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1
5490-5493
6) In terms of I, the average power, denoted by P is 2 2 21 m p P i R I R = = = (7 7) Similarly, we define the rms voltage or effective voltage by V = 2 m v = 0 707 vm (7 8) From Eq
1
5491-5494
7) Similarly, we define the rms voltage or effective voltage by V = 2 m v = 0 707 vm (7 8) From Eq (7
1
5492-5495
707 vm (7 8) From Eq (7 3), we have vm = imR or, 2 2 m m v i R = or, V = IR (7
1
5493-5496
8) From Eq (7 3), we have vm = imR or, 2 2 m m v i R = or, V = IR (7 9) Equation (7
1
5494-5497
(7 3), we have vm = imR or, 2 2 m m v i R = or, V = IR (7 9) Equation (7 9) gives the relation between ac current and ac voltage and is similar to that in the dc case
1
5495-5498
3), we have vm = imR or, 2 2 m m v i R = or, V = IR (7 9) Equation (7 9) gives the relation between ac current and ac voltage and is similar to that in the dc case This shows the advantage of introducing the concept of rms values
1
5496-5499
9) Equation (7 9) gives the relation between ac current and ac voltage and is similar to that in the dc case This shows the advantage of introducing the concept of rms values In terms of rms values, the equation for power [Eq
1
5497-5500
9) gives the relation between ac current and ac voltage and is similar to that in the dc case This shows the advantage of introducing the concept of rms values In terms of rms values, the equation for power [Eq (7
1
5498-5501
This shows the advantage of introducing the concept of rms values In terms of rms values, the equation for power [Eq (7 7)] and relation between current and voltage in ac circuits are essentially the same as those for the dc case
1
5499-5502
In terms of rms values, the equation for power [Eq (7 7)] and relation between current and voltage in ac circuits are essentially the same as those for the dc case It is customary to measure and specify rms values for ac quantities
1
5500-5503
(7 7)] and relation between current and voltage in ac circuits are essentially the same as those for the dc case It is customary to measure and specify rms values for ac quantities For example, the household line voltage of 220 V is an rms value with a peak voltage of vm = 2 V = (1
1
5501-5504
7)] and relation between current and voltage in ac circuits are essentially the same as those for the dc case It is customary to measure and specify rms values for ac quantities For example, the household line voltage of 220 V is an rms value with a peak voltage of vm = 2 V = (1 414)(220 V) = 311 V In fact, the I or rms current is the equivalent dc current that would produce the same average power loss as the alternating current
1
5502-5505
It is customary to measure and specify rms values for ac quantities For example, the household line voltage of 220 V is an rms value with a peak voltage of vm = 2 V = (1 414)(220 V) = 311 V In fact, the I or rms current is the equivalent dc current that would produce the same average power loss as the alternating current Equation (7
1
5503-5506
For example, the household line voltage of 220 V is an rms value with a peak voltage of vm = 2 V = (1 414)(220 V) = 311 V In fact, the I or rms current is the equivalent dc current that would produce the same average power loss as the alternating current Equation (7 7) can also be written as P = V2 / R = I V (since V = I R) Example 7
1
5504-5507
414)(220 V) = 311 V In fact, the I or rms current is the equivalent dc current that would produce the same average power loss as the alternating current Equation (7 7) can also be written as P = V2 / R = I V (since V = I R) Example 7 1 A light bulb is rated at 100W for a 220 V supply
1
5505-5508
Equation (7 7) can also be written as P = V2 / R = I V (since V = I R) Example 7 1 A light bulb is rated at 100W for a 220 V supply Find (a) the resistance of the bulb; (b) the peak voltage of the source; and (c) the rms current through the bulb
1
5506-5509
7) can also be written as P = V2 / R = I V (since V = I R) Example 7 1 A light bulb is rated at 100W for a 220 V supply Find (a) the resistance of the bulb; (b) the peak voltage of the source; and (c) the rms current through the bulb Solution (a) We are given P = 100 W and V = 220 V
1
5507-5510
1 A light bulb is rated at 100W for a 220 V supply Find (a) the resistance of the bulb; (b) the peak voltage of the source; and (c) the rms current through the bulb Solution (a) We are given P = 100 W and V = 220 V The resistance of the bulb is ( ) 2 2 220 V 484 100 W V R =P = = Ω (b) The peak voltage of the source is V 2 311 vm V = = (c) Since, P = I V 100 W 0
1
5508-5511
Find (a) the resistance of the bulb; (b) the peak voltage of the source; and (c) the rms current through the bulb Solution (a) We are given P = 100 W and V = 220 V The resistance of the bulb is ( ) 2 2 220 V 484 100 W V R =P = = Ω (b) The peak voltage of the source is V 2 311 vm V = = (c) Since, P = I V 100 W 0 454A 220 V � � � P I V EXAMPLE 7
1
5509-5512
Solution (a) We are given P = 100 W and V = 220 V The resistance of the bulb is ( ) 2 2 220 V 484 100 W V R =P = = Ω (b) The peak voltage of the source is V 2 311 vm V = = (c) Since, P = I V 100 W 0 454A 220 V � � � P I V EXAMPLE 7 1 Rationalised 2023-24 181 Alternating Current 7
1
5510-5513
The resistance of the bulb is ( ) 2 2 220 V 484 100 W V R =P = = Ω (b) The peak voltage of the source is V 2 311 vm V = = (c) Since, P = I V 100 W 0 454A 220 V � � � P I V EXAMPLE 7 1 Rationalised 2023-24 181 Alternating Current 7 3 REPRESENTATION OF AC CURRENT AND VOLTAGE BY ROTATING VECTORS — PHASORS In the previous section, we learnt that the current through a resistor is in phase with the ac voltage
1
5511-5514
454A 220 V � � � P I V EXAMPLE 7 1 Rationalised 2023-24 181 Alternating Current 7 3 REPRESENTATION OF AC CURRENT AND VOLTAGE BY ROTATING VECTORS — PHASORS In the previous section, we learnt that the current through a resistor is in phase with the ac voltage But this is not so in the case of an inductor, a capacitor or a combination of these circuit elements
1
5512-5515
1 Rationalised 2023-24 181 Alternating Current 7 3 REPRESENTATION OF AC CURRENT AND VOLTAGE BY ROTATING VECTORS — PHASORS In the previous section, we learnt that the current through a resistor is in phase with the ac voltage But this is not so in the case of an inductor, a capacitor or a combination of these circuit elements In order to show phase relationship between voltage and current in an ac circuit, we use the notion of phasors
1
5513-5516
3 REPRESENTATION OF AC CURRENT AND VOLTAGE BY ROTATING VECTORS — PHASORS In the previous section, we learnt that the current through a resistor is in phase with the ac voltage But this is not so in the case of an inductor, a capacitor or a combination of these circuit elements In order to show phase relationship between voltage and current in an ac circuit, we use the notion of phasors The analysis of an ac circuit is facilitated by the use of a phasor diagram
1
5514-5517
But this is not so in the case of an inductor, a capacitor or a combination of these circuit elements In order to show phase relationship between voltage and current in an ac circuit, we use the notion of phasors The analysis of an ac circuit is facilitated by the use of a phasor diagram A phasor* is a vector which rotates about the origin with angular speed w, as shown in Fig
1
5515-5518
In order to show phase relationship between voltage and current in an ac circuit, we use the notion of phasors The analysis of an ac circuit is facilitated by the use of a phasor diagram A phasor* is a vector which rotates about the origin with angular speed w, as shown in Fig 7
1
5516-5519
The analysis of an ac circuit is facilitated by the use of a phasor diagram A phasor* is a vector which rotates about the origin with angular speed w, as shown in Fig 7 4
1
5517-5520
A phasor* is a vector which rotates about the origin with angular speed w, as shown in Fig 7 4 The vertical components of phasors V and I represent the sinusoidally varying quantities v and i
1
5518-5521
7 4 The vertical components of phasors V and I represent the sinusoidally varying quantities v and i The magnitudes of phasors V and I represent the amplitudes or the peak values vm and im of these oscillating quantities
1
5519-5522
4 The vertical components of phasors V and I represent the sinusoidally varying quantities v and i The magnitudes of phasors V and I represent the amplitudes or the peak values vm and im of these oscillating quantities Figure 7
1
5520-5523
The vertical components of phasors V and I represent the sinusoidally varying quantities v and i The magnitudes of phasors V and I represent the amplitudes or the peak values vm and im of these oscillating quantities Figure 7 4(a) shows the voltage and current phasors and their relationship at time t1 for the case of an ac source connected to a resistor i
1
5521-5524
The magnitudes of phasors V and I represent the amplitudes or the peak values vm and im of these oscillating quantities Figure 7 4(a) shows the voltage and current phasors and their relationship at time t1 for the case of an ac source connected to a resistor i e
1
5522-5525
Figure 7 4(a) shows the voltage and current phasors and their relationship at time t1 for the case of an ac source connected to a resistor i e , corresponding to the circuit shown in Fig
1
5523-5526
4(a) shows the voltage and current phasors and their relationship at time t1 for the case of an ac source connected to a resistor i e , corresponding to the circuit shown in Fig 7
1
5524-5527
e , corresponding to the circuit shown in Fig 7 1
1
5525-5528
, corresponding to the circuit shown in Fig 7 1 The projection of voltage and current phasors on vertical axis, i
1
5526-5529
7 1 The projection of voltage and current phasors on vertical axis, i e
1
5527-5530
1 The projection of voltage and current phasors on vertical axis, i e , vm sinw t and im sinw t, respectively represent the value of voltage and current at that instant
1
5528-5531
The projection of voltage and current phasors on vertical axis, i e , vm sinw t and im sinw t, respectively represent the value of voltage and current at that instant As they rotate with frequency w, curves in Fig
1
5529-5532
e , vm sinw t and im sinw t, respectively represent the value of voltage and current at that instant As they rotate with frequency w, curves in Fig 7
1
5530-5533
, vm sinw t and im sinw t, respectively represent the value of voltage and current at that instant As they rotate with frequency w, curves in Fig 7 4(b) are generated
1
5531-5534
As they rotate with frequency w, curves in Fig 7 4(b) are generated From Fig
1
5532-5535
7 4(b) are generated From Fig 7
1
5533-5536
4(b) are generated From Fig 7 4(a) we see that phasors V and I for the case of a resistor are in the same direction
1
5534-5537
From Fig 7 4(a) we see that phasors V and I for the case of a resistor are in the same direction This is so for all times
1
5535-5538
7 4(a) we see that phasors V and I for the case of a resistor are in the same direction This is so for all times This means that the phase angle between the voltage and the current is zero
1
5536-5539
4(a) we see that phasors V and I for the case of a resistor are in the same direction This is so for all times This means that the phase angle between the voltage and the current is zero 7
1
5537-5540
This is so for all times This means that the phase angle between the voltage and the current is zero 7 4 AC VOLTAGE APPLIED TO AN INDUCTOR Figure 7
1
5538-5541
This means that the phase angle between the voltage and the current is zero 7 4 AC VOLTAGE APPLIED TO AN INDUCTOR Figure 7 5 shows an ac source connected to an inductor
1
5539-5542
7 4 AC VOLTAGE APPLIED TO AN INDUCTOR Figure 7 5 shows an ac source connected to an inductor Usually, inductors have appreciable resistance in their windings, but we shall assume that this inductor has negligible resistance
1
5540-5543
4 AC VOLTAGE APPLIED TO AN INDUCTOR Figure 7 5 shows an ac source connected to an inductor Usually, inductors have appreciable resistance in their windings, but we shall assume that this inductor has negligible resistance Thus, the circuit is a purely inductive ac circuit
1
5541-5544
5 shows an ac source connected to an inductor Usually, inductors have appreciable resistance in their windings, but we shall assume that this inductor has negligible resistance Thus, the circuit is a purely inductive ac circuit Let the voltage across the source be v = vm sinw t
1
5542-5545
Usually, inductors have appreciable resistance in their windings, but we shall assume that this inductor has negligible resistance Thus, the circuit is a purely inductive ac circuit Let the voltage across the source be v = vm sinw t Using the Kirchhoff’s loop rule, ∑ε ( )t = 0 , and since there is no resistor in the circuit, d 0 d i v L t − = (7
1
5543-5546
Thus, the circuit is a purely inductive ac circuit Let the voltage across the source be v = vm sinw t Using the Kirchhoff’s loop rule, ∑ε ( )t = 0 , and since there is no resistor in the circuit, d 0 d i v L t − = (7 10) where the second term is the self-induced Faraday emf in the inductor; and L is the self-inductance of FIGURE 7
1
5544-5547
Let the voltage across the source be v = vm sinw t Using the Kirchhoff’s loop rule, ∑ε ( )t = 0 , and since there is no resistor in the circuit, d 0 d i v L t − = (7 10) where the second term is the self-induced Faraday emf in the inductor; and L is the self-inductance of FIGURE 7 4 (a) A phasor diagram for the circuit in Fig 7
1
5545-5548
Using the Kirchhoff’s loop rule, ∑ε ( )t = 0 , and since there is no resistor in the circuit, d 0 d i v L t − = (7 10) where the second term is the self-induced Faraday emf in the inductor; and L is the self-inductance of FIGURE 7 4 (a) A phasor diagram for the circuit in Fig 7 1
1
5546-5549
10) where the second term is the self-induced Faraday emf in the inductor; and L is the self-inductance of FIGURE 7 4 (a) A phasor diagram for the circuit in Fig 7 1 (b) Graph of v and i versus wt
1
5547-5550
4 (a) A phasor diagram for the circuit in Fig 7 1 (b) Graph of v and i versus wt FIGURE 7
1
5548-5551
1 (b) Graph of v and i versus wt FIGURE 7 5 An ac source connected to an inductor
1
5549-5552
(b) Graph of v and i versus wt FIGURE 7 5 An ac source connected to an inductor * Though voltage and current in ac circuit are represented by phasors – rotating vectors, they are not vectors themselves
1
5550-5553
FIGURE 7 5 An ac source connected to an inductor * Though voltage and current in ac circuit are represented by phasors – rotating vectors, they are not vectors themselves They are scalar quantities
1
5551-5554
5 An ac source connected to an inductor * Though voltage and current in ac circuit are represented by phasors – rotating vectors, they are not vectors themselves They are scalar quantities It so happens that the amplitudes and phases of harmonically varying scalars combine mathematically in the same way as do the projections of rotating vectors of corresponding magnitudes and directions
1
5552-5555
* Though voltage and current in ac circuit are represented by phasors – rotating vectors, they are not vectors themselves They are scalar quantities It so happens that the amplitudes and phases of harmonically varying scalars combine mathematically in the same way as do the projections of rotating vectors of corresponding magnitudes and directions The rotating vectors that represent harmonically varying scalar quantities are introduced only to provide us with a simple way of adding these quantities using a rule that we already know
1
5553-5556
They are scalar quantities It so happens that the amplitudes and phases of harmonically varying scalars combine mathematically in the same way as do the projections of rotating vectors of corresponding magnitudes and directions The rotating vectors that represent harmonically varying scalar quantities are introduced only to provide us with a simple way of adding these quantities using a rule that we already know Rationalised 2023-24 Physics 182 the inductor
1
5554-5557
It so happens that the amplitudes and phases of harmonically varying scalars combine mathematically in the same way as do the projections of rotating vectors of corresponding magnitudes and directions The rotating vectors that represent harmonically varying scalar quantities are introduced only to provide us with a simple way of adding these quantities using a rule that we already know Rationalised 2023-24 Physics 182 the inductor The negative sign follows from Lenz’s law (Chapter 6)
1
5555-5558
The rotating vectors that represent harmonically varying scalar quantities are introduced only to provide us with a simple way of adding these quantities using a rule that we already know Rationalised 2023-24 Physics 182 the inductor The negative sign follows from Lenz’s law (Chapter 6) Combining Eqs
1
5556-5559
Rationalised 2023-24 Physics 182 the inductor The negative sign follows from Lenz’s law (Chapter 6) Combining Eqs (7
1
5557-5560
The negative sign follows from Lenz’s law (Chapter 6) Combining Eqs (7 1) and (7
1
5558-5561
Combining Eqs (7 1) and (7 10), we have d sin d vm i v t t L L ω = = (7
1
5559-5562
(7 1) and (7 10), we have d sin d vm i v t t L L ω = = (7 11) Equation (7
1
5560-5563
1) and (7 10), we have d sin d vm i v t t L L ω = = (7 11) Equation (7 11) implies that the equation for i(t), the current as a function of time, must be such that its slope di/dt is a sinusoidally varying quantity, with the same phase as the source voltage and an amplitude given by vm/L
1
5561-5564
10), we have d sin d vm i v t t L L ω = = (7 11) Equation (7 11) implies that the equation for i(t), the current as a function of time, must be such that its slope di/dt is a sinusoidally varying quantity, with the same phase as the source voltage and an amplitude given by vm/L To obtain the current, we integrate di/dt with respect to time: d d d d ti t v L t t m ∫ ∫ = sin( ) ω and get, cos( ) constant vm i t L = − ω + ω The integration constant has the dimension of current and is time- independent
1
5562-5565
11) Equation (7 11) implies that the equation for i(t), the current as a function of time, must be such that its slope di/dt is a sinusoidally varying quantity, with the same phase as the source voltage and an amplitude given by vm/L To obtain the current, we integrate di/dt with respect to time: d d d d ti t v L t t m ∫ ∫ = sin( ) ω and get, cos( ) constant vm i t L = − ω + ω The integration constant has the dimension of current and is time- independent Since the source has an emf which oscillates symmetrically about zero, the current it sustains also oscillates symmetrically about zero, so that no constant or time-independent component of the current exists
1
5563-5566
11) implies that the equation for i(t), the current as a function of time, must be such that its slope di/dt is a sinusoidally varying quantity, with the same phase as the source voltage and an amplitude given by vm/L To obtain the current, we integrate di/dt with respect to time: d d d d ti t v L t t m ∫ ∫ = sin( ) ω and get, cos( ) constant vm i t L = − ω + ω The integration constant has the dimension of current and is time- independent Since the source has an emf which oscillates symmetrically about zero, the current it sustains also oscillates symmetrically about zero, so that no constant or time-independent component of the current exists Therefore, the integration constant is zero
1
5564-5567
To obtain the current, we integrate di/dt with respect to time: d d d d ti t v L t t m ∫ ∫ = sin( ) ω and get, cos( ) constant vm i t L = − ω + ω The integration constant has the dimension of current and is time- independent Since the source has an emf which oscillates symmetrically about zero, the current it sustains also oscillates symmetrically about zero, so that no constant or time-independent component of the current exists Therefore, the integration constant is zero Using − = −   cos( ) sin ω ω t t 2π , we have i i t =m −   sin ω 2π (7
1
5565-5568
Since the source has an emf which oscillates symmetrically about zero, the current it sustains also oscillates symmetrically about zero, so that no constant or time-independent component of the current exists Therefore, the integration constant is zero Using − = −   cos( ) sin ω ω t t 2π , we have i i t =m −   sin ω 2π (7 12) where m m v i = ωL is the amplitude of the current
1
5566-5569
Therefore, the integration constant is zero Using − = −   cos( ) sin ω ω t t 2π , we have i i t =m −   sin ω 2π (7 12) where m m v i = ωL is the amplitude of the current The quantity w L is analogous to the resistance and is called inductive reactance, denoted by XL: XL = w L (7
1
5567-5570
Using − = −   cos( ) sin ω ω t t 2π , we have i i t =m −   sin ω 2π (7 12) where m m v i = ωL is the amplitude of the current The quantity w L is analogous to the resistance and is called inductive reactance, denoted by XL: XL = w L (7 13) The amplitude of the current is, then m m L v i =X (7
1
5568-5571
12) where m m v i = ωL is the amplitude of the current The quantity w L is analogous to the resistance and is called inductive reactance, denoted by XL: XL = w L (7 13) The amplitude of the current is, then m m L v i =X (7 14) The dimension of inductive reactance is the same as that of resistance and its SI unit is ohm (W)
1
5569-5572
The quantity w L is analogous to the resistance and is called inductive reactance, denoted by XL: XL = w L (7 13) The amplitude of the current is, then m m L v i =X (7 14) The dimension of inductive reactance is the same as that of resistance and its SI unit is ohm (W) The inductive reactance limits the current in a purely inductive circuit in the same way as the resistance limits the current in a purely resistive circuit
1
5570-5573
13) The amplitude of the current is, then m m L v i =X (7 14) The dimension of inductive reactance is the same as that of resistance and its SI unit is ohm (W) The inductive reactance limits the current in a purely inductive circuit in the same way as the resistance limits the current in a purely resistive circuit The inductive reactance is directly proportional to the inductance and to the frequency of the current
1
5571-5574
14) The dimension of inductive reactance is the same as that of resistance and its SI unit is ohm (W) The inductive reactance limits the current in a purely inductive circuit in the same way as the resistance limits the current in a purely resistive circuit The inductive reactance is directly proportional to the inductance and to the frequency of the current A comparison of Eqs
1
5572-5575
The inductive reactance limits the current in a purely inductive circuit in the same way as the resistance limits the current in a purely resistive circuit The inductive reactance is directly proportional to the inductance and to the frequency of the current A comparison of Eqs (7
1
5573-5576
The inductive reactance is directly proportional to the inductance and to the frequency of the current A comparison of Eqs (7 1) and (7
1
5574-5577
A comparison of Eqs (7 1) and (7 12) for the source voltage and the current in an inductor shows that the current lags the voltage by p/2 or one-quarter (1/4) cycle
1
5575-5578
(7 1) and (7 12) for the source voltage and the current in an inductor shows that the current lags the voltage by p/2 or one-quarter (1/4) cycle Figure 7
1
5576-5579
1) and (7 12) for the source voltage and the current in an inductor shows that the current lags the voltage by p/2 or one-quarter (1/4) cycle Figure 7 6 (a) shows the voltage and the current phasors in the present case at instant t1
1
5577-5580
12) for the source voltage and the current in an inductor shows that the current lags the voltage by p/2 or one-quarter (1/4) cycle Figure 7 6 (a) shows the voltage and the current phasors in the present case at instant t1 The current phasor I is p/2 behind the voltage phasor V
1
5578-5581
Figure 7 6 (a) shows the voltage and the current phasors in the present case at instant t1 The current phasor I is p/2 behind the voltage phasor V When rotated with frequency w counter- clockwise, they generate the voltage and current given by Eqs
1
5579-5582
6 (a) shows the voltage and the current phasors in the present case at instant t1 The current phasor I is p/2 behind the voltage phasor V When rotated with frequency w counter- clockwise, they generate the voltage and current given by Eqs (7
1
5580-5583
The current phasor I is p/2 behind the voltage phasor V When rotated with frequency w counter- clockwise, they generate the voltage and current given by Eqs (7 1) and (7
1
5581-5584
When rotated with frequency w counter- clockwise, they generate the voltage and current given by Eqs (7 1) and (7 12), respectively and as shown in Fig
1
5582-5585
(7 1) and (7 12), respectively and as shown in Fig 7
1
5583-5586
1) and (7 12), respectively and as shown in Fig 7 6(b)
1
5584-5587
12), respectively and as shown in Fig 7 6(b) Interactive animation on Phasor diagrams of ac circuits containing, R, L, C and RLC series circuits: http://www
1
5585-5588
7 6(b) Interactive animation on Phasor diagrams of ac circuits containing, R, L, C and RLC series circuits: http://www animations
1
5586-5589
6(b) Interactive animation on Phasor diagrams of ac circuits containing, R, L, C and RLC series circuits: http://www animations physics
1
5587-5590
Interactive animation on Phasor diagrams of ac circuits containing, R, L, C and RLC series circuits: http://www animations physics unsw
1
5588-5591
animations physics unsw edu
1
5589-5592
physics unsw edu au//jw/AC