A television set uses
fast-moving electrons inside its picture tube to excite phosphors on the inside
surface of its glass screen. These phosphors then emit light. To get the
electrons moving quickly through the picture tube, the television puts positive
charge on the inside surface of the screen, near the phosphors. The negatively
charged electrons are attracted toward the positively charged screen and
collide with the phosphors at high speed, causing the phosphors to emit light.
This scheme works well and allows you to see a bright image. But putting
positive charge on the inside surface of the glass screen has some interesting
effects on the outside surface of the screen--the part that you can touch.
1. Having the inside surface of the screen positively charged causes charges outside of the television to approach and stick to the outer surface of the screen. What type of charge (positive or negative) sticks to the outside of the screen? Why?
Answer: Negative charge sticks to the outside of the screen.
Why: Because of the large positive charge inside the screen's
glass surface, any negative charges in the air outside the television set are
attracted to the screen's outer surface, where they stick. Any positive charges
in the air are repelled.
2. After the television set has been on for a while, you’ll find that dust from the room is stuck all over its surface. Dust is ordinarily electrically neutral, so why is it attracted to positive charge inside the screen?
Answer: The dust becomes electrically polarized and sticks even
though it is electrically neutral.
Why: Although the dust may not acquire any net charge, the
presence of positive charge inside the glass screen polarizes the
dust--shifting the dust's negative charge toward the screen and the dust's
positive charge away from the screen. This polarization of the dust allows the
dust to be attracted toward the positive charge inside the screen so that the
dust sticks to the outer surface of the screen.
3. The screen’s internal positive charge produces an electric field in front of the television set. In which direction does that electric field point?
Answer: The electric field points directly away from the glass
screen (and into the room in front of the television set).
Why: An electric field points in the direction that a positive
test charge would accelerate if it were at that point in space. Since the
screen harbors positive charge, a positive test particle would be repelled by
the screen. Thus the screen's electric field points away from the screen.
4. When you turn the television set off, it removes the positive charge from the inside of the screen. However, all of the charge that accumulated on the outside surface of the screen while the set was on (see question 1) remains there. Does the outside surface of the screen have a positive, zero, or negative voltage? Why?
Answer: The outside surface of the screen has a negative
voltage because it has an accumulation of negative charges and it would take
work to remove a positive charge from among these negative charges.
Why: While the set was on, it attracted negative charges to the
outside surface of the screen. Once the set was turned off, those negative
charges are still there, but they are no longer held in place by the positive
charges inside the screen. They push each other away. The voltage of the
screen's outer surface is now negative because you would have to do work to
remove a unit of positive charge from this accumulation of negative charge.
Alternatively, the accumulated negative charge has a positive amount of
electrostatic potential energy but it has a negative overall charge, so its
energy per unit of charge is a negative number, hence its voltage is negative.
An interesting novelty toy
consists of a small neon lamp with a metal knob on each end. The lamp is a
glass tube containing neon gas, with two sharp metal wires pointing toward one
another and separated by a small gap. In this toy, each of the neon lamp’s
wires is connected to a knob. You rub your feet on the carpet to build up a
static charge and hold one knob in your hand as you approach a metal pipe or
other large metal object. About the time the second knob touches the metal
object, the neon lamp glows brightly for a second or two. This process uses up
your static charge so you have to rub your feet a second time before trying to
make light again.
5. Suppose that you accumulate a large positive charge by rubbing your feet on a particular carpet. Is your voltage positive, negative, or zero? Why?
Answer: Your voltage is positive because work would be done on
a unit of positive charge leaving you (or equivalently, the charge on you is a
positive amount and has a positive amount of electrostatic potential energy,
for a positive amount of energy per unit of charge).
Why: As you accumulate positive charge, your electrostatic
potential energy rises and each unit of positive charge on you has a positive
amount of electrostatic potential energy. Your voltage is positive. Moreover,
if a unit of charge were to leave you, the electrostatic forces would do work
on it to push it away, so again your voltage is positive.
6. As the second knob touches the metal pipe, which way does the electric field point inside the neon lamp?
Answer: The electric field points from you toward the pipe.
Why: The electric field points in the direction that a positive
test charge would accelerate if it were in that point in space. Since you are
positively charged and the pipe is not, the test charge would accelerate toward
the pipe and the electric field thus points toward the pipe.
7. Neon is a gas and doesn’t normally conduct electricity. What effect causes electric charge to enter the gas and make it conduct electricity?
Answer: A corona discharge.
Why: When lots of positive charge accumulates on the metal point
inside the neon lamp, the individual positive charges push one another off the
metal and into the neon gas. The neon gas becomes electrically charged and
accelerates toward the opposite metal point. Collisions during this charge
transport process cause the neon atoms to receive energy and emit light.
8. You and your twin sibling are both wearing identical clothes and shoes and both rub your feet equally hard on the carpet. The two of you then grasp opposite knobs of the lamp. Will the lamp light up? Why or why not?
Answer: The lamp will not light up because you and your twin
have the same charges.
Why: Since you and your twin are so identical, you both accumulate
exactly the same charges and the same voltages. Positive charge moving from you
to your twin or vice versa won't experience any change in electrostatic
potential energy, so it won't tend to flow between you. The lamp won't light up
because no charge will flow.