Need some hints for a Coulomb's Law question

In summary: Your unknown charge looks to be a bit large (by several orders of magnitude). Maybe you can check or show your calculation.
  • #1
whatisphysics
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0

Homework Statement


Suppose that resting on your non-conducting desktop is a tiny piece of paper(m=0.08g) that carries a charge of +4 x 10-10C. You comb your hair and then slowly lower your comb until when it is 3 cm away from the paper, the paper jumps up to the comb.
a) Give an approximate value for the charge on the comb.
b) Does the comb carry an excess of electrons or protons? How many?

Homework Equations


Coulomb's Law:
F=(1/4*pi*E0)*(|Q1Q2|/r2)
Magnitude of charge of electron and proton: e=1.60217 x 10-19
Q=ne

The Attempt at a Solution


I know that we have to consider w=mg since there is mass involved for the paper. I tried to use Coulomb's Law to find the charge, but I don't know what the Force is between the two, so how can I find charge for the comb? I've converted the given numbers to the right units which are 0.08g = 0.8 x 10-4 and 3cm = 0.03m.
Now, my question is, can anyone give me a hint on how to start this question?
 
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  • #2
What force is required to lift a piece of paper that has mass 0.08 grams?
 
  • #3
gneill said:
What force is required to lift a piece of paper that has mass 0.08 grams?

That would be w=mg! But can I use this force as the force between the two objects? If so, I will then be able to solve for the unknown charge.

Am I on the right track?
 
  • #4
whatisphysics said:
That would be w=mg! But can I use this force as the force between the two objects? If so, I will then be able to solve for the unknown charge.

Am I on the right track?

Yes, yes you are!
 
  • #5
gneill said:
Yes, yes you are!

Thanks so much for the hint! Just one small question, for calculating the number of electrons(excess electrons of comb);I do Q=ne which is:

1.96x10-7C = n*1.60x10-19C
and I will get e = 1.23x1012. [tex]\leftarrow[/tex]This is the number of electrons right?

Thanks again!
 
  • #6
Your unknown charge looks to be a bit large (by several orders of magnitude). Maybe you can check or show your calculation.

Your method for finding the number of electrons transferred looks fine.
 
  • #7
Hey, just had to come back here to say thanks again. I asked my teacher the same question, and it turned out right!

Thanks a bunch.
 

What is Coulomb's Law?

Coulomb's Law is a fundamental law of electrostatics that describes the force between two electrically charged particles. It states that the force is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

How do I use Coulomb's Law to solve a problem?

To use Coulomb's Law, you need to know the magnitude and sign of the charges involved and the distance between them. Then, you can plug these values into the equation F = k(q1*q2)/r^2, where k is the Coulomb's constant, q1 and q2 are the charges, and r is the distance between them.

What are some common mistakes when using Coulomb's Law?

One common mistake is forgetting to convert the distance to meters if it is given in a different unit. Another mistake is using the wrong sign for the charges, which can result in an incorrect answer. Make sure to pay attention to the signs and units when using Coulomb's Law.

Can Coulomb's Law be used for more than two charges?

Yes, Coulomb's Law can be extended to be used for more than two charges. In this case, you would need to calculate the force between each pair of charges and then add them up vectorially to find the total force on each charge.

What are some real-life applications of Coulomb's Law?

Coulomb's Law has many practical applications, including understanding the behavior of electric charges in everyday objects such as hair and clothing. It is also essential in the design of electronic devices, such as capacitors and semiconductors. Additionally, Coulomb's Law is used in the field of particle physics to study the behavior of subatomic particles.

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