Using the data of the previous problem, find the voltage across the external part of the circuit — Physics problems | PhysLabo
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Voltage across the external part of the circuit

Easy 10 grade Electricity / Complete Electrical Circuit
Problem statement

A source with emf $\varepsilon = 12$ V and internal resistance $r = 1$ ohm is connected to a resistor $R = 5$ ohm (the current in the circuit is $I = 2$ A). Find the voltage across the external part of the circuit.

Given
  • $\varepsilon = 12$ V
  • $r = 1$ ohm
  • $R = 5$ ohm
  • $I = 2$ A
  • $U = ?$
Hints
Hint 1 / 2
Voltage across the external part: $U = IR$.
Hint 2 / 2
You can check through the emf: part of the voltage is lost across the internal resistance, $U = \varepsilon - Ir$.
Solution

By Ohm's law for the external part of the circuit:

$$U = IR = 2 \cdot 5 = 10 \text{ V}$$

A second way to check — the voltage drop across the internal resistance:

$$U = \varepsilon - Ir = 12 - 2 \cdot 1 = 10 \text{ V}$$

Answer

$U = 10$ V

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