Using Kirchhoff's law, deduce the value and direction of the current

In summary, according to Kirchhoff's law, the value and direction of current I can be deduced as -2A entering from the top of the diagram. This may seem counterintuitive, but it is due to the nature of Kirchhoff's law and the fact that the positive or negative designation of current is based on the direction of the arrow on the diagram, not the actual direction of the current itself. Therefore, in this case, the current is actually entering the wire from the top, even though the arrow is pointing in the opposite direction. This satisfies Kirchhoff's law, as the current entering and exiting the wire is balanced.
  • #1
haha0p1
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9
Homework Statement
Use Kirchhoff's law to deduce the value and direction of the current I.
Relevant Equations
Current entering a point is equal to current exiting a point
3 A+ 2 A = 7 A+ I
I = -2A
How the current can be in negative direction? If the sign if negative, doesn't it mean that the current will move towards the point P? Also If I am taking the 2A to be positive then Kirchhoff's law isn't satisfied as 5A≠7A
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  • #2
haha0p1 said:
Homework Statement:: Use Kirchhoff's law to deduce the value and direction of the current I.
Relevant Equations:: Current entering a point is equal to current exiting a point

3 A+ 2 A = 7 A+ I
I = -2A
How the current can be in negative direction? If the sign if negative, doesn't it mean that the current will move towards the point P?
The arrow on the diagram indicates the positive direction for current ##I##. It's doesn't imply the current itself is in the positive direction.
 
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  • #3
PeroK said:
The arrow on the diagram indicates the positive direction for current ##I##. It's doesn't imply the current itself is in the positive direction
Even If I take the value of I to be 2 A, Still my current entering the wire and current exiting the wire is not the same as 3+5=7+2
And 5≠9
 
  • #4
haha0p1 said:
Even If I take the value of I to be 2 A
Why would you do that?
 
  • #5
PeroK said:
Why would you do that?
Because The question is asking to find value of I which is coming 2
 
  • #6
haha0p1 said:
Because The question is asking to find value of I which is coming 2
##I \ne 2## as you yourself have already established. And, you seem to know that ##I = -2## but have decided that current must be positive. That puts you in an impossible position in this case.
 
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  • #7
think of it this way:

If you designate a current leaving a node as positive and it mathematically turns out negative via KCL then that current is actually entering.

What’s really happening is that

3A is entering from the right
2A is entering from the bottom
2A is entering from the left

7A is leaving from the top
 
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FAQ: Using Kirchhoff's law, deduce the value and direction of the current

What is Kirchhoff's Current Law (KCL) and how is it used to find the current in a circuit?

Kirchhoff's Current Law (KCL) states that the total current entering a junction in a circuit must equal the total current leaving the junction. To use KCL to find the current, you identify all the junctions in the circuit, write equations that set the sum of currents entering each junction equal to the sum of currents leaving, and solve the resulting system of equations.

What is Kirchhoff's Voltage Law (KVL) and how does it help in determining the current direction?

Kirchhoff's Voltage Law (KVL) states that the sum of all electrical potential differences around any closed loop in a circuit must equal zero. By applying KVL, you can write equations for each loop in the circuit that include the voltage drops and gains. Solving these equations helps determine the magnitude and direction of the current in each branch of the loop.

How do you set up the equations using Kirchhoff's laws for a given circuit?

To set up the equations using Kirchhoff's laws, follow these steps: 1) Identify all junctions and loops in the circuit. 2) Apply KCL to each junction to write current equations. 3) Apply KVL to each loop to write voltage equations. 4) Include Ohm's Law (V = IR) to relate voltages and currents in resistive elements. 5) Solve the system of linear equations to find the unknown currents.

How can you determine the direction of the current using Kirchhoff's laws?

The direction of the current can be assumed initially when setting up the equations. If the solution for the current is positive, the assumed direction is correct. If the solution is negative, the actual current flows in the opposite direction to the assumed one. The consistency in the signs of the current values obtained from solving the equations will indicate the correct directions.

Can Kirchhoff's laws be applied to both AC and DC circuits?

Yes, Kirchhoff's laws can be applied to both AC and DC circuits. For DC circuits, the calculations involve steady-state currents and constant voltages. For AC circuits, the laws are applied to the instantaneous values of currents and voltages, which may involve complex numbers to account for phase differences between voltage and current.

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