past paper question on magnetic fields

Can someone show me how to do this past paper question please. :confused:

The magnitude of the force on a current-carrying conductor in a magnetic field is directly proportional to the magnitude of the current in the conductor.

a.) Describe how you could demonstrate this in a school lab.

At a certain point on the earth's surface the horizontal component of the earth's magnetic field is 1.8x10^(-5) T. A straight piece of conducting wire 2m long, of mass 1.5g lies on a horizontal wooden bench in an east-west direction. When a very large current flows momentarily in the wire, its just sufficient to cause the wire to lift up off the surface of the bench.

b.) Calculate the value of the current and state its direction.
c.) What other noticeable effect will this current produce?

Thanks

Reply 1

Justintabib

dhokes

Can someone show me how to do this past paper question please. :confused:

a) the answer should include a diagram: in that case you dont need to descirbe the arrangement of the appratus.

A diagram showing a metal rod clamped & connected to an ammeter (10A d.c) & variable d.c. power supply by connecting wires. There is magnadur/horseshoe magnets in between the rod (or some part of the rod) & the magnet is placed on an electronic balance. (The diagram & the details of this experiment can be found in any good physics book)
Initially the magnet is placed on the balance & the balance is set to read zero (allowing the weight of the magnet) to determine the force on the rod.
The power supply is switched on & a particular steady current (say 2 A) is passed through the rod.
The value of the force is noted from the balance.
The current is increased in steps upto a maximum of 10 A (using a rheostat) & the corresponding values of forces noted.
A graph of force F against current I is plotted.
The graph is a straight line through the origin verifying F is directly proportional to I for a constant magnetic field B.

(the answer would secure 4-6 marks & I strongly suggest you include a labelled diagram instead of describing how the apparatus has been arranged)

Reply 2

Justintabib

dhokes

At a certain point on the earth's surface the horizontal component of the earth's magnetic field is 1.8x10^(-5) T. A straight piece of conducting wire 2m long, of mass 1.5g lies on a horizontal wooden bench in an east-west direction. When a very large current flows momentarily in the wire, its just sufficient to cause the wire to lift up off the surface of the bench.

b.) Calculate the value of the current and state its direction.
c.) What other noticeable effect will this current produce?

b)for the wire to be just lifted up,
weight of the rod W=force on the rod
m x g = B x I x l
I= (m x g)/ (B x l) = [(1.5/1000)kg x 9.81 N/kg] / [(1.8 x 10^-5 T) x 2 m] = 408.75 A=410 A (2 s.f.)
Direction: applying Fleming's left hand rule, (with F up), current is flowing horizontally from east to west.
c)not sure about it but here's the possible ans:
the connecting wires would burn/melt due to such large current. (this seems to be a noticeable effect for sure!)

Reply 3

dhokes

Thanks a lot mate. How do you know the direction of the field?

Reply 4

cdpuk

dhokes

Thanks a lot mate. How do you know the direction of the field?

I've not read the whole thread but this page may help you
http://wikinotes.cdpuk.net/wiki/Magnetic_flux_density

Diagram of the left hand rule there and a quick explation of F=BIl

-Chris

Reply 5

dhokes

cdpuk

I've not read the whole thread but this page may help you
http://wikinotes.cdpuk.net/wiki/Magnetic_flux_density

Diagram of the left hand rule there and a quick explation of F=BIl

-Chris

k thanks

Reply 6

~Adel~

Hey guys, regarding the direction of current in the second part of the question, do you assume the direction of the earth's horizontal magnetic field is from north to south?