Voltage drop specifies how the supplied energy from a voltage source is reduced as electric current flows through the elements that do not supply voltage (passive elements) of the electrical circuits. The voltage drop across conductors, across connectors and internal resistances of the source is unwanted since the supply energy is lost. The voltage drop across active circuit elements and loads are desired since the supplied power performs efficient work.
The voltage drop formula is given by,
V = I Z
Where,
I = Current in amperes
Z = impedance in Ω
Example 1:
A current of 9A flows through a circuit that carries a resistance of 10 Ω. Determine the voltage drop across the circuit.
Solution:
Given:
Current I = 9A,
Impedance Z = 10Ω
the voltage drop formula is given by
V = IZ
  = 9 × 10
  V= 90 v.
Example 2
A lamp of 15 Ω and 30 Ω are connected in series. A current of 4 A is made to flow through it. Determine the voltage drop.
Solution:
Given:
Resistance Z = (15 + 30) Ω
   Z = 45 Ω,
Current I = 4A
The voltage drop formula is given by,
V = IZ
   = 4 × 45
  V= 180 V
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