A m
SOLVED PROBLEM 27.20
A solenoid 20 cm long is wound with 300 turns of wire and carries a current of 1.5 A. (a) What is the
magnetic intensity H inside the solenoid? (b) What should the permeability of the core at this value of
H be so that the magnetic field inside is 0.6 T? How many times greater than µ 0 is this?
B N 300
(a) H = = I = (1.5A) = 2250 A/m
µ L 0.2m
B 0.6T
(b) = = 2.67 × 10 −4 T·m/A
H 2250 A/m
µ 2.67 × 10 −4 T·m/A
= = 212
µ 0 4π × 10 −7 T·m/A

Multiple-Choice Questions

27.1. There is no interaction between a magnetic field and a
(a) stationary electric charge (c) stationary magnet
(b) moving electric charge (d) moving magnet

27.2. When an observer moves past a stationary electric charge, she detects
(a) only an electric field
(b) only a magnetic field
(c) both an electric and a magnetic field
(d) any of the above, depending on her velocity

27.3. Neglecting the earth’s magnetic field, the direction of the magnetic field below a power cable in which a current is
flowing north is
(a) north (c) south
(b) east (d) west

27.4. The magnetic field lines around a long, straight current are
(a) straight lines parallel to the current
(b) straight lines that radiate from the current like spokes of a wheel
(c) concentric circles around the current
(d) concentric helixes around the current

27.5. The magnetic field inside a current-carrying solenoid

(a)is 0 (c) decreases away from the axis
(b) is uniform (d) increases away from the axis