Download Observation Experiment: Ohm`s Law 3.1 Use the equipment

Zubrzycki: DC Circuits 3
Observation Experiment: Ohm’s Law
3.1 Use the equipment provided to determine the relationship between current, potential
difference, and resistance.
Equipment: rheostat, power source, ammeter, voltmeter, connecting wires
Did you know?
Resistance of a material depends on its resistivity (ρ), length (L), cross-sectional area (A),
and temperature (T). The resistance is directly proportional to the length of the material.
For this experiment we determine the relative resistance of a rheostat using the length of
the rheostat being used.
Observation Experiment 1: Hold the length of the rheostat constant and vary the voltage;
measure the current.
Observation Experiment 2: Hold the voltage constant and vary the length of the rheostat
being used; measure the current.
Build your circuit. DO NOT PLUG THE POWER SUPPLY INTO THE ELECTRICAL
OUTLET. YOU MUST OBTAIN TEACHER’S APPROVAL BEFORE
BEGINNING YOU EXPERIMENTS!
DAMAGE CONTROL:
Do not turn power supply on before approval!
Do not exceed 12 volts on the voltmeter! (stay below 10-12 volts for max.)
For best results, use the center of the rheostat. Vary the length of the rheostat being
used in experiment 2 by intervals of about 2 centimeters.
Include the following in your lab write-up:
 Purpose
 Equipment: List and draw the symbol for each piece of equipment. Describe how to
connect each to the circuit.
 Circuit Diagram (set-up with symbols) and Sketch of set-up.
 Procedure: Describe your experiment fully. Explain how you will vary each variable
with the equipment available.
 Data: Construct your data table(s).
 Graph: Place current on the y-axis for both graphs. Analyze. Re-graph any inverse
relationship to determine the direct relationship. Analyze slopes including units.
 Conclusion: State the relationship between current, resistance, and voltage. Support
with graphs. State a formula that combines them mathematically. Describe current in
terms of resistance, potential difference, and electric energy.
Zubrzycki: DC Circuits 3
The mathematical pattern you found is the basis of the relationship for Ohm’s Law. As
current is proportional to potential difference, a proportionality constant must be included
in the equation. The relationship between current and voltage can be written as follows:
Where the current, I, is the dependent variable, 1/R is the slope of the line on the graph
(where R is the resistance), and ∆V is the potential difference.
Resistance is a physical quantity and is measured in ohms (the symbol is W ).
3.2 Imagine that you have a 9.0-V battery connected by wires to a light bulb. Fill in the
table that follows. Consider that the negative terminal of the battery is at zero potential.
Draw the circuit.
Draw a qualitative electric potential versus position
graph.
∆V (V)
Negative
battery
terminal
Positive One side
of the
battery
bulb
terminal
Other
side of
the bulb
Negative
battery
terminal
3.3 Represent and reason Complete the table that follows for the circuit in your
illustration.
You have a circuit with a 1.5 V battery and
identical resistors in series. Draw a circuit
diagram and label 6 points there with the
letters A, B, C, D, E, and F.
Plot the electric potential-versus-position for the circuit.
Does it matter what the resistance of the identical resistors
is?
∆V (V)
2
1
0
A
B
C
D
E
F
A
Zubrzycki: DC Circuits 3
3.4 Regular Problem You connect a 75 W resistor to a 9 V battery. What is the current
through the resistor?
3.5 Regular Problem You put a light bulb in a circuit with a 9 V battery. The ammeter
shows that the current though it is 0.07A. What is the resistance of the bulb?
3.6 Explain A person accidentally touches a 120 V electric line with one hand while
touching a ground wire with the other hand. Determine the current through the body
when the hands are dry (100,000 W resistance) and when wet (5000 W resistance). Are
either or both currents dangerous? Explain how you know. (For reference: 0.001 A =
faint tingle, 0.005 A = slight shock, 0.006-0.030 A = painful shock, 0.05 – 0.150 A =
Extreme pain, cannot let go. over 1 A = death is likely).
3.7 Regular Problem The light in an automobile draws a current of 1.0 A when
connected to a 12 V battery. What is the light's resistance? (b) What potential difference
is needed to produce a current of 5.0 mA through a 2.0- W resistor?
3.8 Regular Problem If a long wire is connected to the terminals of a 12 V battery, 3.6 x
1019 electrons pass a cross section of the wire each second. Determine its resistance.
3.9 Regular Problem As the potential difference in volts across a thin platinum wire
increases, the current in amperes changes as follows: (I; ∆V) = (0 A; 0 V), (0.112 A; 1.00
V), (0.337 A; 3.00 V), and (0.675 A; 6.10 V). Plot a graph of current as a function of
potential and indicate whether the platinum wire satisfies Ohm's law. If so, what is the
resistance of the wire?
3.10 What factors determine whether a resistor obeys Ohm’s Law? Is a light bulb an
ohmic resistor?