Question

A current is flowing north along a power line. The direction of the magnetic field above it (neglect the earth’s field) is towards:
A. North
B. East
C. South
D. West

Answer 1

Hint: Faraday’s law of induction states that the magnetic field induces around the current carrying conductor in the circular loop. The direction of this magnetic field can be determined by Maxwell's right hand thumb rule. Recall the right hand thumb rule. The curled fingers of the right hand denote the direction of the magnetic field.

Complete answer:
From Faraday’s law of induction, we know that the magnetic field is induced around the current carrying conductor in the circular loop. The direction of this magnetic field can be determined by Maxwell’s right hand thumb rule.Right-hand thumb rule states that if we hold the current carrying conductor in our right hand such that the thumb represents the direction of current flow; then the curled fingers around the conductor denote the direction of the magnetic field.

In this case, we should point the thumb along the north above the power line. We can see the curled fingers above the power line denote the magnetic field towards east as shown in the figure below.

So, the correct answer is option B.

Additional information:
We can also determine the magnetic force exerted on the charged particle above the power line using the Fleming left-hand rule. According to left hand rule, if the forefinger, middle finger and thumb perpendicular to each other such that the forefinger is in the direction of magnetic field, middle finger in the direction of current; then the thumb denotes the direction of the magnetic force.

Note:The right-hand thumb rule is given by Maxwell and not by Fleming. In the above diagram, the direction of the current is not downwards but along the north. The direction of the magnetic field changes at each point but the magnitude of the magnetic field does not change at a specific distance from the conductor.