Electric Current and Resistance Physics. Potential Difference Charges can “lose” potential...
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Transcript of Electric Current and Resistance Physics. Potential Difference Charges can “lose” potential...
Electric Current and Resistance
Physics
Potential DifferenceCharges can “lose” potential energy by
moving from a location at high potential (voltage) to a location at low potential.
Charges will continue to move as long as the potential difference (voltage) is maintained.
Potential Difference =Voltage=EMF
In a battery, a series of chemical reactions occur in which electrons are transferred from one terminal to another. There is a potential difference (voltage) between these poles.
The maximum potential difference a power source can have is called the electromotive force or (EMF), . The term isn't actually a force, simply the amount of energy per charge (J/C or V)
CurrentA sustained flow of electric charge past
a point is called an electric current.Specifically, electric current is the rate
that electric charge passes a point, so
Current = or I = q/tCharge
time
Measuring Current If 1 Coulomb of charge (6.25 x 1018
electrons) passes a point each second, the current is 1 Ampere.
So, 1 Ampere = 1 Coulomb/sec
Ampere (for Andre’ Ampere)
Usually called an ampOpen Circuit – break in the circuit, no current flow
sCA 11
Voltage SourceA battery or electrical outlet is a source
of electric potential or voltage - not charge.
The electrons that move in a conductor are supplied by the conductor - not the voltage source.
The net charge on a current-carrying conductor is zero.
Electromotive ForceAn old-fashioned term for electric
potential or voltage is “electromotive force” or “emf”.
Electrical ResistanceMost materials offer some resistance to
the flow of electric charges through them. This is called electrical resistance.
ResistanceResistance (R) – is defined as the restriction of electron
flow. It is due to interactions that occur at the atomic scale. For example, as electron move through a conductor they are attracted to the protons on the nucleus of the conductor itself. This attraction doesn’t stop the electrons, just slows them down a bit and causes the system to waste energy.
The unit for resistance is the OHM,
ResistanceResistance of a conductor depends on:
Material - Gold is best Length - longer conductors have more
resistance. Cross section - thick wires have less
resistance than thin wires Temperature - higher temperature means
more resistance for most conductors
Ohm’s LawFor many conductors, current depends
on:Voltage - more voltage, more current
Current is proportional to voltageResistance - more resistance, less
current Current is inversely proportional to
resistance
Ohms’ Law In symbols:
V = IRVI R
George Simon Ohm (1787-1854)
The actual values depend on the resistance of the conductor
Called Ohm’s LawR – resistance measured in Ohms ()
I V
IR V
Ohm’s Law“The voltage (potential difference, emf) is directly
related to the current, when the resistance is constant”
IR
IRV
R
R
IV
Resistance
alityproportion ofconstant
Voltage vs. Current
0
1
2
3
4
5
6
7
8
9
10
0 0.2 0.4 0.6 0.8 1
Current(Amps)
Vo
ltag
e(V
)
Voltage(V)
Since R=V/I, the resistance is the SLOPE of a V vs. I graph
R= resista
nce = slo
pe
Direct Current If the voltage is maintained between
two points in a circuit, charge will flow in one direction - from high to low potential. This is called direct current (DC)
Battery-powered circuits are dc circuits.
Alternating Current If the high & low voltage terminals
switch locations periodically, the current will flow “back and forth” in the circuit. This is called alternating current (AC).
Circuits powered by electrical outlets are AC circuits.
AC in the US In the US, current changes direction
120 times per second, for a frequency of 60 cycles per second or 60 Hertz.
Normal outlet voltage in the US is 110-120 volts, although some large household appliances run on 220-240 volts.
Converting AC to DCAC is converted to DC using devices
called diodes, which allow charges to move in only 1 direction.
Speed of Electrons
Electrons in a circuit do not move quickly - they actually “drift” at about 1 mm/s.
It is the electric field that moves quickly - at about the speed of light - through the circuit and carries the energy.
Electrical POWERWe have already learned that POWER is the rate at which work
(energy) is done. Circuits that are a prime example of this as batteries only last for a certain amount of time AND we get charged an energy bill each month based on the amount of energy we used over the course of a month…aka POWER.
POWERIt is interesting to see how certain electrical
variables can be used to get POWER. Let’s take Voltage and Current for example.
Other useful power formulas
These formulas can also be used! They are simply derivations of the POWER formula with different versions of Ohm's law substituted in.
Ways to Wire CircuitsThere are 2 basic ways to wire a circuit. Keep in
mind that a resistor could be ANYTHING ( bulb, toaster, ceramic material…etc)
Series – One after anotherParallel – between a set of junctions and parallel to each other
Schematic SymbolsBefore you begin to understand circuits you need to be able to
draw what they look like using a set of standard symbols understood anywhere in the world For the battery
symbol, the LONG line is considered to be the POSITIVE terminal and the SHORT line , NEGATIVE.The VOLTMETER and
AMMETER are special devices you place IN or AROUND the circuit to measure the VOLTAGE and CURRENT.
Closing the switch establishes a potential difference (voltage) and an electric field in the circuit.
Electrons flow in a net direction away from the (-) terminal.
High Potential
Low Potential
Conventional CurrentBy tradition,
direction in which “positive charges” would flow.
Direction is opposite of electron flow.
The Voltmeter and AmmeterThe voltmeter and ammeter cannot be just placed anywhere in the circuit. They must be used according to their DEFINITION.
Since a voltmeter measures voltage or POTENTIAL DIFFERENCE it must be placed ACROSS the device you want to measure. That way you can measure the CHANGE on either side of the device.
Voltmeter is drawn ACROSS the resistorSince the ammeter measures the current or FLOW it must be placed in such a way as the charges go THROUGH the device.
Current goes THROUGH the ammeter
Voltmeter
Measures the voltage between two points in an electric circuit.
Must be connected in parallel.
AmmeterMeasures electric current.
Must be placed in series.
Simple CircuitWhen you are drawing a
circuit it may be a wise thing to start by drawing the battery first, then follow along the loop (closed) starting with positive and drawing what you see.
Series CircuitIn in series circuit, the resistors
are wired one after another. Since they are all part of the SAME LOOP they each experience the SAME AMOUNT of current. In figure, however, you see that they all exist BETWEEN the terminals of the battery, meaning they SHARE the potential (voltage).
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Totalseries
Series Circuit
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Totalseries
As the current goes through the circuit, the charges must USE ENERGY to get through the resistor. So each individual resistor will get its own individual potential voltage). We call this VOLTAGE DROP.
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Totalseries
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RIRIRIRI
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;Note: They may use the terms “effective” or “equivalent” to mean TOTAL!
ExampleA series circuit is shown to the left. a) What is the total resistance?
b) What is the total current?
c) What is the current across EACH resistor?
d) What is the voltage drop across each resistor?( Apply Ohm's law to each resistor separately)
R(series) = 1 + 2 + 3 = 6
V=IR 12=I(6) I = 2A
They EACH get 2 amps!
V12 V V3=(2)(3)= 6V V2=(2)(2)= 4V
Notice that the individual VOLTAGE DROPS add up to the TOTAL!!
Parallel CircuitIn a parallel circuit, we have
multiple loops. So the current splits up among the loops with the individual loop currents adding to the total current
It is important to understand that parallel circuits will all have some position where the current splits and comes back together. We call these JUNCTIONS.
The current going IN to a junction will always equal the current going OUT of a junction.
Junctions OUTIN
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IIII
:Junctions Regarding
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Parallel CircuitNotice that the JUNCTIONS both touch the POSTIVE and NEGATIVE terminals of the battery. That means you have the SAME potential difference down EACH individual branch of the parallel circuit. This means that the individual voltages drops are equal.
This junction touches the POSITIVE terminal
This junction touches the NEGATIVE terminal
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Totalparallel
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Example
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To the left is an example of a parallel circuit. a) What is the total resistance?
b) What is the total current?
c) What is the voltage across EACH resistor? d) What is the current drop across each resistor? (Apply Ohm's law to each resistor separately)
2.20
3.64 A
8 V each!
9
8
7
8
5
8975 III
IRV
1.6 A 1.14 A 0.90 A
Notice that the individual currents ADD to the total.
Compound (Complex) CircuitsMany times you will have series and parallel in the SAME circuit.
Solve this type of circuit from the inside out.
WHAT IS THE TOTAL RESISTANCE?
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3.33;50
1
100
11
s
PP
R
RR
Compound (Complex) Circuits
3.1133.3380
3.33;50
1
100
11
s
PP
R
RR
T
T
TTT
I
I
RIV
)3.113(120
Suppose the potential difference (voltage) is equal to 120V. What is the total current?
1.06 A
What is the VOLTAGE DROP across the 80 resistor?
80
80
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V
V
RIV
84.8 V
Compound (Complex) Circuits
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R
T
T
T
06.1
8.84
06.1
120
3.113
80
80
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3&2
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8.84120
V
V
VVV
VVV
seriesT
parallelT
What is the VOLTAGE DROP across the 100 and 50 resistor?
35.2 V Each!
50
2.35100
2.35
50
100
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I
III
III
seriesT
parallelT
What is the current across the 100 and 50 resistor?
0.352 A
0.704 A
Add to 1.06A
The End