Title: Elementary Electrostatics
1Elementary Electrostatics
- Rubbing a balloon on a wool jumper makes the
balloon attract your hair - The balloon is said to be charged or to have an
electric charge - Similarly glass rubbed with silk/fur will become
charged - Charged glass will attract a charged balloon
- Two charged balloons will repel each other
2Types and Sources of Electric Charge
Two kinds of electric charge positive
negative
protons
electrons
3Electric Charge
- All ordinary matter contains both positive and
negative charge. - You do not usually notice the charge because most
matter contains the exact same number of positive
and negative charges. - An object is electrically neutral when it has
equal amounts of both types of charge.
4Properties of Electric Charge
- Charge is quantized.
- Positive and Negative charges have same
magnitude - e p 1.6 x 10-19 C
- SI unit of charge is the Coulomb
- Charge is conserved.
5Behaviour of Charges
- Like charges repel
- Unike or opposite charges attract
- Q ne where n is a whole number
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7Seeing the effects of charge the electroscope
- the electroscope is a simple device for observing
the presence of electric charge - it consists of a small piece of metal foil (gold
if possible) suspended from a rod with a metal
ball at its top
- If a negatively charged rod is placed near the
ball, - the electrons move away because of the
repulsion. - The two sides of the metal foil then separate.
8Atoms and Materials
Classification of Materials Conductors,
Insulators, Semiconductors
9Different Methods of Charging
- Friction e.g. rubbing a balloon with wool
- Conduction e.g. touching an electroscope
- Induction e.g. balloon sticking to a wall
10How do we charge an object? Charging by rubbing
- When two neutral objects are rubbed together,
charge is transferred from one to the other and
the objects become oppositely charged. - This is called charging by friction.
- Objects charged by this method will attract each
other.
11How do we charge an object? Charging by rubbing
Conductors charge will distribute around the
surface to try until equilibrium is
achieved. - charge only resides on
surface, not inside.
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13Charge by Cnduction (contact)
14Charge by Induction (no contact)
15How do we charge an object? B. Induction
Inducing a charge without touching.
Conductors
Charge is physically moved in/on the materials.
16http//www.tutorvista.com/content/physics/physics-
iv/electric-charges/charging-induction-animation.p
hp
17Induction (contd)
Insulators
18Fields and forces
- You can think of the field as the way forces are
transmitted between objects. - Charge creates an electric field that creates
forces on other charges.
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20http//qbx6.ltu.edu/s_schneider/physlets/main/efie
ld.shtml
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22Drawing the electric field
23Lightning- outdoor spark
- causes 80 million dollars in damage each year in
the US - On average, kills 85 people a year in the US
- over in a thousandth of a second
- carries up to 200,000 A
- causes the thunder!
24development of a lightning bolt
stepped leader
leader streamer
leader meets streamer
lightning bolt
charge separation
25applications of electrostatics
- Xerox copiers use electrostatic attraction to put
the ink droplets on the paper - electrostatic precipitators use the attraction of
charged dust to remove dust particles from smoke. - can be used to hold balloons on your head
26Removing soot particles
Positive cylinder
Chimney stack
Charging units spray electrons on the
soot particles
soot