Voltage Sources

1
Voltage Sources

• If we want to move charge from one place to
  another, we must apply a force to make it move
• Another way of thinking about this is to say that
  we must give the charges some potential energy

2
Voltage Sources

• We can give an object gravitational potential
  energy by lifting it into the air
• We have to do work on the object to lift it into
  the air
• Doing work uses energy
• Conservation of Energy

3
Voltage Sources
4
Voltage Sources

• We need an pump that pumps charges!
• The simplest charge pump is a battery
• It uses chemical reactions to separate charges
  and thus create electrical potential energy
• More convenient to talk about PE/charge or
  electric potential measured in volts
• So, a battery is a kind of electric pump

5
Voltage Sources

• Another kind of source is a generator
• A generator converts mechanical energy into
  electrical energy
• In any case, what we need is a device to separate
  charges!!!

6
Electric Current

• The movement of charge is called a current
• Metals have some electrons that are not needed to
  bond the atoms together in the solid
• They are pretty free to just roam about the
  material and are not associated with any
  individual metal atom

7
Electric Current

• Compare to the flow of water in a hose

8
Electric Current

• We measure the flow of water in gallons/minute
• We measure the flow of charges in coulombs/sec
• 1 coulomb/sec 1 ampere
• Remember this is 6.25 x 1018 electrons moving
  past a point in a wire per second

9
Electric Current

• In a wire, the electrons actually move quite
  slowly, less than 0.01 meters/sec
• However, their electric field moves at the speed
  of light!!!
• So, we can send signals down a wire very quickly,
  because the information moves at the speed of the
  changing electric field which is at the speed of
  light

10
Electric Current
Electron flow

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11
Electric Current

• In the 1700s people figured out that charges
  could move
• They had two choices
• Positive charge moves
• Negative charge moves
• They guessed WRONG!!!!!
• We know electrons move

12
Electric Current

• Hook up a battery and electrons flow from minus
  terminal to plus terminal
• Ancients defined current as positive charge flow
• Direction of current is from plus to minus

13
DC and AC

• Hook up a battery and we have direct current,
  i.e. current flows ALWAYS in one direction
• Modern generators in power plants reverse the
  polarity of the output terminals 60 times per
  second
• This is called alternating current
• It is what comes out of the wall plug

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DC and AC
15
Electrical Resistance

• So, we hook up a wire between the terminals on a
  battery
• The question is, how much current flows?
• We have lots of free electrons able to move in
  the metal composing the wire
• So, how much charge moves past any point in the
  wire per second?

16
Electrical Resistance

• The amount of current that flows is determined by
  how much resistance there is to the flow of the
  charges
• What makes this resistance?
• Collisions of the electrons with each other and
  with the stationary atoms of the metal
• Atoms are actually vibrating in the lattice

17
Electrical Resistance

• Resistance depends on the structure of the
  material and the temperature
• The higher the temperature, the more the atoms
  vibrate and the more electrons make collisions
  with the atoms
• How much current flows was discovered by George
  Ohm, and we honor him by naming the unit of
  resistance an ohm

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Ohms Law

• Ohms discovery was that current is proportional
  to voltage and inversely proportional to
  resistance

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Ohms Law

• Three forms are convenient

20
Electric Shock

• We all know not to stick our finger into a light
  bulb socket or to drop an electrical appliance
  into our bath tub
• What causes the damage to our bodies?
• It is the amount of current that flows through
  the body that can cause problems
• Human skin has resistance ranging from 100 ohms
  to 500,000 ohms (wet to dry)

21
Electric Shock

• We can use Ohms Law to calculate current based
  on the size of the applied voltage
• You can feel 0.001 ampere (1 milliampere)
• 0.005 amperes hurts
• 0.010 amperes causes muscle spasms
• 0.015 amperes loss of muscle control
• 0.070 amperes disrupts heart rythyms (fatal)

22
Electric Shock

• There must be a potential difference between one
  part of your body and another
• You must become a conductor of electricity
• OK for birds to sit on a 5000 V transmission line
  as long as no part of the bird touches something
  else
• Squirrels get across the transformers