Download Physics 10 Chapter 24 HW Solutions

Physics 10 Chapter 11 HW Solutions
E: 6, 9, 12, 16, 20, 22, 26, 30, 34, 38, 43, 44
P: 1, 2, 6
Exercises
6. A magnet will induce the magnetic domains of a nail or paper clip into alignment. Opposite poles
in the magnet and the iron object are then closest to each other and attraction results (this is similar
to a charged comb attracting bits of electrically neutral paper). A wooden pencil, on the other hand,
does not have magnetic domains that will interact with a magnet.
9. The net force on a compass needle is zero because its north and south poles are pulled in opposite
directions with equal forces in the Earth’s magnetic field. When the needle is not aligned with the
magnetic field, then a pair of torques is produced that turn the needle so that the domains align with
the external magnetic field.
12. The wire is aligned with the magnetic field. For a force to act on a current-carrying wire in a
magnetic field, the wire must be at a nonzero angle to the field. Maximum force occurs when the
wire is at 90 degrees to the field.
16. Just as a nail is magnetized by beating on it, an iron ship is beat upon in its manufacture, making
it a permanent magnet. Its initial magnetic field orientation, which is a factor in subsequent magnetic
measurements, is in effect recorded on the brass plaque.
20. When we write work = force × distance, we really mean the component of force in the direction
of motion multiplied by the distance moved (Chapter 5). Since the magnetic force that acts on a
beam of electrons is always perpendicular to the beam, there is no component of magnetic force
along the instantaneous direction of motion. Therefore a magnetic field can do no work on a charged
particle. (Indirectly, however, a time-varying magnetic field can induce an electric field that can do
work on a charged particle.)
22. Charged particles moving through a magnetic field are deflected most when they move at right
angles to the field lines, and least when they move parallel to the field lines. If we consider cosmic
rays heading toward the Earth from all directions and from great distance, those descending toward
northern Canada will be moving nearly parallel to the magnetic field lines of the Earth. They will not
be deflected very much, and secondary particles they create high in the atmosphere will also stream
downward with little deflection. Over regions closer to the equator like Mexico, the incoming
cosmic rays move more nearly at right angles to Earth’s magnetic field, and many of them are
deflected back out into space before they reach the atmosphere. The secondary particles they create
are less intense at the Earth’s surface. (This “latitude effect” provided the first evidence that cosmic
rays from outer space consist of charged particles—mostly protons, as we now know.)
26. Yes, each will experience a force because each is in the magnetic field generated by the other.
Interestingly, currents in the same direction attract, and currents in opposite directions repel.
30. Part of the Earth’s magnetic field is enclosed in the wide loop of wire imbedded in the road. If
this enclosed field is somehow changed, then in accord with the law of electromagnetic induction, a
pulse of current will be produced in the loop. Such a change is produced when the iron parts of a car
pass over it, momentarily increasing the strength of the field. A practical application is triggering
automobile traffic lights. (When small ac voltages are used in such loops, small “eddy currents” are
induced in metal of any kind that passes over the loop. The magnetic fields so induced are then
detected by the circuit.)
34. Agree with your friend. Any coil of wire spinning in a magnetic field that cuts through magnetic
field lines is a generator.
38. Induction occurs only for a change in the intercepted magnetic field. The galvanometer will
display a pulse when the switch in the first circuit is closed and current in the coil increases from
zero. When the current in the first coil is steady, no current is induced in the secondary and the
galvanometer reads zero. The galvanometer needle will swing in the opposite direction when the
switch is opened and current falls to zero.
43. The voltage impressed across the lamp is 120 V and the current through it is 0.1 A. The first
transformer steps the voltage down to 12 V and the second one steps it back up to 120V. The current
in the secondary of the second transformer, which is the same as the current in the bulb, is 1/10 of
the current in the primary, or 0.1 A.
44. By symmetry, the voltage and current for both primary and secondary are the same. So 12 V is
impressed on the meter, with a current of 1 A ac.
Problems
1. (120 V)/(500 turns) = (12 V)/x, so x = 50 turns.
2. (120 V)/(240 turns) = 6 V/x turns, so x = 12 turns.