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For each homework question and lab report we will make a post, this will probably contain a few tips on what the problems are about and how to solve them. If you are stuck on something then instead of emailing us directly you should post a comment in reply to the relevant post. We will try to guide you through tough points and help you understand the problems and the concepts behind them.
Wednesday, September 1, 2010
Chapter 15, Questions 1-4
These four questions are from SV8 section 15.3. The whole assignment is due on Tuesday, September 7th at 3:30 PM.
12 comments:
Anonymous
said...
I'm having trouble understanding question 4 on webassign. any one have any tips. Ive read up to page 513. Any advice is appreciated!
If when a car is struck my lightning you are safe in the car because the charge, like Faraday's ice pail experiment holds the charge on the outside on the car. However my question is when you get out of a car that has been struck by lightning, what happens? The charge doesn't just stay there. Could you theoretically get a big shock if you touched the metal shell of the car?
10:02 AM: Start by looking at the 6.00 muC charge on the left. it is repelled (force to the left) by the 1.50 muC charge, and attracted by the -2.00 muC charge. Let's work out each of those forces. The 1.50 muC charge exerts a force ke * 6x10^-6 * 1.5*10^-6/(0.03)^2, in the negative x direction. Similar expression for the -2 muC charge, but now it is in the positive x direction, use 2 muC in stead of 1.5 muC, and the distance is 0.05 m. Pay attention to the signs when you combine the forces. Same idea for the other parts.
10:42. Your car might still hold a significant charge, so you could get a shock when you climb out. But if lightning struk your car, it probably also arced over between your car and the ground, essentially discharging it. Have you noticed chains hanging from the bottom of gasolne tankers, to keep them from accumulating a charge?
Hi..this is my first time using web assign..I had been using mastering physics for 121..so I was just wondering..Is this a timed assignment and once I click on the assignment link..do I only have one opportunity to complete it?
Not timed, and you have ten opportunities to work each problem. This was originally set up with 10% credit subtracted for each incorrect attempt, but we have changed it so that there is no penalty.
I am having trouble on question 2 of the homework, first I apply the Force equation to solve the question. we have the values for distance (r) which is 2.0x10^7 m and the Coulomb constant, but as for the two charges I first find there are two moles of protons and electrons, then I multiply each of them with Avacadro constant to get the number of protons and number of electrons. Next I multiply these two numbers separately with 1.6x10^-19 and -1.6x10^-19 to get the charges. After all that I apply the two charge numbers into equation but I got it wrong, please point out where I did wrong. Thanks.
I'm still on the WebAssign learning curve. I thought I configured it to not subtract the partial credit, but maybe that cannot be done once an assignment is launched. We'll sort it out somehow.
I am having a lot of trouble with question 3. I thought it would just be Coulumb's law applied to the two objects next to each other. i.e. F = 8.99E9 (6E-6 * 1.5E-6) / (9E-4), but I am getting the wrong answer. Can someone explain what I am missing?
That's the force that the 1.5 muC charge exerts on the 6.00 muC charge (and pay attention to direction - negative x direction on the 6 muC charge, positive on the 1.5), but there is another charge in the problem. On the 6 muC charge, there is also a force of 8.99E9 * (6E-6 * 2E-2)/(0.05^2) in the positive direction. In all cases, you have to consider force exerted by the other two charges.
12 comments:
I'm having trouble understanding question 4 on webassign. any one have any tips. Ive read up to page 513. Any advice is appreciated!
Question regarding page 515 applying physics 15.3.
If when a car is struck my lightning you are safe in the car because the charge, like Faraday's ice pail experiment holds the charge on the outside on the car. However my question is when you get out of a car that has been struck by lightning, what happens? The charge doesn't just stay there. Could you theoretically get a big shock if you touched the metal shell of the car?
10:02 AM: Start by looking at the 6.00 muC charge on the left. it is repelled (force to the left) by the 1.50 muC charge, and attracted by the -2.00 muC charge. Let's work out each of those forces. The 1.50 muC charge exerts a force ke * 6x10^-6 * 1.5*10^-6/(0.03)^2, in the negative x direction. Similar expression for the -2 muC charge, but now it is in the positive x direction, use 2 muC in stead of 1.5 muC, and the distance is 0.05 m. Pay attention to the signs when you combine the forces. Same idea for the other parts.
10:42. Your car might still hold a significant charge, so you could get a shock when you climb out. But if lightning struk your car, it probably also arced over between your car and the ground, essentially discharging it. Have you noticed chains hanging from the bottom of gasolne tankers, to keep them from accumulating a charge?
Hi..this is my first time using web assign..I had been using mastering physics for 121..so I was just wondering..Is this a timed assignment and once I click on the assignment link..do I only have one opportunity to complete it?
Not timed, and you have ten opportunities to work each problem. This was originally set up with 10% credit subtracted for each incorrect attempt, but we have changed it so that there is no penalty.
I am having trouble on question 2 of the homework, first I apply the Force equation to solve the question. we have the values for distance (r) which is 2.0x10^7 m and the Coulomb constant, but as for the two charges I first find there are two moles of protons and electrons, then I multiply each of them with Avacadro constant to get the number of protons and number of electrons. Next I multiply these two numbers separately with 1.6x10^-19 and -1.6x10^-19 to get the charges. After all that I apply the two charge numbers into equation but I got it wrong, please point out where I did wrong. Thanks.
I think that's the wrong value for the separation between the charges. Radius of Earth is 6.38*10^6 m (6.38E+06), and the separation is twice that.
Professor Stephens,
You had said that there is no 10 percent penalty for each wrong attempt but webassign still seems to be extracting points!
Prof. Stephens, I'm also experiencing the 10% penalty even though you said it was reconfigured not to do so.
I'm still on the WebAssign learning curve. I thought I configured it to not subtract the partial credit, but maybe that cannot be done once an assignment is launched. We'll sort it out somehow.
I am having a lot of trouble with question 3. I thought it would just be Coulumb's law applied to the two objects next to each other. i.e. F = 8.99E9 (6E-6 * 1.5E-6) / (9E-4), but I am getting the wrong answer. Can someone explain what I am missing?
That's the force that the 1.5 muC charge exerts on the 6.00 muC charge (and pay attention to direction - negative x direction on the 6 muC charge, positive on the 1.5), but there is another charge in the problem. On the 6 muC charge, there is also a force of 8.99E9 * (6E-6 * 2E-2)/(0.05^2) in the positive direction. In all cases, you have to consider force exerted by the other two charges.
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