Need help to find I in a circuit (mesh analysis?)

In summary, the student attempted to solve equations using mesh current analysis, but was having difficulty getting the correct current for the loops. They were confused because the current direction was not what they assumed it to be.
  • #1
asdf12312
199
1

Homework Statement


http://tinypic.com/r/3478nle/6
3478nle.jpg

need to find the current I

Homework Equations


was thinking about using mesh current analysis, but our teacher hasn't gone over it yet.

The Attempt at a Solution


no clue, totally stumped on what to do. was thinking mesh analysis and i attempted that, but i had no idea what i was doing after a while.
 
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  • #2
asdf12312 said:

Homework Statement


http://tinypic.com/r/3478nle/6
need to find the current I

Homework Equations


was thinking about using mesh current analysis, but our teacher hasn't gone over it yet.

The Attempt at a Solution


no clue, totally stumped on what to do. was thinking mesh analysis and i attempted that, but i had no idea what i was doing after a while.

Mesh analysis looks like a fine idea. Why don't you show how you got started so we can see how to help you when you get stuck?
 
  • #3
well i assigned currents I1, I2, and I3 to the right, left, and top loops respectively. moving in clockwise direction this what i got:

right loop:
3V+21(I1)-6(I2)-6(13)=0

left loop:
3V-6(I1)+21(I2)-6(I3)=0

top loop:
-6(I1)-6(I2)+21(I3)=0

now the trouble I'm having is the part where I'm supposed to solve all 3 equations simultaneously. my method is getting I1=I2=I3 in the bottom two equations and then plugging them into eq#1 so i can solve for I1 (only current i need to find).

cancelling out I3:
3.5(3V-6(I1)+21(I2)-6(I3)=0)
-6(I1)-6(I2)+21(I3)=0
--
10.5V-21(I1)+73.5(I2)-21(I3)=0
-6(I1)-6(I2)+21(13)=0
--
-27(I1)+67.5(I2)=-10.5
67.5(I2)=27(I1)-10.5
I2=0.4(I1)-0.16

cancelling out 12:
3V-6(I1)+21(I2)-6(I3)=0
3.5(-6(I1)-6(I2)+21(I3)=0)
--
3V-6(I1)+21(I2)-6(I3)=0
-21(I1)-21(12)+73.5(I3)=0
--
-27(I1)+67.5(I3)=-3V
67.5(I3)=27(I1)-3
I3=0.4(I1)-0.044

substituting into eq#1:
21(I1)-6(I2)-6(13)=-3
21(I1)-6(0.4(I1)-0.16)-6(0.4(I1)-0.044)=-3
21(I1)-2.4(I1)+0.96-2.4(I1)+0.26=-3
16.2(I1)=-4.22
I1=-0.26A

but i know i did this wrong...cause i got a negative current.
 
  • #4
Nothing wrong with a negative current, it just means that your assumed current direction for the loop is the opposite of the actual current direction. Your assumed direction was a clockwise mesh current, right? So the actual current is counterclockwise.

Note the direction that was assigned to I in the diagram; It's the opposite of your assumed mesh current. So I = -I1 ...
 
  • #5


I would recommend seeking out additional resources or seeking help from a tutor or classmate. Mesh analysis can be a complex concept and it is important to fully understand it before attempting to apply it to a circuit. Additionally, it may be helpful to review any related material or examples from class or your textbook to gain a better understanding of the concept. Don't be afraid to ask for help or clarification from your teacher as well. Remember, it is always better to fully understand a concept before attempting to apply it.
 

FAQ: Need help to find I in a circuit (mesh analysis?)

What is mesh analysis and why is it important?

Mesh analysis is a method used to solve electrical circuits by analyzing the currents flowing through individual loops or "meshes" within the circuit. It is important because it allows us to determine the current and voltage values at different points in the circuit, which is crucial for designing and troubleshooting electrical systems.

How do you identify the meshes in a circuit?

To identify the meshes in a circuit, you need to first draw a schematic diagram of the circuit. Then, look for any closed loops or paths that do not contain any other loops within them. These closed loops are the meshes of the circuit.

Can you explain the steps for solving a circuit using mesh analysis?

Sure, the steps for solving a circuit using mesh analysis are as follows:

1. Identify the meshes in the circuit.

2. Assign a current variable to flow through each mesh in a clockwise or counterclockwise direction.

3. Write Kirchhoff's Voltage Law equations for each mesh, using Ohm's Law (V=IR) to represent the voltage drops across resistors.

4. Solve the resulting system of equations to find the current values for each mesh.

5. Use the current values to calculate the voltage and power at different points in the circuit.

Are there any limitations to using mesh analysis?

Yes, there are a few limitations to using mesh analysis. It can only be used for circuits with independent current sources and resistors. It also cannot be used for circuits with voltage sources that are not connected to any resistors. Additionally, it can become quite complex for circuits with a large number of meshes.

Can you use mesh analysis for AC circuits?

Yes, mesh analysis can be used for AC circuits as well. However, the voltage drops across resistors must be represented using complex numbers, and the resulting equations may become more complicated to solve compared to DC circuits. It is also important to take into account any reactance in the circuit, such as capacitors or inductors, when using mesh analysis for AC circuits.

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