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Direction of Force on the Wire (A2 Edexcel)

in this question:

received_567250938629190.jpeg

the wire was made to move downards, so the force created by the interaction of the magnetic field of the magnets and the induced magnetic field produced around the wire due to induced current and induced emf, according to Ampere's Law, was directed in the upwards direction on the wire, in order to oppose this motion of the wire being moved downwards according to Lenz's law. And due to Newton's 3rd law, there was an opposite force with same magnitude acting on the magnets in the downward direction, making the balance reading increase.

BUT

what about the direction of force here?:

received_165871399469509.jpeg

there isn't any vertical movement of the wire (up and down) then why is the force on the wire in the upwards direction?
the question states a balance reading. doesn't the reading on the balance hint that :
"the force acting on the wire is in the downward direction"?
(edited 1 year ago)
Original post by Aleksander Krol
in this question:

received_567250938629190.jpeg

the wire was made to move downards, so the force created by the interaction of the magnetic field of the magnets and the induced magnetic field produced around the wire due to induced current and induced emf, according to Ampere's Law, was directed in the upwards direction on the wire, in order to oppose this motion of the wire being moved downwards according to Lenz's law. And due to Newton's 3rd law, there was an opposite force with same magnitude acting on the magnets in the downward direction, making the balance reading increase.

BUT

what about the direction of force here?:

received_165871399469509.jpeg

there isn't any vertical movement of the wire (up and down) then why is the force on the wire in the upwards direction?
the question states a balance reading. doesn't the reading on the balance hint that :
"the force acting on the wire is in the downward direction"?


A current carrying conductor in a magnetic field experiences a force and edexcel uses Fleming's LHR to find the direction of this force. When the magnetic field exerts a force on the wire, by N3 the wire exerts an equal and opposite force on the field. It is the force exerted by the wire on the magnetic field (i.e. magnets) that causes the balance reading to increase. So the force exerted on the wire must be in opposite direction which is upwards. You should use the direction of the force on the wire in LHR, hence the direction of current is from X to Y.
Original post by thomas.rhett
A current carrying conductor in a magnetic field experiences a force and edexcel uses Fleming's LHR to find the direction of this force. When the magnetic field exerts a force on the wire, by N3 the wire exerts an equal and opposite force on the field. It is the force exerted by the wire on the magnetic field (i.e. magnets) that causes the balance reading to increase. So the force exerted on the wire must be in opposite direction which is upwards. You should use the direction of the force on the wire in LHR, hence the direction of current is from X to Y.


i understand what you're trying to explain. but just one thing that's getting me confused is, how do you know that the force that is exerted on the wire by the magnetic field of the magnets is in the "upwards direction"? or is this like a rule that the force exerted on the wire by the magnets is always in the upwards direction?

P.S: Thanks for taking your time to read all of that!!
(edited 1 year ago)
Original post by Aleksander Krol
i understand what you're trying to explain. but just one thing that's getting me confused is, how do you know that the force that is exerted on the wire by the magnetic field of the magnets is in the "upwards direction"? or is this like a rule that the force exerted on the wire by the magnets is always in the upwards direction?

P.S: Thanks for taking your time to read all of that!!


The balance reading "increased" when there is current in the wire from 1.500 N to 1.503 N. Which means, there is an extra 0.003 N "downward" force on the balance. This is because the wire is exerting a downward force on the permanent magnet placed on the balance. So by N3, the force exerted on the wire by the magnetic field must be upwards.

If the force on the wire was downwards, wire would be exerting an upward force on the magnet which would reduce the reaction force needed from the balance on the magnet for forces to be in equilibrium and thus balance reading would decrease.
The current carrying wire is putting a downwards force on the permanent magnets... So the permanent magnets have to be putting an equal size force on the current carrying wire... But in the opposite direction I. E upwards.

If the force on the permanent magnets was upward and reduced the reading on the balance, the force on the wire would have to be downward.
thank you all for helping me out:smile:

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