1. L 25 Electricity & Magnetism [2]
static electricity
the charging process
the van de Graff generator
electrostatic shielding
lightning
batteries and frogs legs
electric circuits

2. review – electric charge
Matter (stuff) has two basic properties
mass  gravitational force
charge  electric and magnetic forces
positive charge
negative charge
electric forces
like charges repel +/+ or - / -
unlike charges attract + / -
charge is measured in Coulombs [C]
all charge is a multiple of the basic unit of charge – we call this e = 1.6 x10-19 C charges cannot be divided into smaller units than this.

3. Conductors and Non- Conductors
Metals (copper, aluminum, iron) are conductors of electricity  that means that charge can move through them
Plastics, wood, ceramics, and glass are non-conductors (or insulators)  they do not let electricity flow through them
You should not stick a metal fork into an electrical outlet!
You could stick a plastic fork into an outlet without electrocuting yourself, but don’t do it!

4. What makes conductors conduct?
Atoms have equal numbers of positive and negative charges, so that a chunk of stuff usually has no net charge  the plusses and minuses cancel each other.
However, when you put a bunch of metal atoms (like copper) together an amazing thing happens  one electron from each atom forgets which atom it belongs to.
All the homeless electrons are free to wander about inside the material

5. Pure water is non-conducting
clean water will not conduct electricity
if salt or acid is added, however, it will conduct electricity
H2O
carbon electrodes

6. A salt water solution is a conductor
When salt NaCl (sodium chloride) is added to water H2O, the NaCl molecule dissociates into a positive ion Na+, and a negative ion Cl- .
Thus the solutions contains both positive and negative ions, both of which can conduct electricity.
Electric current can pass through dirty bath water and through you also!
we are conductors – water + Na+ + Cl–

7. Gas discharges PLASMA High Voltage Source
When a high voltage is applied to a gas-filled tube, the gas
can become ionized, one or more electrons are separated from
each atom. Since positive and negative charges are present
the ionized gas conducts electricity. The gas atoms are excited
and emit light of a color characteristic of the gas.
PLASMA
Gas in
tube
not blood!
High Voltage
Source

8. examples of electrical discharges
fluorescent lamp
the Aurora
neon lights

9. Charging by friction
If you rub plastic with cat’s fur, electrons are rubbed onto the plastic making it negative
if you rub glass or plastic with silk, electrons are rubbed off the glass making it positive
the charge can be transferred to other objects.
only the electrons can be transferred

10. The charging process
an object is charged positive (has a net positive charge ) if electrons are removed from it
an object is charged negative (has a net negative charge) if electrons are transferred to it
charges can be transferred from conductors or non-conductors but they can only move through conductors.

11. Where is the charge? the charge is in atoms
positive  protons
negative  electrons
matter is usually electrically neutral  it has the same amount of positive and negative charge
electrons (not protons) can be transferred from one object to another by rubbing (friction)

12. Non-conductors can be charged too!
Even though non-conductors do not have free electrons meandering about, they can be charged by friction
When you move your comb through your hair, the friction (rubbing) between the comb and hair can pull some of the electrons out of your hair and onto the comb
as a result your comb ends up with a net negative charge and attracts your hair which is now positive.

13. Example
Object A has a charge of -5 C and Object B has a charge of +5 C
If -10 Coulombs of negative charge are transferred from object A to object B. What is the final charge on each object?
ANSWER:
object A has a net charge of +5 C
object B has a net charge of -5 C.
-10 C
-5 C
+5 C A B

14. One Coulomb is a HUGE charge
To get a charge of one Coulomb on an object we would have to remove
6.250 x 1018
electrons from it!
In the capacitor discharge demo, only 0.01 C of charge were involved.

15. Attracting uncharged objects+
A negatively charged
rod will push the
electrons to the far
side leaving the near
side positive.
The force is attractive
because the positive
charges are closer to
the rod than the negative
charges
uncharged
metal sphere

16. You can bend water with charge!
charged rod
The water molecule
has a positive end and
a negative end.
When a negative rod is
brought near the stream
of water, all the positive
ends of the water mole-
cules turn to the right
and are attracted to the
negative rod.
stream of water

17. We can cause the 2 x 4 to move
The Magic Wand
2 x 4
We can cause the 2 x 4 to move
with electric forces

18. Seeing 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.

19. Danger High Voltage !
The van de Graff can charge the sphere to more than 50,000 volts!
This is enough to cause discharges to the surrounding air  ionization or breakdown
The sparks excite air molecules which give off light

20. Making Sparks: The Van de Graff Generator
The van de Graff generator is a device for building up a large electrical charge on a metal sphere.
The charge is generated by friction between a rubber belt and a roller.
the charge on the belt is transferred to the sphere by a brush.

21. Electric Potential  voltage
The amount of charge on a charged sphere can be measured in terms of its
electric potential or voltage
the more charge that is on the sphere, the higher its voltage
electric potential is measured in VOLTS
if I connect a 9 V battery to the sphere and the ground, it will have a potential of 9 V
9 Volt
battery

22. applications 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

23. Removing soot particles
Positive
cylinder
Chimney
stack
Charging units
spray electrons
on the soot
particles
soot

24. Electrostatic shielding

25. Electrostatic shielding
The effect of the high voltage on the van de Graff generator stops on the outside of the metal cage  Homer is SAFE!
Being inside your car during a lightning storm offers you some protection
radio signals cannot penetrate through a metal enclosure
the metal bars (rebar) that reinforce the concrete in walls can also interfere

26. Lightning- outdoor spark
causes 80 million dollars in damage each year in the US
On average, kills 85 people a year in the US
is all over in a thousandth of a second
carries up to 200,000 A
causes the thunder!

27. development of a lightning bolt
charge
separation
stepped
leader
leader &
streamer
leader meets
streamer
lightning
bolt

28. Frog's leg Batteries
in 18th century Luigi Galvani a professor of anatomy at the University of Bologna found that a freshly dissected frog leg hung on a copper hook twitched when touched by an iron scalpel.
The two metals had to be different.
Galvani thought that he had discovered the secret life force

29. Alessandro Volta
Professor of Physics at the University of Pavia realized that the electricity was not in the frog’s leg but the twitching was the result of touching it with two different metals
Volta had discovered the first battery.
Lemon battery

30. Batteries  use chemical energy to produce electricity
two dissimilar metals immersed in a conducting fluid (like an acid for example) cause a chemical reaction which can produce electric current.
acid
zinc
electrode
copper
electrode

31. Inside a Duracell 1.5 Volt battery
Metal Cap
plastic case
DURACELL +
Zinc outer
electrode
Carbon center
electrode
- Bottom
electrode
Electrolyte
paste

32. Current– flow of electric charge
If I connect a battery to the ends of the
copper bar the electrons in the copper will
be pulled toward the positive side of the
battery and will flow around and around.
 this is called current – flow of charge
copper
An electric circuit!
Duracell +

33. Electric current (symbol I)
Electric current is the flow of electric charge q (Coulombs)
It is the amount of charge q that passes a given point in a wire in a time t, I = q ÷ t
Current is measured in amperes
1 ampere (A) = 1 C / 1 s q

34. Potential difference or Voltage (symbol V)
Voltage is what causes charge to move in a conductor
It plays a role similar to pressure in a pipe; to get water to flow there must be a pressure difference between the ends, this pressure difference is produced by a pump
A battery is like a pump for charge, it provides the energy for pushing the charges around a circuit

35. Voltage and current are not the same thing
You can have voltage, but without a path (connection) there is no current. An
electrical
outlet
voltage