High School Physics : Kirchoff's Laws

Study concepts, example questions & explanations for High School Physics

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Example Questions

Example Question #1 : Kirchoff's Laws

Calculate the current through the 10 ohm resistor.

Possible Answers:

Correct answer:

Explanation:

To begin, let us start with the resistor and the  resistor that are in parallel. In parallel we can add resistors through the equation 

This new resistor is now in series with the two  resistors.  In series we can just add these resistors up.

 

This new resistor is now in parallel with the  resistor. In parallel we can add resistors through the equation 

This new resistor is now in series with the  and  resistor.  In series we can just add these resistors up.

Now that we have the equivalent resistance of the circuit.  We can now determine the current flowing out of the battery.

Rearrange to solve for current.

One of the best ways to work through a problem like this is to create a V= IR chart for all the components of the circuit.


Table1

 

We know that the current that is flowing out of the battery is the current that is flowing through both by 5 and 3 Ohm resistors since all of these are in series.  So we can put this information into our chart.

Table2

 

Using Ohm’s Law we can determine the voltage for each of these two resistors.

 

 Table3

 

We can now use Ohm’s law and our voltage to determine the current going through the 14 Ohm resistor.

 

Table4

Next we can analyze the current in the Junction between the 5, 14 and 4 Ohm resistors.  Kirchoff’s laws state that the sum of the current flowing in and out of junction must equal 0.

 

We have  flowing in from the  resistor and  flowing out to go through the  resistor.

 

So the current going through the 4 Ohm resistor is 3 amps.  This will be the same for both 4 ohm resistors as both have the same current going into and out of the junctions near them.  We can then use this to determine the voltage drop across each of the resistors.

 

 

Now let’s add this to our chart.

Table5

 

We can now use Kirchoff’s loop law to determine the voltage across the 10 Ohm resistor.  Let’s analyze the loop that goes from the 5, to the 4 to the 10 back through the 4 and then through 3 Ohm resistor.

We can now use Ohm’s Law to determine the current through the 10 Ohm resistor.

 

 

 

 

Example Question #2 : Kirchoff's Laws

Calculate the current in the 15 Ohm Resistor.

 

Possible Answers:

Correct answer:

Explanation:

To begin, let us start with the  resistor and the  resistor that are in parallel. In parallel we can add resistors through the equation 

This new resistor is now in series with the two  resistors.  In series we can just add these resistors up.

 

This new resistor is now in parallel with the  resistor. In parallel we can add resistors through the equation 

This new resistor is now in series with the  and  resistor.  In series we can just add these resistors up.

Now that we have the equivalent resistance of the circuit.  We can now determine the current flowing out of the battery.

 

Rearrange to solve for current.

 

 

One of the best ways to work through a problem like this is to create a V= IR chart for all the components of the circuit.

Table1

We know that the current that is flowing out of the battery is the current that is flowing through both by 5 and 3 Ohm resistors since all of these are in series.  So we can put this information into our chart.

Table2

Using Ohm’s Law we can determine the voltage for each of these two resistors.

Table3

We can now use Kirchoff’s loop law through the loop of the 5, 3, and 14 Ohm resistor to determine the voltage that is traveling through the 14 Ohm resistor.

We can now use Ohm’s law and our voltage to determine the current going through the 14 Ohm resistor.

We can now add this information to our chart.

Table4

Next we can analyze the current in the Junction between the 5, 14 and 4 Ohm resistors.  Kirchoff’s laws state that the sum of the current flowing in and out of the junction must equal 0.

 

We have 4A flowing in from the 5 Ohm resistor and 1A flowing out to go through the 14 Ohm resistor.

So the current going through the 4 Ohm resistor is 3 amps.  This will be the same for both 4 ohm resistors as both have the same current going into and out of the junctions near them.  We can then use this to determine the voltage drop across each of the resistors.

Now let’s add this to our chart.

 

Table5

We can now use Kirchoff’s loop law to determine the voltage across the 10 Ohm resistor.  Let’s analyze the loop that goes from the 5, to the 4 to the 10 back through the 4 and then through 3 Ohm resistor.

 

We can now use Ohm’s Law to determine the current through the 10 Ohm resistor.

We can now add this information to our chart.

Table6

 

We can now analyze the junction between the 4, 10 and 15 Ohm resistor.

Kirchoff’s laws state that the sum of the current flowing in and out of the junction must equal 0.

 

We have 2A flowing in from the 4 Ohm resistor and 1.2A flowing out to go through the 10 Ohm resistor.

 

 

 

 

 

 

 

 

 

 

Example Question #4 : Kirchoff's Laws

Calculate the current through the 6 ohm resistor.

 

Possible Answers:

Correct answer:

Explanation:

To begin we need to simplify the circuit to get the equivalent resistance.  Let’s start with the 3 and 6 Ohm resistors in parallel.

 

 

Now we can add this resistor to the 4 Ohm resistor as they are in series.

 

We can now determine the current coming out of the battery using Ohm’s Law.

 

Rearrange to solve for current.

 

The current coming out of the battery will be the same current that moves through the 4 Ohm resistor.  So we can determine the voltage drop across the 4 ohm resistor.

 

We can then use Kirchoff’s loop law to determine the voltage drop across the 6 Ohm resistor.  Let’s analyze the loop that goes from the battery to the 4 ohm resistor and through the 6 ohm resistor.

 

Example Question #5 : Kirchoff's Laws

Calculate the voltage drop across the 4 ohm resistor.

 

Possible Answers:

Correct answer:

Explanation:

To begin we need to simplify the circuit to get the equivalent resistance.  Let’s start with the 3 and 6 Ohm resistors in parallel.

 

Now we can add this resistor to the 4 Ohm resistor as they are in series.

 

We can now determine the current coming out of the battery using Ohm’s Law.

 

Rearrange to solve for current.

The current coming out of the battery will be the same current that moves through the 4 Ohm resistor.  So we can determine the voltage drop across the 4 ohm resistor.

 

 

Example Question #7 : Kirchoff's Laws

Kirchoff’s junction rule is an example of 

 

Possible Answers:

Conservation of charge

None of the givens answers

Conservation of momentum

Conservation of energy

Correct answer:

Conservation of charge

Explanation:

Kirchoff’s loop rules states the sum of the current going into and out of the junction must equal 0. In other words, the current going in must equal the current going on.  Current is a measure of the flow of charge.  Therefore, this law is conservation of charge as the number of electrons going into a junction must equal the number of electrons flowing out.

 

Example Question #1 : Kirchoff's Laws

 

 

Which of the equations here is valid for the circuit shown?

 

Possible Answers:

Correct answer:

Explanation:

To answer this question we must consider Kirchoff’s Loop Law.  This law states that the voltage around any loop must equal 0.  In this case there are two different loops at play.  To begin, let’s start on the left with the 2 Volt battery.

 

As we start with the 2 Volt battery, we then move into the 1 Ohm resistor with  going through it.  Ohm’s law states that the voltage is equal to the current times the resistance.  Therefore the voltage through this circuit is  Since this resistor is using the voltage this will be a negative voltage when we sum around the loop.

 

We will continue our loop through the middle of the circuit into the 4 Volt battery.  This battery is facing the opposite direction from our 2 Volt battery and therefore will be a negative when it comes to our equation.  

 

Next is  the 2 Ohm resistor with  going through it.  According to Ohm’s law the voltage being used by this resistor is equal to   


When summarizing all of these parts we get an equation that looks like which simplifies down to  This is one of the equations available to us and therefore there is no need to analyze any other loops.

Example Question #9 : Kirchoff's Laws

 

Calculate the voltage drop across the 14 ohm resistor.

 

Possible Answers:

Correct answer:

Explanation:

To begin, let us start with the  resistor and the  resistor that are in parallel. In parallel we can add resistors through the equation 

This new resistor is now in series with the two 4Ω resistors.  In series we can just add these resistors up.

 

This new resistor is now in parallel with the  resistor. In parallel we can add resistors through the equation 

This new resistor is now in series with the  and  resistor.  In series we can just add these resistors up.

Now that we have the equivalent resistance of the circuit.  We can now determine the current flowing out of the battery.

 

Rearrange to solve for current.

One of the best ways to work through a problem like this is to create a V= IR chart for all the components of the circuit.


Table1

We know that the current that is flowing out of the battery is the current that is flowing through both by 5 and 3 Ohm resistors since all of these are in series.  So we can put this information into our chart.

Table2

 

Using Ohm’s Law we can determine the voltage for each of these two resistors.

 Table3

We can now use Kirchoff’s loop law through the loop of the 5, 3, and 14 Ohm resistor to determine the voltage that is traveling through the 14 Ohm resistor.

 

 

 

Example Question #2 : Kirchoff's Laws

Calculate the voltage drop from point  to point

 

Possible Answers:

Correct answer:

Explanation:

To begin we need to simplify the circuit to get the equivalent resistance.  Let’s start with the 3 and 6 Ohm resistors in parallel.

 

Now we can add this resistor to the 4 Ohm resistor as they are in series.

 

We can now determine the current coming out of the battery using Ohm’s Law.

 

Rearrange to solve for current.

 

The current coming out of the battery will be the same current that moves through the 4 Ohm resistor.  So we can determine the voltage drop across the 4 ohm resistor.

 

We can then use Kirchoff’s loop law to determine the voltage drop from point  to point .

Let’s analyze the loop that goes from the battery to the 4 ohm resistor and through the 3 ohm resistor.

 

Example Question #2 : Kirchoff's Laws

Kirchoff’s loop rule is an example of 

Possible Answers:

Conservation of charge

Conservation of momentum

None of the givens answers

Conservation of energy

Correct answer:

Conservation of energy

Explanation:

Kirchoff’s loop law states that the sum of the voltage around a loop must equal zero.  In other words, the voltage that is being provided by the batteries in the circuit must equal the voltage being used by the objects in the circuit.  Voltage is a measure of the potential difference, or energy within the circuit.  In other words, the battery does a certain amount of work and provides energy to the circuit which is then used by all the parts of the circuit.  Therefore this is an example of conservation of energy.

 

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