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  Main » Electricity » Kirchhoff's Laws » Lesson III.3.1 Electricity

Kirchhoff's Laws - Current and Voltage - Lesson III.3.1

Key Terms:

Kirchhoff's Current Law | Kirchhoff's Voltage Law | Series Circuit |Parallel Circuit |Equivalent (total) resistance |Power (P) |IL, CCT, NUM

Kirchhoff's Laws apply the Law of Conservation of Energy and the Law of Conservation of Charge. Kirchhoff's Laws deal with current and voltage in electrical connections. There are two basic types of connections (circuits).

Series Circuit

  • electrons flow along one path only:
Series Circuit
Parallel Circuit
  • electrons flow along more than one pathway(i.e. alternate branches for current to follow. The total current It will split into branch 1 - I1 and branch 2 - I2):
Parallel Circuit
Kirchhoff's Current Law states….

At any junction in an electric circuit, the total current flowing into the junction is the same as the total current leaving the junction.

Thus …

  • For a series circuit the current at all points will be the same since electrons can flow along only one path.
    Series Circuits
  • For a parallel circuit the total current flowing into a connection must equal the sum of the currents flowing out of the connection.
Parallel Circuits
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Kirchhoff's Voltage Law states

The algebraic sum of the potential difference around any closed path or loop must be zero.
Thus
  • For a series connection the total potential difference is equal to the sum of the potential differences across each component.
Vt = V1 + V2 + V3+....+Vn

Series Circuit
In a circuit, potential difference increases across the terminals of a cell(power supply) and decrease across the electrical components(i.e. resistors, lamps, speakers,..etc.). The increases in potential difference across the terminals must equal the sum of the decrease in potential difference across each component connected in series. This is a result of the Law of Conservation of Energy.

  • For parallel connections the drop in potential difference across all branches are equal.
Vt = V1 = V2 = V3=....Vn
Parallel Circuit
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Kirchhoff's Laws can also be applied to a circuit which is a combination of a series and a parallel connection.

For example:

Combination Circuit

Find I1, I3, R1, R2, R3, V1, and V2

Solution

  1. R1 is in series with the loop and therefore the current passing through R1 is the same amount entering the loop It= I1 = 620 mA.
  2. We don't yet know V1 so we can not calculate R1.
  3. 620 mA enters the parallel connection, 220 mA travels along one path through R2.
    620 mA - 220 mA = 400 mA travels along the other path through R3.
    Therefore I3 = 400 mA
  4. We know the current passing through R3 and the voltage drop accross it, now we can calcuate the resistance.
    R3 = V3 / I3
    R3 = 1.9 V / 0.400 A
    R2 = 4.75 Ohm.
  5. Since the potential difference across the parallel connection is 1.9 V, V2 = 1.9 V.
  6. We know the voltage drop accross R2 and the current passing through it, now we can calcuate the resistance.
    R2 = V2 / I2
    R2 = 1.9 V / 0.220 A
    R2 = 8.64 Ohm.
  7. In the loop containing R1 and the 2nd loop, the sum of the potential difference drops across the components must equal the potential difference increase. Therefore 4.5 V = 1.9 V + V1 and V1 = 2.6 V.
  8. Now we can calculate R1.
    R1 = V1 / I1
    R1 = 2.6 V / 0.620 A
    R1 = 4.19 Ohm.
 

 

 

 

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