Question

Problem 6: A flat rectangular wire loop is positioned next to a long straight current-carrying wire. Both the loop and the wire are in the plane of the page, and the direction of the current is clearly indicated in the figure.

Part (a) Which image best indicates the direction of the magnetic field due to the current in the long straight wire at a point inside the loop? 

Part (b) How does the magnitude of the magnetic field change as the perpendicular distance from the wire, r, increases? 

Part (c) Due to an external force, the loop begins to move towards the top of the page. Which statement best describes the change in the magnitude of the magnetic flux, |φ_B|. and the direction of any induced EMF, ε, as the loop just begins to move? 

Part (d) The magnitude of the electric current in the long straight wires increases steadily. Which statement best describes the change in the magnitude of the magnetic flux, |φ_B|, and the direction of any induced EMF, ε? 

Part (e) Due to an external force, the loop begins to move towards the right of the page. Which statement best describes the change in the magnitude of the magnetic flux, |φ_B|, and the direction of any induced EMF, ε, as the loop just begins to move? 

Part (f) The magnitude of the electric current in the long straight wires decreases monotonically towards zero. Which statement best describes the change in the magnitude of the magnetic flux, Bl, and the direction of any induced EMF, ε?

Part (g) Due to an external force, the loop begins to move towards the bottom of the page. Which statement best describes the change in the magnitude of the magnetic flux, |φ_B|, and the direction of any induced EMF, ε , as the loop just begins to move?

Answer 1

(a)

Magnetic field due to wire is into the page inside the loop

____________

(b)

B = u_oI / 2 $\pi$ r

so,

as 'r' increases, B decreases

_________________

(c)

there is no change in magnetic flux , hence there will be no induced emf

__________________

(d)

if current increases, it means field into the page increases, this means the flux is increasing. Due to this, we need to induce field out of the page to oppose this increase this flux, so emf will be counter-clockwise.

_________________

(e)

as loop moves to the right, the flux into the page decreases, so an external field is induced into the page to oppose the decrease

so,

emf will be induced and is clockwise

______________

(f)

current is decreasing, so flux into the page will decrease

so,

emf is clockwise

___________________

(g)

no change in flux, no induced emf