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1
3890-3893
Rationalised 2023-24 Physics 126 EXAMPLE 4 11 As q à 0, the perpendicular distance between the forces of the couple also approaches zero This makes the forces collinear and the net force and torque zero The torques in Eqs
1
3891-3894
11 As q à 0, the perpendicular distance between the forces of the couple also approaches zero This makes the forces collinear and the net force and torque zero The torques in Eqs (4
1
3892-3895
This makes the forces collinear and the net force and torque zero The torques in Eqs (4 26) and (4
1
3893-3896
The torques in Eqs (4 26) and (4 27) can be expressed as vector product of the magnetic moment of the coil and the magnetic field
1
3894-3897
(4 26) and (4 27) can be expressed as vector product of the magnetic moment of the coil and the magnetic field We define the magnetic moment of the current loop as, m = I A (4
1
3895-3898
26) and (4 27) can be expressed as vector product of the magnetic moment of the coil and the magnetic field We define the magnetic moment of the current loop as, m = I A (4 28) where the direction of the area vector A is given by the right-hand thumb rule and is directed into the plane of the paper in Fig
1
3896-3899
27) can be expressed as vector product of the magnetic moment of the coil and the magnetic field We define the magnetic moment of the current loop as, m = I A (4 28) where the direction of the area vector A is given by the right-hand thumb rule and is directed into the plane of the paper in Fig 4
1
3897-3900
We define the magnetic moment of the current loop as, m = I A (4 28) where the direction of the area vector A is given by the right-hand thumb rule and is directed into the plane of the paper in Fig 4 18
1
3898-3901
28) where the direction of the area vector A is given by the right-hand thumb rule and is directed into the plane of the paper in Fig 4 18 Then as the angle between m and B is q , Eqs
1
3899-3902
4 18 Then as the angle between m and B is q , Eqs (4
1
3900-3903
18 Then as the angle between m and B is q , Eqs (4 26) and (4
1
3901-3904
Then as the angle between m and B is q , Eqs (4 26) and (4 27) can be expressed by one expression (4
1
3902-3905
(4 26) and (4 27) can be expressed by one expression (4 29) This is analogous to the electrostatic case (Electric dipole of dipole moment pe in an electric field E)
1
3903-3906
26) and (4 27) can be expressed by one expression (4 29) This is analogous to the electrostatic case (Electric dipole of dipole moment pe in an electric field E) ττ = p×× E e As is clear from Eq
1
3904-3907
27) can be expressed by one expression (4 29) This is analogous to the electrostatic case (Electric dipole of dipole moment pe in an electric field E) ττ = p×× E e As is clear from Eq (4
1
3905-3908
29) This is analogous to the electrostatic case (Electric dipole of dipole moment pe in an electric field E) ττ = p×× E e As is clear from Eq (4 28), the dimensions of the magnetic moment are [A][L2] and its unit is Am2
1
3906-3909
ττ = p×× E e As is clear from Eq (4 28), the dimensions of the magnetic moment are [A][L2] and its unit is Am2 From Eq
1
3907-3910
(4 28), the dimensions of the magnetic moment are [A][L2] and its unit is Am2 From Eq (4
1
3908-3911
28), the dimensions of the magnetic moment are [A][L2] and its unit is Am2 From Eq (4 29), we see that the torque ttttt vanishes when m is either parallel or antiparallel to the magnetic field B
1
3909-3912
From Eq (4 29), we see that the torque ttttt vanishes when m is either parallel or antiparallel to the magnetic field B This indicates a state of equilibrium as there is no torque on the coil (this also applies to any object with a magnetic moment m)
1
3910-3913
(4 29), we see that the torque ttttt vanishes when m is either parallel or antiparallel to the magnetic field B This indicates a state of equilibrium as there is no torque on the coil (this also applies to any object with a magnetic moment m) When m and B are parallel the equilibrium is a stable one
1
3911-3914
29), we see that the torque ttttt vanishes when m is either parallel or antiparallel to the magnetic field B This indicates a state of equilibrium as there is no torque on the coil (this also applies to any object with a magnetic moment m) When m and B are parallel the equilibrium is a stable one Any small rotation of the coil produces a torque which brings it back to its original position
1
3912-3915
This indicates a state of equilibrium as there is no torque on the coil (this also applies to any object with a magnetic moment m) When m and B are parallel the equilibrium is a stable one Any small rotation of the coil produces a torque which brings it back to its original position When they are antiparallel, the equilibrium is unstable as any rotation produces a torque which increases with the amount of rotation
1
3913-3916
When m and B are parallel the equilibrium is a stable one Any small rotation of the coil produces a torque which brings it back to its original position When they are antiparallel, the equilibrium is unstable as any rotation produces a torque which increases with the amount of rotation The presence of this torque is also the reason why a small magnet or any magnetic dipole aligns itself with the external magnetic field
1
3914-3917
Any small rotation of the coil produces a torque which brings it back to its original position When they are antiparallel, the equilibrium is unstable as any rotation produces a torque which increases with the amount of rotation The presence of this torque is also the reason why a small magnet or any magnetic dipole aligns itself with the external magnetic field If the loop has N closely wound turns, the expression for torque, Eq
1
3915-3918
When they are antiparallel, the equilibrium is unstable as any rotation produces a torque which increases with the amount of rotation The presence of this torque is also the reason why a small magnet or any magnetic dipole aligns itself with the external magnetic field If the loop has N closely wound turns, the expression for torque, Eq (4
1
3916-3919
The presence of this torque is also the reason why a small magnet or any magnetic dipole aligns itself with the external magnetic field If the loop has N closely wound turns, the expression for torque, Eq (4 29), still holds, with m = N I A (4
1
3917-3920
If the loop has N closely wound turns, the expression for torque, Eq (4 29), still holds, with m = N I A (4 30) Example 4
1
3918-3921
(4 29), still holds, with m = N I A (4 30) Example 4 11 A 100 turn closely wound circular coil of radius 10 cm carries a current of 3
1
3919-3922
29), still holds, with m = N I A (4 30) Example 4 11 A 100 turn closely wound circular coil of radius 10 cm carries a current of 3 2 A
1
3920-3923
30) Example 4 11 A 100 turn closely wound circular coil of radius 10 cm carries a current of 3 2 A (a) What is the field at the centre of the coil
1
3921-3924
11 A 100 turn closely wound circular coil of radius 10 cm carries a current of 3 2 A (a) What is the field at the centre of the coil (b) What is the magnetic moment of this coil
1
3922-3925
2 A (a) What is the field at the centre of the coil (b) What is the magnetic moment of this coil The coil is placed in a vertical plane and is free to rotate about a horizontal axis which coincides with its diameter
1
3923-3926
(a) What is the field at the centre of the coil (b) What is the magnetic moment of this coil The coil is placed in a vertical plane and is free to rotate about a horizontal axis which coincides with its diameter A uniform magnetic field of 2T in the horizontal direction exists such that initially the axis of the coil is in the direction of the field
1
3924-3927
(b) What is the magnetic moment of this coil The coil is placed in a vertical plane and is free to rotate about a horizontal axis which coincides with its diameter A uniform magnetic field of 2T in the horizontal direction exists such that initially the axis of the coil is in the direction of the field The coil rotates through an angle of 90° under the influence of the magnetic field
1
3925-3928
The coil is placed in a vertical plane and is free to rotate about a horizontal axis which coincides with its diameter A uniform magnetic field of 2T in the horizontal direction exists such that initially the axis of the coil is in the direction of the field The coil rotates through an angle of 90° under the influence of the magnetic field (c) What are the magnitudes of the torques on the coil in the initial and final position
1
3926-3929
A uniform magnetic field of 2T in the horizontal direction exists such that initially the axis of the coil is in the direction of the field The coil rotates through an angle of 90° under the influence of the magnetic field (c) What are the magnitudes of the torques on the coil in the initial and final position (d) What is the angular speed acquired by the coil when it has rotated by 90°
1
3927-3930
The coil rotates through an angle of 90° under the influence of the magnetic field (c) What are the magnitudes of the torques on the coil in the initial and final position (d) What is the angular speed acquired by the coil when it has rotated by 90° The moment of inertia of the coil is 0
1
3928-3931
(c) What are the magnitudes of the torques on the coil in the initial and final position (d) What is the angular speed acquired by the coil when it has rotated by 90° The moment of inertia of the coil is 0 1 kg m2
1
3929-3932
(d) What is the angular speed acquired by the coil when it has rotated by 90° The moment of inertia of the coil is 0 1 kg m2 FIGURE 4
1
3930-3933
The moment of inertia of the coil is 0 1 kg m2 FIGURE 4 19 (a) The area vector of the loop ABCD makes an arbitrary angle q with the magnetic field
1
3931-3934
1 kg m2 FIGURE 4 19 (a) The area vector of the loop ABCD makes an arbitrary angle q with the magnetic field (b) Top view of the loop
1
3932-3935
FIGURE 4 19 (a) The area vector of the loop ABCD makes an arbitrary angle q with the magnetic field (b) Top view of the loop The forces F1 and F2 acting on the arms AB and CD are indicated
1
3933-3936
19 (a) The area vector of the loop ABCD makes an arbitrary angle q with the magnetic field (b) Top view of the loop The forces F1 and F2 acting on the arms AB and CD are indicated Rationalised 2023-24 127 Moving Charges and Magnetism EXAMPLE 4
1
3934-3937
(b) Top view of the loop The forces F1 and F2 acting on the arms AB and CD are indicated Rationalised 2023-24 127 Moving Charges and Magnetism EXAMPLE 4 12 EXAMPLE 4
1
3935-3938
The forces F1 and F2 acting on the arms AB and CD are indicated Rationalised 2023-24 127 Moving Charges and Magnetism EXAMPLE 4 12 EXAMPLE 4 11 Solution (a) From Eq
1
3936-3939
Rationalised 2023-24 127 Moving Charges and Magnetism EXAMPLE 4 12 EXAMPLE 4 11 Solution (a) From Eq (4
1
3937-3940
12 EXAMPLE 4 11 Solution (a) From Eq (4 16) B = µ0RNI 2 Here, N = 100; I = 3
1
3938-3941
11 Solution (a) From Eq (4 16) B = µ0RNI 2 Here, N = 100; I = 3 2 A, and R = 0
1
3939-3942
(4 16) B = µ0RNI 2 Here, N = 100; I = 3 2 A, and R = 0 1 m
1
3940-3943
16) B = µ0RNI 2 Here, N = 100; I = 3 2 A, and R = 0 1 m Hence, = × × × − − 4 10 10 2 10 5 1 (using p ´ 3
1
3941-3944
2 A, and R = 0 1 m Hence, = × × × − − 4 10 10 2 10 5 1 (using p ´ 3 2 = 10) = 2 × 10–3 T The direction is given by the right-hand thumb rule
1
3942-3945
1 m Hence, = × × × − − 4 10 10 2 10 5 1 (using p ´ 3 2 = 10) = 2 × 10–3 T The direction is given by the right-hand thumb rule (b) The magnetic moment is given by Eq
1
3943-3946
Hence, = × × × − − 4 10 10 2 10 5 1 (using p ´ 3 2 = 10) = 2 × 10–3 T The direction is given by the right-hand thumb rule (b) The magnetic moment is given by Eq (4
1
3944-3947
2 = 10) = 2 × 10–3 T The direction is given by the right-hand thumb rule (b) The magnetic moment is given by Eq (4 30), m = N I A = N I p r2 = 100 × 3
1
3945-3948
(b) The magnetic moment is given by Eq (4 30), m = N I A = N I p r2 = 100 × 3 2 × 3
1
3946-3949
(4 30), m = N I A = N I p r2 = 100 × 3 2 × 3 14 × 10–2 = 10 A m2 The direction is once again given by the right-hand thumb rule
1
3947-3950
30), m = N I A = N I p r2 = 100 × 3 2 × 3 14 × 10–2 = 10 A m2 The direction is once again given by the right-hand thumb rule (c) t = m × B [from Eq
1
3948-3951
2 × 3 14 × 10–2 = 10 A m2 The direction is once again given by the right-hand thumb rule (c) t = m × B [from Eq (4
1
3949-3952
14 × 10–2 = 10 A m2 The direction is once again given by the right-hand thumb rule (c) t = m × B [from Eq (4 29)] = m B sin q Initially, q = 0
1
3950-3953
(c) t = m × B [from Eq (4 29)] = m B sin q Initially, q = 0 Thus, initial torque ti = 0
1
3951-3954
(4 29)] = m B sin q Initially, q = 0 Thus, initial torque ti = 0 Finally, q = p/2 (or 90º)
1
3952-3955
29)] = m B sin q Initially, q = 0 Thus, initial torque ti = 0 Finally, q = p/2 (or 90º) Thus, final torque tf = m B = 10 ´ 2 = 20 N m
1
3953-3956
Thus, initial torque ti = 0 Finally, q = p/2 (or 90º) Thus, final torque tf = m B = 10 ´ 2 = 20 N m (d) From Newton’s second law, I where I is the moment of inertia of the coil
1
3954-3957
Finally, q = p/2 (or 90º) Thus, final torque tf = m B = 10 ´ 2 = 20 N m (d) From Newton’s second law, I where I is the moment of inertia of the coil From chain rule, d d d d d d d d   t t        Using this, I d sin d  m B     Integrating from q = 0 to q = p/2, Example 4
1
3955-3958
Thus, final torque tf = m B = 10 ´ 2 = 20 N m (d) From Newton’s second law, I where I is the moment of inertia of the coil From chain rule, d d d d d d d d   t t        Using this, I d sin d  m B     Integrating from q = 0 to q = p/2, Example 4 12 (a) A current-carrying circular loop lies on a smooth horizontal plane
1
3956-3959
(d) From Newton’s second law, I where I is the moment of inertia of the coil From chain rule, d d d d d d d d   t t        Using this, I d sin d  m B     Integrating from q = 0 to q = p/2, Example 4 12 (a) A current-carrying circular loop lies on a smooth horizontal plane Can a uniform magnetic field be set up in such a manner that the loop turns around itself (i
1
3957-3960
From chain rule, d d d d d d d d   t t        Using this, I d sin d  m B     Integrating from q = 0 to q = p/2, Example 4 12 (a) A current-carrying circular loop lies on a smooth horizontal plane Can a uniform magnetic field be set up in such a manner that the loop turns around itself (i e
1
3958-3961
12 (a) A current-carrying circular loop lies on a smooth horizontal plane Can a uniform magnetic field be set up in such a manner that the loop turns around itself (i e , turns about the vertical axis)
1
3959-3962
Can a uniform magnetic field be set up in such a manner that the loop turns around itself (i e , turns about the vertical axis) (b) A current-carrying circular loop is located in a uniform external magnetic field
1
3960-3963
e , turns about the vertical axis) (b) A current-carrying circular loop is located in a uniform external magnetic field If the loop is free to turn, what is its orientation of stable equilibrium
1
3961-3964
, turns about the vertical axis) (b) A current-carrying circular loop is located in a uniform external magnetic field If the loop is free to turn, what is its orientation of stable equilibrium Show that in this orientation, the flux of Rationalised 2023-24 Physics 128 EXAMPLE 4
1
3962-3965
(b) A current-carrying circular loop is located in a uniform external magnetic field If the loop is free to turn, what is its orientation of stable equilibrium Show that in this orientation, the flux of Rationalised 2023-24 Physics 128 EXAMPLE 4 12 the total field (external field + field produced by the loop) is maximum
1
3963-3966
If the loop is free to turn, what is its orientation of stable equilibrium Show that in this orientation, the flux of Rationalised 2023-24 Physics 128 EXAMPLE 4 12 the total field (external field + field produced by the loop) is maximum (c) A loop of irregular shape carrying current is located in an external magnetic field
1
3964-3967
Show that in this orientation, the flux of Rationalised 2023-24 Physics 128 EXAMPLE 4 12 the total field (external field + field produced by the loop) is maximum (c) A loop of irregular shape carrying current is located in an external magnetic field If the wire is flexible, why does it change to a circular shape
1
3965-3968
12 the total field (external field + field produced by the loop) is maximum (c) A loop of irregular shape carrying current is located in an external magnetic field If the wire is flexible, why does it change to a circular shape Solution (a) No, because that would require ttttt to be in the vertical direction
1
3966-3969
(c) A loop of irregular shape carrying current is located in an external magnetic field If the wire is flexible, why does it change to a circular shape Solution (a) No, because that would require ttttt to be in the vertical direction But ttttt = I A × B, and since A of the horizontal loop is in the vertical direction, t would be in the plane of the loop for any B
1
3967-3970
If the wire is flexible, why does it change to a circular shape Solution (a) No, because that would require ttttt to be in the vertical direction But ttttt = I A × B, and since A of the horizontal loop is in the vertical direction, t would be in the plane of the loop for any B (b) Orientation of stable equilibrium is one where the area vector A of the loop is in the direction of external magnetic field
1
3968-3971
Solution (a) No, because that would require ttttt to be in the vertical direction But ttttt = I A × B, and since A of the horizontal loop is in the vertical direction, t would be in the plane of the loop for any B (b) Orientation of stable equilibrium is one where the area vector A of the loop is in the direction of external magnetic field In this orientation, the magnetic field produced by the loop is in the same direction as external field, both normal to the plane of the loop, thus giving rise to maximum flux of the total field
1
3969-3972
But ttttt = I A × B, and since A of the horizontal loop is in the vertical direction, t would be in the plane of the loop for any B (b) Orientation of stable equilibrium is one where the area vector A of the loop is in the direction of external magnetic field In this orientation, the magnetic field produced by the loop is in the same direction as external field, both normal to the plane of the loop, thus giving rise to maximum flux of the total field (c) It assumes circular shape with its plane normal to the field to maximise flux, since for a given perimeter, a circle encloses greater area than any other shape
1
3970-3973
(b) Orientation of stable equilibrium is one where the area vector A of the loop is in the direction of external magnetic field In this orientation, the magnetic field produced by the loop is in the same direction as external field, both normal to the plane of the loop, thus giving rise to maximum flux of the total field (c) It assumes circular shape with its plane normal to the field to maximise flux, since for a given perimeter, a circle encloses greater area than any other shape 4
1
3971-3974
In this orientation, the magnetic field produced by the loop is in the same direction as external field, both normal to the plane of the loop, thus giving rise to maximum flux of the total field (c) It assumes circular shape with its plane normal to the field to maximise flux, since for a given perimeter, a circle encloses greater area than any other shape 4 9
1
3972-3975
(c) It assumes circular shape with its plane normal to the field to maximise flux, since for a given perimeter, a circle encloses greater area than any other shape 4 9 2 Circular current loop as a magnetic dipole In this section, we shall consider the elementary magnetic element: the current loop
1
3973-3976
4 9 2 Circular current loop as a magnetic dipole In this section, we shall consider the elementary magnetic element: the current loop We shall show that the magnetic field (at large distances) due to current in a circular current loop is very similar in behaviour to the electric field of an electric dipole
1
3974-3977
9 2 Circular current loop as a magnetic dipole In this section, we shall consider the elementary magnetic element: the current loop We shall show that the magnetic field (at large distances) due to current in a circular current loop is very similar in behaviour to the electric field of an electric dipole In Section 4
1
3975-3978
2 Circular current loop as a magnetic dipole In this section, we shall consider the elementary magnetic element: the current loop We shall show that the magnetic field (at large distances) due to current in a circular current loop is very similar in behaviour to the electric field of an electric dipole In Section 4 6, we have evaluated the magnetic field on the axis of a circular loop, of a radius R, carrying a steady current I
1
3976-3979
We shall show that the magnetic field (at large distances) due to current in a circular current loop is very similar in behaviour to the electric field of an electric dipole In Section 4 6, we have evaluated the magnetic field on the axis of a circular loop, of a radius R, carrying a steady current I The magnitude of this field is [(Eq
1
3977-3980
In Section 4 6, we have evaluated the magnetic field on the axis of a circular loop, of a radius R, carrying a steady current I The magnitude of this field is [(Eq (4
1
3978-3981
6, we have evaluated the magnetic field on the axis of a circular loop, of a radius R, carrying a steady current I The magnitude of this field is [(Eq (4 15)], ( ) 2 0 3/2 2 2 2 µ = + I R B x R and its direction is along the axis and given by the right-hand thumb rule (Fig
1
3979-3982
The magnitude of this field is [(Eq (4 15)], ( ) 2 0 3/2 2 2 2 µ = + I R B x R and its direction is along the axis and given by the right-hand thumb rule (Fig 4
1
3980-3983
(4 15)], ( ) 2 0 3/2 2 2 2 µ = + I R B x R and its direction is along the axis and given by the right-hand thumb rule (Fig 4 12)
1
3981-3984
15)], ( ) 2 0 3/2 2 2 2 µ = + I R B x R and its direction is along the axis and given by the right-hand thumb rule (Fig 4 12) Here, x is the distance along the axis from the centre of the loop
1
3982-3985
4 12) Here, x is the distance along the axis from the centre of the loop For x >> R, we may drop the R2 term in the denominator
1
3983-3986
12) Here, x is the distance along the axis from the centre of the loop For x >> R, we may drop the R2 term in the denominator Thus, 2 0 3 2 IR B x µ = Note that the area of the loop A = pR2
1
3984-3987
Here, x is the distance along the axis from the centre of the loop For x >> R, we may drop the R2 term in the denominator Thus, 2 0 3 2 IR B x µ = Note that the area of the loop A = pR2 Thus, 0 3 2 IA B x =µ π As earlier, we define the magnetic moment m to have a magnitude IA, m = I A
1
3985-3988
For x >> R, we may drop the R2 term in the denominator Thus, 2 0 3 2 IR B x µ = Note that the area of the loop A = pR2 Thus, 0 3 2 IA B x =µ π As earlier, we define the magnetic moment m to have a magnitude IA, m = I A Hence, B ≃ µ0m 3 2 πx π 0 3 2 4 x =µ m [4
1
3986-3989
Thus, 2 0 3 2 IR B x µ = Note that the area of the loop A = pR2 Thus, 0 3 2 IA B x =µ π As earlier, we define the magnetic moment m to have a magnitude IA, m = I A Hence, B ≃ µ0m 3 2 πx π 0 3 2 4 x =µ m [4 31(a)] The expression of Eq
1
3987-3990
Thus, 0 3 2 IA B x =µ π As earlier, we define the magnetic moment m to have a magnitude IA, m = I A Hence, B ≃ µ0m 3 2 πx π 0 3 2 4 x =µ m [4 31(a)] The expression of Eq [4
1
3988-3991
Hence, B ≃ µ0m 3 2 πx π 0 3 2 4 x =µ m [4 31(a)] The expression of Eq [4 31(a)] is very similar to an expression obtained earlier for the electric field of a dipole
1
3989-3992
31(a)] The expression of Eq [4 31(a)] is very similar to an expression obtained earlier for the electric field of a dipole The similarity may be seen if we substitute, 0 1/0 µ ε → Rationalised 2023-24 129 Moving Charges and Magnetism e m→ p (electrostatic dipole) B→ E (electrostatic field) We then obtain, 3 0 2 4 e εx = π p E which is precisely the field for an electric dipole at a point on its axis