Download HW18 - University of St. Thomas

Physics 112 HW18/7
DUE 31 October 2014
ILB01. A straight, vertical wire carries a current of 1.20 A downward in a region between the poles of a large
superconducting electromagnet, where the magnetic field has magnitude B = 0.588 T and is horizontal. What
are the magnitude and direction of the magnetic force on a 1.00 cm section of the wire that is in this uniform
magnetic field, if the magnetic field direction is
a) East;
b) South;
c) 30o south of west?
ILB02. A thin, 50.0 cm long metal bar with mass 750 g and negligible resistance rests on, but is not attached to,
two metallic supports in a uniform 0.450 T field, as shown in the figure. A battery and a 25.0 Ω resistor in
series are connected to the supports.
a) Which point, a or b, should be the positive terminal of the battery in order for the magnetic force on the bar
to point up (opposite that of gravity)?
b) What is the largest voltage the battery can have without breaking the circuit at the supports?
c) The battery voltage has the maximum value calculated in part (a). If the resistor suddenly gets partially
short-circuited, decreasing its resistance to 2.0 Ω, find the initial acceleration of the bar.
a
battery
25.0 Ω
b
B = 0.450 T
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µ01. (Wolfson, Ch. 26 #38) A single-turn square wire loop 5.0 cm on a side carries a 450-mA current.
a) What is the magnetic dipole moment of the loop?
b) If the loop is in a uniform 1.4-T magnetic field with its dipole moment vector at 40° to the field direction,
what is the magnitude of the torque it experiences?
µ02. (Wolfson, Ch. 26 #39) An electric motor contains a 250-tum circular coil 6.2 cm in diameter. If it is to
develop a maximum torque of 1.2 N·m at a current of 3.3 A, what should be the magnetic field strength?
µ03. (Wolfson, Ch. 26 #56) A simple electric motor consists of a 100-tum coil 3.0 cm in diameter, mounted
between the poles of a magnet that produces a 0.12-T field. When a 5.0-A current flows in the coil, what are
a) its magnetic dipole moment and
b) the maximum torque developed by the motor?
µ04. A bar of cobalt 1 cm wide by 20 cm long is to feel a maximum torque of 10 N·m when placed in a 5 T
magnetic field.
a) Calculate how thick this bar must be in order to make this happen.
b) Each electron has an effective µ of 9.27 × 10-24 J/T. How many electrons must be lined up inside the cobalt
bar in order to produce this magnetic moment?
µ05. (Wolfson, Ch. 26 #79) A wire of length L carries a current I.
a) Find an expression for the magnetic dipole moment that results when the wire is wound into an N-turn
circular coil. NOTE: The more turns you have, the smaller the area enclosed.
b) For what integer value of N is this moment a maximum?