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3390-3393
4), we find that there is an upward force F, of magnitude IlB, For mid-air suspension, this must be balanced by the force due to gravity: m g = I lB m g B I l = 0 2 9 8 0
1
3391-3394
For mid-air suspension, this must be balanced by the force due to gravity: m g = I lB m g B I l = 0 2 9 8 0 65 T 2 ×1
1
3392-3395
2 9 8 0 65 T 2 ×1 5 = = × Note that it would have been sufficient to specify m/l, the mass per unit length of the wire
1
3393-3396
8 0 65 T 2 ×1 5 = = × Note that it would have been sufficient to specify m/l, the mass per unit length of the wire The earth’s magnetic field is approximately 4 × 10–5 T and we have ignored it
1
3394-3397
65 T 2 ×1 5 = = × Note that it would have been sufficient to specify m/l, the mass per unit length of the wire The earth’s magnetic field is approximately 4 × 10–5 T and we have ignored it Example 4
1
3395-3398
5 = = × Note that it would have been sufficient to specify m/l, the mass per unit length of the wire The earth’s magnetic field is approximately 4 × 10–5 T and we have ignored it Example 4 2 If the magnetic field is parallel to the positive y-axis and the charged particle is moving along the positive x-axis (Fig
1
3396-3399
The earth’s magnetic field is approximately 4 × 10–5 T and we have ignored it Example 4 2 If the magnetic field is parallel to the positive y-axis and the charged particle is moving along the positive x-axis (Fig 4
1
3397-3400
Example 4 2 If the magnetic field is parallel to the positive y-axis and the charged particle is moving along the positive x-axis (Fig 4 4), which way would the Lorentz force be for (a) an electron (negative charge), (b) a proton (positive charge)
1
3398-3401
2 If the magnetic field is parallel to the positive y-axis and the charged particle is moving along the positive x-axis (Fig 4 4), which way would the Lorentz force be for (a) an electron (negative charge), (b) a proton (positive charge) FIGURE 4
1
3399-3402
4 4), which way would the Lorentz force be for (a) an electron (negative charge), (b) a proton (positive charge) FIGURE 4 4 Solution The velocity v of particle is along the x-axis, while B, the magnetic field is along the y-axis, so v × B is along the z-axis (screw rule or right-hand thumb rule)
1
3400-3403
4), which way would the Lorentz force be for (a) an electron (negative charge), (b) a proton (positive charge) FIGURE 4 4 Solution The velocity v of particle is along the x-axis, while B, the magnetic field is along the y-axis, so v × B is along the z-axis (screw rule or right-hand thumb rule) So, (a) for electron it will be along –z axis
1
3401-3404
FIGURE 4 4 Solution The velocity v of particle is along the x-axis, while B, the magnetic field is along the y-axis, so v × B is along the z-axis (screw rule or right-hand thumb rule) So, (a) for electron it will be along –z axis (b) for a positive charge (proton) the force is along +z axis
1
3402-3405
4 Solution The velocity v of particle is along the x-axis, while B, the magnetic field is along the y-axis, so v × B is along the z-axis (screw rule or right-hand thumb rule) So, (a) for electron it will be along –z axis (b) for a positive charge (proton) the force is along +z axis EXAMPLE 4
1
3403-3406
So, (a) for electron it will be along –z axis (b) for a positive charge (proton) the force is along +z axis EXAMPLE 4 2 Charged particles moving in a magnetic field
1
3404-3407
(b) for a positive charge (proton) the force is along +z axis EXAMPLE 4 2 Charged particles moving in a magnetic field Interactive demonstration: http://www
1
3405-3408
EXAMPLE 4 2 Charged particles moving in a magnetic field Interactive demonstration: http://www phys
1
3406-3409
2 Charged particles moving in a magnetic field Interactive demonstration: http://www phys hawaii
1
3407-3410
Interactive demonstration: http://www phys hawaii edu/~teb/optics/java/partmagn/index
1
3408-3411
phys hawaii edu/~teb/optics/java/partmagn/index html Rationalised 2023-24 Physics 112 4
1
3409-3412
hawaii edu/~teb/optics/java/partmagn/index html Rationalised 2023-24 Physics 112 4 3 MOTION IN A MAGNETIC FIELD We will now consider, in greater detail, the motion of a charge moving in a magnetic field
1
3410-3413
edu/~teb/optics/java/partmagn/index html Rationalised 2023-24 Physics 112 4 3 MOTION IN A MAGNETIC FIELD We will now consider, in greater detail, the motion of a charge moving in a magnetic field We have learnt in Mechanics (see Class XI book, Chapter 6) that a force on a particle does work if the force has a component along (or opposed to) the direction of motion of the particle
1
3411-3414
html Rationalised 2023-24 Physics 112 4 3 MOTION IN A MAGNETIC FIELD We will now consider, in greater detail, the motion of a charge moving in a magnetic field We have learnt in Mechanics (see Class XI book, Chapter 6) that a force on a particle does work if the force has a component along (or opposed to) the direction of motion of the particle In the case of motion of a charge in a magnetic field, the magnetic force is perpendicular to the velocity of the particle
1
3412-3415
3 MOTION IN A MAGNETIC FIELD We will now consider, in greater detail, the motion of a charge moving in a magnetic field We have learnt in Mechanics (see Class XI book, Chapter 6) that a force on a particle does work if the force has a component along (or opposed to) the direction of motion of the particle In the case of motion of a charge in a magnetic field, the magnetic force is perpendicular to the velocity of the particle So no work is done and no change in the magnitude of the velocity is produced (though the direction of momentum may be changed)
1
3413-3416
We have learnt in Mechanics (see Class XI book, Chapter 6) that a force on a particle does work if the force has a component along (or opposed to) the direction of motion of the particle In the case of motion of a charge in a magnetic field, the magnetic force is perpendicular to the velocity of the particle So no work is done and no change in the magnitude of the velocity is produced (though the direction of momentum may be changed) [Notice that this is unlike the force due to an electric field, qE, which can have a component parallel (or antiparallel) to motion and thus can transfer energy in addition to momentum
1
3414-3417
In the case of motion of a charge in a magnetic field, the magnetic force is perpendicular to the velocity of the particle So no work is done and no change in the magnitude of the velocity is produced (though the direction of momentum may be changed) [Notice that this is unlike the force due to an electric field, qE, which can have a component parallel (or antiparallel) to motion and thus can transfer energy in addition to momentum ] We shall consider motion of a charged particle in a uniform magnetic field
1
3415-3418
So no work is done and no change in the magnitude of the velocity is produced (though the direction of momentum may be changed) [Notice that this is unlike the force due to an electric field, qE, which can have a component parallel (or antiparallel) to motion and thus can transfer energy in addition to momentum ] We shall consider motion of a charged particle in a uniform magnetic field First consider the case of v perpendicular to B
1
3416-3419
[Notice that this is unlike the force due to an electric field, qE, which can have a component parallel (or antiparallel) to motion and thus can transfer energy in addition to momentum ] We shall consider motion of a charged particle in a uniform magnetic field First consider the case of v perpendicular to B The perpendicular force, q v × B, acts as a centripetal force and produces a circular motion perpendicular to the magnetic field
1
3417-3420
] We shall consider motion of a charged particle in a uniform magnetic field First consider the case of v perpendicular to B The perpendicular force, q v × B, acts as a centripetal force and produces a circular motion perpendicular to the magnetic field The particle will describe a circle if v and B are perpendicular to each other (Fig
1
3418-3421
First consider the case of v perpendicular to B The perpendicular force, q v × B, acts as a centripetal force and produces a circular motion perpendicular to the magnetic field The particle will describe a circle if v and B are perpendicular to each other (Fig 4
1
3419-3422
The perpendicular force, q v × B, acts as a centripetal force and produces a circular motion perpendicular to the magnetic field The particle will describe a circle if v and B are perpendicular to each other (Fig 4 5)
1
3420-3423
The particle will describe a circle if v and B are perpendicular to each other (Fig 4 5) If velocity has a component along B, this component remains unchanged as the motion along the magnetic field will not be affected by the magnetic field
1
3421-3424
4 5) If velocity has a component along B, this component remains unchanged as the motion along the magnetic field will not be affected by the magnetic field The motion in a plane perpendicular to B is as before a circular one, thereby producing a helical motion (Fig
1
3422-3425
5) If velocity has a component along B, this component remains unchanged as the motion along the magnetic field will not be affected by the magnetic field The motion in a plane perpendicular to B is as before a circular one, thereby producing a helical motion (Fig 4
1
3423-3426
If velocity has a component along B, this component remains unchanged as the motion along the magnetic field will not be affected by the magnetic field The motion in a plane perpendicular to B is as before a circular one, thereby producing a helical motion (Fig 4 6)
1
3424-3427
The motion in a plane perpendicular to B is as before a circular one, thereby producing a helical motion (Fig 4 6) You have already learnt in earlier classes (See Class XI, Chapter 4) that if r is the radius of the circular path of a particle, then a force of m v2 / r, acts perpendicular to the path towards the centre of the circle, and is called the centripetal force
1
3425-3428
4 6) You have already learnt in earlier classes (See Class XI, Chapter 4) that if r is the radius of the circular path of a particle, then a force of m v2 / r, acts perpendicular to the path towards the centre of the circle, and is called the centripetal force If the velocity v is perpendicular to the magnetic field B, the magnetic force is perpendicular to both v and B and acts like a centripetal force
1
3426-3429
6) You have already learnt in earlier classes (See Class XI, Chapter 4) that if r is the radius of the circular path of a particle, then a force of m v2 / r, acts perpendicular to the path towards the centre of the circle, and is called the centripetal force If the velocity v is perpendicular to the magnetic field B, the magnetic force is perpendicular to both v and B and acts like a centripetal force It has a magnitude q v B
1
3427-3430
You have already learnt in earlier classes (See Class XI, Chapter 4) that if r is the radius of the circular path of a particle, then a force of m v2 / r, acts perpendicular to the path towards the centre of the circle, and is called the centripetal force If the velocity v is perpendicular to the magnetic field B, the magnetic force is perpendicular to both v and B and acts like a centripetal force It has a magnitude q v B Equating the two expressions for centripetal force, m v 2/r = q v B, which gives r = m v / qB (4
1
3428-3431
If the velocity v is perpendicular to the magnetic field B, the magnetic force is perpendicular to both v and B and acts like a centripetal force It has a magnitude q v B Equating the two expressions for centripetal force, m v 2/r = q v B, which gives r = m v / qB (4 5) for the radius of the circle described by the charged particle
1
3429-3432
It has a magnitude q v B Equating the two expressions for centripetal force, m v 2/r = q v B, which gives r = m v / qB (4 5) for the radius of the circle described by the charged particle The larger the momentum, the larger is the radius and bigger the circle described
1
3430-3433
Equating the two expressions for centripetal force, m v 2/r = q v B, which gives r = m v / qB (4 5) for the radius of the circle described by the charged particle The larger the momentum, the larger is the radius and bigger the circle described If w is the angular frequency, then v = w r
1
3431-3434
5) for the radius of the circle described by the charged particle The larger the momentum, the larger is the radius and bigger the circle described If w is the angular frequency, then v = w r So, w = 2p n = q B/ m [4
1
3432-3435
The larger the momentum, the larger is the radius and bigger the circle described If w is the angular frequency, then v = w r So, w = 2p n = q B/ m [4 6(a)] which is independent of the velocity or energy
1
3433-3436
If w is the angular frequency, then v = w r So, w = 2p n = q B/ m [4 6(a)] which is independent of the velocity or energy Here n is the frequency of rotation
1
3434-3437
So, w = 2p n = q B/ m [4 6(a)] which is independent of the velocity or energy Here n is the frequency of rotation The independence of n from energy has important application in the design of a cyclotron (see Section 4
1
3435-3438
6(a)] which is independent of the velocity or energy Here n is the frequency of rotation The independence of n from energy has important application in the design of a cyclotron (see Section 4 4
1
3436-3439
Here n is the frequency of rotation The independence of n from energy has important application in the design of a cyclotron (see Section 4 4 2)
1
3437-3440
The independence of n from energy has important application in the design of a cyclotron (see Section 4 4 2) The time taken for one revolution is T= 2p/w º 1/n
1
3438-3441
4 2) The time taken for one revolution is T= 2p/w º 1/n If there is a component of the velocity parallel to the magnetic field (denoted by v||), it will make the particle FIGURE 4
1
3439-3442
2) The time taken for one revolution is T= 2p/w º 1/n If there is a component of the velocity parallel to the magnetic field (denoted by v||), it will make the particle FIGURE 4 5 Circular motion FIGURE 4
1
3440-3443
The time taken for one revolution is T= 2p/w º 1/n If there is a component of the velocity parallel to the magnetic field (denoted by v||), it will make the particle FIGURE 4 5 Circular motion FIGURE 4 6 Helical motion Rationalised 2023-24 113 Moving Charges and Magnetism EXAMPLE 4
1
3441-3444
If there is a component of the velocity parallel to the magnetic field (denoted by v||), it will make the particle FIGURE 4 5 Circular motion FIGURE 4 6 Helical motion Rationalised 2023-24 113 Moving Charges and Magnetism EXAMPLE 4 3 Example 4
1
3442-3445
5 Circular motion FIGURE 4 6 Helical motion Rationalised 2023-24 113 Moving Charges and Magnetism EXAMPLE 4 3 Example 4 3 What is the radius of the path of an electron (mass 9 × 10-31 kg and charge 1
1
3443-3446
6 Helical motion Rationalised 2023-24 113 Moving Charges and Magnetism EXAMPLE 4 3 Example 4 3 What is the radius of the path of an electron (mass 9 × 10-31 kg and charge 1 6 × 10–19 C) moving at a speed of 3 ×107 m/s in a magnetic field of 6 × 10–4 T perpendicular to it
1
3444-3447
3 Example 4 3 What is the radius of the path of an electron (mass 9 × 10-31 kg and charge 1 6 × 10–19 C) moving at a speed of 3 ×107 m/s in a magnetic field of 6 × 10–4 T perpendicular to it What is its frequency
1
3445-3448
3 What is the radius of the path of an electron (mass 9 × 10-31 kg and charge 1 6 × 10–19 C) moving at a speed of 3 ×107 m/s in a magnetic field of 6 × 10–4 T perpendicular to it What is its frequency Calculate its energy in keV
1
3446-3449
6 × 10–19 C) moving at a speed of 3 ×107 m/s in a magnetic field of 6 × 10–4 T perpendicular to it What is its frequency Calculate its energy in keV ( 1 eV = 1
1
3447-3450
What is its frequency Calculate its energy in keV ( 1 eV = 1 6 × 10–19 J)
1
3448-3451
Calculate its energy in keV ( 1 eV = 1 6 × 10–19 J) Solution Using Eq
1
3449-3452
( 1 eV = 1 6 × 10–19 J) Solution Using Eq (4
1
3450-3453
6 × 10–19 J) Solution Using Eq (4 5) we find r = m v / (qB) = 9 ×10–31 kg × 3 × 107 m s–1 / ( 1
1
3451-3454
Solution Using Eq (4 5) we find r = m v / (qB) = 9 ×10–31 kg × 3 × 107 m s–1 / ( 1 6 × 10–19 C × 6 × 10–4 T) = 28 × 10–2 m = 28 cm n = v / (2 pr) = 17×106 s–1 = 17×106 Hz =17 MHz
1
3452-3455
(4 5) we find r = m v / (qB) = 9 ×10–31 kg × 3 × 107 m s–1 / ( 1 6 × 10–19 C × 6 × 10–4 T) = 28 × 10–2 m = 28 cm n = v / (2 pr) = 17×106 s–1 = 17×106 Hz =17 MHz E = (½ )mv 2 = (½ ) 9 × 10–31 kg × 9 × 1014 m2/s2 = 40
1
3453-3456
5) we find r = m v / (qB) = 9 ×10–31 kg × 3 × 107 m s–1 / ( 1 6 × 10–19 C × 6 × 10–4 T) = 28 × 10–2 m = 28 cm n = v / (2 pr) = 17×106 s–1 = 17×106 Hz =17 MHz E = (½ )mv 2 = (½ ) 9 × 10–31 kg × 9 × 1014 m2/s2 = 40 5 ×10–17 J ≈ 4×10–16 J = 2
1
3454-3457
6 × 10–19 C × 6 × 10–4 T) = 28 × 10–2 m = 28 cm n = v / (2 pr) = 17×106 s–1 = 17×106 Hz =17 MHz E = (½ )mv 2 = (½ ) 9 × 10–31 kg × 9 × 1014 m2/s2 = 40 5 ×10–17 J ≈ 4×10–16 J = 2 5 keV
1
3455-3458
E = (½ )mv 2 = (½ ) 9 × 10–31 kg × 9 × 1014 m2/s2 = 40 5 ×10–17 J ≈ 4×10–16 J = 2 5 keV move along the field and the path of the particle would be a helical one (Fig
1
3456-3459
5 ×10–17 J ≈ 4×10–16 J = 2 5 keV move along the field and the path of the particle would be a helical one (Fig 4
1
3457-3460
5 keV move along the field and the path of the particle would be a helical one (Fig 4 6)
1
3458-3461
move along the field and the path of the particle would be a helical one (Fig 4 6) The distance moved along the magnetic field in one rotation is called pitch p
1
3459-3462
4 6) The distance moved along the magnetic field in one rotation is called pitch p Using Eq
1
3460-3463
6) The distance moved along the magnetic field in one rotation is called pitch p Using Eq [4
1
3461-3464
The distance moved along the magnetic field in one rotation is called pitch p Using Eq [4 6 (a)], we have p = v||T = 2pm v|| / q B [4
1
3462-3465
Using Eq [4 6 (a)], we have p = v||T = 2pm v|| / q B [4 6(b)] The radius of the circular component of motion is called the radius of the helix
1
3463-3466
[4 6 (a)], we have p = v||T = 2pm v|| / q B [4 6(b)] The radius of the circular component of motion is called the radius of the helix 4
1
3464-3467
6 (a)], we have p = v||T = 2pm v|| / q B [4 6(b)] The radius of the circular component of motion is called the radius of the helix 4 4 MAGNETIC FIELD DUE TO A CURRENT ELEMENT, BIOT-SAVART LAW All magnetic fields that we know are due to currents (or moving charges) and due to intrinsic magnetic moments of particles
1
3465-3468
6(b)] The radius of the circular component of motion is called the radius of the helix 4 4 MAGNETIC FIELD DUE TO A CURRENT ELEMENT, BIOT-SAVART LAW All magnetic fields that we know are due to currents (or moving charges) and due to intrinsic magnetic moments of particles Here, we shall study the relation between current and the magnetic field it produces
1
3466-3469
4 4 MAGNETIC FIELD DUE TO A CURRENT ELEMENT, BIOT-SAVART LAW All magnetic fields that we know are due to currents (or moving charges) and due to intrinsic magnetic moments of particles Here, we shall study the relation between current and the magnetic field it produces It is given by the Biot-Savart’s law
1
3467-3470
4 MAGNETIC FIELD DUE TO A CURRENT ELEMENT, BIOT-SAVART LAW All magnetic fields that we know are due to currents (or moving charges) and due to intrinsic magnetic moments of particles Here, we shall study the relation between current and the magnetic field it produces It is given by the Biot-Savart’s law Fig
1
3468-3471
Here, we shall study the relation between current and the magnetic field it produces It is given by the Biot-Savart’s law Fig 4
1
3469-3472
It is given by the Biot-Savart’s law Fig 4 7 shows a finite conductor XY carrying current I
1
3470-3473
Fig 4 7 shows a finite conductor XY carrying current I Consider an infinitesimal element dl of the conductor
1
3471-3474
4 7 shows a finite conductor XY carrying current I Consider an infinitesimal element dl of the conductor The magnetic field dB due to this element is to be determined at a point P which is at a distance r from it
1
3472-3475
7 shows a finite conductor XY carrying current I Consider an infinitesimal element dl of the conductor The magnetic field dB due to this element is to be determined at a point P which is at a distance r from it Let q be the angle between dl and the displacement vector r
1
3473-3476
Consider an infinitesimal element dl of the conductor The magnetic field dB due to this element is to be determined at a point P which is at a distance r from it Let q be the angle between dl and the displacement vector r According to Biot-Savart’s law, the magnitude of the magnetic field dB is proportional to the current I, the element length |dl|, and inversely proportional to the square of the distance r
1
3474-3477
The magnetic field dB due to this element is to be determined at a point P which is at a distance r from it Let q be the angle between dl and the displacement vector r According to Biot-Savart’s law, the magnitude of the magnetic field dB is proportional to the current I, the element length |dl|, and inversely proportional to the square of the distance r Its direction* is perpendicular to the plane containing dl and r
1
3475-3478
Let q be the angle between dl and the displacement vector r According to Biot-Savart’s law, the magnitude of the magnetic field dB is proportional to the current I, the element length |dl|, and inversely proportional to the square of the distance r Its direction* is perpendicular to the plane containing dl and r Thus, in vector notation, d I d r B r ∝ × l 3 = × µ0 3 4π I d lr r [4
1
3476-3479
According to Biot-Savart’s law, the magnitude of the magnetic field dB is proportional to the current I, the element length |dl|, and inversely proportional to the square of the distance r Its direction* is perpendicular to the plane containing dl and r Thus, in vector notation, d I d r B r ∝ × l 3 = × µ0 3 4π I d lr r [4 11(a)] where m0/4p is a constant of proportionality
1
3477-3480
Its direction* is perpendicular to the plane containing dl and r Thus, in vector notation, d I d r B r ∝ × l 3 = × µ0 3 4π I d lr r [4 11(a)] where m0/4p is a constant of proportionality The above expression holds when the medium is vacuum
1
3478-3481
Thus, in vector notation, d I d r B r ∝ × l 3 = × µ0 3 4π I d lr r [4 11(a)] where m0/4p is a constant of proportionality The above expression holds when the medium is vacuum FIGURE 4
1
3479-3482
11(a)] where m0/4p is a constant of proportionality The above expression holds when the medium is vacuum FIGURE 4 7 Illustration of the Biot-Savart law
1
3480-3483
The above expression holds when the medium is vacuum FIGURE 4 7 Illustration of the Biot-Savart law The current element I dl produces a field dB at a distance r
1
3481-3484
FIGURE 4 7 Illustration of the Biot-Savart law The current element I dl produces a field dB at a distance r The Ä sign indicates that the field is perpendicular to the plane of this page and directed into it
1
3482-3485
7 Illustration of the Biot-Savart law The current element I dl produces a field dB at a distance r The Ä sign indicates that the field is perpendicular to the plane of this page and directed into it * The sense of dl × r is also given by the Right Hand Screw rule : Look at the plane containing vectors dl and r
1
3483-3486
The current element I dl produces a field dB at a distance r The Ä sign indicates that the field is perpendicular to the plane of this page and directed into it * The sense of dl × r is also given by the Right Hand Screw rule : Look at the plane containing vectors dl and r Imagine moving from the first vector towards second vector
1
3484-3487
The Ä sign indicates that the field is perpendicular to the plane of this page and directed into it * The sense of dl × r is also given by the Right Hand Screw rule : Look at the plane containing vectors dl and r Imagine moving from the first vector towards second vector If the movement is anticlockwise, the resultant is towards you
1
3485-3488
* The sense of dl × r is also given by the Right Hand Screw rule : Look at the plane containing vectors dl and r Imagine moving from the first vector towards second vector If the movement is anticlockwise, the resultant is towards you If it is clockwise, the resultant is away from you
1
3486-3489
Imagine moving from the first vector towards second vector If the movement is anticlockwise, the resultant is towards you If it is clockwise, the resultant is away from you Rationalised 2023-24 Physics 114 EXAMPLE 4
1
3487-3490
If the movement is anticlockwise, the resultant is towards you If it is clockwise, the resultant is away from you Rationalised 2023-24 Physics 114 EXAMPLE 4 5 The magnitude of this field is, 0 2 d sin d 4 I l r µ θ = π B [4
1
3488-3491
If it is clockwise, the resultant is away from you Rationalised 2023-24 Physics 114 EXAMPLE 4 5 The magnitude of this field is, 0 2 d sin d 4 I l r µ θ = π B [4 11(b)] where we have used the property of cross-product
1
3489-3492
Rationalised 2023-24 Physics 114 EXAMPLE 4 5 The magnitude of this field is, 0 2 d sin d 4 I l r µ θ = π B [4 11(b)] where we have used the property of cross-product Equation [4