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490-493
Example 1 7 An electron falls through a distance of 1 5 cm in a uniform electric field of magnitude 2 0 × 104 N C–1 [Fig
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7 An electron falls through a distance of 1 5 cm in a uniform electric field of magnitude 2 0 × 104 N C–1 [Fig 1
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5 cm in a uniform electric field of magnitude 2 0 × 104 N C–1 [Fig 1 10(a)]
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0 × 104 N C–1 [Fig 1 10(a)] The direction of the field is reversed keeping its magnitude unchanged and a proton falls through the same distance [Fig
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1 10(a)] The direction of the field is reversed keeping its magnitude unchanged and a proton falls through the same distance [Fig 1
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10(a)] The direction of the field is reversed keeping its magnitude unchanged and a proton falls through the same distance [Fig 1 10(b)]
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The direction of the field is reversed keeping its magnitude unchanged and a proton falls through the same distance [Fig 1 10(b)] Compute the time of fall in each case
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1 10(b)] Compute the time of fall in each case Contrast the situation with that of ‘free fall under gravity’
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10(b)] Compute the time of fall in each case Contrast the situation with that of ‘free fall under gravity’ FIGURE 1
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Compute the time of fall in each case Contrast the situation with that of ‘free fall under gravity’ FIGURE 1 10 Solution In Fig
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Contrast the situation with that of ‘free fall under gravity’ FIGURE 1 10 Solution In Fig 1
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FIGURE 1 10 Solution In Fig 1 10(a) the field is upward, so the negatively charged electron experiences a downward force of magnitude eE where E is the magnitude of the electric field
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10 Solution In Fig 1 10(a) the field is upward, so the negatively charged electron experiences a downward force of magnitude eE where E is the magnitude of the electric field The acceleration of the electron is ae = eE/me where me is the mass of the electron
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1 10(a) the field is upward, so the negatively charged electron experiences a downward force of magnitude eE where E is the magnitude of the electric field The acceleration of the electron is ae = eE/me where me is the mass of the electron Starting from rest, the time required by the electron to fall through a distance h is given by 2 2 e e e h m h t a e E = = For e = 1
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10(a) the field is upward, so the negatively charged electron experiences a downward force of magnitude eE where E is the magnitude of the electric field The acceleration of the electron is ae = eE/me where me is the mass of the electron Starting from rest, the time required by the electron to fall through a distance h is given by 2 2 e e e h m h t a e E = = For e = 1 6 × 10–19C, me = 9
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The acceleration of the electron is ae = eE/me where me is the mass of the electron Starting from rest, the time required by the electron to fall through a distance h is given by 2 2 e e e h m h t a e E = = For e = 1 6 × 10–19C, me = 9 11 × 10–31 kg, E = 2
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506-509
Starting from rest, the time required by the electron to fall through a distance h is given by 2 2 e e e h m h t a e E = = For e = 1 6 × 10–19C, me = 9 11 × 10–31 kg, E = 2 0 × 104 N C–1, h = 1
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507-510
6 × 10–19C, me = 9 11 × 10–31 kg, E = 2 0 × 104 N C–1, h = 1 5 × 10–2 m, te = 2
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508-511
11 × 10–31 kg, E = 2 0 × 104 N C–1, h = 1 5 × 10–2 m, te = 2 9 × 10–9s In Fig
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0 × 104 N C–1, h = 1 5 × 10–2 m, te = 2 9 × 10–9s In Fig 1
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510-513
5 × 10–2 m, te = 2 9 × 10–9s In Fig 1 10 (b), the field is downward, and the positively charged proton experiences a downward force of magnitude eE
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511-514
9 × 10–9s In Fig 1 10 (b), the field is downward, and the positively charged proton experiences a downward force of magnitude eE The acceleration of the proton is ap = eE/mp where mp is the mass of the proton; mp = 1
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512-515
1 10 (b), the field is downward, and the positively charged proton experiences a downward force of magnitude eE The acceleration of the proton is ap = eE/mp where mp is the mass of the proton; mp = 1 67 × 10–27 kg
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513-516
10 (b), the field is downward, and the positively charged proton experiences a downward force of magnitude eE The acceleration of the proton is ap = eE/mp where mp is the mass of the proton; mp = 1 67 × 10–27 kg The time of fall for the proton is EXAMPLE 1
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The acceleration of the proton is ap = eE/mp where mp is the mass of the proton; mp = 1 67 × 10–27 kg The time of fall for the proton is EXAMPLE 1 7 Rationalised 2023-24 18 Physics EXAMPLE 1
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67 × 10–27 kg The time of fall for the proton is EXAMPLE 1 7 Rationalised 2023-24 18 Physics EXAMPLE 1 8 EXAMPLE 1
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The time of fall for the proton is EXAMPLE 1 7 Rationalised 2023-24 18 Physics EXAMPLE 1 8 EXAMPLE 1 7 –7 2 2 1 3 10 s p p p h m h t
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7 Rationalised 2023-24 18 Physics EXAMPLE 1 8 EXAMPLE 1 7 –7 2 2 1 3 10 s p p p h m h t a e E = = = × Thus, the heavier particle (proton) takes a greater time to fall through the same distance
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8 EXAMPLE 1 7 –7 2 2 1 3 10 s p p p h m h t a e E = = = × Thus, the heavier particle (proton) takes a greater time to fall through the same distance This is in basic contrast to the situation of ‘free fall under gravity’ where the time of fall is independent of the mass of the body
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7 –7 2 2 1 3 10 s p p p h m h t a e E = = = × Thus, the heavier particle (proton) takes a greater time to fall through the same distance This is in basic contrast to the situation of ‘free fall under gravity’ where the time of fall is independent of the mass of the body Note that in this example we have ignored the acceleration due to gravity in calculating the time of fall
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a e E = = = × Thus, the heavier particle (proton) takes a greater time to fall through the same distance This is in basic contrast to the situation of ‘free fall under gravity’ where the time of fall is independent of the mass of the body Note that in this example we have ignored the acceleration due to gravity in calculating the time of fall To see if this is justified, let us calculate the acceleration of the proton in the given electric field: p p e E a m = 19 4 1 27 (1 6 10 C) (2 0 10 N C ) 1 67 10 kg
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This is in basic contrast to the situation of ‘free fall under gravity’ where the time of fall is independent of the mass of the body Note that in this example we have ignored the acceleration due to gravity in calculating the time of fall To see if this is justified, let us calculate the acceleration of the proton in the given electric field: p p e E a m = 19 4 1 27 (1 6 10 C) (2 0 10 N C ) 1 67 10 kg − − − × × × = × 12 –2 1 9 10 m s
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522-525
Note that in this example we have ignored the acceleration due to gravity in calculating the time of fall To see if this is justified, let us calculate the acceleration of the proton in the given electric field: p p e E a m = 19 4 1 27 (1 6 10 C) (2 0 10 N C ) 1 67 10 kg − − − × × × = × 12 –2 1 9 10 m s = × which is enormous compared to the value of g (9
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523-526
To see if this is justified, let us calculate the acceleration of the proton in the given electric field: p p e E a m = 19 4 1 27 (1 6 10 C) (2 0 10 N C ) 1 67 10 kg − − − × × × = × 12 –2 1 9 10 m s = × which is enormous compared to the value of g (9 8 m s–2), the acceleration due to gravity
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− − − × × × = × 12 –2 1 9 10 m s = × which is enormous compared to the value of g (9 8 m s–2), the acceleration due to gravity The acceleration of the electron is even greater
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525-528
= × which is enormous compared to the value of g (9 8 m s–2), the acceleration due to gravity The acceleration of the electron is even greater Thus, the effect of acceleration due to gravity can be ignored in this example
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8 m s–2), the acceleration due to gravity The acceleration of the electron is even greater Thus, the effect of acceleration due to gravity can be ignored in this example Example 1
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The acceleration of the electron is even greater Thus, the effect of acceleration due to gravity can be ignored in this example Example 1 8 Two point charges q1 and q2, of magnitude +10–8 C and –10–8 C, respectively, are placed 0
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Thus, the effect of acceleration due to gravity can be ignored in this example Example 1 8 Two point charges q1 and q2, of magnitude +10–8 C and –10–8 C, respectively, are placed 0 1 m apart
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Example 1 8 Two point charges q1 and q2, of magnitude +10–8 C and –10–8 C, respectively, are placed 0 1 m apart Calculate the electric fields at points A, B and C shown in Fig
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8 Two point charges q1 and q2, of magnitude +10–8 C and –10–8 C, respectively, are placed 0 1 m apart Calculate the electric fields at points A, B and C shown in Fig 1
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531-534
1 m apart Calculate the electric fields at points A, B and C shown in Fig 1 11
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Calculate the electric fields at points A, B and C shown in Fig 1 11 FIGURE 1
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1 11 FIGURE 1 11 Solution The electric field vector E1A at A due to the positive charge q1 points towards the right and has a magnitude 9 2 -2 8 1A 2 (9 10 Nm C ) (10 C) (0
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11 FIGURE 1 11 Solution The electric field vector E1A at A due to the positive charge q1 points towards the right and has a magnitude 9 2 -2 8 1A 2 (9 10 Nm C ) (10 C) (0 05m) E − × × = = 3
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535-538
FIGURE 1 11 Solution The electric field vector E1A at A due to the positive charge q1 points towards the right and has a magnitude 9 2 -2 8 1A 2 (9 10 Nm C ) (10 C) (0 05m) E − × × = = 3 6 × 104 N C–1 The electric field vector E2A at A due to the negative charge q2 points towards the right and has the same magnitude
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11 Solution The electric field vector E1A at A due to the positive charge q1 points towards the right and has a magnitude 9 2 -2 8 1A 2 (9 10 Nm C ) (10 C) (0 05m) E − × × = = 3 6 × 104 N C–1 The electric field vector E2A at A due to the negative charge q2 points towards the right and has the same magnitude Hence the magnitude of the total electric field EA at A is EA = E1A + E2A = 7
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05m) E − × × = = 3 6 × 104 N C–1 The electric field vector E2A at A due to the negative charge q2 points towards the right and has the same magnitude Hence the magnitude of the total electric field EA at A is EA = E1A + E2A = 7 2 × 104 N C–1 EA is directed toward the right
1
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6 × 104 N C–1 The electric field vector E2A at A due to the negative charge q2 points towards the right and has the same magnitude Hence the magnitude of the total electric field EA at A is EA = E1A + E2A = 7 2 × 104 N C–1 EA is directed toward the right Rationalised 2023-24 Electric Charges and Fields 19 The electric field vector E1B at B due to the positive charge q1 points towards the left and has a magnitude 9 2 –2 8 1B 2 (9 10 Nm C ) (10 C) (0
1
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Hence the magnitude of the total electric field EA at A is EA = E1A + E2A = 7 2 × 104 N C–1 EA is directed toward the right Rationalised 2023-24 Electric Charges and Fields 19 The electric field vector E1B at B due to the positive charge q1 points towards the left and has a magnitude 9 2 –2 8 1B 2 (9 10 Nm C ) (10 C) (0 05 m) E − × × = = 3
1
540-543
2 × 104 N C–1 EA is directed toward the right Rationalised 2023-24 Electric Charges and Fields 19 The electric field vector E1B at B due to the positive charge q1 points towards the left and has a magnitude 9 2 –2 8 1B 2 (9 10 Nm C ) (10 C) (0 05 m) E − × × = = 3 6 × 104 N C–1 The electric field vector E2B at B due to the negative charge q2 points towards the right and has a magnitude 9 2 –2 8 2B 2 (9 10 Nm C ) (10 C) (0
1
541-544
Rationalised 2023-24 Electric Charges and Fields 19 The electric field vector E1B at B due to the positive charge q1 points towards the left and has a magnitude 9 2 –2 8 1B 2 (9 10 Nm C ) (10 C) (0 05 m) E − × × = = 3 6 × 104 N C–1 The electric field vector E2B at B due to the negative charge q2 points towards the right and has a magnitude 9 2 –2 8 2B 2 (9 10 Nm C ) (10 C) (0 15 m) E − × × = = 4 × 103 N C–1 The magnitude of the total electric field at B is EB = E1B – E2B = 3
1
542-545
05 m) E − × × = = 3 6 × 104 N C–1 The electric field vector E2B at B due to the negative charge q2 points towards the right and has a magnitude 9 2 –2 8 2B 2 (9 10 Nm C ) (10 C) (0 15 m) E − × × = = 4 × 103 N C–1 The magnitude of the total electric field at B is EB = E1B – E2B = 3 2 × 104 N C–1 EB is directed towards the left
1
543-546
6 × 104 N C–1 The electric field vector E2B at B due to the negative charge q2 points towards the right and has a magnitude 9 2 –2 8 2B 2 (9 10 Nm C ) (10 C) (0 15 m) E − × × = = 4 × 103 N C–1 The magnitude of the total electric field at B is EB = E1B – E2B = 3 2 × 104 N C–1 EB is directed towards the left The magnitude of each electric field vector at point C, due to charge q1 and q2 is 9 2 –2 8 1C 2C 2 (9 10 Nm C ) (10 C) (0
1
544-547
15 m) E − × × = = 4 × 103 N C–1 The magnitude of the total electric field at B is EB = E1B – E2B = 3 2 × 104 N C–1 EB is directed towards the left The magnitude of each electric field vector at point C, due to charge q1 and q2 is 9 2 –2 8 1C 2C 2 (9 10 Nm C ) (10 C) (0 10 m) E E − × × = = = 9 × 103 N C–1 The directions in which these two vectors point are indicated in Fig
1
545-548
2 × 104 N C–1 EB is directed towards the left The magnitude of each electric field vector at point C, due to charge q1 and q2 is 9 2 –2 8 1C 2C 2 (9 10 Nm C ) (10 C) (0 10 m) E E − × × = = = 9 × 103 N C–1 The directions in which these two vectors point are indicated in Fig 1
1
546-549
The magnitude of each electric field vector at point C, due to charge q1 and q2 is 9 2 –2 8 1C 2C 2 (9 10 Nm C ) (10 C) (0 10 m) E E − × × = = = 9 × 103 N C–1 The directions in which these two vectors point are indicated in Fig 1 11
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10 m) E E − × × = = = 9 × 103 N C–1 The directions in which these two vectors point are indicated in Fig 1 11 The resultant of these two vectors is 1 2 cos cos 3 3 π π = + C c c E E E = 9 × 103 N C–1 EC points towards the right
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548-551
1 11 The resultant of these two vectors is 1 2 cos cos 3 3 π π = + C c c E E E = 9 × 103 N C–1 EC points towards the right 1
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11 The resultant of these two vectors is 1 2 cos cos 3 3 π π = + C c c E E E = 9 × 103 N C–1 EC points towards the right 1 8 ELECTRIC FIELD LINES We have studied electric field in the last section
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550-553
The resultant of these two vectors is 1 2 cos cos 3 3 π π = + C c c E E E = 9 × 103 N C–1 EC points towards the right 1 8 ELECTRIC FIELD LINES We have studied electric field in the last section It is a vector quantity and can be represented as we represent vectors
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551-554
1 8 ELECTRIC FIELD LINES We have studied electric field in the last section It is a vector quantity and can be represented as we represent vectors Let us try to represent E due to a point charge pictorially
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552-555
8 ELECTRIC FIELD LINES We have studied electric field in the last section It is a vector quantity and can be represented as we represent vectors Let us try to represent E due to a point charge pictorially Let the point charge be placed at the origin
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553-556
It is a vector quantity and can be represented as we represent vectors Let us try to represent E due to a point charge pictorially Let the point charge be placed at the origin Draw vectors pointing along the direction of the electric field with their lengths proportional to the strength of the field at each point
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554-557
Let us try to represent E due to a point charge pictorially Let the point charge be placed at the origin Draw vectors pointing along the direction of the electric field with their lengths proportional to the strength of the field at each point Since the magnitude of electric field at a point decreases inversely as the square of the distance of that point from the charge, the vector gets shorter as one goes away from the origin, always pointing radially outward
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555-558
Let the point charge be placed at the origin Draw vectors pointing along the direction of the electric field with their lengths proportional to the strength of the field at each point Since the magnitude of electric field at a point decreases inversely as the square of the distance of that point from the charge, the vector gets shorter as one goes away from the origin, always pointing radially outward Figure 1
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556-559
Draw vectors pointing along the direction of the electric field with their lengths proportional to the strength of the field at each point Since the magnitude of electric field at a point decreases inversely as the square of the distance of that point from the charge, the vector gets shorter as one goes away from the origin, always pointing radially outward Figure 1 12 shows such a picture
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557-560
Since the magnitude of electric field at a point decreases inversely as the square of the distance of that point from the charge, the vector gets shorter as one goes away from the origin, always pointing radially outward Figure 1 12 shows such a picture In this figure, each arrow indicates the electric field, i
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558-561
Figure 1 12 shows such a picture In this figure, each arrow indicates the electric field, i e
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559-562
12 shows such a picture In this figure, each arrow indicates the electric field, i e , the force acting on a unit positive charge, placed at the tail of that arrow
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560-563
In this figure, each arrow indicates the electric field, i e , the force acting on a unit positive charge, placed at the tail of that arrow Connect the arrows pointing in one direction and the resulting figure represents a field line
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561-564
e , the force acting on a unit positive charge, placed at the tail of that arrow Connect the arrows pointing in one direction and the resulting figure represents a field line We thus get many field lines, all pointing outwards from the point charge
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562-565
, the force acting on a unit positive charge, placed at the tail of that arrow Connect the arrows pointing in one direction and the resulting figure represents a field line We thus get many field lines, all pointing outwards from the point charge Have we lost the information about the strength or magnitude of the field now, because it was contained in the length of the arrow
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563-566
Connect the arrows pointing in one direction and the resulting figure represents a field line We thus get many field lines, all pointing outwards from the point charge Have we lost the information about the strength or magnitude of the field now, because it was contained in the length of the arrow No
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564-567
We thus get many field lines, all pointing outwards from the point charge Have we lost the information about the strength or magnitude of the field now, because it was contained in the length of the arrow No Now the magnitude of the field is represented by the density of field lines
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565-568
Have we lost the information about the strength or magnitude of the field now, because it was contained in the length of the arrow No Now the magnitude of the field is represented by the density of field lines E is strong near the charge, so the density of field lines is more near the charge and the lines are closer
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566-569
No Now the magnitude of the field is represented by the density of field lines E is strong near the charge, so the density of field lines is more near the charge and the lines are closer Away from the charge, the field gets weaker and the density of field lines is less, resulting in well-separated lines
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567-570
Now the magnitude of the field is represented by the density of field lines E is strong near the charge, so the density of field lines is more near the charge and the lines are closer Away from the charge, the field gets weaker and the density of field lines is less, resulting in well-separated lines Another person may draw more lines
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568-571
E is strong near the charge, so the density of field lines is more near the charge and the lines are closer Away from the charge, the field gets weaker and the density of field lines is less, resulting in well-separated lines Another person may draw more lines But the number of lines is not important
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569-572
Away from the charge, the field gets weaker and the density of field lines is less, resulting in well-separated lines Another person may draw more lines But the number of lines is not important In fact, an infinite number of lines can be drawn in any region
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570-573
Another person may draw more lines But the number of lines is not important In fact, an infinite number of lines can be drawn in any region FIGURE 1
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571-574
But the number of lines is not important In fact, an infinite number of lines can be drawn in any region FIGURE 1 12 Field of a point charge
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572-575
In fact, an infinite number of lines can be drawn in any region FIGURE 1 12 Field of a point charge EXAMPLE 1
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573-576
FIGURE 1 12 Field of a point charge EXAMPLE 1 8 Rationalised 2023-24 20 Physics It is the relative density of lines in different regions which is important
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574-577
12 Field of a point charge EXAMPLE 1 8 Rationalised 2023-24 20 Physics It is the relative density of lines in different regions which is important We draw the figure on the plane of paper, i
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575-578
EXAMPLE 1 8 Rationalised 2023-24 20 Physics It is the relative density of lines in different regions which is important We draw the figure on the plane of paper, i e
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576-579
8 Rationalised 2023-24 20 Physics It is the relative density of lines in different regions which is important We draw the figure on the plane of paper, i e , in two- dimensions but we live in three-dimensions
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577-580
We draw the figure on the plane of paper, i e , in two- dimensions but we live in three-dimensions So if one wishes to estimate the density of field lines, one has to consider the number of lines per unit cross-sectional area, perpendicular to the lines
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578-581
e , in two- dimensions but we live in three-dimensions So if one wishes to estimate the density of field lines, one has to consider the number of lines per unit cross-sectional area, perpendicular to the lines Since the electric field decreases as the square of the distance from a point charge and the area enclosing the charge increases as the square of the distance, the number of field lines crossing the enclosing area remains constant, whatever may be the distance of the area from the charge
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579-582
, in two- dimensions but we live in three-dimensions So if one wishes to estimate the density of field lines, one has to consider the number of lines per unit cross-sectional area, perpendicular to the lines Since the electric field decreases as the square of the distance from a point charge and the area enclosing the charge increases as the square of the distance, the number of field lines crossing the enclosing area remains constant, whatever may be the distance of the area from the charge We started by saying that the field lines carry information about the direction of electric field at different points in space
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580-583
So if one wishes to estimate the density of field lines, one has to consider the number of lines per unit cross-sectional area, perpendicular to the lines Since the electric field decreases as the square of the distance from a point charge and the area enclosing the charge increases as the square of the distance, the number of field lines crossing the enclosing area remains constant, whatever may be the distance of the area from the charge We started by saying that the field lines carry information about the direction of electric field at different points in space Having drawn a certain set of field lines, the relative density (i
1
581-584
Since the electric field decreases as the square of the distance from a point charge and the area enclosing the charge increases as the square of the distance, the number of field lines crossing the enclosing area remains constant, whatever may be the distance of the area from the charge We started by saying that the field lines carry information about the direction of electric field at different points in space Having drawn a certain set of field lines, the relative density (i e
1
582-585
We started by saying that the field lines carry information about the direction of electric field at different points in space Having drawn a certain set of field lines, the relative density (i e , closeness) of the field lines at different points indicates the relative strength of electric field at those points
1
583-586
Having drawn a certain set of field lines, the relative density (i e , closeness) of the field lines at different points indicates the relative strength of electric field at those points The field lines crowd where the field is strong and are spaced apart where it is weak
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584-587
e , closeness) of the field lines at different points indicates the relative strength of electric field at those points The field lines crowd where the field is strong and are spaced apart where it is weak Figure 1
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585-588
, closeness) of the field lines at different points indicates the relative strength of electric field at those points The field lines crowd where the field is strong and are spaced apart where it is weak Figure 1 13 shows a set of field lines
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586-589
The field lines crowd where the field is strong and are spaced apart where it is weak Figure 1 13 shows a set of field lines We can imagine two equal and small elements of area placed at points R and S normal to the field lines there
1
587-590
Figure 1 13 shows a set of field lines We can imagine two equal and small elements of area placed at points R and S normal to the field lines there The number of field lines in our picture cutting the area elements is proportional to the magnitude of field at these points
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588-591
13 shows a set of field lines We can imagine two equal and small elements of area placed at points R and S normal to the field lines there The number of field lines in our picture cutting the area elements is proportional to the magnitude of field at these points The picture shows that the field at R is stronger than at S
1
589-592
We can imagine two equal and small elements of area placed at points R and S normal to the field lines there The number of field lines in our picture cutting the area elements is proportional to the magnitude of field at these points The picture shows that the field at R is stronger than at S To understand the dependence of the field lines on the area, or rather the solid angle subtended by an area element, let us try to relate the area with the solid angle, a generalisation of angle to three dimensions