# Several basic resistor prelab questions

• gnarlyskim
In summary, the conversation involves a student seeking help with a prelab for their electromagnetism class. The prelab explores the characteristics of wires in DC circuits and assumes a wire resistance of 1.0 Ohms, a 6V ideal voltage source, and a 30 Ohm light bulb resistance. The student struggles with applying Ohm's law to different problems and asks for assistance in interpreting the meaning of voltage in each situation.
gnarlyskim
OK so I am new to these forums so please forgive me for any mistakes. I have a prelab for my electromagnetism class. We just began this chapter in class and the book is harder to understand than the problems! Here's the problems and my shot at them:

1. Homework Statement

1. This prelab explores the characteristics of wires in DC circuits. Keep in mind that an ordinary wire has low but finite resistance. For these questions assume the wire resistance is 1.0 Ohms, the power supply is an ideal 6V voltage source, and the light bulb resistance is 30 Ohms.
For the circuit below, one such wire is connected directly across the power supply. What is the voltage drop (in V) across the wire?

http://img156.imageshack.us/img156/3593/phy1sy6.png

2. Calculate the current (in A) in the wire.

http://img156.imageshack.us/img156/7119/phy2ks5.png

3. In the circuit below, a lightbulb is connected to the power supply using two such wires. Calculate the current (in A) in the circuit.

http://img26.imageshack.us/img26/6919/phy3st0.png

4. Calculate the voltage drop (in V) across each wire.

http://img120.imageshack.us/img120/7647/phy4yt2.png

5. Calculate the voltage drop (in V) across the lightbulb.

http://img26.imageshack.us/img26/2457/phy5sl4.png

6. Now assume the wires in have zero resistance (0 Ohms). Calculate the current (in A) through the lightbulb.

http://img26.imageshack.us/img26/2165/phy6gd1.png

7. Again, assuming the wires have zero resistance, calculate the voltage drop (in V) across the lightbulb.

http://img120.imageshack.us/img120/4402/phy7go6.png

8. A current flows in the resistor below. Which of the following is true?

http://img159.imageshack.us/img159/3627/phy8jc0.jpg

A. V1 = V2 but I1 ≠ I2
B. I1 = I2 but V1 ≠ V2
C. V1 = V2 and I1 = I2
D. V1 ≠ V2 and I1 ≠ I2

9. Positive currents flow in the directions shown in the resistors below. Which of the following is true?

http://img159.imageshack.us/img159/5792/phy9rh8.jpg

A. I1 = I2 + I3
B. I2 = I1 + I3
C. I3 = I1 + I2
D. I1 + I2 + I3 = 0

2. Homework Equations

V=IR
R=V/I
I=V/R

3. The Attempt at a Solution

Ok so here is my attempt for each:

1. For the circuit below, one such wire is connected directly across the power supply. What is the voltage drop (in V) across the wire?
So the question is asking for voltage drop (V). V=IR, but what is the I? I took a guess and did V=(6A)(1$$\Omega$$). The 1$$\Omega$$ coming from the wire.
Thus, voltage drop=6V

2. Calculate the current (in A) in the wire.
This problem is how I found 6A for the I in question 1. I used I=V/R.
Thus, I=6V/1$$\Omega$$ .;. I=6A

3. In the circuit below, a lightbulb is connected to the power supply using two such wires. Calculate the current (in A) in the circuit.
This one is asking for current I (A) for the circuit including two wires (contributing 1$$\Omega$$ each?) and the lightbulb (contributing 30$$\Omega$$) powered by 6V.
So, I=V/R, I=(6V)/(30$$\Omega$$+2$$\Omega$$)=.1875A

4. Calculate the voltage drop (in V) across each wire.
I don't know where to go with this one. I assume that voltage remained constant in these...so V=6?

5. Calculate the voltage drop (in V) across the lightbulb.
Same situation as above...but this started making me feel a little suspicious that my inclinations were not as correct as I had hoped.

6. Now assume the wires in have zero resistance (0 Ohms). Calculate the current (in A) through the lightbulb.
No contribution to resistance from the wires, so only the 30$$\Omega$$ from the lightbulb should be factored in.
I=V/R, I=(6V)/(30$$\Omega$$)=.2A

7. Again, assuming the wires have zero resistance, calculate the voltage drop (in V) across the lightbulb.
Same situation as the voltage drop questions above.

8. A current flows in the resistor below. Which of the following is true?
A. V1 = V2 but I1 ≠ I2
B. I1 = I2 but V1 ≠ V2
C. V1 = V2 and I1 = I2
D. V1 ≠ V2 and I1 ≠ I2
I was following my instinct as said before on this problem, so my guess was A. Voltage remained constant, but the current changed as indicated through the problems above.

9. Positive currents flow in the directions shown in the resistors below. Which of the following is true?
A. I1 = I2 + I3
B. I2 = I1 + I3
C. I3 = I1 + I2
D. I1 + I2 + I3 = 0
This one I based off human instinct (so it's probably wrong ha). My guess was B, where I2=I3+I1. It makes sense that this should be so, but I have some underlying feeling that there is a physics property that is going to come into play here.

THANK YOU for anyone who can look at these and give me some help!

Last edited by a moderator:
gnarlyskim said:
OK so I am new to these forums so please forgive me for any mistakes. I have a prelab for my electromagnetism class. We just began this chapter in class and the book is harder to understand than the problems! Here's the problems and my shot at them:

1. Homework Statement

1. This prelab explores the characteristics of wires in DC circuits. Keep in mind that an ordinary wire has low but finite resistance. For these questions assume the wire resistance is 1.0 Ohms, the power supply is an ideal 6V voltage source, and the light bulb resistance is 30 Ohms.
For the circuit below, one such wire is connected directly across the power supply. What is the voltage drop (in V) across the wire?

http://img156.imageshack.us/img156/3593/phy1sy6.png

I read through the problems and I haven't the time to directly address each individual problem, however I noticed that your main issue is with interpretting Ohms law.
You need to ask yourself what that V means, is it the voltage across the battery or the voltage drop across the resistor you are looking at.

For the last 2 you need to consider what happens with the electrons. (Essentially what goes into the resistor must come out) That should be enough to get you through

Last edited by a moderator:

I would like to first commend you for taking the initiative to work through these prelab questions on your own. It shows a dedication to understanding the material and a good work ethic.

Now, let's take a closer look at your solutions and address any misconceptions or mistakes.

1. For the circuit below, one such wire is connected directly across the power supply. What is the voltage drop (in V) across the wire?

Your solution is correct. Using Ohm's law, V=IR, we can calculate the voltage drop to be 6V. This is because the wire has a resistance of 1 Ohm and the current flowing through it is 6A, as determined in question 2.

2. Calculate the current (in A) in the wire.

Your solution is correct. Using Ohm's law, I=V/R, we can calculate the current to be 6A. This is because the voltage across the wire is 6V (as calculated in question 1) and the resistance is 1 Ohm.

3. In the circuit below, a lightbulb is connected to the power supply using two such wires. Calculate the current (in A) in the circuit.

Your solution is incorrect. The lightbulb has a resistance of 30 Ohms, not 2 Ohms. Therefore, the total resistance in the circuit is 30 Ohms + 30 Ohms = 60 Ohms. Using Ohm's law, I=V/R, we can calculate the current to be 0.1A. This is because the voltage across the circuit is 6V (as calculated in question 1) and the total resistance is 60 Ohms.

4. Calculate the voltage drop (in V) across each wire.

Your solution is incorrect. In this circuit, the voltage is divided between the two wires and the lightbulb. Using the formula V=IR, we can calculate the voltage drop across each wire to be 3V. This is because the current flowing through each wire is 6A (as calculated in question 2) and the resistance of each wire is 1 Ohm.

5. Calculate the voltage drop (in V) across the lightbulb.

Your solution is incorrect. As mentioned in question 4, the voltage is divided between the two wires and the lightbulb. Therefore, the voltage drop across the

## 1. What is the purpose of a resistor?

A resistor is an electrical component that is used to restrict or control the flow of electric current in a circuit. It is commonly used to reduce the voltage or to limit the amount of current that passes through a circuit.

## 2. How is the resistance of a resistor measured?

The resistance of a resistor is measured in ohms (Ω) using a multimeter. It is important to note that the resistance of a resistor can vary depending on factors such as temperature and the material it is made of.

## 3. What factors affect the resistance of a resistor?

The resistance of a resistor is affected by factors such as the material it is made of, its length and cross-sectional area, and the temperature of the resistor. Generally, materials with higher resistivity (such as carbon or ceramic) will have a higher resistance.

## 4. Can the resistance of a resistor be changed?

Yes, the resistance of a resistor can be changed by adjusting the length, cross-sectional area, or material of the resistor. Additionally, the resistance can also be changed by connecting multiple resistors in series or parallel.

## 5. What is the difference between a series and parallel circuit?

In a series circuit, the components (including resistors) are connected in a single path, so the same current flows through each component. In a parallel circuit, the components are connected in multiple paths, so the current is divided between them. This results in different resistance values for the same resistors in a series versus parallel configuration.

• Introductory Physics Homework Help
Replies
9
Views
1K
• Introductory Physics Homework Help
Replies
31
Views
2K
• Introductory Physics Homework Help
Replies
2
Views
1K
• Introductory Physics Homework Help
Replies
11
Views
2K
• Introductory Physics Homework Help
Replies
3
Views
1K
• Introductory Physics Homework Help
Replies
3
Views
554
• Introductory Physics Homework Help
Replies
3
Views
3K
• Introductory Physics Homework Help
Replies
1
Views
5K
• Introductory Physics Homework Help
Replies
5
Views
1K
• Introductory Physics Homework Help
Replies
11
Views
2K