# A few problems

magnetism and electric fields

Hello all. I'm new to this, but I would really appreciate any help, I'm absolutely terrible at physics....

1) In a trial of Millikan's oil drop experiment, we find that the electric field required to suspend a tiny oil particle with mass 1.46x10^-14 kg is 2.98x10^5 N/C upward. We can infer that the oil particle

a) is missing three electrons
b) is missing two electrons
c) has three extra electrons
d) has two extra electrons

I worked through this problem to the best of my ability and figured that q=4.8x10^-19 when it should equal 1.6x10^-14. So that means there are either three extra or three missing, but I can't seem to figure out which one. I thought three extra, but I could be wrong. Any hints?

3) An electrons 'falls' toward a proton from far away. If it started with zero velocity, how fast is it moving when it gets to a typical atomic distance of 1.0x10^-10 m from the proton?

a) 3.2x10^6 m/s
b) 2.2x10^6 m/s
c) 1.6x10^6 m/s
d) 1.1x10^6 m/s

I used the conservation of energy, PE=KE, which gave me m*a*d/q=.5m*v^2. For some reason I got 1.1x10^5, which is close to d but I have a feeling I did something wrong.

3) The Earth's magnetic field above the Earth's equator is about 10^-4 T northward. If a proton is initially moving upward at 5x10^5 m/s, it will

a) curve east, making a circle with r=50m
b) curve west, making a circle with r=50m
c) curve east, making a circle with r=3 cm
d) curve west, making a circle with r=3 cm
e) keep moving in a straight path

For this question, I got answer a using the equation r=mv/qB. I'm pretty sure that part is right, but what I don't know is which way it will move. I think I'm confusing myself with the right hand rule. Any suggestions would be awesome.

4) I have a 500-turn circular coil of wire, 0.10 m in diameter, in a 0.025 T magnetic field. Initially the plane of the coil is perpendicular to B. If I flip the coil 180 degrees in 1/30 of a second, what is the average voltage created across the ends of the coil?

a) -6.0V
b) -12V
c) -3.0V
d)-24V

I got c for this one. I used found the flux to be 1.96x10^-4 and then I found V by using V=-N(flux)/time. I got -2.94, but is this the right way to solve this type of problem?

Thank you to anyone who can help me!!!!

Last edited:

OlderDan
Homework Helper
Hello all. I'm new to this, but I would really appreciate any help, I'm absolutely terrible at physics....

1) In a trial of Millikan's oil drop experiment, we find that the electric field required to suspend a tiny oil particle with mass 1.46x10^-14 kg is 2.98x10^5 N/C upward. We can infer that the oil particle

a) is missing three electrons
b) is missing two electrons
c) has three extra electrons
d) has two extra electrons

I worked through this problem to the best of my ability and figured that q=4.8x10^-19 when it should equal 1.6x10^-14. So that means there are either three extra or three missing, but I can't seem to figure out which one. I thought three extra, but I could be wrong. Any hints?
The electric field is upward. Gravity acts downward. Which way does the electric force have to be?

OlderDan
Homework Helper
3) An electrons 'falls' toward a proton from far away. If it started with zero velocity, how fast is it moving when it gets to a typical atomic distance of 1.0x10^-10 m from the proton?

a) 3.2x10^6 m/s
b) 2.2x10^6 m/s
c) 1.6x10^6 m/s
d) 1.1x10^6 m/s

I used the conservation of energy, PE=KE, which gave me m*a*d/q=.5m*v^2. For some reason I got 1.1x10^5, which is close to d but I have a feeling I did something wrong.
Conservation of energy is the correct approach, but I do not recognize m*a*d/q. What is the electrical potential energy of two charges sparated by a distance r?

OlderDan
Homework Helper
3) The Earth's magnetic field above the Earth's equator is about 10^-4 T northward. If a proton is initially moving upward at 5x10^5 m/s, it will

a) curve east, making a circle with r=50m
b) curve west, making a circle with r=50m
c) curve east, making a circle with r=3 cm
d) curve west, making a circle with r=3 cm
e) keep moving in a straight path

For this question, I got answer a using the equation r=mv/qB. I'm pretty sure that part is right, but what I don't know is which way it will move. I think I'm confusing myself with the right hand rule. Any suggestions would be awesome.
There are various right hand rules. The one I prefer is:
fingers of the right hand start in the direction of v and curl through the smalles angle to the direction of B; thumb points in the direction of the force.

OlderDan
Homework Helper
4) I have a 500-turn circular coil of wire, 0.10 m in diameter, in a 0.025 T magnetic field. Initially the plane of the coil is perpendicular to B. If I flip the coil 180 degrees in 1/30 of a second, what is the average voltage created across the ends of the coil?

a) -6.0V
b) -12V
c) -3.0V
d)-24V

I got c for this one. I used found the flux to be 1.96x10^-4 and then I found V by using V=-N(flux)/time. I got -2.94, but is this the right way to solve this type of problem?

Thank you to anyone who can help me!!!!
You started with the flux through the loop in one direction and ended wtih it in the opposite direction, so what was the change?