The diagrams show five possible orientations of a magnetic dipole mu in a unifor

Question

The diagrams show five possible orientations of a magnetic dipole mu in a uniform magnetic field B. For which of these does the magnetic torque on the dipole have the greatest magnitude? A) III B) II C) IV D) I E) V An iron is marked "120 volt, 600 watt". What is the resistance of the iron? A) 0.2 Ohm B) 240 Ohm C) 5 Ohm D) 2 Ohm E) 24 Ohm A wire is 1 m long and 1 times 10^-6 m^2 in cross-sectional area. When connected to a potential difference of 3 V, a current of 5 A exists in the wire. The resistivity of this wire is: A) 1.0 times 10^-6 Ohm middot m B) 1.67 times 10^-7 Ohm middot m C) 3 times 10^-7 Ohm middot m D) 0.6 times 10^-6 Ohm middot m E) 5 times 10^-7 Ohm middot m

Explanation / Answer

6. Torque on magnetic dipole, t = mu x B [ where mu is the dipole moment and B is the magnetic frield]
hence the magnitude of the torque depends on the theta of the angle between the dipole and the magnetic field

so, Case 1 has the highest torque, at theta = 90 deg
so answer is part D) I

7. Given, V = 120 V
P = 600 W
now, P = VI
so, I = P/V = 600/120 = 5 A
but from ohms law
V = IR
R = V/I = 120/5 = 24 ohm s

8. length of wire, l = 1 m
cross sectional area, A = 10^-6 m^2
VOltage, V = 3 V
current, i = 5A
resistance of wire = R
R = rho*l/A { rho is resistivity of wire]}
but fro mohms law, V = iR
so, rho = AV/il = 10^-6*3/5*1 = 6*10^-7 mho

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