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    What am I over looking on this question? The answer on the mark scheme is 'A' but how can you get a negative reading?

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    You can get a negative reading because the balance was zeroed after something was put on it.
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    Knew it would be something stupid I was over looking -.-
    Thank you.
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    (Original post by ashboot)
    Knew it would be something stupid I was over looking -.-
    Thank you.
    Haha, no problem, physics always has those little annoying things about it!
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    The important point in this question is about the force on the wire due to the current in it passing through the magnetic field of the magnets.
    F=BLI
    and as B and L don't change, it's the change in the current that causes this change in the force.
    If you reverse the current you reverse the force.
    If you double the current you double the force.
    That's why, if you reverse and double the current, you get a force twice as large in the opposite direction, giving a double negative reading.
    The positive reading would be the case where the force (of the wire) on the scale pushes downwards, and the negative reading implies the force on the scale is pushing upwards.
 
 
 
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