Fall ’12 PHY 122
Homework Solutions #7
HW7: Ch. 26 P 34, 36 Ch.27 Q 2, 4, 8,
18 P 2, 8, 17, 19, 37
Chapter 26 Problem 34
Determine the magnitudes and directions of the currents in each
resistor shown in Fig. 26–52. The batteries have emfs of E1 = 9.0V
and E2 = 12.0V and the resistors have values of R1=25Ω, R2 = 48Ω
and R3=35Ω. (a) Ignore internal resistance of the batteries. (b)
Assume each battery has internal resistance
r=1.0Ω.
Chapter 26 Problem 36
(a)Determine the currents I1,I2 and I3 in Fig. 26–53. Assume the
internal resistance of each battery is r=1.0Ω (b) What is the terminal
voltage of the 6.0-V battery?
Chapter 27 Question 2
Draw the magnetic field lines around a straight section of wire
carrying a current horizontally to the left.
Solution
The magnetic field lines are concentric circles around the wire.
With the current running to the left, the field is directed
counterclockwise when looking from the left end. So, the field
goes into the page above the wire and comes out of the page
below the wire.
Chapter 27 Question 4
In the relation F=I l x B which pairs of the vectors (F, l,B) are always
at 90°? Which can be at other angles?
Solution
F is always perpendicular to both B and l. B and l can be at any
angle with respect to each other.
Chapter 27 Question 8
A positively charged particle in a nonuniform magnetic field follows
the trajectory shown in Fig. 27–35. Indicate the direction of the
magnetic field at points near the path, assuming the path is always in
the plane of the page, and indicate the relative magnitudes of the
field in each region.
Solution
In the areas where the particle’s path is curving up towards the top
of the page, the magnetic field is directed into the page. Where the
particle’s path curves downward towards the bottom of the page,
the magnetic field is directed out of the page. Where the particle is
moving in a straight line, the magnetic field direction is parallel or
anti-parallel to the particle’s velocity. The strength of the magnetic
field is greatest where the radius of curvature of the path is the
smallest.
Chapter 27 Question 18
How could you tell whether moving electrons in a certain region of
space are being deflected by an electric field or by a magnetic field
(or by both)?
Solution
If the direction of the velocity of the electrons is changing but their
speed is not, then they are being deflected by a magnetic field only,
and their path will be circular or helical. If the speed of the electrons
is changing but the direction is not, then they are being accelerated
by an electric field only. If both speed and direction are changing,
the particles are possibly being deflected by both magnetic and
electric fields, or they are being deflected by an electric field that is
not parallel to the initial velocity of the particles. In the latter case,
the component of the electron velocity antiparallel to the field
direction will continue to increase, and the component of the
electron velocity perpendicular to the field direction will remain
constant. Therefore, the electron will asymptotically approach a
straight path in the direction opposite the field direction. If the
particles continue with a circular component to their path, there
must be a magnetic field present.
Chapter 27 Problem 2
Calculate the magnitude of the magnetic force on a 240-m length of
wire stretched between two towers and carrying a 150-A current.
The Earth’s magnetic field of 5.0x10-5T makes an angle of 68° with
the wire.
Chapter 27 Problem 8
A long wire stretches along the x axis and carries a 3.0-A current to
the right (+x). The wire is in a uniform magnetic field B = (0.20i –
0.36j + 0.25k)T. Determine the components of the force on the wire
per cm of length.
Chapter 27 Problem 17
Determine the direction of B for each case in Fig. 27–43, where F
represents the maximum magnetic force on a positively charged
particle moving with velocity v.
Chapter 27 Problem 19
A doubly charged helium atom whose mass is 6.63 x 10-27 kg is
accelerated by a voltage of 2700 V. (a) What will be its radius of
curvature if it moves in a plane perpendicular to a uniform 0.340-T
field? (b) What is its period of revolution?
Chapter 27 Problem 37
A circular coil 18.0 cm in diameter and containing twelve loops lies
flat on the ground. The Earth’s magnetic field at this location has
magnitude 5.50 x 10-5 T and points into the Earth at an angle of
66.0° below a line pointing due north. If a 7.10-A clockwise current
passes through the coil, determine (a) the torque on the coil, and (b)
which edge of the coil rises up, north, east, south, or west.