# A few problems

jekch85
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:

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?

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?

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.