Question

Right hand thumb rule can be used to find direction of magnetic field due to:
A. Current carrying straight wire
B. Current carrying circular loop
C. Current carrying solenoid
D. All of the above

Answer 1

Hint: Right hand thumb rule is used very often to understand the directions of vectors in three dimensions. In physics also, it is used to find directions and current and magnetic fields. Using the same concept, we will solve the question.

Formula used:
$B=\dfrac{{{\mu }_{0}}}{2\pi {{r}^{2}}}\left( I\times r \right)$

Complete step by step answer:
Now, in question we are asked to find the direction of the magnetic field and for that vector form of equation of magnetic field is given by,
$B=\dfrac{{{\mu }_{0}}}{2\pi {{r}^{2}}}\left( I\times r \right)$
Where, B is magnetic field, I is current and r is distance at which magnetic field is acting.
Now, from the expression it can be said that the magnetic field acts in the direction perpendicular to the direction of current passing in the wire.
Now, to find the direction of the magnetic field using the direction of current in the wire we will use the Right Hand Thumb rule. Now, we will go step by step to find the direction.
Now, consider a thin wire and the current is flowing in the direction as shown in the figure below,

Step 1: Place your thumb in the direction of the current and keep the fingers open.
Step 2: Now, wrap your fingers around the wire like you are holding the wire. By doing this the fingers will be pointing towards your wrist. The direction of fingers is the direction of magnetic field. It is shown in the figure below,

In the same way for, solenoid also the direction of magnetic field is perpendicular to direction of current in the wire which can be seen in the equation given as,
$dB=\dfrac{{{\mu }_{0}}}{4\pi }\dfrac{Ids\times r}{{{r}^{3}}}$
The above equation is an expression of Biot-Savart's law, which states that the direction of magnetic field is always perpendicular to the direction of current flow.
Hence, we can say that Right hand Thumb rule can be used to find the direction of magnetic field due to current carrying straight wire, solenoid and current in circular loop.

So, the correct answer is “Option D”.

Note:
While using Right Hand Thumb rule students should know which quantity is given and which we have to find. Suppose instead of direction of current flow, if we are given the direction of the magnetic field then we have to use the rule accordingly. Students might make mistakes in placing the hand or fingers in the direction of the quantity provided, so students must practice more for this to solve such questions easily. The above problem can be solved just by using the Biot- Savart’s law for direction of current and magnetic field in wire of any shape.