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Ohm’s Law
A German physicist, Georg S. Ohm, developed this mathematical relationship, which is present in most circuits.
This relationship is known as Ohm's law. This relationship states that if the voltage (energy) in a circuit increases,
so does the current (flow of charges). If the resistance increases, the current flow decreases. In a typical circuit
example, a battery provides the voltage, a light bulb (or resistor) provides the resistance, and the current flows
through the entire circuit, including the wires. All work must be shown in order to receive full credit!
Circuit Example
𝑰 (𝒄𝒖𝒓𝒓𝒆𝒏𝒕 𝒊𝒏 𝒂𝒎𝒑𝒔) =
𝑽 (𝒑𝒐𝒕𝒆𝒏𝒕𝒊𝒂𝒍 𝒅𝒊𝒇𝒇𝒆𝒓𝒆𝒏𝒄𝒆 𝒊𝒏 𝒗𝒐𝒍𝒕𝒔)
𝑹 (𝒓𝒆𝒔𝒊𝒔𝒕𝒂𝒏𝒄𝒆 𝒊𝒏 𝒐𝒉𝒎𝒔)
*Keep in mind that the equation can be rearranged to have
the variables in different locations.
𝑰 =
𝑽
𝑹
=>
𝑹 =
𝑽
𝑰
=>
𝑽 = 𝑰𝑹
Hint: I = 4 Amps
http://zone.ni.com/devzone/cda/tut/p/id/3037
Answer and solve the following questions.
1)
Which of the following will cause the current through an electrical circuit to increase? Choose all that apply.
a. decrease the voltage
b. 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)
If the resistance of a circuit were tripled, then the current through the circuit would be ____.
a. one-third as much
b. three times as much
c. unchanged
d. ... nonsense! There would be no way to make such a prediction.
4)
A light bulb has a resistance of 4 ohms and a current of 2 A. What is the voltage across the bulb?
Ans: 4 ohms x 2 A = 8 V
5)
A MP3 player uses two 1.5 V batteries. If the current in the MP3 player is 2 A, what is its resistance?
Ans: (2) (1.5 V) / 2 A = 1.5 ohms
6)
You have a large flashlight that takes 4 D-cell batteries. If the resistance of the light bulb in the flashlight is 3
ohms, what is the current of the light bulb? (Hint: A D-cell battery has 1.5 volts.)
Ans: (4)(1.5 V) / 3 ohms = 2 amps
Physics
In the figure to the right is a nanowire (the tiny white strip that sits across
all the electrodes). In order to measure the resistance across the nanowire,
special tools are used to probe the electrodes (metal contacts that extend
from the nanowire).
7)
If the wires are nanosized, what range do you think the electric
current flow would be in? (Hint: Think of the wire as a hose and
current is the amount of water going through it.) Choose all that
apply.
a. mega 106
b. kilo
103
c. milli 10-3
d. micro 10-6
e. nano 10-9
f. pico
10-12
8)
Use the Ohm's law equation to provide numerical answers to the following questions:
a. A nanowire has a voltage drop of 0.119 volts across it. If the current flowing through the wire is measured
as 4.99 x 10-10 amps, the wire must have a resistance of ________ ohms.
Ans: 0.119 V / 4.99 x 10-10 amps = 2.23 x 108 ohms.
b. An nanowire with a resistance of 0.21 ohms will allow a current of 7.36x10-10 amps to flow through it if a
voltage drop of ________ Volts is impressed across the device. (Impressed across just means the voltage
applied to the object.)
Ans: 0.21 ohms x 7.36x10-10 amps = 1.56 x 10-10 Volts
9)
Use the Ohm's law equation to determine the missing values in the following circuits.
Part A Ans:
I = 2 Amp
R = 3 Ohms
Ohms
Part B Ans:
V=6V
R = 6V/1 Amp=6
Part C Ans:
I = 2 Amp
V = (2 Amp)(2 Ohms)
= 4 Volts
Part C Ans:
V = (1 Amp)(2 Ohms) = 2 Volts
Physics
10) Using the provided data on nanowires, plot the I-V curve. (I -> current on the y-axis and V -> voltage on the
x-axis). Draw a line connecting all the points and describe what type of line or curve you have.) Referring to
Ohm’s Law, does this make sense? Why or why not? Turn in answer with graph.
IV Curve
7.00E-09
6.00E-09
5.00E-09
Current (I)
4.00E-09
Series1
3.00E-09
2.00E-09
1.00E-09
0.00E+00
0.00E+00
-1.00E-09
5.00E+00
1.00E+01
1.50E+01
2.00E+01
2.50E+01
3.00E+01
Voltage (V)
Answer: It is a linear curve. The graph should look similar to the one above, except the one above covers more
points. The graph should make sense because as you continue to increase the voltage, the amount of current
continues to increase using the same resistance (same nanowire) for measured values.
Physics Joke:
Q: What is the name of the first electricity detective?.........A: Sherlock Ohms
Adapted from
http://www.physicsclassroom.com/Class/circuits/u9l3c.cfm
Physics First Classroom
http://jupiterscientific.org/sciinfo/jokes/physicsjokes.html
Physics