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BPhO by topic – electric field

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This topic comes up a lot. Make sure you are familiar with the formulae for electrostatics, and possibly SHM and binomial approximation. All Non-BPhO questions come from 1000 Solved Problems in Classical Physics by Kamal. Any concerns, leave a comment.

Non-BPhO questions to warm up

These non-BPhO questions give you an insight into the type of electric field questions that can be asked in Section 2.

Hint 11.5: which quantities are conserved when the oil drops coalesce?

Hint 11.6: resolve forces or otherwise.

Hint 11.8: “infinite” probably indicates some sort of pattern.

Hint 11.9: find the differential equation for the motion of the particle in order to find omega and hence find T. You might want to use binomial approximation.
Hint 11.10: analyze the forces acting on one of the three charges at the corners.
Hint 11.14: what forces are acting on the oil droplet?
Hint 11.15: equate gravitational force and electrical force.
Hint 11.23: Electric field is a vector.

My solutions are as follows:

Check out Millikan’s oil drop experiment. This has come up in BPhO before.
Easy one.
Easy if you spot the pattern. For PAT, you must remember the formulae for summing.

BPhO 2016 R1 Section Q4

BPhO 2015 R1 Section 2 Q2

Hint Part (a): kinematics equations. Which quantity do you have to equate for x and y components of the motion?

Hint Part (b): which component of the velocity remains unchanged?

Hint Part (c): upon entering the final field, the z component of the velocity is zero, which means…

My solution is as follows:

Equate time for the two components of the motion. You don’t have to follow my solution. As long as you get the idea it’s fine.
The z component is zero initially, which means that the final z component is the same as the final y component of the velocity because the two fields are identical.

BPhO 2011 R1 Section 2 Q9

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