Electrical Resistance
Student Extras
Visit The Physics Classroom's Flickr Galleries and enjoy a photo overview of the topic of electric circuits.
The Calculator PadImprove your problem-solving skills with problems, answers and solutions from The Calculator Pad.
Ohm's Law
Explore the effect of varying resistance and voltage upon the current in a circuit with this interactive Java applet.
Ohm Zone
Use this virtual playground of switches, cells, wires, resistors and more to explore circuit principles. Best of all, you won't get shocked.
Teacher's Guide
Looking for a lab that coordinates with this page? Try the Voltage-Current-Resistance Lab from The Laboratory.
Curriculum CornerLearning requires action. Give your students this sense-making activity from The Curriculum Corner.
Curriculum CornerPractice makes perfect with this computational activity from The Curriculum Corner.
Ohm's Law
This interactive Java applet by Walter Fendt simulates Ohm's law. Students can alter the resistance or the voltage and observe its affect upon the current.
Cirque du Circuit: A Unit on Electric Circuits for High School Physics
This is a 10-day multimedia unit that explores the fundamentals of electric circuits in a web-based, interactive format; it demonstrates the use of Compadre filing cabinets.
Molecular Expressions: Java Tutorial - Ohm's Law
This Java applet provides an opportunity to modify the voltage and resistance of a simple circuit and to observe the effect on current.
Treasures from TPFPresent physics concepts with a historical context. Let The Physics Front's treasure box of catalogued resources help.
Treasures from TPFNeed ideas? Need help? Explore The Physics Front's treasure box of catalogued resources on Electricity and Electrical Energy.
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 • RIn 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.
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)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.
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(a.k.a. voltage) |
V |
V
= PE
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I = |
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or V / |
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PE
/ t
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• L / A
R = |
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PE |
PE
= V
• Q
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W • s |
(Note the unit symbol C represents the unit Coulombs.)
The next section of Lesson 3 will involve taking another look at the quantity power. A new equation for power will be introduced by combining two (or more) of the equations in the above table.
Check
Your Understanding
1. Which of the following will cause the current through an electrical circuit to decrease? Choose all that apply.
a. decrease the voltageb. decrease the resistance
c. increase the voltage
d. increase the resistance
2. A certain electrical circuit contains a battery with three cells, wires and a light bulb. Which of the following would cause the bulb to shine less brightly? Choose all that apply.
a. increase the voltage of the battery (add another cell)b. decrease the voltage of the battery (remove a cell)
c. decrease the resistance of the circuit
d. increase the resistance of the circuit
3. You have likely been warned to avoid contact with electrical appliances or even electrical outlets when your hands are wet. Such contact is more dangerous when your hands are wet (vs. dry) because wet hands cause ____.
a. the voltage of the circuit to be higherb. the voltage of the circuit to be lower
c. your resistance to be higher
d. your resistance to be lower
e. the current through you to be lower
4. If the resistance of a circuit were tripled, then the current through the circuit would be ____.
a. one-third as muchb. three times as much
c. unchanged
d. ... nonsense! There would be no way to make such a prediction.
5. If the voltage across a circuit is quadrupled, then the current through the circuit would be ____.
a. one-fourth as muchb. four times as much
c. unchanged
d. ... nonsense! There would be no way to make such a prediction.
6. A circuit is wired with a power supply, a resistor and an ammeter (for measuring current). The ammeter reads a current of 24 mA (milliAmps). Determine the new current if the voltage of the power supply was ...
a. ... increased by a factor of 2 and the resistance was held constant.b. ... increased by a factor of 3 and the resistance was held constant.
c. ... decreased by a factor of 2 and the resistance was held constant.
d. ... held constant and the resistance was increased by a factor of 2.
e. ... held constant and the resistance was increased by a factor of 4.
f. ... held constant and the resistance was decreased by a factor of 2.
g. ... increased by a factor of 2 and the resistance was increased by a factor of 2.
h. ... increased by a factor of 3 and the resistance was decreased by a factor of 2.
i. ... decreased by a factor of 2 and the resistance was increased by a factor of 2.
7. Use the Ohm's law equation to provide numerical answers to the following questions:
a. An electrical device with a resistance of 3.0b. When a voltage of 120 V is impressed across an electric heater, a current of 10.0 amps will flow through the heater if the resistance is ________
c. A flashlight that is powered by 3 Volts and uses a bulb with a resistance of 60
8. Use the Ohm's law equation to determine the missing values in the following circuits.
9. Refer to question 8 above. In the circuits of diagrams A and B, what method was used to control the current in the circuits? And in the circuits of diagrams C and D, what method was used to control the current in the circuits?