Electromagnetism

[Integrator741]

Hello,
I am trying to study some electromagnetism from “Electromagnetics for Engineers” by Fawwaz T. Ulaby.
In a topic about “Static and Dynamic Fields” it says:
“Consider a small section of a beam of charged particles that are moving at a constant velocity. The moving charges constitute a DC current. The electric field due to that section of the beam is determined by the total charge q contained in that section. The magnetic field does not depend on q, but rather on the rate of charge (current) flowing through that section.”
I don’t get it… So what is the difference between Magnetic field and Electric field? From what I understand they are both dependent on the same thing – charge. So charge contained in that section is not equal to the rate of charge flowing through that section??
Because I imagine charge flowing through some section – ok. Let’s say we stop the flow so wouldn’t rate of charge through that section be equal to the total charge in that section???
Also I got a question about “Coulomb’s force law”.
“Estimate the charge present on a statically charged body, capable of picking up small piece of paper (0.1g) from a distance of 1cm. How many electrons does this correspond to?” – I read the entire section about Coulomb’s force law, didn’t find anything related to this question… Or I just simply don’t get it.
Please help.

 

[davenn]

I don’t get it… So what is the difference between Magnetic field and Electric field?

have you tried googling definitions of each of them ?

as a hint, for one of them, the charge doesn't have to be moving

 

[Integrator741]

have you tried googling definitions of each of them ?

as a hint, for one of them, the charge doesn't have to be moving

Thanks, I ll do that!

Maybe you can help me to answer that question?

 

[Arouse1973]

Hello
This is homework. We are not being mean when we ask you to do a little work yourself. We will help you if you are really stuck. Have a look and tell us what you think electric and magnetic fields are.
Adam

 

[Merlin3189]

“Consider a small section of a beam of charged particles that are moving at a constant velocity. The moving charges constitute a DC current. The electric field due to that section of the beam is determined by the total charge q contained in that section. The magnetic field does not depend on q, but rather on the rate of charge (current) flowing through that section.”
I don’t get it… So what is the difference between Magnetic field and Electric field? From what I understand they are both dependent on the same thing – charge. So charge contained in that section is not equal to the rate of charge flowing through that section??
Because I imagine charge flowing through some section – ok. Let’s say we stop the flow so wouldn’t rate of charge through that section be equal to the total charge in that section???

Would it be too much help to ask what the OP understands by the word "rate"? Since he thinks "rate of charge flow... through" is the same as "total charge in" , then it seems he is not grasping the idea of rate. I don't know if that's something we can't explain, but it does seem to be a basic concept not peculiar to electronics.

Say, I had $500 in my bank account and am paying money in from my salary at a rate of $1000 a month. If my spending on rent and bills came to $800 a month (the rate of money going out of my account) then the net amount in my account would increase at the rate of $200 a month. In 6 months time, the rate of money coming in is still $1000 a month, the rate of money going out is $800 a month, but the total amount in my account is now $1700. If my expenditure goes up to match my income, so that I now have money coming in at a rate of $1000 a month and going out at a rate of $1000 a month, my total amount in the account would stay at $1700.
The rate of flow of money affects the total amount, but is not the same as it.

Would it also be allowable to make an analogy between electrons flowing in a beam through a tube and water flowing through a tube? And to ask him to think about the amount of water in a small length of tube, or the rate of flow of water through that tube? For eg. if the small length of tube had a volume of 1cm^3 (ie. area of 1cm^2 and length 1cm), and the water could flow at a rate of 1cm^3 per second, or faster at 2cm^3 per second, or faster at 3cm^3 per second etc, ar when the tap is turned off, there would be no flow (0 cm^3 per second) but still 1cm^3 of water static in the tube.

But simplest of all might be to simply rewrite the text in the quote, so that it is easier to read?

 

[Arouse1973]

Would it be too much help to ask what the OP understands by the word "rate"? Since he thinks "rate of charge flow... through" is the same as "total charge in" , then it seems he is not grasping the idea of rate. I don't know if that's something we can't explain, but it does seem to be a basic concept not peculiar to electronics.

Say, I had $500 in my bank account and am paying money in from my salary at a rate of $1000 a month. If my spending on rent and bills came to $800 a month (the rate of money going out of my account) then the net amount in my account would increase at the rate of $200 a month. In 6 months time, the rate of money coming in is still $1000 a month, the rate of money going out is $800 a month, but the total amount in my account is now $1700. If my expenditure goes up to match my income, so that I now have money coming in at a rate of $1000 a month and going out at a rate of $1000 a month, my total amount in the account would stay at $1700.
The rate of flow of money affects the total amount, but is not the same as it.

Would it also be allowable to make an analogy between electrons flowing in a beam through a tube and water flowing through a tube? And to ask him to think about the amount of water in a small length of tube, or the rate of flow of water through that tube? For eg. if the small length of tube had a volume of 1cm^3 (ie. area of 1cm^2 and length 1cm), and the water could flow at a rate of 1cm^3 per second, or faster at 2cm^3 per second, or faster at 3cm^3 per second etc, ar when the tap is turned off, there would be no flow (0 cm^3 per second) but still 1cm^3 of water static in the tube.

But simplest of all might be to simply rewrite the text in the quote, so that it is easier to read?

Yes I agree with that. Maybe include the wording an amount of something that passes a certain point in a specific time period.
Adam