Question: A charged ball of mass 0.32 g hovers 2.0 cm above a charged plate that has a charge of −4.72 × 10-7 C. Calculate the magnitude of the charge on the ball.
Hey A.Rose!
I'm gonna start off by saying that: I am not super confident in physics right now! It is just one of those subjects I have put on the back burner, and have been meaning to work on. So, I would take my reply with a grain of salt (just incase I have no clue what I'm talking about).
Anywho, I don't remember mass ever coming into play for questions like these, so I believe that is just an added (and unnecessary) piece of information trying to throw you off. So, I would ignore the 0.32g. If you are a bit rusty, as you said, and have no clue whether or not the mass is necessary, your go to is the formula sheet
If you look under the "electromagnetism" section there are no formulas involving mass. So you can just rule out that bit of information.
But lets just have a look at what we think is important:
- the ball hovers 2cm above charged plate
- plate has a charge of -4.72x10
-7 C
So now we can head to the formula sheet and see what we can use.
You'll see that there are two formulas that look very good:
F = 1/4πɛ
0 x Qq/r
2 AND E = 1/4πɛ
0 x q/r
2 (keeping in mind that 1/4πɛ
0 = k = 9x10
9 - also be careful with units, I'm fairly certain that the distance should be in meters so just make sure you do any necessary conversions)
Because we aren't looking for the force we can rule out the first formula - leaving us with the second one.
Therefore, I would start off by using: E = 1/4πɛ
0 x q/r
2 (Once again, I could be entirely wrong. But this is what I would do anyway... I hope I'm not wrong tho and I hope this helps
- good luck!)