L 25 Electricity

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 dont 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

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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
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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
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
9
Charging by friction

• If you rub plastic with cats 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.

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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)

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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.

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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.

-5 C
5 C
B
A
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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
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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
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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.

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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

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Making SparksThe 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.

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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
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Electrostatic shielding
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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
stepped leader
leader streamer
leader meets streamer
lightning bolt
charge separation
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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

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Alessandro Volta

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

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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.

zinc electrode
copper electrode
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Inside a Duracell 1.5 Volt battery
Metal Cap
plastic case

Zinc outer electrode
Carbon center electrode
- Bottom electrode
Electrolyte paste
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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
An electric circuit!
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

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

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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