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Course: Physics library > Unit 12

Lesson 1: Ohm's law and circuits with resistors
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Ohm's law and circuits with resistors
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Kirchhoff's laws

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Kirchhoff's Laws describe current in a node and voltage around a loop. These two laws are the foundation of advanced circuit analysis. Written by Willy McAllister.
Kirchhoff's Laws for current and voltage lie at the heart of circuit analysis. With these two laws, plus the equations for individual component (resistor, capacitor, inductor), we have the basic tool set we need to start analyzing circuits.
This article assumes you are familiar with the definitions of node, distributed node, branch, and loop.
You may want to have a pencil and paper nearby to work the example problems.

Currents into a node

Try to reason through this example by yourself, before we talk about the theory. The schematic below shows four branch currents flowing in and out of a distributed node. The various currents are in milliamps, mA. One of the currents, i, is not known.
Problem 1: What is i?
Choose 1 answer:

Here's another example, this time with variable names instead of numerical values. This node happens to have 5 branches. Each branch might (or might not) carry a current, labeled i1toi5.
All the arrows are drawn pointing in. This choice of direction is arbitrary. Arrows pointing inward is as good a choice as any at this point. The arrows establish a reference direction for what we choose to call a positive current.
Look at branch current i1.
Where does it go?
The first thing i1 does is flow into the node (represented by the black dot).
Then what?
Here's two things i1 can't do: The flowing charge in i1 can't stay inside the node. (The node does not have a place to store charge). And i1's charge can't jump off the wires into thin air. Charge just doesn't do that under normal circumstances.
What's left?: The current has to flow out of the node through one or more of the other branches.
For our example node, we would write this as,
i1+i2+i3+i4+i5=0
If i1 is a positive current flowing into the node, then one or more of the other currents must be flowing out. Those outgoing currents will have a −negative sign.
This observation about currents flowing in a node is nicely captured in general form as Kirchhoff's Current Law.

Kirchhoff's Current Law

Kirchhoff's Current Law says that the sum of all currents flowing into a node equals the sum of currents flowing out of the node. It can be written as,
∑iin=∑iout

Kirchhoff's Current Law - concept checks

Currents are in milliamps, mA.
Problem 2: What is i5?
Choose 1 answer:

Problem 3: What is i3 in this distributed node?
Choose 1 answer:

Voltage around a loop

Below is a circuit with four resistors and a voltage source. We will solve this from scratch using Ohm's Law. Then we will look at the result and make some observations. The first step in solving the circuit is to compute the current. Then we will compute the voltage across the individual resistors.
We recognize this as a series circuit, so there is only one current flowing, i, through all five elements. To find i, the four series resistors can be reduced to a single equivalent resistor:
Rseries=100+200+300+400=1000Ω
Using Ohm's Law, the current is:
i=VRseries=20V1000Ω=0.020A=20mA
Now we know the current. Next we find the voltages across the four resistors. Go back to the original schematic and add voltage labels to all five elements:
Apply Ohm's Law four more times to find the voltage across each resistor:
vR1=iR
vR1=20mA⋅100Ω=+2V
vR2=20mA⋅200Ω=+4V
vR3=20mA⋅300Ω=+6V
vR4=20mA⋅400Ω=+8V
We know the current and all voltages. The circuit is now solved.
We can write the voltages for the resistors and the source on the schematic. These five voltages are referred to as element voltages. (The circuit nodes get names, a to e, so we can talk about them.)
Let's do a quick check. Add up the voltages across the resistors,
2V+4V+6V+8V=20V
The individual resistor voltages add up to the source voltage. This makes sense, and confirms our calculations.
Now we add up the voltages again, using a slightly different procedure: by "going around the loop." There's no new science here, we are just rearranging the same computation.

Procedure: Add element voltages around a loop

Step 1: Pick a starting node.
Step 2: Pick a direction to travel around the loop (clockwise or counterclockwise).
Step 3: Walk around the loop.
Include element voltages in a growing sum according to these rules:
  • When you encounter a new element, look at the voltage sign as you enter the element.
  • If the sign is +, then there will be a voltage drop going through the element. Subtract the element voltage.
  • If the sign is −, then there will be a voltage rise going through the element. Add the element voltage.
Step 4: Continue around the loop until you reach the starting point, including element voltages all the way around.

Apply the loop procedure

Let's follow the procedure step-by-step.
  1. Start at the lower left at node a.
  2. Walk clockwise.
A copy of the solved circuit.
  1. The first element we come to is the voltage source. The first voltage sign we encounter is a − minus sign, so there is going to be a voltage rise going through this element. Consulting the procedure step 3., we initialize the loop sum by adding the source voltage.
vloop=+20V going through the voltage source, to node b.
The next element we encounter is the 100Ω resistor. Its nearest voltage sign is +. Consult the procedure again, and this time we subtract the element voltage from the growing sum.
vloop=+20V−2V going through the 100Ω resistor, to node c.
Keep going. Next we visit the 200Ω resistor, and again we first encounter a + sign, so we subtract this voltage.
vloop=+20V−2V−4V going through the 200Ω resistor, to node d.
We complete the loop with the addition of two more elements,
vloop=+20V−2V−4V−6V through the 300Ω resistor, to node e.
vloop=+20V−2V−4V−6V−8V after the 400Ω resistor.
(Check the circuit diagram, make sure I got the last two − signs correct.)
  1. Done. We made it back home to node a. What does this expression for vloop add up to?
vloop=+20V−2V−4V−6V−8V=0
The sum of voltages going around the loop is 0. The starting and ending node is the same, so the starting and ending voltage is the same. On your "walk" you went up voltage rises and down voltage drops, and they all cancel out when you get back to where you started. This happens because electric force is conservative. There isn't a net gain or loss of energy if you return to the same place you started.
We'll do another example, this time with variable names instead of numerical values. The following familiar schematic is labeled with voltages and node names. The voltage polarity on the resistors is arranged in a way you might not expect, with all the arrows pointing in the same direction around the loop. This reveals a cool property of loops.
Let's take a walk around the loop, adding up voltages as we go. Our starting point is node a in the lower left corner. Our walk goes clockwise around the loop (an arbitrary choice, either way works).
Starting at node a, going up, we first encounter a minus sign on the voltage source, which says there is going to be a voltage rise of vab volts going through the voltage source. Because it is a voltage rise, this element voltage gets a + sign when we include it in the loop sum.
Continue around the loop from node b to c to d to e, and finish back home at node a. Append resistor voltages to the loop sum along the way. The polarity labels on all the resistors are arranged so we encounter a − sign as we approach each resistor. So the resistor voltages all go into the loop sum with a + sign. The final loop sum looks like this:
+vab+vR1+vR2+vR3+vR4
What does this add up to? Let's reason it out.
The loop starts and ends at the same node, so the starting and ending voltages are identical. We went around the loop, adding voltages, and we end up back at the same voltage. That means the voltages have to add to zero. For our example loop, we would write this as,
vab+vR1+vR2+vR3+vR4=0
This observation about voltages around a loop is nicely captured in general form as Kirchhoff's Voltage Law.

Kirchhoff's Voltage Law

Kirchhoff's Voltage Law: The sum of voltages around a loop is zero.
Kirchhoff's Voltage Law can be written as,
∑nvn=0
where n counts the element voltages around the loop.
You can also state Kirchhoff's Voltage Law another way: The sum of voltage rises equals the sum of voltage drops around a loop.
∑vrise=∑vdrop
Kirchhoff's Voltage Law has some nice properties:
  • You can trace a loop starting from any node. Walk around the loop and end up back at the starting node, the sum of voltages around the loop adds up to zero.
  • You can go around the loop in either direction, clockwise or counterclockwise. Kirchhoff's Voltage Law still holds.
  • If a circuit has multiple loops, Kirchhoff's Voltage Law is true for every loop.

Voltages all positive?

If you are wondering: how can the element voltages all be positive if they have to add up to zero? It's okay. The voltage arrows and polarity signs are just reference directions for voltage. When the circuit analysis is complete, one or more of the element voltages around the loop will be negative with respect to its voltage arrow. The signs of the actual voltages always sort themselves out during calculations.

Kirchhoff's Voltage Law - concept check

Problem 4: What is vR3?
Reminder: Check the first sign of each element voltage as you walk around the loop.
Choose 1 answer:

Summary

We were introduced to two new friends.
Kirchhoff's Current Law for branch currents at a node,
∑nin=0
Kirchhoff's Voltage Law for element voltages around a loop,
∑nvn=0
Our new friends sometimes go by their initials, KCL and KVL.
And we learned it's important to pay close attention to voltage and current signs if we want correct answers. This is a tedious process that requires attention to detail. It is a core skill of a good electrical engineer.

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  • blobby green style avatar for user Abhishek Kumar Sharma
    Posted 8 years ago. Direct link to Abhishek Kumar Sharma's post “what is the algebraic sum...”
    what is the algebraic sum?
    (2 votes)
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  • blobby green style avatar for user Christine Sharrer
    Posted 8 years ago. Direct link to Christine Sharrer's post “How is this affected if t...”
    How is this affected if there are multiple batteries, and therefore multiple currents? What if the resistors are in parallel?
    (5 votes)
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    • spunky sam orange style avatar for user Willy McAllister
      Posted 8 years ago. Direct link to Willy McAllister's post “Kirchhoff's Laws work for...”
      Good Answer
      Kirchhoff's Laws work for every circuit, no matter the number of batteries or resistor configuration. KCL tells you about the sum of currents at each specific node in the circuit. KVL tells you about the sum of voltage rises/drops around every loop of a circuit. KVL and KCL aren't fooled by multiple voltage or current sources, or parallel resistors. They always work.
      (14 votes)
  • leaf green style avatar for user dgdosen
    Posted 7 years ago. Direct link to dgdosen's post “When I see the words "vol...”
    When I see the words "voltage on each resistor" - what does that mean? that voltage is getting eaten up or used by the resistor? Is it wasting energy?
    (10 votes)
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    • spunky sam orange style avatar for user Willy McAllister
      Posted 7 years ago. Direct link to Willy McAllister's post “The phrases "Voltage on" ...”
      The phrases "Voltage on" and "Voltage across" mean the same thing. One end of the resistor is at a higher potential than the other end. The difference in potential is called the "voltage across" the resistor. It is the nature of resistors that when they have a voltage across them, a current flows. When current flows, there is energy being dissipated. If the circuit is doing something useful, then we say "Nice job using the energy! I like this song." If the circuit is not useful, then we might say "What a waste of energy, how sad".
      (2 votes)
  • blobby green style avatar for user Yash Trivedi
    Posted 8 years ago. Direct link to Yash Trivedi's post “How do we know that in ki...”
    How do we know that in kirchoff's voltage law we have to add or subtract the voltage
    (9 votes)
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  • piceratops seedling style avatar for user Daniel
    Posted 8 years ago. Direct link to Daniel's post “what is voltage?”
    what is voltage?
    (3 votes)
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    • piceratops seed style avatar for user jboling5
      Posted 8 years ago. Direct link to jboling5's post “It is the electric potent...”
      Good Answer
      It is the electric potential between two points. You probably know potential energy, like when a bowling ball is sitting on a skyscraper it has the potential energy stored from gravity because it could fall. Voltage is the same thing but with electrons (negatively charged) that have potential because they will try and move to a postive (or "less" negative) area.
      (12 votes)
  • marcimus pink style avatar for user mahimakashyap222
    Posted 7 years ago. Direct link to mahimakashyap222's post “Where does the current fl...”
    Where does the current flow when it enters the node.? What if all the current (arrows) are pointing inward, how is the sum of current zero in that case?
    (4 votes)
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  • leaf green style avatar for user drakshaspodia
    Posted 7 years ago. Direct link to drakshaspodia's post “is kirchhoff's law applic...”
    is kirchhoff's law applicable for ac circuits?
    (3 votes)
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    • purple pi purple style avatar for user APDahlen
      Posted 7 years ago. Direct link to APDahlen's post “Hello Drakshaspodia, Yes...”
      Hello Drakshaspodia,

      Yes, both KVL and KCL apply to AC circuits.

      You will find AC circuit are more interesting than DC circuits as capacitors and inductors change impedance as frequency changes. Think of impedance as the resistance to an AC signal.

      Regards,

      APD
      (5 votes)
  • aqualine seed style avatar for user silkyjhajharia2304
    Posted 7 years ago. Direct link to silkyjhajharia2304's post “what do we understand fro...”
    what do we understand from the arrows in the voltage diagrams
    what is voltage polarity
    (2 votes)
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    • purple pi purple style avatar for user APDahlen
      Posted 7 years ago. Direct link to APDahlen's post “Hello Here is how you m...”
      Hello

      Here is how you make the arrows.

      1) Assume a direction of current flow.

      2) Walk around the loop in the chosen direction of current flow. For sake of argument lets start at the 200 Ω resistor.

      2) As you walk along the circuit you will encounter a component. Ask yourself if the voltage as you enter this component is greater than or less than the exit terminal.

      3) If the voltage on the terminal you entered is greater than the exit terminal then the entrance terminal gets a “+”. And the other terminal gets a “-.”

      For a resister you don’t need to think very hard. If you are traveling in the direction of the current then the voltage on the terminal you enter will be higher than the exit.

      The battery is more interesting. As we travel we will enter the battery on the negative terminal. When we look across we see the positive terminal. Consequently the entrance terminal gets a ‘-’ and the exit terminal gets a “+” symbol.

      One last point - if you were tr travel all the way around this loop the sum of the voltages is equal to zero. This property is essential to you study of electronics. We give it the special name KCL (Kirchhoff's Voltage Law).

      Regards,

      APD

      P.S. You could have assumed the opposite direction for the current and everything would have mathematically worked out...
      (5 votes)
  • marcimus pink style avatar for user Muddassir Ahmed
    Posted 7 years ago. Direct link to Muddassir Ahmed's post “In circuit diagrams for e...”
    In circuit diagrams for example the circuit diagram before KVL(having Vab,VR1,VR2...) how we can give + and - signs to a voltage I mean how we can give polarity signs to a component
    (3 votes)
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    • spunky sam orange style avatar for user Willy McAllister
      Posted 7 years ago. Direct link to Willy McAllister's post “When you apply voltage to...”
      When you apply voltage to a circuit and let currents start to flow, each component will have some current and there will also be a voltage appearing between/across the terminals of the component (by virtue of Ohm's Law). When we put voltage labels on components we are just giving names to those voltages so we can talk about them between ourselves and in equations.

      You name a voltage by calling it something and putting + and - signs on it. You might be worried that the signs could be "wrong". Don't worry. We just give them names and signs so we have something to talk about. It's like saying that the top of a map is North. Just a reference direction. If (after we do the analysis) the voltage ends up being the other way around, it just gets a negative value. Just like if I'm walking South, I could say I'm walking -North.
      (2 votes)
  • blobby green style avatar for user cptlaurion
    Posted 7 years ago. Direct link to cptlaurion's post “Isn't the answer to the s...”
    Isn't the answer to the second question for i5 "-6 mA"? The answer key suggests "+6 mA" but this would give you a summation of +12 mA. I believe the description below the problem states the same.
    (2 votes)
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    • piceratops seedling style avatar for user Islam Hamed
      Posted 7 years ago. Direct link to Islam Hamed's post “Hi cptlaurion, the answer...”
      Hi cptlaurion,
      the answer key says "-6mA" going into the node OR "+6mA" going out of the node, they have the same meaning. the "+ve" sign means that your arbitrary direction you suggested is true and the "-ve" sign means that your arbitrary direction you suggested is false and the true current direction is in the opposite one.
      I hope you got what you need from my answer.
      (4 votes)