Air Cleaners and Xerox Machines 1
Electric Charges and Forces
What is electric charge? How do we measure it (units)?
o It is a theoretical concept which we use to describe the relative number of protons and electrons on a
If there are more negative charges (electrons) than positive ones (protons), it is negatively
If there are more positive charges than negative ones, it is positively charged
o We measure charge in coulombs - a proton is +1 C and an electron is -1 C
What did Benjamin Franklin discover with glass and rubber that related to charge?
o He discovered that rubbing glass with silk gave it the characteristics of a charged object (+)
o Also, rubbing rubber with fur gave it the characteristics of a different kind of charged object (-)
o NOTABLY, he was wrong in terms of electrons - the glass actually gained electrons, so it should have
been considered to be negatively charged (and vice versa with the rubber)
Explain what Coulomb's Law is.
o Coulomb's Law describes the nature of the attraction between two charged objects - like charges repel,
opposite charges attract
o Furthermore, the strength of this attraction is affected by:
Magnitude of charges on objects
Distance between objects (the square of it)
Imagine two light bulb-battery systems, where one battery is 1.5 V and the other is 100 V. What differences
will we see in the light bulbs, and why?
o The light bulb in the 100 V system will glow brighter - this is because the charges coming out of a 100 V
battery have more potential energy than the 1.5 V battery, and so they can provide more of this energy
to the light bulb
What if the systems were both 1.5 V batteries, except one of them had TWO batteries connected in series?
o The one with the batteries connected in series would shine brighter because the idea is that whenever
charge goes through a battery, it GAINS potential energy (it is like lifting a ball to a 2 m height, then
lifting it 2 more m to a 4 m height)
Electric Charges and Forces - Selected Demonstrations
What happened in class when we rubbed glass and rubber, then put them near the balls hanging from a
o They caused the balls to move back and forth, due to the forces of attraction and repulsion created by
the electric charges (recall Coulomb's Law)
Explain how the machine worked where you turn a crank and every so often there would be a flash of
o Turning the crank was doing some rubbing so that charge built up on one plate of a capacitor
o When there was too much charge on there for the capacitor to handle, it discharged through the air to
the other plate
How does the "electrostatic bell" work?
o It was a ball in the middle of two plates, one of which was positively charged and the other negative
o Thus when we set the ball on one plate (say negative), negative charge will be transferred onto there
and it will swing towards the positive plate
o When it gets there, it will gain positive charged and be attracted back to the positive plate Air Cleaners and Xerox Machines 2
Dust and Dust Filters
What is dust? Discuss the 3 different types.
o Dust is basically NEUTRAL particles floating in the air
o There are 3 types:
Regular dust: rock, dirt, organic matter, etc.
Soot: carbon that has been burned imperfectly - oily, greasy, etc.
Ash: powdery non-combustible residue of a fire (which has been burned perfectly, I guess)
Firstly, what kind of forces is the dust in the air subject to?
o It is subject to all the traditional forces which any other material would experience: air resistance, air
currents, gravity, buoyancy, etc.
How do traditional air cleaners work? What are some drawbacks?
o They are just a filter with very small holes so that when the air passes through, the large parts of dust
contained in it are stopped
o However, there are drawbacks:
The small particles of dust get through
The filter gets clogged eventually with the large particles
Give a brief overview (more later) on how an electronic air cleaner might work.
o The concept is very simple - if we can put negative charges onto the dust particles in the air, then force
all the air through the electronic air cleaner where the WALLS of the cleaner are POSITIVELY charged,
then the dust will stick to the walls due to electrostatic attraction
Electronic Air Filters in More Detail
Explain what a corona discharge is (it will be important for later).
o The idea is that if we can aggregate enough negative charge within a very confined area, the repulsion
between the electrons will be so great that eventually the electrons will jump off on their own into the
o Some examples of this:
If we connect a power supply to a wire - they build up on the end of the wire
If we connect a power supply to a pin - the charges build up on the tip of the pin