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## Electric Potential and Electric Circuits

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Title: Electric Potential and Electric Circuits

1
Electric Potential and Electric Circuits
2
Electric Potential
• Total electrical potential energy divided by the
charge
• Electric potential Electric potential energy
• Charge

3
VOLT
• SI unit for electric potential VOLT
• Named after Allesandro Volta (1745-1827)
• 1 volt 1 joule/coulomb

4
Voltage
• Same as volts
• One volt takes 1 joule of energy per coulomb to
• 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

5
Capacitors
• Electrical storage
• Uses
• In computers to store 1 and 0
• In keyboards
• Photoflash bulb
• Giant lasers
• Electric fences

6
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.

7
Formula
• C Q/V
• C capacitance (Farads or Coulombs/Volt)
• Q charge (coulombs)
• V voltage difference (volts)

8
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

9
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

10
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

11
Formula for Current
• I ?Q/?t
• I current (amperes or amps)
• Q charge (coulombs)
• T time (seconds)
• 1 amp 1 coulomb/second

12
Voltage Sources
• Electron pump
• Needs to slowly release the electrons
• Not all at once like in a shock
• Dry cells chemical energy
• Wet cells chemical energy
• Generator converts mechanical energy
• Voltage provides pressure to move electrons
between terminals

13
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

14
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

15
Drawing Currents
Water line
Electric circuit
hose
resistance
valve
switch
pump
battery
16
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

17
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

18
Ohms Law Formula
• VIR
• V voltage (volts)
• I Current (amps)
• R resistance (ohms O)

19
Resistors
• Current is regulated by resistors
• Ranges from few ohms to millions

20
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

21
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

22
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
23
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
1000 O, what would you feel on a 24 volt battery?
• Current 24 V/ 1000 O 0.024 A

24
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

25
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

26
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27
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

28
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

29
Diode
• One way valve in a circuit
• Only allows electrons to move one way
• Can be used to convert AC to DC current

30
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

31
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32
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

33
Lighting a bulb which one works?
Must have a closed loop!!
34
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

35
Types of Circuits
• Series single pathway for all electrons to flow
• Parallel branched pathway for electrons to flow

36
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

37
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

38
• If one device fails, the whole circuit ceases

39
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

40
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

41
Schematic Diagrams
Resistance
Battery (2)
http//www.rkm.com.au/ANIMATIONS/animation-electri
cal-circuit.html
42
Combined Compound Circuits
43
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

44
Another Example
• ---------
• 8O
• -------- ?
-----------
• 8O 8O 4O
• ---------
• 8O
• ? --------
• 12O

45