Electric potential question (check my work)

Click For Summary
The discussion centers on calculating the velocity of electrons in a beam directed at a copper sphere and determining the charge required to repel the electrons. The user attempts to solve for velocity using the equation 1/2mv^2 = -qΔV but arrives at an implausibly high speed of 2.19 x 10^33 m/s, which exceeds the speed of light. Participants point out that this result is incorrect and highlight the need for a proper approach to the calculations. The user expresses uncertainty about how to proceed with the second part of the problem regarding the charge on the sphere. Clarification and guidance on both parts of the question are requested.
AnthroMecha
Messages
26
Reaction score
0

Homework Statement



A beam of electrons with energy 1.20 keV (per electron) is directed towards a solid copper sphere of radius 1.25cm.
a) Determine the velocity of the electrons in the beam.
b) The beam continues to send electrons into the sphere until it is charged up enough to repel the electrons. What is the charge on the sphere at which it stops further electrons from hitting it?

Homework Equations



1/2mv^2=-qΔV
mass of an electron=9.12X10^-31kg
elementary charge=1.602X10^-19 C

The Attempt at a Solution



Not sure if I'm going part a right:
.5(9.12X10^-31kg)v=(-1.602X10^-19 C)(1.2KeV)

Solving for v (velocity) I get 2.19 X 10^33 m/s

Does this look right?

Not even sure what to do to answer part b.

Any help is appreciated!
 
Physics news on Phys.org
Sorry, I have no time to check it fully, but 10^33 if way to much for a speed. It's 10^25 times higher than the speed of light!
 
Innoko said:
Sorry, I have no time to check it fully, but 10^33 if way to much for a speed. It's 10^25 times higher than the speed of light!

Yeah, I knew that answer sounded ridiculous.
 

Thread 'Magnitude of buoyant force in fluids of different densities'

The book claims the answer is that all the magnitudes are the same because "the gravitational force on the penguin is the same". I'm having trouble understanding this. I thought the buoyant force was equal to the weight of the fluid displaced. Weight depends on mass which depends on density. Therefore, due to the differing densities the buoyant force will be different in each case? Is this incorrect?
View full post »

Similar threads

Why Is Work Positive When Done Against the Electric Field?
  • · Replies 22 ·
Replies
22
Views
4K
Electron brought to rest by the E-field, potential difference question
  • · Replies 4 ·
Replies
4
Views
6K
Charge on a grounded conducting sphere in a uniform electric field, after ungrounding and movement
  • · Replies 1 ·
Replies
1
Views
1K
Electric field between two parallel plates
  • · Replies 58 ·
2
Replies
58
Views
5K
Potential difference between two points in an electric field
  • · Replies 1 ·
Replies
1
Views
2K
Poisson's equation: Calculating the Laplacian of an electric potential
  • · Replies 3 ·
Replies
3
Views
1K
Kinetic and potential energy of a particle attracted by charged sphere
  • · Replies 6 ·
Replies
6
Views
2K
Conceptual Issue RE: electric potential difference
  • · Replies 1 ·
Replies
1
Views
1K
How electric potential boundary condition works
  • · Replies 5 ·
Replies
5
Views
3K
Electrical Potential Energy and Potential
  • · Replies 2 ·
Replies
2
Views
2K