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#Post#: 14--------------------------------------------------
Electrical Resistance
By: Dietrech Date: October 21, 2013, 9:29 am
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[font=comic sans ms]Ohm's Law
There are certain formulas in Physics that are so powerful and
so pervasive that they reach the state of popular knowledge. A
student of Physics has written such formulas down so many times
that they have memorized it without trying to. Certainly to the
professionals in the field, such formulas are so central that
they become engraved in their minds. In the field of Modern
Physics, there is E = m • c2. In the field of Newtonian
Mechanics, there is Fnet = m • a. In the field of Wave
Mechanics, there is v = f • . And in the field of current
electricity, there is V = I • R.
The predominant equation which pervades the study of electric
circuits is the equation
V = I • R
In words, the electric potential difference between two points
on a circuit (V) is equivalent to the product of the current
between those two points (I) and the total resistance of all
electrical devices present between those two points (R). Through
the rest of this unit of The Physics Classroom, this equation
will become the most common equation which we see. Often
referred to as the Ohm's law equation, this equation is a
powerful predictor of the relationship between potential
difference, current and resistance.
Ohm's Law as a Predictor of Current
The Ohm's law equation can be rearranged and expressed as
As an equation, this serves as an algebraic recipe for
calculating the current if the electric potential difference and
the resistance are known. Yet while this equation serves as a
powerful recipe for problem solving, it is much more than that.
This equation indicates the two variables that would affect the
amount of current in a circuit. The current in a circuit is
directly proportional to the electric potential difference
impressed across its ends and inversely proportional to the
total resistance offered by the external circuit. The greater
the battery voltage (i.e., electric potential difference), the
greater the current. And the greater the resistance, the less
the current. Charge flows at the greatest rates when the battery
voltage is increased and the resistance is decreased. In fact, a
twofold increase in the battery voltage would lead to a twofold
increase in the current (if all other factors are kept equal).
And an increase in the resistance of the load by a factor of two
would cause the current to decrease by a factor of two to
one-half its original value.
The table below illustrates this relationship both qualitatively
and quantitatively for several circuits with varying battery
voltages and resistances.
Circuit
Diagram
Battery
Voltage
(V)
Total
Resistance
()
Current
(Amps)
1.
1.5 V
3
0.50 Amp
2.
3.0 V
3
1 Amp
3.
4.5 V
3
1.5 Amp
4.
1.5 V
6
0.25 Amp
5.
3.0 V
6
0.5 Amp
6.
4.5 V
6
0.75 Amp
7.
4.5 V
9
0.50 Amp
Rows 1, 2 and 3 illustrate that the doubling and the tripling of
the battery voltage leads to a doubling and a tripling of the
current in the circuit. Comparing rows 1 and 4 or rows 2 and 5
illustrates that the doubling of the total resistance serves to
halve the current in the circuit.
Because the current in a circuit is affected by the resistance,
resistors are often used in the circuits of electrical
appliances to affect the amount of current that is present in
its various components. By increasing or decreasing the amount
of resistance in a particular branch of the circuit, a
manufacturer can increase or decrease the amount of current in
that branch. Kitchen appliances such as electric mixers and
light dimmer switches operate by altering the current at the
load by increasing or decreasing the resistance of the circuit.
Pushing the various buttons on an electric mixer can change the
mode from mixing to beating by reducing the resistance and
allowing more current to be present in the mixer. Similarly,
turning a dial on a dimmer switch can increase the resistance of
its built-in resistor and thus reduce the current.
The diagram below depicts a couple of circuits containing a
voltage source (battery pack), a resistor (light bulb) and an
ammeter (for measuring current). In which circuit does the light
bulb have the greatest resistance? Click the Check Answers
button to see if you are correct.
The Ohm's law equation is often explored in physics labs using a
resistor, a battery pack, an ammeter, and a voltmeter. An
ammeter is a device used to measure the current at a given
location. A voltmeter is a device equipped with probes that can
be touched to two locations on a circuit to determine the
electric potential difference across those locations. By
altering the number of cells in the battery pack, the electric
potential difference across the external circuit can be varied.
The voltmeter can be used to determine this potential difference
and the ammeter can be used to determine the current associated
with this V. A battery can be added to the battery pack and the
process can be repeated several times to yield a set of I-V
data. A plot of I versus V will yield a line with a slope that
is equivalent to the reciprocal of the resistance of the
resistor. This can be compared to the manufacturers stated value
to determine the accuracy of the lab data and the validity of
the Ohm's law equation.
Quantities, Symbols, Equations and Units!
The tendency to give attention to units is an essential trait of
any good physics student. Many of the difficulties associated
with solving problems may be traced back to the failure to give
attention to units. As more and more electrical quantities and
their respective metric units are introduced in this unit of The
Physics Classroom tutorial, it will become increasingly
important to organize the information in your head. The table
below lists several of the quantities that have been introduced
thus far. The symbol, the equation and the associated metric
units are also listed for each quantity. It would be wise to
refer to this list often or even to make your own copy and add
to it as the unit progresses. Some students find it useful to
make a fifth column in which the definition of each quantity is
stated.
Quantity
Symbol
Equation(s)
Standard Metric Unit
Other Units
Potential Difference
(a.k.a. voltage)
V
V = PE / Q
V = I • R
Volt (V)
J / C
Current
I
I = Q / t
I = V / R
Amperes (A)
Amp or C / s
or V /
Power
P
P = PE / t
(more to come)
Watt (W)
J / s
Resistance
R
R = • L / A
R = V / I
Ohm ()
V / A
Energy
E or PE
PE = V • Q
PE = P • t
Joule (J)
V • C or
W • s
(Note the unit symbol C represents the unit Coulombs.)
[/font]
#Post#: 16--------------------------------------------------
Re: Electrical Resistance
By: Kalindu Date: October 21, 2013, 9:36 am
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are all these coming tomorrow man :o :o :o :o :o :o :o :o :o :o
#Post#: 17--------------------------------------------------
Re: Electrical Resistance
By: Dietrech Date: October 21, 2013, 9:54 am
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Lol Kalindu, I posted some extra work as well to help
understand, not all of it is coming for the test.
#Post#: 40--------------------------------------------------
Re: Electrical Resistance
By: Dammaberlin Date: April 27, 2015, 7:40 am
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Criticism is wonderful.
#Post#: 48--------------------------------------------------
Re: Electrical Resistance
By: Lewisgowin Date: May 18, 2015, 2:52 am
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I think that is good enough.
#Post#: 72--------------------------------------------------
Re: Electrical Resistance
By: Sunyanicha Date: March 14, 2017, 1:49 am
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good comments.
#Post#: 76--------------------------------------------------
Re: Electrical Resistance
By: Taranut Date: January 5, 2018, 9:08 pm
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It helped me a lot.
#Post#: 123--------------------------------------------------
Re: Electrical Resistance
By: Fairyfrys Date: January 10, 2019, 4:28 am
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#Post#: 128--------------------------------------------------
Re: Electrical Resistance
By: Coconut Date: January 16, 2019, 11:12 pm
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