magnetic fields mutiple choice

hey can some one pls help explain this question to me , the answer should be A btw , thank you

The diagram shows a rigidly-clamped straight horizontal current-carrying wire held mid-way
between the poles of a magnet on a top-pan balance. The wire is perpendicular to the magnetic
field direction.
The balance, which was zeroed before the switch was closed, read 161 g after the switch was
closed. When the current is reversed and doubled, what would be the new reading on the
balance?
A
−322 g
B
−161 g
C
zero
D
322 g

Reply 1

Joinedup

I can't see the diagram but I presume the setup looks something like the one here https://www.bbc.co.uk/bitesize/guides/zc3dxfr/revision/2

the relevant equation is F=BIL

from the text of the question I gather that:
the magnetic flux density is unchanged
the length of conductor is unchanged
the current is reversed and doubled... which is the same as saying it is multiplied by -2

so what happens to the force?

Reply 2

1234kelly

Omg omg thanks so much !! I attached the photos of the diagram just in case sorry abou that . But why is the unit g ? And a balance can’t measure force ?

7DA97D7E-D900-48EE-8540-EEAFBA6D3720.jpg507.5KB

Reply 3

Joinedup

Original post by 1234kelly

Omg omg thanks so much !! I attached the photos of the diagram just in case sorry abou that . But why is the unit g ? And a balance can’t measure force ?

g will be gram i.e. 1/1000 of a kg

you'd normally use a pan balance to measure weight force - but here the weight has been subtracted by zeroing the balance so it's going to measure the vertical electromagnetic force