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. 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.

4. 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)

5. Charging by friction (triboelectric effect)
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

6. 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.

7. 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 and object B has a net charge of -5 C.
Note that the net charge (=0) is the same before and after A B
-10 C
+5 C
-5 C A B
–5 C
+5 C

8. 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

9. Can attract nonconductors also
Even though nonconductors do not have free electrons that can move around, the molecules can be polarized – the positive and negative charges can be separated slightly + +

10. 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

11. You can bend water with charge!
The water molecule
has a positive end and
a negative end; it is
always polarized
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.
charged rod O
- -
H H
stream of water

12. 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.

13. 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

14. 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

15. Lightning- outdoor spark
causes 80 million dollars in damage each year in the US
On average, kills 90 people a year in the US
lasts only a thousandth of a second
carries up to 200,000 A
causes the thunder!

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

17. Electrostatic shielding

18. 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

19. Conductors and Non- Conductors
Metals (copper, aluminum, iron) are conductors of electricity  they allow current (moving free electrons) to pass through them Plastics, wood, ceramics, and glass are non-conductors (or insulators)  they do not let electricity flow through them  they have no free electrons to move around

20. 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

21. 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–

22. 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

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

24. 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

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