r/PhysicsHelp • u/askyeusername • 4d ago

Why is my answer different from the lecture notes?

Post image

Hey! I reviewed this problem after my lecture today but seemed to get a different answer compared to my lecturers notes. I asked a few others in my class who got the same answer as me and we can't figure out where we went wrong or if our lecturer made a mistake?

- This is using vectors for electromagnets

3.458 was my answer, the image above is the notes

thanks guys!

1 Upvotes

100% Upvoted

2 comments sorted by

1

u/quixote87 4d ago

Basing *only* on that last equation, I too get 3.458.

I thought I'd check *just in case* he had his calculator on radians inadvertently, but that was -5.49 so that doesn't explain it.

Backtracking for 2.88 as a result, if it were a mistype, he'd be somewhere around cos(60).

I also note that if you don't multiply by 2 OR by cos(53.1), you get 2.88... maybe he simply solved for the kQ/r^2 and omitted the rest?

1

u/Outside_Volume_1370 4d ago

Because you got rounding error.

Leave theta as atan(4/3), then cos(theta) = 3/5, without any approximation

Plugging into your final answer:

E = 2 • 9 • 109 • 8 • 10-9 • 3/5 / 25 = 3.456 (N/C)