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Electric Charge

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 0:10
  • Question 2 1:03
  • Question 3 1:32
  • Question 4 2:12
  • Question 5 3:01
  • Question 6 3:49
  • Question 7 4:24
  • Question 8 4:50
  • Question 9 5:32
  • Question 10 5:55
  • Question 11 6:26

Transcription: Electric Charge

Hi everyone and welcome back to

In this mini-lesson, we are going to do the first page of the APlusPhysics worksheet on electric charge.0002

So let us dive right in.0007

The diagram below represents two electrically charged identical-sized metal spheres (A) and (B).0011

If the spheres are brought into contact, which sphere will have a net gain of electrons?0016

If they are brought into contact, what we are going to have is we are going to have a transfer of charge so that we have 1.5 × 10-7 on each of these.0021

We will now have 1.5 × 10-7 C here and 1.5 × 10-7 C here, assuming they are conducting.0032

Now, if we go from +2 to 1.5 × 10-7 C, then that means this charge became less, it must have had more electrons on it, so we had a net flow of electrons from (B) to (A).0043

Which one has a net gain of electrons? The answer must be Number 1 -- (A) only.0057

Number 2 -- Metal sphere (A) has a charge of -2 units and an identical metal sphere (B) has charge of -4 units.0063

If they are brought into contact with each other and then separated, the charge on each sphere will be...?0070

Well, when they are brought into contact with each other, the charges are free to move, so if they are identical they are going to spread out as much as they can, which means you will have -3 units on each sphere and if you pull them apart and they can no longer move, you are still left with -3 units on each one. Answer 3.0075

Number 3 -- If an object has a net negative charge of 4 C, the object possesses...0093

If it is negative, it must have more electrons than protons, so we can get rid of choices 3 and 4 and let us convert that 4 C into elementary charges to see how many more electrons we have than protons.0097

We have 4 C and let us multiply that by -- well, we know that the 1 elementary charge has a charge of 1.6 × 10-19 C, so that is going to give us 2.5 × 1019 elementary charges -- Answer 2.0109

Number 4 -- A lightweight sphere hangs by an insulating thread. A student wishes to determine if the sphere is neutral or electrostatically charged.0133

She has a negatively charged hard rubber rod in a positively charged glass rod.0142

She does not touch the sphere with the rods, but runs test by bringing them near the sphere one at a time.0146

What test result would prove that the sphere is neutral?0152

Well, a neutral object can be attracted by positive and by negative objects, so I would say the test that would prove it is neutral is if it is attracted by both the negative rod and the positive rod.0155

In similar fashion, describe the test result that would prove that the sphere is positively charged.0179

The way to prove that something is charged is going to be with repulsion.0187

If you bring the positive rod near the sphere and you observe the repulsion, then the sphere must be positively charged.0190

So use repulsion to prove something is charged -- positive, positive and you will get repulsion, so the positive rod near the sphere -- if they repel, the sphere must also be positive.0218

Number 6 -- Oil droplets may gain electrical charges as they are projected through a nozzle.0229

Which quantity of charge is not possible on an oil droplet?0234

If you recall, the elementary charge is going to be our fundamental unit of charge, so we can only have charges that are in integer multiples of the elementary charge.0237

Elementary charge is 1.6 × 10-19 C, so we could have that value; we could have twice that (3.2), but there is no way to get 2.6 × 10-19 by multiplying this by an integer, so 4 is the one that could not occur.0247

Number 7 -- A positive test charge is placed between the electron (E) and a proton (P) as shown in the diagram below.0264

When the test charge is released, it will move toward -- well the positive test charge will be attracted by that electron and it will be repelled by that proton, so in both cases we are going to have a net force toward (d).0272

Taking a look at Number 8 -- A metal sphere has a net negative charge of 1.1 × 10-6 C.0289

Approximately, how many more electrons than protons are on the sphere?0296

Let us convert that charge to elementary charges.0300

1.1 × 10-6 C -- and we will multiply that by -- we know that 1 elementary charge is equal to 1.6 × 10-19 C...0304 our coulombs will cancel out and we should get something right around 6.9 × 1012 elementary charges, so the number of more electrons than protons on the sphere is Answer 3, 6.9 × 1012.0315

Number 9 -- A positively charged glass rod attracts object (X). The net charge of (X) may be...0333

If a positively charged glass rod attracts it, (X) could be negative or it could be neutral, so it may be 0 or negative -- Correct answer for 9 must be Number 1.0339

Number 10 -- The charge to mass ratio of an electron.0356

Charge of an electron divided by the mass of an electron -- the charge on an electron is 1.6 × 10-19 C and the mass of an electron is 9.11 × 10-31 kg...0359 what we are going to end up with here is about 1.76 × 1011 C/kg -- Answer Number 3.0373

One more here -- What is the magnitude of the charge in coulombs of a lithium nucleus containing 3 protons and 4 neutrons.0385

From the 3 protons, we get a charge of +3 elementary charges and the neutrons have no charge, so that is going to be 3 × 1.6 × 10-19 C or 4.8 × 10-19 C.0394

Hopefully, this went pretty well for you. If it did not, take a few minutes and go back and look over our longer lesson on electric charge and if it did go well -- Excellent -- go ahead and keep moving forward.0413

Thanks so much for your time everybody and make it a great day!0424