Electrical Principles - Chapter 1: Ohm’s Law, Power Law and RC circuits
Publish Date: Mar 27, 2013
Overview
The Electrical Principles/Fundamentals series present the basic theories and concepts taught at entry level electronics courses
both 2 year and 4 year institutions. This series of content provides examples to professors to enable them to easily teach conce
to students, who can develop a solid underlying knowledge of electronics using the NI solution. This series focuses on some of
basic theory as well as providing the NI Multisim circuits to enable practical implementation end experimentation as homework
students.
Table of Contents
1.
2.
3.
4.
5.
6.
7.
In this Chapter
Example Courses
Ohm’s Law
Power Law
Example Problem
Suggested NI Solution
References
1. In this Chapter
We begin this chapter by exploring a couple of the basic rules of circuit analysis: Ohm’s Law and Power Law then apply them t
simple RC circuit and calculate the current, voltage, and power dissipation. We will use the NI Multisim circuit teaching
environment to verify our calculated results with example circuits that can be used by any educator or student.
If you do not have NI Multisim installed on your computer, you can download a free 30 day evaluation at
http://www.ni.com/multisim/try/ (http://www.ni.com/multisim/try/)
2. Example Courses
Listed below are example courses that teach this concept at their schools.
Course Name
School
Learn More
Electrical
Principles
Conestoga College
http://www.conestogac.on.ca/fulltime/0071.jsp
Electronic
Technology 1
Macomb
Community College
http://www.macomb.edu/noncms/Search/Courses/coursekey.asp?coursekey=ELEC-11
3. Ohm’s Law
Ohm’s Law states that the voltage drop across a device is directly proportional to the current passing through it. The constant o
proportionality is its resistance.
V = I x R [1]
4. Power Law
The Power Law states that the power dissipated in a device is inversely proportional to the squared value of the voltage across
P = V2/R [2]
It can also be stated as the power dissipated in a device is directly proportional to the squared value of the current going throug
it:
P = I2 x R [2]
Also, the power law can be dictated simply as:
P = V x I [2]
5. Example Problem
Let us now examine the below circuit and apply the above laws to determine the different currents and voltages as well as the
power dissipated.
STEP 1: Open circuit file “rc_circuit.ms12” using NI Multisim. You will see the circuit below [3].
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Answer Sub-Step 1: Consider the “Immediately after closing switch” phase
Assume the capacitors are completely uncharged.
Immediately after we close the switch, the capacitors will remain uncharged (since they do not charge instantaneously) and
therefore the voltage drop across them will be equal to zero; i.e. current will pass through them freely as in a simple wire. [3,
STEP 2: Open Circuit file “rc_after_closing.ms12” which shows the following circuit:
Since there is a short circuit, the current will not pass through the R1 and R3 branches.
Answer Sub-Step 2: Calculate the voltage drop in the above circuit
Using Ohm’s Law: lculate the power dissipated in the above circuit
Using the Power Law in R2 : P = I2 x R = 32 x 4 = 36W
Answer Sub-Step 4: Consider the “Steady State” phase of the above circuit
In this case the above circuit representation is no longer valid. The capacitors when fully charged will not allow any more cur
to go through and therefore will act as open circuits, and all the resistors in series will those capacitors will then behave as
simple wires. [3, 4]
STEP 3: Open circuit file “rc_steady_state.ms12”. You will notice the following circuit:
Answer Sub-Step 5: Calculate the voltage drop for the above circuit
Since no current is passing through the capacitor C1, we will consider the left loop only and can calculate:
Rseries = R1 + R2 = 6 Ω
Therefore: V = I x R and I = 12 / 6 = 2 A
Answer Sub-Step 6: Calculate the power dissipated in the above circuit
The power dissipated in the R1 : P = I2 x R = 22 x 2 = 8 W
And for R2 : P = 16W
STEP 4: Re-open circuit file “rc_after_closing.ms12” then double-click on the multimeter to open the front panel and choose the
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STEP 4: Re-open circuit file “rc_after_closing.ms12” then double-click on the multimeter to open the front panel and choose the
“A” button to measure the current. Then, simulate the circuit by clicking on the run button or choosing “Simulation>>Run
Simulation”.
You will notice that the current passing through the circuit is indeed 3 A which confirms the correct value of the calculated po
as well.
STEP 5: Re-open circuit file “rc_steady_state.ms12” then again double-click on the multimeter to open the front panel and choo
the “A” button to measure the current. Then, simulate the circuit by clicking on the run button or choosing “Simulation>>Run
Simulation”.
Thus we have verified that the calculated current value is correct and therefore the power dissipated across the two resistors
also correct.
6. Suggested NI Solution
National Instruments offers a number of products that combine to provide a scalable and powerful teaching platform for educato
The solution includes:
NI Multisim circuit teaching environment: Combining an intuitive circuit definition environment, with powerful SPICE simulation
technology, educators can use NI Multisim to easily teach the ins-and-outs of circuits in a safe environment.
NI ELVIS teaching and measurement platform allows educators to provide students with a compact, all-in-one unit for their
measurement and analysis needs. Combining an oscilloscope, function generator, DMM, bode analyzer and 8 other instrument
into a small platform; it simplifies the laboratory experience for students and lab instructors.
7. References
[1] Hoppe, Patrick. Wisconsin Technical College System. “Wisc-Online”. Ohm’s Law: The Relationship of Voltage, Resistance,
Current.
[http://www.wisc-online.com/Objects/ViewObject.aspx?ID=DCE8104]. (05/02/2013)
[2] Georgia State University, Department of Physics and Astrology. “DC Circuits”. DC Electric Power.
[http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elepow.html#c1]. (05/02/2013)
[3] Brightstorm. “RC Circuits”. RC Circuits.
[http://www.brightstorm.com/science/physics/electricity/rc-circuits/]. (18/01/2013)
[4] W. G. Olgham, University of California. “RC Circuits”. Charging and discharging in RC circuits.
[http://www-inst.eecs.berkeley.edu/~ee42/sp01/LectNotes/Lect8.PDF]. (05/02/2013)
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