EF 152 – Physics for Engineers
Lab 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 – Physics for Engineers
Task 1. Learning how to work with a Multimeter
Measuring voltage:
The voltage between two points is a short name for the electrical force that would drive an electric
current between those points. The symbol for voltage (battery) is shown at the right and the units are
Volts (V).
Set the multimeter to DCV (direct current volts). Record the reading for
each of the following cases when connected to a D-cell battery. Hold the
common or ground lead (COM) on the negative end of the battery and the
VΩmA end on the positive end of the battery. What is the voltage of the
battery? ____________
Now put the scale on 2000m and connect COM lead to positive end of the
battery and the VΩmA lead on the negative end of the battery. What do
you read? ____________
Scale
Reading
200 m
2000 m
20
200
1000
Measuring resistance:
The electrical resistance of an object is a measure of its opposition to the passage of an electric
current. The symbol for a resistor is shown at the right and units are Ohms (Ω).
Choose three different resistors from your packet and calculate their resistance using the following chart.
Calculated
resistance
(Ω)
Measured
resistance
(Ω)
Resistor 1
Resistor 2
Resistor 3
Check your calculations using the multimeter. Set the multimeter to Ω and choose the appropriate resistance
range. For example, the 20K range measures resistances up to 20,000 Ω.
Note that to measure resistance using a multimeter the component must be removed from the circuit altogether.
Measuring current:
Just as to measure flow of water (such as with a Venturi meter), to measure current we need to place the meter in
the circuit.
 Put one D cell battery in the circuit board.
 Use a wire to connect from the negative end of the battery to one end of a light bulb.
 Move the VΩmA lead of the multimeter to the 10ADC plug and switch the multimeter to 10A.
 Put the 10A lead of the multimeter on the positive spring connection of the battery and the COM lead on
the other side the light bulb. Notice what happens to the light bulb. Record the reading of the meter.
This is the current in Amperes (A) or amps for short.
 **Move the 10A lead back to VΩmA plug for the rest of the lab.**
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EF 152 – Physics for Engineers
Task 2. Ohm’s Law
Select the 100Ω, the 330Ω, and the 560Ω resistors. Set up the circuit as
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
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EF 152 – Physics for Engineers
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.








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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