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Electric CurrentsChapter 34
What is an electric current?What makes current flow?
An Electric Current is simply electrons traveling through a conductor
like currents in a river
Why does the water in a river flow?
PE
Gravity is pulling it down, but a general rule of thumb is that an object has a tendency to move towards positions of lower energy.
Water will stop flowing when there is no PE difference
This is also when the pressures are equal on both sides
Water will stop flowing when there is no PE difference
In order to keep water flowing we would need to maintain a difference in height
The water would have to be continuously pumped to keep it flowing
An electrical current is made up of little charged particles (electrons).
What tells you the Potential Energy of an electron. (Hint J/C)
Electric Potential (Volts)
Electrons travel from high to low voltages
-
2.0 V
+- +- +- +
1.5 V1.0 V0.5 V0.0 V
As electrons flow the potential difference is reduced.
No current flow when there is no potential difference.
0 V
In order to keep current flowing we need an electron pump.
1.5 V
A battery keeps current flowing by maintaininga voltage difference (until it “dies”)
e-
electrons are produced when the zinc “corrodes” at the anode
electrons used up at the cathode
Chemical reactions in a battery pump electrons around a loop
How would you measure the flowrate of water?
Volume
time
gallon
minute
The rate at which electrons “flow” is measured in current
I =Qt
Electrical Current: Amperes or Amps for short
the amount of charge that flows through a wire (C)
per time (s)
1 ampere =1 Coulomb
1 second
What does as 15 amp circuit mean
One light bulb draws 1/3 amp
Another draws 2/3 amp.
How many more electrons pass through the second as compared to the first in one second?
The greater the potential difference (pressure) the faster the flow of water.
The bigger the electric potential difference, the faster the flow of electrons.
As Voltage Current
Voltage can be thought of as electrical pressure
How does the size of the hole affect flowrate?
A small hole provides more RESISTANCE to water flow.
Electrical RESISTANCE of something (like a wire) is how much it resists electrons flowing through it.
It is measure in OHMS
The Electrical Resistance of an object depends on 4 things
1.) THE MATERIAL
Copper wire Glass rod
Which will have the higher resistance?
The Electrical Resistance of an object depends on 4 things
2.) The Length
Which will have the higher resistance?
Short Long
The Electrical Resistance of an object depends on 4 things
3.) The thickness
Which will have the higher resistance?
THICK
THIN
The Electrical Resistance of an object depends on 4 things
4.) The Temperature
The hotter an object the more it resists current.
When some materials are cooled WAY DOWN, they actually lose all resistance and become SUPERCONDUCTORS
How would the resistance be affected if the wire was stretched out.
As Resistance Current
Resistance is measure in Ohms
switch
resistor
battery
+-
valve
Tube
pump
switch
resistor
battery
+-
valve
Tube
pumpwater flow e- flow
pressure drop acrossvoltage drop across
Ohm’s Law
I = VR
Current (amps)
Electrical Potential(Volts)
Resistance(Ohms)
Ohm’s Law
I = VR
As Resistance Current
As Voltage Current
A 9-volt battery is place on your wet tongue which has a resistance of about 3000 ohms. How many amps run through your tongue?
Amperage Chart
.001 A mild shock.005 A hurts
.015 A loss of muscle control
.100 A probable death
Dry skin has a resistance of about 500,000 WET skin has a resistance
of about 3,000
If you touch a 120 V circuit
low currenthigh current
Why is the bird safely perched on a power line
A technician installs a bird diverter to keep condors away from points which may be grounded.
What resistance would you want, in order to flow 15 Amps through a 120 V circuit?
How much current flows across a light bulb with a resistance of 25 Ohms connected to a 12 V batter?
Voltage from a 12 V battery (Direct Current or DC)
Current
T
Current 0
Time
AC or alternating current
The US use AC current at 60 Hz,what does that mean
Time
What happens to the current that flow through the wire?
At times the current is moving:left or right or not at all
Current 0
Are there any electrons in a copper wire which is not connected to a battery?
e-
In a neutral wire, an electron moves randomly but doesn’t really go anywhere
-+
Path of an electron in an electric field
drift speed
-+
Path of an electron in an electric field
drift speed
An electron has a total speed of about 1,500,000 m/sbut usually has a drift speed of about .0001 m/s
With a drift speed of about .0001 m/s
Why does it not take several minutes for the lights to turn on, when you hit the switch?
The wires are already full of electrons, when one enters one exits.
-+
The electron bumps into atoms in its path
This slows its drift speed, & is also why a wire gets hotwhen current passes through it.
The hotter the wire, the faster the random motion of the electron is.
The more frequently bumps into an atom, and loses drift speed.
(the greater the resistance)
Temp
Electron motion applet
-+
Path of an electron in an electric field
Which way would field lines point?
The electric company does not sell electrons
all they do is jiggle the electrons in your wires which requires ENERGY
When you get electric shock, its your own electrons that are shocking you!
In order for current to flow, a circuit must be made
Which circuits will have current?
Which light bulb(s) will light?
Which light bulb(s) will light?
BOOK Problems
Page 546 2-16, 26 - 32
9V battery9 V
0 V
No resistance = No energy loss
EnergyEnergyEnergyEnergyEnergy
Electric POWER
Power =Energytime
If you were looking for the power of a waterfall,
what 2 factors would determine it
the height of the falls,the change in PE of the water
the flowrate of the water
the change in PE of the electronthe flowrate of electrons
P =
For electric power, it is very similar
ICurrentC/s
VVoltageJ/C
PowerJ/s(Watt, W)
9V battery
EnergyEnergyEnergyEnergyEnergyEnergy
The voltage is the same so each electron gives the same amount of energy, but more electrons pass through each second
I P
120 V battery
Energy
Even if the current is the same, the Power increases because each electron drops off more energy!
V PEnergyEnergyEnergyEnergyEnergy
What power is used by a 2 amp toaster on a 120 Vcircuit?
A 60 watt bulb is made to flow .50 A, what voltage is it intended to be plugged into?
If you were to connect 2 resistors together (or anything else) there are two ways to do it.
parallel
series
Which will be harder to push water through
Twice as hard
When resistor are positioned in series the total resistance just adds up
4 4
Total or EQUIVALENT resistance
8
4 8 12
4 8 12
8 V
What is the total current running through each circuit4
2.0 A
8 V
4 4
1.0 A
8 V
Because the current can only go in 1 path, the current through both resistors is the same
4 4
1.0 A
1.0 A 1.0 A1.0 A
Same flow rate everywhere
Pressure drops after resistance
100 Pa
100 Pa
50 Pa
0 Pa
8 V
Voltage drops after each resistor
4 4
1.0 A
0 V
8 V
8 V
Voltage drop is given by Ohms Law
4 4
1.0 A
V = IR
4 V drop 4 V drop
24 V
What is the voltage drop across the first resistor?
4 8
What is the voltage between the resistors?What is the voltage drop across the 2nd resistor?
EnergyEnergy
EnergyEnergy
EnergyEnergy
EnergyEnergy
A lamp with a resistance of 360 and one that is 144 are connected in series to a 120 V circuit. Which burns brighter?
9 V
3 light bulbs each with a resistance of 2 , are connected to a 9V battery. What is the current and voltage at each point along the circuit
Which bulb is the brightest?
9 V
What happens if one bulb burns out?
9 V
What happens if one bulb burns out?
9 V
What happens to the brightness of a bulb, if another is connected in series.
9 V
?
9 V
The bulbs are dimmer, the same voltage with more resistance means…
9 V
less current
There are Twice the paths open to the water
The resistance is only 1/2 , so twice as much water will flow
Which will be easier to pump water through?
4 4
4
Resistors can also be added in Parallel
Which circuit has the higher Resistance to current flow?
4
4
4
Equivalent Resistance
4
4
4
4
2
4/3
4
4
4
4
4
4
As more resistors are added in PARALLEL, the equivalent resistance DROPS
R1
R2
R3=
REQ
REQ
1= R1
1+ R2
1+ R3
1+ etc…
4
4
4
Equivalent Resistance
4
4
4
4
2
4/3
1= 4
1+4
1
1= 4
1+4
1+4
1
What is the equivalent resistance ofa 10 ohm and 20 ohm resistor in series and parallel?
Page 559 3, 4, 5, 6, 9, 10, 11
due next class
4
8
What is the equivalent resistance
9 V
What is the total current out of the battery?
REQ=2.7
Is the current the same through each resistor?
I=3.3 A
4
8
Which resistor has the greatest voltage drop?
9 V
9 V 0 V
4
8
9 V
Which resistor will have the greatest current?
Will the flow rate be the same through both tubes?
Both tubes are fed the same pressure
4
8
9 V
Which resistor will consume the greatest power?
8
24
9 V
What would happen the the Voltage, Current, and Resistance if another resistor was added in parallel?
4
9 V
9 V
How does the brightness change if another bulb is added in series?
What changed?
½ Resistance
Twice current
battery drains twice as fast
9 V
What if one burns out?
9 V
What if one burns out?
A light bulb with a thick filament and one with a thin filament are connected in parallel to a battery.Which will burn brighter?
9 V
Hot From Power company
Circuit Breakeror fuse (15 A)
to other circuit breakers
7 AFridge unused outlet
8 AmpMicrowave
Hot From Power company
Circuit Breakeror fuse (15 A)
to other circuit breakers
7 AmpFridge
8 AmpMicrowave
1 Amplight
Circuit Breakeror fuse (15 A)
to other circuit breakers
7 AmpFridge
8 AmpMicrowave
1 Amplight
Hot From Power company
What is the equivalent resistance of the circuit below
4 2
6
4
4 4
4
4 4
What is the equivalent resistance of the circuit below
How much current would flow from a 100 V batttery?
Book problemspage 560: 16, 17, 27, 28, 29
12 V
20 80
40
I1
What is the current out of the battery?
Which of these resistors will have more current?
4
15
10
4
20
40
24 V
4
6
10
2
What is the equivalent resistance of the circuit below
20
40
Book problems: p561 29-32
9V battery
Energy
9V battery
Energy
24 V
What is the voltage drop across the first resistor?
4 8
What is the voltage between the resistors?What is the voltage drop across the 2nd resistor?
EnergyEnergy
8
4
9 V
Energy
8
4
9 V
Energy
Energy
