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Coulomb's Law

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Question 1 0:14
  • Question 2 0:47
  • Question 3 1:25
  • Question 4 2:25
  • Question 5 3:01

Transcription: Coulomb's Law

Hi folks and welcome back to

In this mini-lesson, we are going to go through the first page of the APlusPhysics worksheet on Coulombs Law and you can find a link to that worksheet down below the video.0002

With that, let us dive in and look at question Number 1.0011

Which graph best represents the electrostatic force between an alpha particle with a charge of +2 elementary charges and a positively charged nucleus as a function of their distance of separation?0015

The electric force is (k)(q1)(q2)/r2.0026

Remember we have this inverse-square Law as a function of distance.0033

As things get further apart, we get a smaller electric force and that goes down as the square of the distance, so our correct answer must be Number 1.0036

Number 2 -- In the diagram below, we have two positively charged spheres (A) and (B) of masses (ma) and (mb) and they are located a distance (d) apart.0048

Which diagram best represents the directions of the gravitational force (Fg) and the electrostatic force (FE), acting on sphere (A) due to the mass and charge of sphere (B)?0056

Well (A) is going to fill an attractive force to be due to gravity -- gravity only attracts, but because they are both positive, it is going to be repelled due to the electrostatic force, the coulombic force (FE), so the answer that shows this best is Number 3.0066

Number 3 -- What is the magnitude of the electrostatic force between two electrons separated by a distance of 1 × 10-8 m?0086

Well the electric force is (k)(q1)(q2)/r2, so that is going to be 9 × 109 N-m2/C2k.0095

Our first charge is the charge of an electron, 1.6 × 10-19 C, our second charge is also 1.6 × 10-19 C, and that is all divided by the square of the distance between them, 1 × 10-8 m2.0108

Therefore, the electric force -- when I plug this into my calculator and check it out, I get something right around 2.3 × 10-12 N -- Answer Number 3.0126

All right, let us take a look at Number 4 -- Two metal spheres (A) and (B) possess charges of 1 microcoulombs and 2 microcoulombs respectively.0144

In the diagram below, Arrow (F) represents the electrostatic force exerted on sphere (B) by sphere (A).0152

Which arrow represents the magnitude and direction of the electrostatic force exerted on sphere (A) by sphere (B)?0160

Well, this is just really a Newton's Third Law problem.0166

If we have a force of (A) on (B) to the right at length (F), we must have the exact same force in the opposite direction on (A) or answer Number 1.0169

One more here -- Two small identical metal spheres, (A) and (B) on insulated stands are each given a charge of 2 × 10-6 C.0181

The distance between the spheres is 2 × 10-1 m.0191

Calculate the magnitude of the electrostatic force that the charge on sphere (A) exerts on the charge on sphere (B).0194

Well the electrostatic force again is (k)(q1)(q2)/r2, so that is going to be 9 × 109 N-m2/C2 × 2 × 10-6 C (charge)...0201

...and our other charge, 2 × 10-6 C, all divided by the square of the distance between them, 2 × 10-1 m2.0217

When I put all of that in my calculator, I come up with a value of about 0.9 N.0228

All right. Hopefully that gets you through page 1 of the Coulombs Law worksheet without any problems.0237

If you struggled there a little bit, go back and check the previous video, where we talk about the electrostatic force and if it went great, then you are probably ready to move on to AP level questions.0242

Thanks so much for your time everyone and make it a great day!0250