How Do Electric Charge and Fields Influence Forces and Charges?

[Dx]

Hello,

I have some missed test questions that I still don't understand. Can anyone please explain these.

1)
Two chrged objects atrract each other with a force F. What happens to the F between them if one is doubled, the other is tripled and the separation distance between their centers is reduced to 1/4 its original value? the force is now equal to

16F
3/8F
96F

2)What is the charge on 1kg of protons?

1C
9.6 x 10^7 C
6x10^26C

3)A copper penny has mass 3g. A total of 4x10^12 electrons are transfored from one nuetral penny to another. if the electrostatic force of attraction between the pennies is equal to the weight of a penny, what is the separation between them?

31cm
35cm
37cm

4) How can a negatively charged rod charge an electroscope negatively? Is it by conduction because its not induction.

can anyone gimme their answer and how they derived it since I missed it I provided the remaining test answers to help you choose from. Please any help is appreciated as always.

Thanks!
Dx

 

[quantumdude]

As you know, I don't like posting complete solutions here, especially when the poster hasn't shown any work. It doesn't teach you anything when someone shows you how to solve a problem. So, I am going to give you hints so that you can figure them out for yourself.

Originally posted by Dx
1)
Two chrged objects atrract each other with a force F. What happens to the F between them if one is doubled, the other is tripled and the separation distance between their centers is reduced to 1/4 its original value? the force is now equal to

16F
3/8F
96F

First, note that all 3 changes serve to increase the force, so you can eliminate choice #2 immediately.

Second, take the expression for Coulomb's law:

F=kq₁q₂/r²

Now let q₁-->2q₁
and q₂-->3q₂
and r-->(1/4)r

Make the substitutions in Coulomb's law and answer the question.

2)What is the charge on 1kg of protons?

1C
9.6 x 10^7 C
6x10^26C

Easy as pie.

You need the mass and charge of a proton, and from there it's a simple unit conversion.

3)A copper penny has mass 3g. A total of 4x10^12 electrons are transfored from one nuetral penny to another. if the electrostatic force of attraction between the pennies is equal to the weight of a penny, what is the separation between them?

31cm
35cm
37cm

They give you the magnitude of the electric force. It equals m_pennyg. They also give you the charge on each penny. Use Coulomb's law:

F=kq₁q₂/r²

The only unknown is r, and that is what they are asking for.

4) How can a negatively charged rod charge an electroscope negatively? Is it by conduction because its not induction.

There's no hint I can think of that doesn't give it away. You'll have to do the research on it.

 

[frankR]

I don't understand the first question.

Is it asking: What is the force between the two chrages if the charge of one particle is double and the other trippled and their distance of separation cut by a quarter.

Set it up:

F = k * q^2/r^2
Ff = k * 6q^2/(1/4r)^2

Mentor Edit: Rest of solution removed.

I apprciate anyone who helps out in this forum, but let's stick to giving hints. A few complete solutions to problems have been posted lately, and I'm cracking down!

 

[Dx]

Originally posted by frankR
I don't understand the first question.

Is it asking: What is the force between the two chrages if the charge of one particle is double and the other trippled and their distance of separation cut by a quarter.

Set it up:

F = k * q^2/r^2
Ff = k * 6q^2/(1/4r)^2

Mentor Edit: Rest of solution removed.

I apprciate anyone who helps out in this forum, but let's stick to giving hints. A few complete solutions to problems have been posted lately, and I'm cracking down!

You did it exactly as i did, I got 24F and it was marked wrong on my test.

Please understand I am serious about cheating also and don't believe in it fellas, please understand your help is appreciated and ALL I AM ASKING is how you solved it. Thats it! Besides I can't learn if I don't know what's going on and I hate typing all my work just to get a few sentences of "HINTS". So ty to understand I am not about cheating I just want to understand why I missed those questions and I appreciate everyones help. Tom, you always do a great job of explaining, i thank you, sir.
TY!
Dx

 

[quantumdude]

Originally posted by Dx
Please understand I am serious about cheating also and don't believe in it fellas, please understand your help is appreciated and ALL I AM ASKING is how you solved it.

Cheating is not the issue. The issue is that, if we simply hand the solution and an explanation to you, you aren't learning anything. You have to figure these things out for yourself, and if you post your work we can help you with that (This is the Homework Help forum, after all).

Post your work, and we will tell you where you went wrong.

 

[Dx]

I see! But that's sure a lot of typing I must do.
I'll comply from this day forward.

 

[frankR]

Originally posted by Dx
You did it exactly as i did, I got 24F and it was marked wrong on my test.

Dx

Ummm:

What's 6/(1/4)²?



Opps! Edit

 

[frankR]

Originally posted by frankR
Mentor Edit: Rest of solution removed.

I apprciate anyone who helps out in this forum, but let's stick to giving hints. A few complete solutions to problems have been posted lately, and I'm cracking down!

Ummmm: Sorry, I wasn't really thinking about that. I usually don't post whole solutions.