Electric Potential and Electric Circuits

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Electric Potential and Electric Circuits
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Electric Potential

• Total electrical potential energy divided by the
  charge
• Electric potential Electric potential energy
• Charge

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VOLT

• SI unit for electric potential VOLT
• Named after Allesandro Volta (1745-1827)
• 1 volt 1 joule/coulomb

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Voltage

• Same as volts
• One volt takes 1 joule of energy per coulomb to
  add it to the conductor
• Rub a balloon in your hair
• It may take several thousand volts
• HOWEVER it is very few coulombs less than one
  millionth
• SO the energy it very low

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Capacitors

• Electrical storage
• Uses
• In computers to store 1 and 0
• In keyboards
• Photoflash bulb
• Giant lasers
• Electric fences

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Simple Capacitors

• Conducting plates separated by a small distance
• Connected to battery which moves electrons from
  battery to plate
• Complete when voltage on plates battery voltage
• Advantage Discharged in large quantity
• Can be dangerous even when appliances are not
  powered.

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Formula

• C Q/V
• C capacitance (Farads or Coulombs/Volt)
• Q charge (coulombs)
• V voltage difference (volts)

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Electric Terms

• Voltage electric pressure
• Current flow of charge
• Resistance Restrains the electron flow
• DC (Direct current) flows in one direction
• AC (Alternating current) flows back and forth
• Power Rate at which energy is transferred

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Flow of Charge

• When ends of electric conductor (example-wire)
  have different potential energy the charge flows
• Flow continues until it reaches the same
  potential
• Examples
• Shock
• Flashlight

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Electric Current

• Flow of electric charge
• Electrons carry the charge
• Protons are bound to atoms
• In solutions, ions can carry the charge
• Measured in amperes (amps)
• SI unit (A)
• Amps 1 coulomb/second
• Example - 5 amps wire carries 5 coulombs of
  charge pass a point in 1 second
• When there is no current, the wire has no charge

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Formula for Current

• I ?Q/?t
• I current (amperes or amps)
• Q charge (coulombs)
• T time (seconds)
• 1 amp 1 coulomb/second

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Voltage Sources

• Electron pump
• Needs to slowly release the electrons
• Not all at once like in a shock
• Steady flow
• Dry cells chemical energy
• Wet cells chemical energy
• Generator converts mechanical energy
• Voltage provides pressure to move electrons
  between terminals

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Generators

• Average home outlets are AC
• Pressure of 120 volts
• This means there is 120 joules of energy forcing
  the coulombs through the wire
• Voltage does not FLOW through the wire
• The electrons are pushed by the voltage

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Electric Resistance

• Current depends on
• Voltage
• Resistance
• Resistance slows the flow
• Depends on
• Conductivity of material silver best
• Thickness more thick, less resistance
• Length of wire - longer, more resistance
• Temperature hotter, more resistance

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Drawing Currents
Water line
Electric circuit
hose
resistance
valve
switch
pump
battery
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Ohms Law

• Current in a circuit is directly proportional to
  the voltage and inversely proportional to the
  resistance
• Current voltage/resistance
• I V/R
• Units ? 1 ampere 1 volt/ohm
• Double the voltage double the current
• Increase the resistance decrease the current

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Typical resistance

• Cord less than 1 ohm
• 100 W light bulb 100 ohms
• Iron 15 ohms
• Electric toaster 15-20 ohms
• Low resistance larger current which produces
  lots of heat

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Ohms Law Formula

• VIR
• V voltage (volts)
• I Current (amps)
• R resistance (ohms O)

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Resistors

• Radios and TV electronics
• Current is regulated by resistors
• Ranges from few ohms to millions

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Question

• What is the resistance of an electric frying pan
  that draws a 12 ampere current when connected to
  a 120-volt circuit?
• R V/I 120 V/12 A 10 O
• How much current is drawn by a lamp that has a
  resistance of 100 O when a voltage of 50 V is
  impressed across it?
• I V/R 50 V/ 100 O 0.5 amps

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Electric Shock

• What effects us current or voltage?
• Damage comes from current through the body
• Current depends on voltage and resistance in body
• Resistance varies from 100 O (covered in salt
  water) to 500,000 O (very dry skin)
• Usually cant feel 12 volts and 24 volts would
  tingle
• If wet 24 volts could be very uncomfortable

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Effect of Various Electric Current
Current in Amps Effect
0.001 Can be felt
0.005 Painful
0.010 Spasms
0.015 Loss of muscle control
0.070 Probably fatal (if through the heart) for more than one second
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Questions

• If resistance of your body were 100,000 ohms,
  what would be the current in your body when you
  touched the terminals of a 12-volt battery?
• Current V/R 12 v/100,000 O 0.00012 A
• If your skin were wet, your resistance would be
  1000 O, what would you feel on a 24 volt battery?
• Current 24 V/ 1000 O 0.024 A

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Shocked

• 120 V in house
• normally our shoes provide resistance
• Standing barefoot in bathtub very little
  resistance to ground dont use appliances in
  bathtub!!
• Birds sitting on wire
• Same potential on both sides of body
• Only get it if they touch wires with different
  potential

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Grounded

• Most appliances are grounded
• Helps prevent potential differences
• All ground wires are connected together
• Provides a direct route to the ground
• What causes shock current or voltage?
• The initial cause is the voltage, but the current
  does the damage

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Types of current

• DC Direct current
• Flow of charge is always in one direction
• Source batteries (wet or dry)
• Move from negative terminal to positive
• AC Alternating current
• Charge moves back and forth in the line
• In North America frequency is 60 cycles per
  second 60 Hz

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Voltage

• North America
• Small appliances 110-120 volts
• Large appliances 220 volts
• Obtain by combining voltage
• Europe 220 volts (more efficient)
• Electrons do NOT travel from power plants, they
  oscillate in place
• They sell energy, you provide the electrons in
  your wiring

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Diode

• One way valve in a circuit
• Only allows electrons to move one way
• Can be used to convert AC to DC current

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Power

• Rate at which electric energy is converted to
  another energy form
• Electric power current x voltage
• Units 1 watt 1 amp x 1 volt
• 1 kilowatt 1000 watts
• 1 kilowatt hour amount of energy consumed in 1
  hour at rate of 1 kilowatt

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Calculations

• How much power is used by a calculator that
  operates on 8 volts and 0.1 amps? If it is used
  for one hour, who much energy does it use?
• Power A x V 0.1 A x 8 V 0.8 W
• Energy Power x time 0.8 W x 1 hour 0.8
  watt-hours or 0.0008 kilowatt-hours
• FYI Speed of e- in wire 0.01 cm/s

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Lighting a bulb which one works?
Must have a closed loop!!
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Electric Circuits

• Need
• Pathway
• Voltage source (battery or AC)
• Resistance (light, fan, bell, etc)
• Optional
• switch stops and starts current
• Capacitor hold a large voltage for quick
  release
• Diode one-way valve
• Transformer and capacitors controls current flow

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Types of Circuits

• Series single pathway for all electrons to flow
• Parallel branched pathway for electrons to flow

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Series circuit

• When switch is closed, current runs through all
  lamps in same path
• Open when one break occurs, all flow stops
• Break from switch, broken wire, burned out
  light-bulb

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Series Circuit Features

• Single pathway same current
• Total resistance sum of individual resistance
• Current voltage/ total resistance (Ohms law)
• Voltage drop across each device depends on
  resistance of the device

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Disadvantage of Series

• If one device fails, the whole circuit ceases

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Parallel Circuits

• Devices are connected to the same points in the
  circuit
• Each resistor has its own path to the voltage
  source
• If one device fails, it does not interupt the
  other devices

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Features of Parallel Circuits

• Each device connects directly to the voltage
  source. Therefore the voltage is the same
  across each device.
• Total current divides between devices. The one
  with the lowest resistance gets the most current
  (Ohms law)
• Total current sum of current in the devices
• As number of devices increases, resistance
  decreases

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Schematic Diagrams
Resistance
Battery (2)
http//www.rkm.com.au/ANIMATIONS/animation-electri
cal-circuit.html
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Combined Compound Circuits
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Calculating Current in Compound Circuits

• When resistors are in
• Series
• The resistance equal sum of devices
• -----------------
  -----------
• 8O 8O 16 O
• Parallel (2 devices with same resistance)
• The resistance equal half of one device
• ----------
  ---------
• 8O
  4 O
• -----------
• 8O

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Another Example

• ---------
• 8O
• -------- ?
  -----------
• 8O 8O 4O
• ---------
• 8O
• ? --------
• 12O

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Overloading

• The more devices on a line, the more current it
  draws as resistance lowers
• Overloaded line carrying more than a safe
  amount of current
• Fuses put in a current in SERIES
• Cuts the line if it overheats
• Overheating caused by
• Short circuit (cuts the resistance)
• Too many devices on a line