Question

Which point of the battery terminal is positive?

A. A
B. B
C. Either A or B
D. Cannot be found

Answer 1

This question can be solved by considering the equilibrium condition to oppose the mass acting downward. After this, by using the right-hand rule one can determine the direction of the induced current. This is possible because the direction of the magnetic field and force can be determined from the figure. Following the determination of the direction of the current by using the fact that the current flows from the positive to the negative direction in an electrical circuit, the positive terminal of the battery can be found.

Complete step by step answer:
To obtain an equilibrium condition there should be an upward force acting against the weight mg which is in the downward direction.

Now using the right-hand rule, we can determine the direction of the current flowing through the circuit. Thereby we can determine which terminal of the battery is positive.
Right-hand rule: hold your forefinger, middle finger, and thumb of your right hand in right angles with each other. In this rule the forefinger indicates the magnetic field direction, the thumb denotes the direction of the applied force and the forefinger denotes the direction of the current.
From the figure, we observe that the cross indicates that the direction of the magnetic field is directed inwards and from the equilibrium concept we have determined that the force is acting upwards. Hence by using the right-hand rule, we can determine that the induced current is flowing from P to A to B through the resistor and then to Q. That is the current is flowing in the clockwise direction.
We know that current in an electrical circuit flows from the positive direction to the negative direction. Hence since we know that the current is flowing in a clockwise direction, the positive terminal of the battery is B.
From the solution, we have determined that the positive terminal point is B.
This option is the correct answer.
We have determined the positive terminal to be B
This option is not correct as we have determined the positive terminal to be B.

Note: Other than the right-hand rule there is a left-hand rule also. This rule is used to determine the direction of motion in an electric motor. In the left-hand rule, the forefinger, thumb, and middle finger of the left hand should be positioned in such a way that all of the fingers are perpendicular to each other. In this rule the forefinger indicates the direction of the field, the middle finger represents current and the thumb shows the direction of the force.