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KIRCHHOFF’S VOLTAGE LAW

Kirchhoff’s Voltage law

The algebraic sum of the potential difference in any closed loop is equal to zero. The total sum of all E,m,f in a closed loop is equal to the total sum of the voltage (IR) drops in the loop.

From the diagram( fig 1) above

Where

    are the internal resistances of the cells  

Positive and Negative E.m.f  with Kirchhoff law

See figure 2 above to know when to use -E or +E

Positive and Negative IR with Kirchhoff law

See figure 3 above to know when to use -IR  or +IR

Example

Calculate the current across the 3Ω resistor in fig 4 above. What is the voltage across ab ?

Solution

The two 10Ω resistors are parallel   10‖10 =5Ω

Total voltage = 12-4=8v

8=3I+2I+4I+5I

8 =14I            

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