TBR Phys Sec 8 Pass 1 and 2

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stitchattack

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Passage 1 Q4: For the circuit in Figure 2, which way does the induced current flow?
A. counterclockwise.

I understand that the force of electron is downward. Does it mean, if the electron points downward, it's clockwise and if it's upward, it's counterclockwise? I know current is flow of positive charge so it'll be the other direction opp of electron. I'm not getting how the clockwise/counterclockwise works since we're talking about downward/upward directions.

Passage 2 Q8: In order for an oil drop of mass m, radius r, and charge density, to be suspended between the plates, the magnitude and direction of the electric field must be:
A. 3mg/4pirho^3p; downward

I understand how to find the first part of the question through TBR's explanation since more massive = stronger force required but the explanation for the second part confused me. Gravity points downwards so electric field must point upward. Why is the ans downward? Is it because we're working with electrons here so normally for positive charge, the electric field would point upward but since it's electron, it'll be the opposite so downward?

Passage 2 Q11: If the voltage across the plates were reversed, what would happen to the oil drops between the plates?
I need help understanding the concept here which sounds like it's contradicting what's said in Q8. So here, it's explained that electric F must pt upward to oppose the gravitational F downward. The only reason why electric F would point downward is IF the voltage were reversed. If the voltage was not reversed, the electric F would pt upward? Having trouble tying concepts explained in Q8 and Q11.

Thanks for your help!

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I understand that the force of electron is downward. Does it mean, if the electron points downward, it's clockwise and if it's upward, it's counterclockwise? I know current is flow of positive charge so it'll be the other direction opp of electron. I'm not getting how the clockwise/counterclockwise works since we're talking about downward/upward direction

You can use the right hand rule for current to figure this out, thumb points down and your fingers are curling clockwise, thumb points up and your fingers are curling counterclockwise.

http://www.passmyexams.co.uk/GCSE/physics/images/rightHandRule.jpg
 
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Passage 2 Q8: In order for an oil drop of mass m, radius r, and charge density, to be suspended between the plates, the magnitude and direction of the electric field must be:
A. 3mg/4pirho^3p; downward

I understand how to find the first part of the question through TBR's explanation since more massive = stronger force required but the explanation for the second part confused me. Gravity points downwards so electric field must point upward. Why is the ans downward? Is it because we're working with electrons here so normally for positive charge, the electric field would point upward but since it's electron, it'll be the opposite so downward?

It depends on the charge on the oil drop I am assuming this is negative?
 
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Passage 2 Q11: If the voltage across the plates were reversed, what would happen to the oil drops between the plates?
I need help understanding the concept here which sounds like it's contradicting what's said in Q8. So here, it's explained that electric F must pt upward to oppose the gravitational F downward. The only reason why electric F would point downward is IF the voltage were reversed. If the voltage was not reversed, the electric F would pt upward? Having trouble tying concepts explained in Q8 and Q11.

So what is their answer here? Electric field FORCE on the oil drop must point upwards, the electric field direction required to cause an upwards electric field force is downwards if we have negative charges, because electric field force is in the opposite direction as the field for negative charges. If the voltage between the plates is reversed then the electric field direction is reversed, so it depends on the initial conditions in the passage.
 
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Thanks for the clarification..it seems I was confused between electric field and electric FORCE.
 
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