A long straight wire carries a current of 48.8 A. An electron, travelling at , is 5.20 cm from the wire. We have to calculate the force that acts on the electron if the electron velocity is directed (a) toward the wire, (b) parallel to the current, and (c) at right angles to the direction defined by (a) and (b).

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We fix a coordinate system as shown in the figure. Flow of current in the long straight wire is in direction. Magnitude of the magnetic field at a distance of 5.20 cm from the wire will be

Direction of the vector is along the tangent to the circumference of the circle and therefore for answering the problem we fix it as

.

We now consider the three parts of the problem one-by-one.

(a)

Electron moving with speed is travelling toward the wire. Its velocity will be

.

Its charge is

.

Force on a charge, e, moving with velocity in magnetic field is given by

Therefore,

Force on the electron is in the same direction as that of the current in the wire.

(b)

Electron is moving parallel to the current. Its velocity will be

Therefore,

The direction of the force is radially outward.

(c)

Direction at right angles to and is . Therefore, the force on the electron will be zero as the velocity and the magnetic field are in the same direction.