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Electric- Kirchoff.

posted Aug 6, 2015, 1:45 AM by Upali Salpadoru   [ updated Aug 7, 2016, 3:16 PM ]

Kirchhoff’s current Law.

Gustav Kirchhoff - Prussian. 1824 – 1887

This really is an extension of the Ohm’s law we use to find out the relationship between current, resistance and voltage. When there are too many connections this is not sufficient. The  two laws of Kirchhoff come to rescue.

First Law (Current Law)

The sum of current entering into a junction is equal to the sum of the current leaving the junction.

Giving a positive value for the currents entering and a negative value to currents leaving and add them up the answer should be zero.

In the diagram here arrows indicate the direction of the current.  Ampere readings are also given except in one. The diagram shows how you can use the Kirchhoff ‘s law to get the direction and value of the Blue wire.

KL 1.jpg

Fig.1 Current at a junction.

Kirchhoff’s Voltage Law.

In a circuit the voltages of the cells in a closed loop add up to zero.

We can follow this well if we use a sign convention.

Fig.2 How many circuits or loops are in this diagram?

You may be right. There are three loops shown in three colours.

1.       Green loop. Keep your finger at the point marked as the start. Then move it ina clock-wise direction along the circuit adding the voltages algebraically. That means if you meet a + you have to add , if you meet a minus subtract it.

Let us note the values:

                      + 6v  - 4.5v – 1.5v = 0

2.        Lets consider the red loop.

              +1.5v+2.5v -4v = 0

3.        Outer most blue loop

                + 6v – 4.5v +2.5v  - 4v =0

In the circuits given the rectangles indicate resistors, the circles ammeters and the squares voltmeters.

Question 1.0


1.       What is the voltage across the 4Ω  resistor.

2.       Using Kirchhoff’s voltage law find the  voltage of the bulb.

Question 2.0

Q 1.jpg

Find the following.

1.       The current from A to B?

2.       Reading on V1.

3.       Reading on V2

4.       Voltage of cell V3

5.       Voltage V4.

Question 3.0

Q 2.jpg

Find the following:-

  1. Current in A1. In loop

  2. Current in A2.

  3. Current in A3.

  4. Voltage V1.

  5. Voltage V2.

  6. Voltage V3.

Question 4.0

Q 3.jpg

      Find the following:-

1.       The combined Resistance of the parallel resistors in the ‘C’ loop.

2.       Resistance of the loop ABCDE.

3.       The current in ’a’.

4.       The current in ‘b’.

5.       Voltage in ‘c’.

6.       The current in ’d’.

7.       The voltage in ‘e’.

                     For answers turn to Physics Answer page.