Tbr physics ch 8 #35

[yjj8817]

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I'm not quite getting this one. They said the answer is A.

 

[sazerac]

What direction will a positively charged particle go? A negatively charged one?

Notice how close the lines are on the left, versus being so spread out on the right? How does that affect field strength?

Finally, read the (awkwardly written) question again. They aren't asking about instantaneous acceleration magnitude, but rather the average acceleration magnitude after the particle has been released for a short while.

 

[amy_k]

A positively charged particle will accelerate in the same direction as the field, a negatively charged one will accelerate in the opposite direction as the field. That leaves B or D as possible answers.

According to F = ma, acceleration is inversely proportional to mass given the same force magnitude, in this case the electric field force. So the positive low mass particle has the greatest average magnitude of acceleration in the field at A.

 

[yjj8817]

I'm confused why the average acceleration would be different when you go in opposite directions.

 

[Cawolf]

I'm confused why the average acceleration would be different when you go in opposite directions.

The closer the field lines are to each other (increased density), the stronger the field. A negatively charged particle will travel in the opposite direction than the field points, towards the denser field lines. It will experience an increased force and therefore greater acceleration. The positively charged particle will move parallel to the field lines, towards the less dense region of arrows, experiencing less acceleration.

 

[amy_k]

@Cawolf doesn't that indicate that a negative charge would experience greater acceleration?

 

[Cawolf]

@Cawolf doesn't that indicate that a negative charge would experience greater acceleration?

Yes, the negative charge will experience a greater acceleration.

Your post before mine says "So the positive low mass particle has the greatest average magnitude of acceleration in the field at A."

 

[sazerac]

A positively charged particle will accelerate in the same direction as the field, a negatively charged one will accelerate in the opposite direction as the field. That leaves B or D as possible answers.

According to F = ma, acceleration is inversely proportional to mass given the same force magnitude, in this case the electric field force. So the positive low mass particle has the greatest average magnitude of acceleration in the field at A.

Just so we are all clear -

The positively charged particle will feel a force to the right, a negatively charged particle will feel a force to the left. This does not eliminate any answers.

For any given force, there will be more acceleration when the mass is smaller. This eliminates C and D.

As a positively charged particle goes to the right, it will experience less and less force (and gain less and less acceleration) because the field lines are further apart. As a negatively charged particle goes to the left, it will experience more and more fore (and gain more and more acceleration) because the field lines are getting closer together.

A positive particle or a negative particle would experience the same force and same instant acceleration at point A, assuming they have the same mass. But the question is asking "over time, which particle will experience the greatest average acceleration". Thus the answer can only be A.

 

[amy_k]

@Cawolf, yes sorry I meant the negative one which is A. I read the answers incorrectly because the original poster had circled B. The negative particles acceleration is increasing over time whereas the positive particles acceleration is decreasing over time.

 

[Cawolf]

@Cawolf, yes sorry I meant the negative one which is A. I read the answers incorrectly because the original poster had circled B. The negative particles acceleration is increasing over time whereas the positive particles acceleration is decreasing over time.

Awesome. I just wanted to make sure they got the correct information.