Transcript Electric Potential and Electric Circuits

Electric Potential and
Electric Circuits
Electric Potential
Total electrical potential energy divided by
the charge
Electric potential = Electric potential energy
Charge
VOLT
SI unit for electric
potential – VOLT
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Named after Allesandro
Volta (1745-1827)
1 volt = 1 joule/coulomb
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
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It may take several thousand volts
HOWEVER – it is very few coulombs – less
than one millionth
SO – the energy it very low
Capacitors
Electrical storage
Uses
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In computers to
store “1” and “0”
In keyboards
Photoflash bulb
Giant lasers
Electric fences
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.
Formula
C= Q/V
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C – capacitance (Farads or Coulombs/Volt)
Q – charge (coulombs)
V – voltage difference (volts)
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
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
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Shock
Flashlight
Electric Current
Flow of electric charge
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Electrons carry the charge
Protons are bound to atoms
In solutions, ions can carry the charge
Measured in amperes (amps)
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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
Formula for Current
I = ΔQ/Δt
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I – current (amperes or amps)
Q – charge (coulombs)
T – time (seconds)
1 amp = 1 coulomb/second
Voltage Sources
Electron pump
Needs to slowly release the
electrons
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Not all at once – like in a shock
Steady flow
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Dry cells – chemical energy
Wet cells – chemical energy
Generator – converts
mechanical energy
Voltage provides “pressure”
to move electrons between
terminals
Generators
Average home outlets are
AC
Pressure of 120 volts
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This means there is 120
joules of energy forcing the
coulombs through the wire
Voltage does not FLOW
through the wire
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The electrons are pushed by
the voltage
Electric Resistance
Current depends on
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Voltage
Resistance
Resistance – slows the flow
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Depends on
Conductivity of material – silver best
Thickness – more thick, less resistance
Length of wire - longer, more resistance
Temperature – hotter, more resistance
Drawing Currents
Ohm’s Law
Current in a circuit is directly
proportional to the voltage and
inversely proportional to the
resistance
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Current = voltage/resistance
I = V/R
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Units 
1 ampere = 1 volt/ohm
Double the voltage – double the
current
Increase the resistance –
decrease the current
Typical resistance
Cord – less than 1 ohm
100 W light bulb – 100 ohms
Iron – 15 ohms
Electric toaster – 15-20 ohms
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Low resistance = larger current which
produces lots of heat
Ohm’s Law Formula
V=IR
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V – voltage (volts)
I – Current (amps)
R – resistance (ohms – Ω)
Resistors
Radios and TV electronics
Current is regulated by resistors
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Ranges from few ohms to millions
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 Ω
How much current is drawn by a lamp that
has a resistance of 100 Ω when a voltage
of 50 V is impressed across it?
I = V/R = 50 V/ 100 Ω = 0.5 amps
Electric Shock
What effects us – current or voltage?
Damage comes from current through the
body
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Current depends on voltage and resistance in
body
Resistance varies from 100 Ω (covered in salt
water) to 500,000 Ω (very dry skin)
Usually can’t feel 12 volts and 24 volts
would tingle
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If wet – 24 volts could be very uncomfortable
Effect of Various Electric Current
Current in Amps
0.001
Effect
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
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 Ω = 0.00012 A
If your skin were wet, your resistance
would be 1000 Ω, what would you feel on
a 24 volt battery?
Current = 24 V/ 1000 Ω = 0.024 A
Shocked
120 V in house
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normally our shoes provide resistance
Standing barefoot in bathtub – very little
resistance to ground – don’t use appliances in
bathtub!!
Birds sitting on wire
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Same potential on both sides of body
Only get it if they touch wires with different
potential
Grounded
Most appliances are grounded
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Helps prevent potential differences
All ground wires are connected together
Provides a direct route to the ground
What causes shock – current or voltage?
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The initial cause is the voltage, but the current
does the damage
Types of current
DC – Direct current
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Flow of charge is always in one direction
Source – batteries (wet or dry)
Move from negative terminal to positive
AC – Alternating current
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Charge moves back and forth in the line
In North America – frequency is 60 cycles per
second – 60 Hz
Voltage
North America
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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
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They sell energy, you provide the electrons in
your wiring
Diode
One way valve in a circuit
Only allows electrons to move one way
Can be used to convert AC to DC current
Power
Rate at which electric energy is converted
to another energy form
Electric power = current x voltage
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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
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
Lighting a bulb – which one works?
Must have a
closed loop!!
Electric Circuits
Need
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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
Types of Circuits
Series – single
pathway for all
electrons to flow
Parallel – branched
pathway for electrons
to flow
Series circuit
When switch is closed, current runs
through all lamps in same path
Open – when one break occurs, all flow
stops
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Break from switch, broken wire, burned out
light-bulb
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
Disadvantage of Series
If one device fails, the whole circuit ceases
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
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
(Ohm’s law)
Total current = sum of current in the devices
As number of devices increases, resistance
decreases
Schematic Diagrams
Resistance
Battery (2)
http://www.rkm.com.au/ANIMATIONS/animation-electrical-circuit.html
Combined Compound Circuits
Calculating Current in Compound
Circuits
When resistors are in
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Series
The resistance equal sum of devices
-----^^^^^-----^^^^^^------- = -----^^^^^-----8Ω
8Ω
16 Ω
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Parallel (2 devices with same resistance)
The resistance equal half of one device
----^^^^^-----=
----^^^^----8Ω
4Ω
----^^^^^------8Ω
Another Example
----^^^^^----8Ω
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----^^^^^---8Ω
----^^^^^-----^^^^^-8Ω
4Ω
----^^^^^----8Ω
 ---^^^^^----12Ω
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
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Cuts the line if it overheats
Overheating caused by
Short circuit (cuts the resistance)
Too many devices on a line