How Do Magnetism and Electric Fields Interact in Physics Problems?

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SUMMARY

This discussion focuses on the interaction between magnetism and electric fields in various physics problems. Key topics include Millikan's oil drop experiment, conservation of energy for an electron's motion towards a proton, the effect of Earth's magnetic field on a moving proton, and calculating induced voltage in a circular coil. The participants clarify concepts such as electric field strength, magnetic flux, and the right-hand rule for determining the direction of forces. The correct answers and methodologies for each problem are emphasized, providing clear insights into the physics involved.

PREREQUISITES
  • Understanding of electric fields and forces, specifically in the context of Millikan's oil drop experiment.
  • Knowledge of conservation of energy principles in physics.
  • Familiarity with magnetic fields and their effects on charged particles.
  • Ability to apply Faraday's law of electromagnetic induction for calculating induced voltage.
NEXT STEPS
  • Study the principles of Millikan's oil drop experiment and its implications for charge quantization.
  • Learn about the conservation of energy in electrostatics, focusing on potential and kinetic energy relationships.
  • Explore the right-hand rule and its applications in determining the motion of charged particles in magnetic fields.
  • Investigate Faraday's law of electromagnetic induction and practice calculating induced voltages in various scenarios.
USEFUL FOR

Students of physics, educators teaching electromagnetism, and anyone interested in understanding the fundamental interactions between electric and magnetic fields.

jekch85
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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:
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jekch85 said:
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?
 
jekch85 said:
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?
 
jekch85 said:
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.
 
jekch85 said:
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?
 

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