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Transcript
L 25 Electricity & Magnetism [2]
„
review – electric charge
• Matter has two basic properties
static electricity
• mass Æ gravitational force
• charge Æ electric and magnetic forces
– the charging process
– the van de Graff generator
– electrostatic shielding
– positive charge
– negative charge
• electric forces
lightning
„ batteries and frogs legs
„ electric circuits
• like charges repel +/+ or - / • unlike charges attract + / -
„
• charge is measured in Coulombs [C]
1
One Coulomb is a HUGE charge
• all charge is a multiple of the basic unit of charge
– we call this e = 1.6 x10-19 C Îcharges cannot
be divided into smaller units than this.
2
Where is the charge?
• the charge is in atoms
• To get a charge of one Coulomb
on an object we would have to
remove
– positive Æ protons
– negative Æ electrons
• matter is usually electrically neutral Æ it
has the same amount of positive and
negative charge
• electrons (not protons) can be transferred
from one object to another by rubbing
(friction)
6.250 x 1018
electrons from it!
• In the capacitor discharge demo, only
0.01 C of charge were involved.
3
Charging by friction
(triboelectric effect)
4
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.
• 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
5
6
1
Example
Attracting uncharged objects
• Initially, 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?
-5 C
-10 C
+
+
+
+5 C
• 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
+
A
B
• ANSWER: Removing -5 C from A leaves it with no net charge.
Removing -5 more leaves it with a net +5C. So, object A has a
net charge of +5 C and object B has a net charge of -5 C.
+5 C
uncharged
metal sphere
–5 C
A
B
• Note that the net charge (= 0) is the same before and after.
7
8
Can attract nonconductors also
The Magic Wand
• 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
++
++
2x4
++
++
We can cause the 2 x 4 to move
with electric forces
9
10
Seeing the effects of charge:
the electroscope
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 molecules turn to the right
and are attracted to the
negative rod.
charged rod
O
-+
H
stream of water
+
H
11
• 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.
12
The two sides of the metal foil then separate.
2
Danger High Voltage !
Electric Potential Î 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 Æ
ionization or breakdown
• The sparks excite air
molecules which give off
light
• 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
9 Volt
battery
VOLTS
• if I connect a 9 V battery to
the sphere and the ground, it
will have a potential of 9 V
13
Lightning- big outdoor spark
• National Weather Service:
about 25 million lightning
flashes each year in the US
• NWS reports that over the last
30 years, on average, 58
people were killed each year
• causes 100 million dollars in
damage each year in the US
• lasts only a thousandth of a
second, with up to 200,000 A
(typical hairdryer uses 10 A)
• causes the thunder!
15
14
development of a lightning bolt
charge
separation
stepped
leader
leader &
streamer
leader meets
streamer
lightning
bolt 16
Electrostatic shielding
Electrostatic shielding
• The effect of the high voltage on the van
de Graff generator stops on the outside of
the metal cage Î 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
17
18
3
Conductors and Non- Conductors
Metals (copper, aluminum, iron) are
conductors of electricity Æ they allow
current (moving free electrons) to pass
through them
Pure water is non-conducting
• clean water will not conduct electricity
• if salt or acid is added, however, it will
conduct electricity
Plastics, wood, ceramics, and glass are
non-conductors (or insulators) Æ they do
not let electricity flow through them Æ they
have no free electrons to move around
H2 O
19
20
Gas discharges
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–
carbon electrodes
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
21
examples of electrical discharges
not blood!
High Voltage
Source
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
the Aurora
fluorescent lamp
neon lights
23
24
4
Removing soot particles
Positive
cylinder
Chimney
stack
soot
Charging units
spray electrons
on the soot
particles
25
5