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Here are the 12 ideas covered by the MC questions.
(in no particular order)
1. When a free electron passes through a wire in
a DC circuit, the electric potential energy of
the electron at the start of the wire has been
converted into thermal energy.
2. It takes a few hours for a free electron to
pass through 1.0 m of metal wire in an ordinary
DC circuit.
3. Electric utility companies tranmit electric power
at high voltages to reduce energy loss due to
thermal energy production in the transmission
lines; the high voltage means low current in the
transmission line and thus less thermal energy
production.
4. A capacitor is charged by moving negative charges
from one plate to the other; the plate from which
the negative charges are taken becomes positive.
5. In a simple DC circuit with a battery, two wires,
and a bulb, the voltage drop across either wire is
much smaller than the voltage drop across the bulb.
6. For a current coming out of the page, the B field
makes counterclockwise loops about the current; so
if the current comes out in the upper right-hand
corner, then at the upper left hand corner the
field will be towards the bottom of the page.
7. In the figure, the B field is leftward on the
and the negative electron is going upwards on
page, so the magnetic force is into the page;
your left hand with fingers as B and thumb as
velocity.
page,
the
use
the
8. An air transformer, with the coils close together,
works because nature resists a CHANGING magnetic
field; so there will be a current in the secondary
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coil just after the switch in the primary is opened
or closed.
9. Both gamma rays and radio waves are EM waves, so
both travel at the speed of light.
10. After passing through a Polaroid grid with the
absorbing grid E and W, a northbound light beam
will have its E field alternately up and down,
and its B field alternately E and W (all E fields,
or parts of E fields, which were E and W will
have been absorbed, and the radiated B is always
perpendicular to the radiated E field -- both
are perpendicular to the direction of travel).
11. EM wave intensity diminishes with distance from
a source solely because the wave energy is spread
out over a larger area.
12. The combined masses of carbon12 and a free neutron
are larger than the mass of a carbon13 nucleus.
The nuclear potential energy of the neutron is much
less when the neutron is close to the rest of the
12 nucleons, so as to form carbon13; by E=mc^2,
less energy also means less mass.
Problem 1:
Range of motion of an electron in a 60 Hz
circuit.
This is part (a) of the final question of our
HW Activity "Electromagnetic Waves".
Find the time for the electrons to go back and forth once
by taking the reciprocal of the frequency (a simple unit
argument will do for reasoning). The electrons cover their
full range in half a cycle. Multiply the time for half a
cycle by the given average speed (a unit argument, or a
distance-equals-speed-times-time argument, will do for
reasoning).
Problem 2:
Amount of falling water needed to produce a given
amount of energy in city resistors.
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Essentially question 4 of our HW Activity "Induction".
Use Conservation of Energy. No engines are involved. At
the start the water is effectively at the top of its intake
chute, ready to fall. At the end, the water has fallen and
turned the generators, and has no more KE. All the electric
energy created by the turning generators has appeared as
thermal energy in the resistors of the city. Find the
amount of thermal energy using units (#heaters*(J/s)/heater*
#seconds). Setting the PE at the start equal to this
thermal energy allows you to solve for the mass of the
falling water.
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