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Biot-Savart Law

Biot-Savart law gives the magnetic induction due to an infinitesimal current element.

  • Let AB be a conductor of an arbitrary shape carrying a current I, and P be a point in the vacuum at which the field is to be determined. 
  • Let us divide the conductor into infinitesimal current elements. 
  • Let Biot Savart Law and its Applications | Physics Class 12 - NEET be a displacement vector from the element to the point P.

According to `Biot-Savart Law', the magnetic field induction at P due to the current element Biot Savart Law and its Applications | Physics Class 12 - NEET is given by

Biot Savart Law and its Applications | Physics Class 12 - NEET or Biot Savart Law and its Applications | Physics Class 12 - NEET

Where k is a proportionality constant.

Here Biot Savart Law and its Applications | Physics Class 12 - NEET vector points in the direction of current i.      

Biot Savart Law and its Applications | Physics Class 12 - NEET

In S.I. units,

Biot Savart Law and its Applications | Physics Class 12 - NEET 

The equation above is the vector form of the Biot-Savart Law. The magnitude of the field induction at P is given by:

Biot Savart Law and its Applications | Physics Class 12 - NEET,

where θ is the angle between Biot Savart Law and its Applications | Physics Class 12 - NEET and Biot Savart Law and its Applications | Physics Class 12 - NEET.

If the medium is other than air or vacuum, the magnetic induction is:

Biot Savart Law and its Applications | Physics Class 12 - NEET 

where μr is the relative permeability of the medium and is a dimensionless quantity.

Applications of Biot-Savrat Law

Magnetic Field at an Axial Point of a Circular Point

Consider a circular loop of radius R and carrying a steady current i. We have to find out the magnetic field at the axial point P, which is at a distance x from the center of the loop.

Biot Savart Law and its Applications | Physics Class 12 - NEET

Consider an element i Biot Savart Law and its Applications | Physics Class 12 - NEET of the loop as shown in the figure, and the distance of point P from the current element is r. The magnetic field at P due to this current element from the equation (1) can be given by,

Biot Savart Law and its Applications | Physics Class 12 - NEET

In the case of a point on the axis of a circular coil, as for every current element, there is a symmetrically situated opposite element, the components of the field perpendicular to the axis cancel each other while along the axis add up.

Therefore, Biot Savart Law and its Applications | Physics Class 12 - NEET = Biot Savart Law and its Applications | Physics Class 12 - NEET

Here, q is the angle between the current element id and, which is everywhere and

sin Biot Savart Law and its Applications | Physics Class 12 - NEET = Biot Savart Law and its Applications | Physics Class 12 - NEET = Biot Savart Law and its Applications | Physics Class 12 - NEET

Therefore, B = Biot Savart Law and its Applications | Physics Class 12 - NEET Biot Savart Law and its Applications | Physics Class 12 - NEET

or, B = Biot Savart Law and its Applications | Physics Class 12 - NEET

or, B = Biot Savart Law and its Applications | Physics Class 12 - NEET 

Biot Savart Law and its Applications | Physics Class 12 - NEET

If the coil has N turns, then

B = Biot Savart Law and its Applications | Physics Class 12 - NEET

Direction of  Biot Savart Law and its Applications | Physics Class 12 - NEETThe direction of the magnetic field at a point the axis of a circular coil is along the axis and its orientation can be obtained by using the right-hand thumb rule. If the figures are curled along the current, the stretched thumb will point toward the magnetic field.

Biot Savart Law and its Applications | Physics Class 12 - NEET Biot Savart Law and its Applications | Physics Class 12 - NEET

The magnetic field will be out of the page for anticlockwise current while into the page for clockwise current as shown in the figure given. Now consider some special cases involving the application of equation above.

Case I: Field at the center of the coil

In this case distance of the point P from the center (x) = 0, the magnetic field

B = Biot Savart Law and its Applications | Physics Class 12 - NEET = Biot Savart Law and its Applications | Physics Class 12 - NEET 
Biot Savart Law and its Applications | Physics Class 12 - NEET

Case II: Field at a point far away from the center

It means x >> R, B = Biot Savart Law and its Applications | Physics Class 12 - NEET

Magnetic Field at the Center of a Current Arc

Consider an arc of radius R carrying current I and subtending an angle f at the center.

According to Biot-Savart Law, the magnetic field induction at the point P is given by

B = Biot Savart Law and its Applications | Physics Class 12 - NEET

Here, dl = Rdθ

Therefore, B = Biot Savart Law and its Applications | Physics Class 12 - NEET
   Biot Savart Law and its Applications | Physics Class 12 - NEET

or, B = Biot Savart Law and its Applications | Physics Class 12 - NEET 

It `l' is the length of the circular arc, we have

B = Biot Savart Law and its Applications | Physics Class 12 - NEET 

Consider some special cases involving the application of the equation above.

Case I: If the loop is semicircular

Biot Savart Law and its Applications | Physics Class 12 - NEET

In this case Biot Savart Law and its Applications | Physics Class 12 - NEET= π, so

B = Biot Savart Law and its Applications | Physics Class 12 - NEET

and will be out of the page for anticlockwise current while into the page for clockwise current as shown in the figure.

Case II: If the loop is a full circle with N turns

In this case Biot Savart Law and its Applications | Physics Class 12 - NEET = 2p, so

B = Biot Savart Law and its Applications | Physics Class 12 - NEET 
Biot Savart Law and its Applications | Physics Class 12 - NEET

and will be out of the page for anticlockwise current while into the page for clockwise current as shown in the figure.

Example 1: Two-wire loop PQRSP formed by joining two semicircular wires of radii Rand Rcarries a current i as shown in the figure given below. What is the magnetic field induction at the center O in cases (A) and (B)?

Biot Savart Law and its Applications | Physics Class 12 - NEET Biot Savart Law and its Applications | Physics Class 12 - NEET

Solution: (a) As point O is along the length of the straight wires, the field at O due to them will be zero and hence the magnetic field is only due to semicircular portions

Therefore, Biot Savart Law and its Applications | Physics Class 12 - NEET

or, Biot Savart Law and its Applications | Physics Class 12 - NEET = Biot Savart Law and its Applications | Physics Class 12 - NEET out of the page

(b) Biot Savart Law and its Applications | Physics Class 12 - NEET = Biot Savart Law and its Applications | Physics Class 12 - NEET into the page

Example 2: A battery is connected between two points A and B on the circumference of a uniform conducting ring of radius r and resistance R as shown in the figure given below. One of the arcs AB of the ring subtends an angle q at the center. What is the value of the magnetic field at the center due to the current in the ring?

Biot Savart Law and its Applications | Physics Class 12 - NEET Biot Savart Law and its Applications | Physics Class 12 - NEET

Sol. (a) As the field due to the arc at the center is given by

B = Biot Savart Law and its Applications | Physics Class 12 - NEET

Therefore, B = Biot Savart Law and its Applications | Physics Class 12 - NEET

But (VA - VB) = i1R1 = i2R2

or, i2 = i1Biot Savart Law and its Applications | Physics Class 12 - NEET = i1 Biot Savart Law and its Applications | Physics Class 12 - NEET [Therefore, R µ L]

i2 = i1Biot Savart Law and its Applications | Physics Class 12 - NEET [Therefore, L = rq]

Therefore, BR = Biot Savart Law and its Applications | Physics Class 12 - NEET

i.e., the field at the center of the coil is zero and is independent of θ.

Example 3: A charge of one coulomb is placed at one end of a nonconducting rod of length 0.6m. The rod is rotated in a vertical plane about a horizontal axis passing through the other end of the rod with an angular frequency 104π rad/s. Find the magnetic field at a point on the axis of rotation at a distance of 0.8 m from the center of the path.

Now half of the charge is removed from one end and placed on the other end. The rod is rotated in a vertical plane about a horizontal axis passing through the mid-point of the rod with the same angular frequency. Calculate the magnetic field at a point on the axis at a distance of 0.4 m from the center of the rod.

Solution: As the revolving charge q is equivalent to a current

i = qf = q × Biot Savart Law and its Applications | Physics Class 12 - NEET = 1 × Biot Savart Law and its Applications | Physics Class 12 - NEET = 5 × 103 A

Now B = Biot Savart Law and its Applications | Physics Class 12 - NEET   Biot Savart Law and its Applications | Physics Class 12 - NEET

Therefore, B = 10-7 × Biot Savart Law and its Applications | Physics Class 12 - NEET = 1.13 × 10-3 T

If half of the charge is placed at the other end and the rod is rotated at the same frequency, the equivalent current.

i' = Biot Savart Law and its Applications | Physics Class 12 - NEET = qf = i = 5 × 103 A
  Biot Savart Law and its Applications | Physics Class 12 - NEET

In this case, R' = 0.3 m and x' = 0.4 m

Therefore, B' = 10-7 × Biot Savart Law and its Applications | Physics Class 12 - NEET = 2.3 × 10-3T

Magnetic Field on a Current-Carrying Conductor

When Wire is of Finite Length 

  • Consider a straight wire segment carrying a current i and there is a point P at which the magnetic field is to be calculated as shown in the figure. 
  • This wire segment makes angles θ1 and θ2 at that point with normal OP. 
  • Consider an element of length dy at a distance y from O and the distance of this element from point P is r and the line joining P to Q makes an angle q with the direction of current as shown in the figure. Using Biot-Savart Law magnetic field at point P due to small current element is given by

Biot Savart Law and its Applications | Physics Class 12 - NEET

  • As every element of the wire contributes to Biot Savart Law and its Applications | Physics Class 12 - NEET in the same direction, we have
    Biot Savart Law and its Applications | Physics Class 12 - NEET

Biot Savart Law and its Applications | Physics Class 12 - NEET 

From the triangle OPQ, as shown in the diagram, we have

y = d tan Biot Savart Law and its Applications | Physics Class 12 - NEET

or dy = d sec2Biot Savart Law and its Applications | Physics Class 12 - NEETdBiot Savart Law and its Applications | Physics Class 12 - NEET

and is the same triangle,

r = d sec Biot Savart Law and its Applications | Physics Class 12 - NEETand q = (90º - Biot Savart Law and its Applications | Physics Class 12 - NEET), where Biot Savart Law and its Applications | Physics Class 12 - NEETis the angle between line OP and PQ

Now equation (i) can be written in this form

Biot Savart Law and its Applications | Physics Class 12 - NEET

or Biot Savart Law and its Applications | Physics Class 12 - NEET 

Note: θ1 & θ2 must be taken with the sign.
Biot Savart Law and its Applications | Physics Class 12 - NEET

In the case shown in the figure,

B at A = Biot Savart Law and its Applications | Physics Class 12 - NEET

The direction of Biot Savart Law and its Applications | Physics Class 12 - NEETThe direction of the magnetic field is determined by the cross product of the vector Biot Savart Law and its Applications | Physics Class 12 - NEET with Biot Savart Law and its Applications | Physics Class 12 - NEET. Therefore, at point P, the direction of the magnetic field due to the whole conductor will be perpendicular to the plane of the paper and going into the plane.

Right-hand Thumb Rule: The direction of B at point P due to a long, straight wire can be found by the right-hand thumb rule. The direction of the magnetic field is perpendicular to the plane containing the wire and perpendicular to the point. The orientation of the magnetic field is given by the direction of the curl fingers if we stretch the thumb along the wire in the direction of the current. Refer figure.

Biot Savart Law and its Applications | Physics Class 12 - NEET Biot Savart Law and its Applications | Physics Class 12 - NEET

Conventionally, the direction of the field perpendicular to the plane of the paper is represented by Biot Savart Law and its Applications | Physics Class 12 - NEET if into the page and by Biot Savart Law and its Applications | Physics Class 12 - NEETif out of the page.

Now consider some special cases involving the application of equation (3)

Case 1: When the point P is on the perpendicular bisector

In this case angle θ1 = θ2, using the result of equation (3), the magnetic field is

Biot Savart Law and its Applications | Physics Class 12 - NEET

Biot Savart Law and its Applications | Physics Class 12 - NEET

where Biot Savart Law and its Applications | Physics Class 12 - NEET

Case 2: If the wire is infinitely long 

f the wire is infinitely long then the magnetic field at `P' (as shown in the figure) is given by (using q1 = q2 = 90° and the formula `B' due to straight wire)

B = Biot Savart Law and its Applications | Physics Class 12 - NEET ⇒ B µ Biot Savart Law and its Applications | Physics Class 12 - NEET   Biot Savart Law and its Applications | Physics Class 12 - NEET

The direction of Biot Savart Law and its Applications | Physics Class 12 - NEET at various is as shown in the figure. The magnetic lines of force will be concentric circles around the wire (as shown earlier)

Case 3: When the point lies along the length of the wire (but not on it)

Biot Savart Law and its Applications | Physics Class 12 - NEET

If the point P is along the length of the wire (but not one it), then as Biot Savart Law and its Applications | Physics Class 12 - NEET and Biot Savart Law and its Applications | Physics Class 12 - NEET will either be parallel or antiparallel, i.e., q = 0 or p, so Biot Savart Law and its Applications | Physics Class 12 - NEETand hence using equation (1)

Biot Savart Law and its Applications | Physics Class 12 - NEET

Example 4: Calculate the magnetic field induction at a point distance, Biot Savart Law and its Applications | Physics Class 12 - NEET metre from a straight wire of length `a' metre carrying a current of I amp. The point is on the perpendicular bisector of the wire.

Solution: B = Biot Savart Law and its Applications | Physics Class 12 - NEET [sinq1 + sinq2]

= 10-7Biot Savart Law and its Applications | Physics Class 12 - NEET                      Biot Savart Law and its Applications | Physics Class 12 - NEET

= Biot Savart Law and its Applications | Physics Class 12 - NEET

Perpendicular to the plane of figure (inward).

Example 5: Find the resultant magnetic field at `C' in the figure shown.

Biot Savart Law and its Applications | Physics Class 12 - NEET

Solution: It is clear that `B' at `C' due to all the wires being directed Ä. Also, B at `C due to PQ and SR being the same. Also due to QR and PS being the same

Therefore, Bres = 2(BPQ + BSP)

BPQ = (sin 60° + sin 60°)

BSP = Biot Savart Law and its Applications | Physics Class 12 - NEET (sin 30° + sin 30°)

⇒ Bres = Biot Savart Law and its Applications | Physics Class 12 - NEET = Biot Savart Law and its Applications | Physics Class 12 - NEET

Example 6: The figure shows a square loop made from a uniform wire. Find the magnetic field at the center of the square if a battery is connected between points A and C.

Biot Savart Law and its Applications | Physics Class 12 - NEET

Solution: The current will be equally divided at A. The fields at the center due to the currents in the wires AB and DC will be equal in magnitude and opposite in direction. The result of these two fields will be zero. Similarly, the resultant of the fields due to the wires AD and BC will be zero. Hence, the net field at the center will be zero.

Example 7: In the figure shown there are two parallel long wires (placed in the plane of the paper) carrying currents 2 I and I consider points A, C, D on the line perpendicular to both the wires and also in the plane of the paper. The distances are mentioned.

Biot Savart Law and its Applications | Physics Class 12 - NEET

Find (i) Biot Savart Law and its Applications | Physics Class 12 - NEET at A, C, D

(ii) position of point on line A C D where Biot Savart Law and its Applications | Physics Class 12 - NEET is zero.

Solution: (i) Let us call Biot Savart Law and its Applications | Physics Class 12 - NEET due to (1) and (2) as Biot Savart Law and its Applications | Physics Class 12 - NEET and Biot Savart Law and its Applications | Physics Class 12 - NEET respectively. Then

at A: Biot Savart Law and its Applications | Physics Class 12 - NEET is Biot Savart Law and its Applications | Physics Class 12 - NEET and Biot Savart Law and its Applications | Physics Class 12 - NEET is Biot Savart Law and its Applications | Physics Class 12 - NEET

B1 = Biot Savart Law and its Applications | Physics Class 12 - NEET and B2 = Biot Savart Law and its Applications | Physics Class 12 - NEET

Therefore, Bres = B1 - B2 = Biot Savart Law and its Applications | Physics Class 12 - NEET Biot Savart Law and its Applications | Physics Class 12 - NEET Ans.

at C: Biot Savart Law and its Applications | Physics Class 12 - NEET is Biot Savart Law and its Applications | Physics Class 12 - NEET and Biot Savart Law and its Applications | Physics Class 12 - NEET also Biot Savart Law and its Applications | Physics Class 12 - NEET

Therefore, Bres = B1 + B2

Biot Savart Law and its Applications | Physics Class 12 - NEET + Biot Savart Law and its Applications | Physics Class 12 - NEET = Biot Savart Law and its Applications | Physics Class 12 - NEET = Biot Savart Law and its Applications | Physics Class 12 - NEET Biot Savart Law and its Applications | Physics Class 12 - NEET Ans.

Biot Savart Law and its Applications | Physics Class 12 - NEET
Therefore, Bres = 0 Ans.

(ii) It is clear from the above solution that B = 0 at point `D'.

The document Biot Savart Law and its Applications | Physics Class 12 - NEET is a part of the NEET Course Physics Class 12.
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FAQs on Biot Savart Law and its Applications - Physics Class 12 - NEET

1. What is the Biot-Savart law?
Ans. The Biot-Savart law is a fundamental principle in electromagnetism that describes the magnetic field produced by a current-carrying wire. It states that the magnetic field at a point due to an infinitesimally small current element is directly proportional to the magnitude of the current, the length of the current element, the sine of the angle between the current element and the line connecting the point to the element, and inversely proportional to the square of the distance between the point and the element.
2. How can the Biot-Savart law be applied to calculate the magnetic field?
Ans. To calculate the magnetic field at a point due to a current-carrying wire using the Biot-Savart law, one needs to integrate the contributions of all the infinitesimally small current elements along the wire. The integral involves the product of the current, the length of the current element, the sine of the angle between the element and the line connecting the point to the element, and the inverse square of the distance between the point and the element. By summing up these contributions, the total magnetic field at the point can be determined.
3. Can the Biot-Savart law be used to calculate the magnetic field of a solenoid or a coil?
Ans. Yes, the Biot-Savart law can be used to calculate the magnetic field of a solenoid or a coil. By considering the contributions of the individual current elements along the length of the solenoid or coil, one can apply the Biot-Savart law to determine the magnetic field at any point inside or outside the solenoid or coil.
4. How does the Biot-Savart law relate to Ampere's law?
Ans. The Biot-Savart law and Ampere's law are closely related concepts in electromagnetism. While the Biot-Savart law calculates the magnetic field produced by a current-carrying wire, Ampere's law provides a convenient way to calculate the magnetic field using the concept of closed loops. Ampere's law states that the line integral of the magnetic field around a closed loop is equal to the product of the current passing through the loop and the permeability of free space. In certain situations, Ampere's law can simplify the calculations of the magnetic field by exploiting symmetries or choosing appropriate closed loops.
5. What are some practical applications of the Biot-Savart law?
Ans. The Biot-Savart law has various practical applications, including the design and analysis of electromagnets, magnetic field calculations in electric motors and generators, magnetic field mapping in MRI machines, and the study of magnetic fields produced by current-carrying conductors in different configurations. It is also used in the field of magnetohydrodynamics to study the behavior of plasma and magnetic fields in fusion reactors.
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