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2490-2493
1 Charges +Q and –Q put at the ends of a metallic cylinder The electrons will drift because of the electric field created to neutralise the charges The current thus will stop after a while unless the charges +Q and –Q are continuously replenished FIGURE 3
1
2491-2494
The electrons will drift because of the electric field created to neutralise the charges The current thus will stop after a while unless the charges +Q and –Q are continuously replenished FIGURE 3 2 Illustrating the relation R = rl/A for a rectangular slab of length l and area of cross-section A
1
2492-2495
The current thus will stop after a while unless the charges +Q and –Q are continuously replenished FIGURE 3 2 Illustrating the relation R = rl/A for a rectangular slab of length l and area of cross-section A Rationalised 2023-24 Physics 84 identical to the first and the same current I flows through both
1
2493-2496
FIGURE 3 2 Illustrating the relation R = rl/A for a rectangular slab of length l and area of cross-section A Rationalised 2023-24 Physics 84 identical to the first and the same current I flows through both The potential difference across the ends of the combination is clearly sum of the potential difference across the two individual slabs and hence equals 2V
1
2494-2497
2 Illustrating the relation R = rl/A for a rectangular slab of length l and area of cross-section A Rationalised 2023-24 Physics 84 identical to the first and the same current I flows through both The potential difference across the ends of the combination is clearly sum of the potential difference across the two individual slabs and hence equals 2V The current through the combination is I and the resistance of the combination RC is [from Eq
1
2495-2498
Rationalised 2023-24 Physics 84 identical to the first and the same current I flows through both The potential difference across the ends of the combination is clearly sum of the potential difference across the two individual slabs and hence equals 2V The current through the combination is I and the resistance of the combination RC is [from Eq (3
1
2496-2499
The potential difference across the ends of the combination is clearly sum of the potential difference across the two individual slabs and hence equals 2V The current through the combination is I and the resistance of the combination RC is [from Eq (3 3)], 2 2 C V R R =I = (3
1
2497-2500
The current through the combination is I and the resistance of the combination RC is [from Eq (3 3)], 2 2 C V R R =I = (3 4) since V/I = R, the resistance of either of the slabs
1
2498-2501
(3 3)], 2 2 C V R R =I = (3 4) since V/I = R, the resistance of either of the slabs Thus, doubling the length of a conductor doubles the resistance
1
2499-2502
3)], 2 2 C V R R =I = (3 4) since V/I = R, the resistance of either of the slabs Thus, doubling the length of a conductor doubles the resistance In general, then resistance is proportional to length, R ∝l (3
1
2500-2503
4) since V/I = R, the resistance of either of the slabs Thus, doubling the length of a conductor doubles the resistance In general, then resistance is proportional to length, R ∝l (3 5) Next, imagine dividing the slab into two by cutting it lengthwise so that the slab can be considered as a combination of two identical slabs of length l, but each having a cross sectional area of A/2 [Fig
1
2501-2504
Thus, doubling the length of a conductor doubles the resistance In general, then resistance is proportional to length, R ∝l (3 5) Next, imagine dividing the slab into two by cutting it lengthwise so that the slab can be considered as a combination of two identical slabs of length l, but each having a cross sectional area of A/2 [Fig 3
1
2502-2505
In general, then resistance is proportional to length, R ∝l (3 5) Next, imagine dividing the slab into two by cutting it lengthwise so that the slab can be considered as a combination of two identical slabs of length l, but each having a cross sectional area of A/2 [Fig 3 2(c)]
1
2503-2506
5) Next, imagine dividing the slab into two by cutting it lengthwise so that the slab can be considered as a combination of two identical slabs of length l, but each having a cross sectional area of A/2 [Fig 3 2(c)] For a given voltage V across the slab, if I is the current through the entire slab, then clearly the current flowing through each of the two half-slabs is I/2
1
2504-2507
3 2(c)] For a given voltage V across the slab, if I is the current through the entire slab, then clearly the current flowing through each of the two half-slabs is I/2 Since the potential difference across the ends of the half-slabs is V, i
1
2505-2508
2(c)] For a given voltage V across the slab, if I is the current through the entire slab, then clearly the current flowing through each of the two half-slabs is I/2 Since the potential difference across the ends of the half-slabs is V, i e
1
2506-2509
For a given voltage V across the slab, if I is the current through the entire slab, then clearly the current flowing through each of the two half-slabs is I/2 Since the potential difference across the ends of the half-slabs is V, i e , the same as across the full slab, the resistance of each of the half-slabs R1 is 1 2 2
1
2507-2510
Since the potential difference across the ends of the half-slabs is V, i e , the same as across the full slab, the resistance of each of the half-slabs R1 is 1 2 2 ( /2) V V R R I I = = = (3
1
2508-2511
e , the same as across the full slab, the resistance of each of the half-slabs R1 is 1 2 2 ( /2) V V R R I I = = = (3 6) Thus, halving the area of the cross-section of a conductor doubles the resistance
1
2509-2512
, the same as across the full slab, the resistance of each of the half-slabs R1 is 1 2 2 ( /2) V V R R I I = = = (3 6) Thus, halving the area of the cross-section of a conductor doubles the resistance In general, then the resistance R is inversely proportional to the cross-sectional area, 1 R ∝A (3
1
2510-2513
( /2) V V R R I I = = = (3 6) Thus, halving the area of the cross-section of a conductor doubles the resistance In general, then the resistance R is inversely proportional to the cross-sectional area, 1 R ∝A (3 7) Combining Eqs
1
2511-2514
6) Thus, halving the area of the cross-section of a conductor doubles the resistance In general, then the resistance R is inversely proportional to the cross-sectional area, 1 R ∝A (3 7) Combining Eqs (3
1
2512-2515
In general, then the resistance R is inversely proportional to the cross-sectional area, 1 R ∝A (3 7) Combining Eqs (3 5) and (3
1
2513-2516
7) Combining Eqs (3 5) and (3 7), we have l R ∝A (3
1
2514-2517
(3 5) and (3 7), we have l R ∝A (3 8) and hence for a given conductor l R =ρA (3
1
2515-2518
5) and (3 7), we have l R ∝A (3 8) and hence for a given conductor l R =ρA (3 9) where the constant of proportionality r depends on the material of the conductor but not on its dimensions
1
2516-2519
7), we have l R ∝A (3 8) and hence for a given conductor l R =ρA (3 9) where the constant of proportionality r depends on the material of the conductor but not on its dimensions r is called resistivity
1
2517-2520
8) and hence for a given conductor l R =ρA (3 9) where the constant of proportionality r depends on the material of the conductor but not on its dimensions r is called resistivity Using the last equation, Ohm’s law reads I l V I R ρA = × = (3
1
2518-2521
9) where the constant of proportionality r depends on the material of the conductor but not on its dimensions r is called resistivity Using the last equation, Ohm’s law reads I l V I R ρA = × = (3 10) Current per unit area (taken normal to the current), I/A, is called current density and is denoted by j
1
2519-2522
r is called resistivity Using the last equation, Ohm’s law reads I l V I R ρA = × = (3 10) Current per unit area (taken normal to the current), I/A, is called current density and is denoted by j The SI units of the current density are A/m2
1
2520-2523
Using the last equation, Ohm’s law reads I l V I R ρA = × = (3 10) Current per unit area (taken normal to the current), I/A, is called current density and is denoted by j The SI units of the current density are A/m2 Further, if E is the magnitude of uniform electric field in the conductor whose length is l, then the potential difference V across its ends is El
1
2521-2524
10) Current per unit area (taken normal to the current), I/A, is called current density and is denoted by j The SI units of the current density are A/m2 Further, if E is the magnitude of uniform electric field in the conductor whose length is l, then the potential difference V across its ends is El Using these, the last equation reads GEORG SIMON OHM (1787–1854) Georg Simon Ohm (1787– 1854) German physicist, professor at Munich
1
2522-2525
The SI units of the current density are A/m2 Further, if E is the magnitude of uniform electric field in the conductor whose length is l, then the potential difference V across its ends is El Using these, the last equation reads GEORG SIMON OHM (1787–1854) Georg Simon Ohm (1787– 1854) German physicist, professor at Munich Ohm was led to his law by an analogy between the conduction of heat: the electric field is analogous to the temperature gradient, and the electric current is analogous to the heat flow
1
2523-2526
Further, if E is the magnitude of uniform electric field in the conductor whose length is l, then the potential difference V across its ends is El Using these, the last equation reads GEORG SIMON OHM (1787–1854) Georg Simon Ohm (1787– 1854) German physicist, professor at Munich Ohm was led to his law by an analogy between the conduction of heat: the electric field is analogous to the temperature gradient, and the electric current is analogous to the heat flow Rationalised 2023-24 Current Electricity 85 E l = j r l or, E = j r (3
1
2524-2527
Using these, the last equation reads GEORG SIMON OHM (1787–1854) Georg Simon Ohm (1787– 1854) German physicist, professor at Munich Ohm was led to his law by an analogy between the conduction of heat: the electric field is analogous to the temperature gradient, and the electric current is analogous to the heat flow Rationalised 2023-24 Current Electricity 85 E l = j r l or, E = j r (3 11) The above relation for magnitudes E and j can indeed be cast in a vector form
1
2525-2528
Ohm was led to his law by an analogy between the conduction of heat: the electric field is analogous to the temperature gradient, and the electric current is analogous to the heat flow Rationalised 2023-24 Current Electricity 85 E l = j r l or, E = j r (3 11) The above relation for magnitudes E and j can indeed be cast in a vector form The current density, (which we have defined as the current through unit area normal to the current) is also directed along E, and is also a vector j (ººººº j E/E)
1
2526-2529
Rationalised 2023-24 Current Electricity 85 E l = j r l or, E = j r (3 11) The above relation for magnitudes E and j can indeed be cast in a vector form The current density, (which we have defined as the current through unit area normal to the current) is also directed along E, and is also a vector j (ººººº j E/E) Thus, the last equation can be written as, E = jr (3
1
2527-2530
11) The above relation for magnitudes E and j can indeed be cast in a vector form The current density, (which we have defined as the current through unit area normal to the current) is also directed along E, and is also a vector j (ººººº j E/E) Thus, the last equation can be written as, E = jr (3 12) or, j = s E (3
1
2528-2531
The current density, (which we have defined as the current through unit area normal to the current) is also directed along E, and is also a vector j (ººººº j E/E) Thus, the last equation can be written as, E = jr (3 12) or, j = s E (3 13) where s º1/r is called the conductivity
1
2529-2532
Thus, the last equation can be written as, E = jr (3 12) or, j = s E (3 13) where s º1/r is called the conductivity Ohm’s law is often stated in an equivalent form, Eq
1
2530-2533
12) or, j = s E (3 13) where s º1/r is called the conductivity Ohm’s law is often stated in an equivalent form, Eq (3
1
2531-2534
13) where s º1/r is called the conductivity Ohm’s law is often stated in an equivalent form, Eq (3 13) in addition to Eq
1
2532-2535
Ohm’s law is often stated in an equivalent form, Eq (3 13) in addition to Eq (3
1
2533-2536
(3 13) in addition to Eq (3 3)
1
2534-2537
13) in addition to Eq (3 3) In the next section, we will try to understand the origin of the Ohm’s law as arising from the characteristics of the drift of electrons
1
2535-2538
(3 3) In the next section, we will try to understand the origin of the Ohm’s law as arising from the characteristics of the drift of electrons 3
1
2536-2539
3) In the next section, we will try to understand the origin of the Ohm’s law as arising from the characteristics of the drift of electrons 3 5 DRIFT OF ELECTRONS AND THE ORIGIN OF RESISTIVITY As remarked before, an electron will suffer collisions with the heavy fixed ions, but after collision, it will emerge with the same speed but in random directions
1
2537-2540
In the next section, we will try to understand the origin of the Ohm’s law as arising from the characteristics of the drift of electrons 3 5 DRIFT OF ELECTRONS AND THE ORIGIN OF RESISTIVITY As remarked before, an electron will suffer collisions with the heavy fixed ions, but after collision, it will emerge with the same speed but in random directions If we consider all the electrons, their average velocity will be zero since their directions are random
1
2538-2541
3 5 DRIFT OF ELECTRONS AND THE ORIGIN OF RESISTIVITY As remarked before, an electron will suffer collisions with the heavy fixed ions, but after collision, it will emerge with the same speed but in random directions If we consider all the electrons, their average velocity will be zero since their directions are random Thus, if there are N electrons and the velocity of the ith electron (i = 1, 2, 3,
1
2539-2542
5 DRIFT OF ELECTRONS AND THE ORIGIN OF RESISTIVITY As remarked before, an electron will suffer collisions with the heavy fixed ions, but after collision, it will emerge with the same speed but in random directions If we consider all the electrons, their average velocity will be zero since their directions are random Thus, if there are N electrons and the velocity of the ith electron (i = 1, 2, 3, N ) at a given time is vi, then 1 0 1 N i i v = =∑ N (3
1
2540-2543
If we consider all the electrons, their average velocity will be zero since their directions are random Thus, if there are N electrons and the velocity of the ith electron (i = 1, 2, 3, N ) at a given time is vi, then 1 0 1 N i i v = =∑ N (3 14) Consider now the situation when an electric field is present
1
2541-2544
Thus, if there are N electrons and the velocity of the ith electron (i = 1, 2, 3, N ) at a given time is vi, then 1 0 1 N i i v = =∑ N (3 14) Consider now the situation when an electric field is present Electrons will be accelerated due to this field by a= – E e m (3
1
2542-2545
N ) at a given time is vi, then 1 0 1 N i i v = =∑ N (3 14) Consider now the situation when an electric field is present Electrons will be accelerated due to this field by a= – E e m (3 15) where –e is the charge and m is the mass of an electron
1
2543-2546
14) Consider now the situation when an electric field is present Electrons will be accelerated due to this field by a= – E e m (3 15) where –e is the charge and m is the mass of an electron Consider again the ith electron at a given time t
1
2544-2547
Electrons will be accelerated due to this field by a= – E e m (3 15) where –e is the charge and m is the mass of an electron Consider again the ith electron at a given time t This electron would have had its last collision some time before t, and let ti be the time elapsed after its last collision
1
2545-2548
15) where –e is the charge and m is the mass of an electron Consider again the ith electron at a given time t This electron would have had its last collision some time before t, and let ti be the time elapsed after its last collision If vi was its velocity immediately after the last collision, then its velocity Vi at time t is − = + E V v i i i e t m (3
1
2546-2549
Consider again the ith electron at a given time t This electron would have had its last collision some time before t, and let ti be the time elapsed after its last collision If vi was its velocity immediately after the last collision, then its velocity Vi at time t is − = + E V v i i i e t m (3 16) since starting with its last collision it was accelerated (Fig
1
2547-2550
This electron would have had its last collision some time before t, and let ti be the time elapsed after its last collision If vi was its velocity immediately after the last collision, then its velocity Vi at time t is − = + E V v i i i e t m (3 16) since starting with its last collision it was accelerated (Fig 3
1
2548-2551
If vi was its velocity immediately after the last collision, then its velocity Vi at time t is − = + E V v i i i e t m (3 16) since starting with its last collision it was accelerated (Fig 3 3) with an acceleration given by Eq
1
2549-2552
16) since starting with its last collision it was accelerated (Fig 3 3) with an acceleration given by Eq (3
1
2550-2553
3 3) with an acceleration given by Eq (3 15) for a time interval ti
1
2551-2554
3) with an acceleration given by Eq (3 15) for a time interval ti The average velocity of the electrons at time t is the average of all the Vi’s
1
2552-2555
(3 15) for a time interval ti The average velocity of the electrons at time t is the average of all the Vi’s The average of vi’s is zero [Eq
1
2553-2556
15) for a time interval ti The average velocity of the electrons at time t is the average of all the Vi’s The average of vi’s is zero [Eq (3
1
2554-2557
The average velocity of the electrons at time t is the average of all the Vi’s The average of vi’s is zero [Eq (3 14)] since immediately after any collision, the direction of the velocity of an electron is completely random
1
2555-2558
The average of vi’s is zero [Eq (3 14)] since immediately after any collision, the direction of the velocity of an electron is completely random The collisions of the electrons do not occur at regular intervals but at random times
1
2556-2559
(3 14)] since immediately after any collision, the direction of the velocity of an electron is completely random The collisions of the electrons do not occur at regular intervals but at random times Let us denote by t, the average time between successive collisions
1
2557-2560
14)] since immediately after any collision, the direction of the velocity of an electron is completely random The collisions of the electrons do not occur at regular intervals but at random times Let us denote by t, the average time between successive collisions Then at a given time, some of the electrons would have spent FIGURE 3
1
2558-2561
The collisions of the electrons do not occur at regular intervals but at random times Let us denote by t, the average time between successive collisions Then at a given time, some of the electrons would have spent FIGURE 3 3 A schematic picture of an electron moving from a point A to another point B through repeated collisions, and straight line travel between collisions (full lines)
1
2559-2562
Let us denote by t, the average time between successive collisions Then at a given time, some of the electrons would have spent FIGURE 3 3 A schematic picture of an electron moving from a point A to another point B through repeated collisions, and straight line travel between collisions (full lines) If an electric field is applied as shown, the electron ends up at point B¢ (dotted lines)
1
2560-2563
Then at a given time, some of the electrons would have spent FIGURE 3 3 A schematic picture of an electron moving from a point A to another point B through repeated collisions, and straight line travel between collisions (full lines) If an electric field is applied as shown, the electron ends up at point B¢ (dotted lines) A slight drift in a direction opposite the electric field is visible
1
2561-2564
3 A schematic picture of an electron moving from a point A to another point B through repeated collisions, and straight line travel between collisions (full lines) If an electric field is applied as shown, the electron ends up at point B¢ (dotted lines) A slight drift in a direction opposite the electric field is visible Rationalised 2023-24 Physics 86 time more than t and some less than t
1
2562-2565
If an electric field is applied as shown, the electron ends up at point B¢ (dotted lines) A slight drift in a direction opposite the electric field is visible Rationalised 2023-24 Physics 86 time more than t and some less than t In other words, the time ti in Eq
1
2563-2566
A slight drift in a direction opposite the electric field is visible Rationalised 2023-24 Physics 86 time more than t and some less than t In other words, the time ti in Eq (3
1
2564-2567
Rationalised 2023-24 Physics 86 time more than t and some less than t In other words, the time ti in Eq (3 16) will be less than t for some and more than t for others as we go through the values of i = 1, 2
1
2565-2568
In other words, the time ti in Eq (3 16) will be less than t for some and more than t for others as we go through the values of i = 1, 2 N
1
2566-2569
(3 16) will be less than t for some and more than t for others as we go through the values of i = 1, 2 N The average value of ti then is t (known as relaxation time)
1
2567-2570
16) will be less than t for some and more than t for others as we go through the values of i = 1, 2 N The average value of ti then is t (known as relaxation time) Thus, averaging Eq
1
2568-2571
N The average value of ti then is t (known as relaxation time) Thus, averaging Eq (3
1
2569-2572
The average value of ti then is t (known as relaxation time) Thus, averaging Eq (3 16) over the N-electrons at any given time t gives us for the average velocity vd ( ) ( ) ( ) ≡ = − E v V v d i i i average average average e t m 0 – τ τ = = − E E e e m m (3
1
2570-2573
Thus, averaging Eq (3 16) over the N-electrons at any given time t gives us for the average velocity vd ( ) ( ) ( ) ≡ = − E v V v d i i i average average average e t m 0 – τ τ = = − E E e e m m (3 17) This last result is surprising
1
2571-2574
(3 16) over the N-electrons at any given time t gives us for the average velocity vd ( ) ( ) ( ) ≡ = − E v V v d i i i average average average e t m 0 – τ τ = = − E E e e m m (3 17) This last result is surprising It tells us that the electrons move with an average velocity which is independent of time, although electrons are accelerated
1
2572-2575
16) over the N-electrons at any given time t gives us for the average velocity vd ( ) ( ) ( ) ≡ = − E v V v d i i i average average average e t m 0 – τ τ = = − E E e e m m (3 17) This last result is surprising It tells us that the electrons move with an average velocity which is independent of time, although electrons are accelerated This is the phenomenon of drift and the velocity vd in Eq
1
2573-2576
17) This last result is surprising It tells us that the electrons move with an average velocity which is independent of time, although electrons are accelerated This is the phenomenon of drift and the velocity vd in Eq (3
1
2574-2577
It tells us that the electrons move with an average velocity which is independent of time, although electrons are accelerated This is the phenomenon of drift and the velocity vd in Eq (3 17) is called the drift velocity
1
2575-2578
This is the phenomenon of drift and the velocity vd in Eq (3 17) is called the drift velocity Because of the drift, there will be net transport of charges across any area perpendicular to E
1
2576-2579
(3 17) is called the drift velocity Because of the drift, there will be net transport of charges across any area perpendicular to E Consider a planar area A, located inside the conductor such that the normal to the area is parallel to E (Fig
1
2577-2580
17) is called the drift velocity Because of the drift, there will be net transport of charges across any area perpendicular to E Consider a planar area A, located inside the conductor such that the normal to the area is parallel to E (Fig 3
1
2578-2581
Because of the drift, there will be net transport of charges across any area perpendicular to E Consider a planar area A, located inside the conductor such that the normal to the area is parallel to E (Fig 3 4)
1
2579-2582
Consider a planar area A, located inside the conductor such that the normal to the area is parallel to E (Fig 3 4) Then because of the drift, in an infinitesimal amount of time Dt, all electrons to the left of the area at distances upto |vd|Dt would have crossed the area
1
2580-2583
3 4) Then because of the drift, in an infinitesimal amount of time Dt, all electrons to the left of the area at distances upto |vd|Dt would have crossed the area If n is the number of free electrons per unit volume in the metal, then there are n Dt |vd|A such electrons
1
2581-2584
4) Then because of the drift, in an infinitesimal amount of time Dt, all electrons to the left of the area at distances upto |vd|Dt would have crossed the area If n is the number of free electrons per unit volume in the metal, then there are n Dt |vd|A such electrons Since each electron carries a charge –e, the total charge transported across this area A to the right in time Dt is –ne A|vd|Dt
1
2582-2585
Then because of the drift, in an infinitesimal amount of time Dt, all electrons to the left of the area at distances upto |vd|Dt would have crossed the area If n is the number of free electrons per unit volume in the metal, then there are n Dt |vd|A such electrons Since each electron carries a charge –e, the total charge transported across this area A to the right in time Dt is –ne A|vd|Dt E is directed towards the left and hence the total charge transported along E across the area is negative of this
1
2583-2586
If n is the number of free electrons per unit volume in the metal, then there are n Dt |vd|A such electrons Since each electron carries a charge –e, the total charge transported across this area A to the right in time Dt is –ne A|vd|Dt E is directed towards the left and hence the total charge transported along E across the area is negative of this The amount of charge crossing the area A in time Dt is by definition [Eq
1
2584-2587
Since each electron carries a charge –e, the total charge transported across this area A to the right in time Dt is –ne A|vd|Dt E is directed towards the left and hence the total charge transported along E across the area is negative of this The amount of charge crossing the area A in time Dt is by definition [Eq (3
1
2585-2588
E is directed towards the left and hence the total charge transported along E across the area is negative of this The amount of charge crossing the area A in time Dt is by definition [Eq (3 2)] I Dt, where I is the magnitude of the current
1
2586-2589
The amount of charge crossing the area A in time Dt is by definition [Eq (3 2)] I Dt, where I is the magnitude of the current Hence, v ∆ = + ∆ d I t n e A t (3
1
2587-2590
(3 2)] I Dt, where I is the magnitude of the current Hence, v ∆ = + ∆ d I t n e A t (3 18) Substituting the value of |vd| from Eq
1
2588-2591
2)] I Dt, where I is the magnitude of the current Hence, v ∆ = + ∆ d I t n e A t (3 18) Substituting the value of |vd| from Eq (3
1
2589-2592
Hence, v ∆ = + ∆ d I t n e A t (3 18) Substituting the value of |vd| from Eq (3 17) 2 E τ ∆ = ∆ e A I t n t m (3