# Finding an open-cicuit voltage, why is resistor in series ignored?

• justin___
justin___
Homework Statement
The circuit you should use to find the open-circuit voltage, voc, is shown here. Note that the resistor to the right of terminals a and b has been removed to create the open circuit. Using any circuit analysis technique you like, find the open-circuit voltage.
Relevant Equations
KCL
I found how to get the solution to this question (the answer is 200V), but I don't understand why we ignore the 30kOhm resistor when using analysing the circuit. Because it is in series with the open voltage, wouldn't there be some voltage drop across the resistor that would affect the open-circuit voltage?

Thanks

Hint:
Can you write the equation for the voltage drop across a resistor given its value and the current thru it?

Upon inspection of that equation, the light bulb above your head will likely come on.

Cheers,
Tom

justin___
Tom.G said:
Hint:
Can you write the equation for the voltage drop across a resistor given its value and the current thru it?

Upon inspection of that equation, the light bulb above your head will likely come on.

Cheers,
Tom
Thanks for the hint. So because the 30kOhm resistor is in series with an open circuit, no current flows through it, therefore there is no voltage drop and we can ignore it from the equation?

Tom.G and DaveE

## What is an open-circuit voltage?

An open-circuit voltage is the voltage measured across the terminals of a component or a circuit when no load is connected, meaning there is no current flowing through the circuit. It represents the maximum potential difference that the source can provide.

## Why is a resistor in series ignored when calculating open-circuit voltage?

When calculating the open-circuit voltage, the resistor in series is ignored because there is no current flowing through the circuit. Since the voltage drop across a resistor is dependent on the current (V = IR), and the current is zero in an open-circuit condition, the voltage drop across the resistor is also zero. Therefore, it does not affect the open-circuit voltage.

## Does ignoring the series resistor affect the accuracy of the open-circuit voltage measurement?

No, ignoring the series resistor does not affect the accuracy of the open-circuit voltage measurement because the voltage drop across the resistor is zero when there is no current flow. The open-circuit voltage is solely determined by the source voltage.

## How do you measure open-circuit voltage in practice?

To measure open-circuit voltage in practice, you use a voltmeter connected across the terminals of the source or the component. Ensure that no load is connected to the circuit, so that the current is zero and the reading on the voltmeter reflects the true open-circuit voltage.

## Can the presence of a series resistor affect other parameters of the circuit when it is not open-circuit?

Yes, the presence of a series resistor can affect other parameters of the circuit when it is not open-circuit. When a load is connected and current flows through the circuit, the resistor will cause a voltage drop proportional to the current (V = IR), which will affect the total voltage available to the load and the overall performance of the circuit.

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