Voltage, Electric field, Electric force

[deleted783484]

If the voltage across the plates were reversed what would happen to the oil drops between the plates from the Millikan oil droplet experiment?

A- They would accelerate upward
B- They would accelerate downward
C- They would move with constant velocity upward
D- They would move with constant velocity downward

Answer is B " in order for the drops to be suspended, the electric force must point upward to oppose the gravitational force downward. By reversing the voltage, the direction of the Electric field and therefore the direction of the electric force would also be reversed. Now the electric force and gravitation force act in same direction downward, so the droplet will be accelerated downward with a magnitude of acceleration greater than g.

I'm confused how do I relate the direction of gravitational force and electric force? I know that for the electric field the direction is opposite to the electric force for a negative charge but I don't know how to figure out the gravitational force

Thank you so much!

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[aldol16]

It doesn't matter what your frame of reference is as long as you keep the same frame of reference throughout the problem. The laws of physics apply in all frames of reference - thanks to Einstein. So let's define up as up and down as down such that gravity points down. When the particles are suspended, the electric force points up, directly counteracting gravity. Since the forces then cancel out (assuming same magnitude), the particles experience no acceleration. But if you were to reverse the direction of the electric force, now it faces down and so does gravity. There's no more an upward force opposing the downward motion of the particles and they accelerate downward.

 

[deleted783484]

It doesn't matter what your frame of reference is as long as you keep the same frame of reference throughout the problem. The laws of physics apply in all frames of reference - thanks to Einstein. So let's define up as up and down as down such that gravity points down. When the particles are suspended, the electric force points up, directly counteracting gravity. Since the forces then cancel out (assuming same magnitude), the particles experience no acceleration. But if you were to reverse the direction of the electric force, now it faces down and so does gravity. There's no more an upward force opposing the downward motion of the particles and they accelerate downward.

Thank you so much for always responding to my questions!

 

[BerkReviewTeach]

If you look at Figure 1 of the passage (on page 28), then you'll see a single particle suspended in roughly the middle of that vertical field. For the time being, ignore the field and just consider the stationary particle. An object at rest stays at rest until a net force acts upon it. So the two forces acting on the droplet must be equal and opposite. If you were to reverse the voltage of the battery (such that the anode is on top and cathode is on bottom, opposite of what Figure 1 shows), then direction the electric field would reverse, which in turn would reverse the direction of the electric force. In that case, you would now have an electric force in the same direction as gravity (downward), resulting in a net force double that of mg. In essence, rather than canceling the gravitational force, the electric force would instead add to it.

Even if you didn't see those force vectors in your mind, you can get this answer correct by considering the choices. All of the choices describe the previously stationary droplet loving, which means it must have experienced some sort of acceleration from rest. Having an acceleration prevents the droplet from moving at a constant velocity, which eliminates choices C and D. Choice B is your more intuitive choice, because gravity will remain there no matter what you do to the electric force, and gravity will accelerate the droplet down if there is not an opposing force.

The important thing on this question, and the majority of our questions, is that you develop a method for solving them intuitively and quickly. So while the first paragraph of my reply describes the physics, the second paragraph should be one that you take to heart as well.