EF 152 – Physics for Engineers
Spring, 2014
Recitation 4.3 Ohm’s Law
Objectives:
 Learn to use a basic multimeter
 Learn Ohm’s Law through discovery based learning
There are three basic properties of electric circuits:
 potential – units of volts
 current – units of amps
 resistance – units of Ohms
We will measure each of these quantities in this lab, and also determine the relationship between them.
An analogy with fluid flow is often used. The potential is like a water pump; it gives energy to the system.
The current is like the flow rate of the fluid. Resistance is like a constriction in the pipe, or friction. It is something
that takes energy out of the system.
Analogies to electric circuits:
Title Voltage Current Resistance Current & Flow rate Laws Ground Fluids Pressure = Energy/Volume • A closed faucet has pressure but no flow Thermal
Temperature = Energy/k
• k is Boltzmann’s constant • An isolated body has temperature, but no heat flow Volume flow rate = Volume/Time Heat flow rate = Heat/time
Resistance represented by a Resistance is provided by severe constriction or obstruction insulation, or thermal will produce a pressure drop; resistance Resistance of a wire is represented by pressure loss in the pipe or hose. Poiseuille’s Law: Heat flow:
ΔT VolumeFlowrate
Heat flow rate
Conservation of Liquid: Conservation:  There is no net pressure change  There is no net temperature in any closed loop path change (change in internal energy) in a closed cycle.  A reservoir serves as a pressure Absolute zero serves as temperature reference. reference.  A reservoir can supply water to a circuit. Once the pipe is filled with water, the pump can circulate the water without further use of the reservoir. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir.html
http://faraday.physics.utoronto.ca/IYearLab/Intros/DCI/Flash/WaterAnalogy.html
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Electric Circuit Voltage = Energy/Charge • A free electrical outlet has voltage but no current Current = Charge/Time Resistance represented by a “resistor” will produce a potential drop Ohms Law: Current
Conservation of Charge:  There is no net potential change in any closed loop path  A ground serves as a voltage reference.  A ground can supply charge to a circuit. EF 152 – Phy
ysics for Engiineers
Sp
pring, 2014
Task 1. Learrning how to work with a Multimeter
M
Measuring vo
oltage:
The voltage between
b
two points is a sh
hort name for the electrical force that wo
ould drive an electric
current betwe
een those poiints. The sym
mbol for voltag
ge (battery) is shown at the
e right and the
e units are
Volts (V).
Set the multim
meter to DCV
V (direct curre
ent volts). Record the read
ding for
each of the fo
ollowing case
es when connected to a D-cell battery. Hold the
common or ground
g
lead (C
COM) on the negative end of the batteryy and the
VΩmA end on the positive
e end of the battery. What is the voltage
e of the
battery? ____
_________
Now put the scale
s
on 2000
0m and conne
ect COM lead
d to positive e
end of the
battery and th
he VΩmA lea
ad on the nega
ative end of th
he battery. W
What do
you read? __
___________
_
Scale
Read
ding
200 m
2000 m
20
200
1000
Measuring re
esistance:
The electrica
al resistance of
o an object is
s a measure of
o its oppositio
on to the passsage of an ele
ectric
current. The symbol for a resistor is sh
hown at the rig
ght and units are Ohms (Ω
Ω).
Choose three
e different res
sistors from yo
our packet an
nd calculate th
heir resistancce using the fo
ollowing chartt.
Calcculated
resiistance
(Ω)
M
Measured
re
esistance
(Ω)
Resiistor 1
Resiistor 2
Resiistor 3
Check your calculations
c
using the multimeter. Set th
he multimeterr to Ω and cho
oose the app
propriate resisstance
range. For example,
e
the 20K
2 range me
easures resistances up to 20,000 Ω.
Note that to measure
m
resis
stance using a multimeter the compone
ent must be re
emoved from the circuit alttogether.
Measuring cu
urrent:
Just as to me
easure flow off water (such as with a Venturi meter), tto measure current we nee
ed to place th
he meter in
the circuit.
o D cell batttery in the cirrcuit board.
 Put one
 Use a wire to conn
nect from the negative end
d of the batterry to one end of a light bulb.
e the VΩmA lead of the mu
ultimeter to th
he 10ADC plu
ug and switch the multimetter to 10A.
 Move
 Put the 10A lead of
o the multime
eter on the po
ositive spring connection o
of the battery and the COM
M lead on
the other
o
side the light bulb. Notice what ha
appens to the light bulb. R
Record the rea
ading of the m
meter.
This is the currentt in Amperes (A) or amps for
f short.
ove the 10A le
ead back to VΩmA
V
plug forr the rest of th
he lab.**
 **Mo
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EF 152 – Physics for Engineers
Spring, 2014
Task 2. Ohm’s Law
Select the 100Ω, the 330Ω, and the 560Ω resistors. Set up the circuit at
shown at the right. Set the multimeter to the 20m scale for DCA. Measure
the current for each of the resistors. Fill in the following table. The voltage
is the voltage you measured for the battery. In recording the current,
remember the meter is reading in milliamps. Record your values in amps,
not milliamps.
Resistance
Voltage (V)
Current (A)
V/R
100Ω
330Ω
560Ω
Calculate the ratio of the voltage to the resistance, and record in the last column of the table. Compare the third
column to the fourth column of the table. Based on this comparison, can you determine a relationship between
the voltage, current, and resistance? This is Ohm’s Law. Write it in the box.
OHMS’ LAW
Task 3. Lights in Circuits
Put both D-cell batteries in the circuit board. Use a short length of wire to connect the positive from one battery to
the negative of the second battery.
 Hook up a single light bulb and note the brightness. Sketch the connections that you made in the form of
a circuit diagram using the standard symbols shown below.
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EF 152 – Physics for Engineers
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Spring, 2014
Use additional wires as needed to connect a second light into
the circuit. Once you have achieved success, sketch the
connections that you made in the form of a circuit diagram.
Is your original light the same brightness, or was it brighter or
dimmer that it was? Can you explain any differences in the
brightness, or the fact that it is the same?
If one of the light bulbs is unscrewed, does the other bulb go
out or does it stay on? Why or why not?
If you could characterize the light bulbs as being in series or
parallel circuit, which would it be?
Devise another way of connecting two lights into the same
circuit; try it out. Sketch the circuit diagram.
Is your original light the same brightness, or was it brighter or
dimmer that it was? Can you explain any differences in the
brightness, or the fact that it is the same?
If one of the light bulbs is unscrewed, does the other bulb go
out or does it stay on? Why or why not?
If you could characterize the light bulbs as being in series or
parallel circuit, which would it be?
What are the apparent rules for the operation of lights in series? In parallel?
Upload pictures of your circuit diagrams to the drop box to obtain credit for this
recitation.
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