How to Solve Electrostatics Problems Using Coulomb's Law

In summary: Thanks for the help. In summary, I'm seeking help with a couple of problems involving electrostatics. I've drawn diagrams for both questions. In Q1 the positive charge experiences an attractive force towards the other charges, but do i drop the signs in the equations? still unsure. For Q2 the two charges experience a repulsive force, the two fields must equal zero in between the charges, closer to Q. Still confused. ThanksMaverick280857 for the great advice. I've had success! Thanks for the help.
  • #1
Callisto
41
0
Hi,
I'm seeking help with a couple of problems involving electrostatics.

Q1)Calculate the electric field at the centre of a square 0.1m on a side if one corner is occupied by a charge 0.2micro C, and the other three corners are occupied by the charge of -0.9micro C

Q2) A positive test charge Q is placed at the origin of coordinates, and a charge 6Q is fixed to the x-axis at x=2m, find the location of the place along the x-axis where the electric field due to these is zero.

In Q1 I calculate the forces F12, F13, F14which=F12 using Coulombs Law.
Then taking the vector sum of F12 and F14 and adding F13 gives the electric field at the centre of the square. I'm getting confused with the signs in the equations, what am I doing wrong?

In Q2 I add the to E-fields together and set the sum to zero and solve for x,
ie; E1+E2=kQ/x^2+k6Q/(x+2)^2=0
Is this right, or am I doing something really stupid?
Any help with these problems would be much appreciated, as I've spent sometime on them and I've gotten nowhere.

Thanks Heaps
Callisto :confused:
 
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  • #2
Use an X-Y frame (axes)

Hint #1 (for both questions): Write the vector components of the force/field in terms of a X-Y reference frame (i, j components), to avoid confusion with the signs.

Hint #2 (for both questions): Before writing the vector sum, write the individual force/field vectors and convince yourself of their directions (as expressed by your equations with what they must be, logically).

Cheers
Vivek
 
  • #3
Thanks Vivek,
I've drawn diagrams for both questions.
In Q1 the positive charge experiences an attractive force towards the other charges,
Do i drop the signs in the equations? still unsure.
For Q2 the two charges experience a repulsive force, the two fields must equal zero in between the charges, closer to Q. Still confused.
Thanks
 
  • #4
I'll add a few comments to maverick280857's excellent advice.
Callisto said:
In Q1 I calculate the forces F12, F13, F14which=F12 using Coulombs Law.
Then taking the vector sum of F12 and F14 and adding F13 gives the electric field at the centre of the square. I'm getting confused with the signs in the equations, what am I doing wrong?
First off, you should be calculating the contribution each charge makes to the electric field at the center, not the forces between the charges. The electric field from a point charge is: [itex]E = kQ/r^2[/itex]. To find the direction of each field contribution, realize that the field from a positive charge points away from the source charge, but the field from a negative charge points towards the source charge. Draw them as vectors and add them using components.

In Q2 I add the to E-fields together and set the sum to zero and solve for x,
ie; E1+E2=kQ/x^2+k6Q/(x+2)^2=0
Is this right, or am I doing something really stupid?
No, it's not right. Realize that for a point between x = 0 and x = 2 the field from each charge points in the opposite direction: E1 points to the right, so E1 = kQ/x^2; E2 points to the left, so E2 = -k6Q/(x+2)^2.

Don't blindly rely on formulas to tell you the direction of the field. Use your knowledge of how the field must point.
 
  • #5
Thanks Heaps,Vivek and Doc Al.

Ive had Success!
 

1. What is electric charge?

Electric charge is a fundamental property of matter that causes it to experience a force in the presence of an electric field. It can be either positive or negative, and is measured in units of coulombs (C).

2. 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 between two charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

3. How is electric charge measured?

Electric charge is measured using a device called an electrometer, which can measure the amount of charge on an object. It is typically measured in coulombs (C), with one coulomb equal to the charge of 6.24 x 10^18 electrons.

4. What is the SI unit of electric charge?

The SI unit of electric charge is the coulomb (C). It is defined as the amount of charge that passes through a point in a conductor in one second when a current of one ampere is flowing through it.

5. How is Coulomb's law used in real-life applications?

Coulomb's law is used in many real-life applications, such as in designing electronic circuits, calculating the force between charged particles in a particle accelerator, and determining the strength of electric fields in medical devices like defibrillators. It is also used in the study of lightning and the development of lightning protection systems.

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