Lab 3 Ohm’s Law
Introduction
This lab examines the behavior of resistors and light bulb as related to Ohm’s law. It also looks at
the behaviors of simple circuits through the measurement of currents and voltages.
Equipment
Power supply, DMM, resistors, light bulb, assorted wires and clips..
Background
Most materials have a certain amount of electrical resistance.
When a voltage (potential
difference) is applied to an object and the subsequent current generated through that object is
related linearly to the applied voltage, that object is said to be ohmic. This is called Ohm’s law.
The proportionality of constant is the resistance of the object.
V = IR
Here, V is the potential difference or the voltage, I is the current, and R is the resistance.
There are two types of simple resistor circuits: the series-resistor circuit and the parallel-resistor
circuit. In a series-resistor circuit, resistors are connected end to end. The current must pass
through one resistor first then the other.
In a parallel-resistor circuit, the current can pass through both resistors at the same time.
Each type of resistor configuration has its own voltage and current characteristics.
Experiment: Ohm’s Law for a Resistor
Let’s test the resistor to see how it follows Ohm’s law. Select a resistor that is about 1,000 Ω.
Build the following circuit with the power supply (left). Attach a voltmeter and an ammeter to the
circuit simultaneously (right). You will need 2 DMM’s.
the circuit
the circuit with meters
V
A
Apply a voltage from the power supply from 0 V to 8 V in increments of 0.5 V. You don’t have to
set the voltage to an exact values since we are measuring it. For each applied voltage, measure
the corresponding current.
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Using Excel, plot the voltage (Y) vs the current (X). Is data linear? If so, find the resistance from
the slope. According to Ohm's law, what does the slope represent and what is its unit?
Compare the resistance from the graph to the resistance as measured directly using the DMM
ohmmeter. What is the percent error between them based on the ohmmeter measurement as the
accepted value?
Experiment: Ohm’s Law for a Light Bulb
Replace the resistor from the previous circuit with the light bulb.
the circuit
the circuit with meters
V
A
Again, measure the voltage and the generated current over the range from 0 V to 8 V.
Plot the voltage vs the current. Is the light bulb an ohmic device; that is, is the data linear? If so,
what is its resistance? If not, estimate the range of the light bulb’s resistance again using the
slope of the graph?
Experiment: Series-Resistor Circuit
Build the following series circuit with two resistors that are about 1 kΩ and 2 kΩ. Set the power
supply to 5 V. Measure the voltage across the power supply. Measure the voltages across both
resistors.
R1
R2
Next, measure the current entering each resistor and exiting the resistors.
R1
R2
R1
R2
R1
R2
A
A
A
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Questions
What is the relationship between the voltages across the resistors in a series circuit and the power
supply voltage?
What is the relationship between the currents going in and out of the resistors in a series circuit?
Parallel-Resistor Circuit
Build the following parallel circuit using the resistors above. Set the power supply to 5 V.
Measure the voltage across the power supply. Measure the voltages across both resistors.
R1
R2
Next, measure the current going into the parallel part of the circuit and measure the current
entering each resistor.
R1
R1
R1
A
A
A
R2
R2
R2
Questions
What is the relationship between the voltages across the resistors in a parallel resistor circuit and
the power supply voltage?
What is the relationship between the currents through the resistors in a parallel circuit and the
current going into parallel resistors?
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