# Solving Old Midterm Problem: Voltage Across 9kΩ Resistor

• Engineering
• gfd43tg
In summary, the conversation discusses a problem with finding the voltage across a resistor and the solution posted, which involves finding the current as a function of time and multiplying it by the resistor value. The solution also emphasizes the importance of defining symbols correctly and ensuring they are non-changing values. Additionally, an error is pointed out in the expression for i at t=0.
gfd43tg
Gold Member

I am working on the second problem of an old midterm, and I cannot figure out how they got that answer for the voltage across that 9kΩ resistor. I said the current going through the resistor is the same as that going through the inductor, so I figured I would find the current as a function of time, then just multiply by the resistor value.

I posted the solution as well

#### Attachments

• 2.2 attempt 1.pdf
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• Fall 2011 MT2.pdf
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• Fall 2011 MT 2 Solutions.pdf
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When you present a symbolic expression as an answer, you have to be sure that all symbols are defined. Also you want each symbol to be a non changing value. IL is a dynamic value. While your answer of Vo=IL*R3 is correct. it not the simplest. Find IL as a function of the input current, L, R1, R2, and R3

When t=0 the exp(-t/T) term equals 1.

So the expression you have for i becomes i(0) = i + ( i - i0 )

This does not look right. You have a sign wrong, or something.

Last edited:

## 1. What is the purpose of solving old midterm problems?

Solving old midterm problems can help students prepare for future exams by reinforcing their understanding of key concepts and identifying areas where they may need more practice.

## 2. How do you calculate the voltage across a 9kΩ resistor?

The voltage across a resistor is determined by multiplying the current flowing through the resistor by its resistance. In this case, the voltage across a 9kΩ resistor can be calculated using the formula V = IR, where V is the voltage in volts, I is the current in amps, and R is the resistance in ohms.

## 3. What is the significance of a 9kΩ resistor in a circuit?

A resistor is a component that is used to limit the flow of current in a circuit. A 9kΩ resistor, with its high resistance, can help control the amount of current flowing through a circuit and prevent damage to other components.

## 4. Can the voltage across a 9kΩ resistor change?

Yes, the voltage across a 9kΩ resistor can change if the current or resistance in the circuit changes. Additionally, the voltage may also change if the resistor itself is damaged or has a different tolerance level than expected.

## 5. What are some common mistakes when solving for the voltage across a 9kΩ resistor?

One common mistake is forgetting to convert units. The voltage should be calculated in volts, so the current must be in amps and the resistance in ohms. Another mistake is not taking into account the direction of the current flow. Lastly, incorrect substitutions in the formula can also lead to incorrect results.

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