An electron is released from rest in a region of space where an electric field E

Question

An electron is released from rest in a region of space where an electric field E points up the page and a magnetic field B points into the page. Which path best represents the motion of the electron after it is released? Three long parallel wires carry the same current I_1 = I_2 = I_3, but I_1 is opposite to I_2 and I_3. All the wires lie in the same plane, and I_2 lies directly between I_1 and I_3, a distance R from both. What the direction of the net force on I_3 due to the other two wires? (a) no net force (b) to the right (c) to the left (d) into the page (e) out of the page A loop of wire moves toward a straight wire carrying a current I, as shown in the figure to the right. The direction of the induced current in the loop is _____, which results in a net force on the loop _____. (a) clockwise, to the left (b) counterclockwise, to the left (c) clockwise, to the right (d) counterclockwise, to the right (e) no induced current, no force The primary side of a transformer has a voltage of 200 V with a current of 5 A. If the primary side has 100 loops and the current in the secondary side is 50 A, the secondary has _____ loops and a voltage of _____ (a) 10, 20 V (b) 10, 2000 V (c) 100, 20 V (d) 1000, 20 V (e) 1000, 2000 V A sample of some unknown substance is repelled by a permanent ferromagnet. This material must be _____ and its relative permeability must be _____ (a) diamagnetic, 1 (e) ferromagnetic, > 1

Explanation / Answer

1)

As the electron starts off from rest, the magntic force on it = 0 but the electric force will be directed towards vertically down.

Now, as it gains velocity, magnetic force acts on it towards left (direction is given by the vector multiplication of v and B : F = q(v x B ))

So, it wll move towards left

So, the correct path is (e)

2)

Force on I3 due to I1 will be directed towards right as both the currents are opposite the force is repulsive

Now, force on I3 due to I2 will be directed towards left (force is repulsive for currents in same direction)

But force on I3 due to I2 is greater than I1 (as I2 is closer to I3 than I1)

So, net force on I3 will be directed towards left <---- answer

3)

Magnetic field through the loop is directed into the loop and as the loop moves closer, the flux increases

So, the induced magnetic field will be opposite to the magnetic field due to the wire (given by Lenz's Law) .

So, for this direction of the induced magnetic field the induced current will be directed counterclockwise.

Due to this the net force will be towards right (as on the left side of the loop, the current will be directed opposite to the wire and thus the force will be towards right)

4)

For an ideal transformer,

Vp/Vs = Np/Ns = Is/Ip

Thus ,

200/Vs = 100/Ns = 50/5

So, 100/Ns = 10

So, Ns = 10 <-----answer

and thus, Vs = 200/10 = 20 V <-----answer

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