.net barcode reader open source Figure 25-4 An electric current is generated in a wire loop as the loop rotates in Objective-C

Encode QR Code in Objective-C Figure 25-4 An electric current is generated in a wire loop as the loop rotates

Figure 25-4 An electric current is generated in a wire loop as the loop rotates
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Section 251 Electric Current from Changing Magnetic Fields
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I max 1 Current I 2 3 I max 0 I 4 0 Time I max
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Figure 25-5 The cross-sectional view of a rotating wire loop shows the position of the loop when maximum current is generated (a) When the loop is vertical, the current is zero (b) The current varies with time as the loop rotates (c) The variation of EMF with time can be shown with a similar graph
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Figure 25-6 Only the segments bc and ad have current induced through them This can be shown using the fourth right-hand rule
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v I (induced) b
c
Axis
The current is greatest when the motion of the loop is perpendicular to the magnetic field; that is, when the loop is in the horizontal position, as shown in Figure 25-5a In this position, the component of the loop s velocity perpendicular to the magnetic field is greatest As the loop rotates from the horizontal to the vertical position, as shown in Figure 25-5b, it moves through the magnetic field lines at an ever-increasing angle Thus, it cuts through fewer magnetic field lines per unit of time, and the current decreases When the loop is in the vertical position, the wire segments move parallel to the field and the current is zero As the loop continues to turn, the segment that was moving up begins to move down and reverses the direction of the current in the loop This change in direction takes place each time the loop turns through 180 The current changes smoothly from zero to some maximum value and back to zero during each half-turn of the loop Then it reverses direction A graph of current versus time is shown in Figure 25-5c Does the entire loop contribute to the induced EMF Look at Figure 25-6, where all four sides of the loop are depicted in the magnetic field If the fourth right-hand rule is applied to segment ab, the direction of the induced current is toward the side of the wire The same applies to segment cd Thus, no current is induced along the length of the wire in ab or cd But in segment bc, the direction of the induced current is from b to c, and in segment ad, the current is from d to a Because the conducting loop is rotating in a circular motion, the relative angle between a point on the loop and the magnetic field constantly changes The electromotive force can be calculated by the electromotive force equation given earlier, EMF BLv(sin ), except that L is now the length of segment bc The maximum voltage is induced when a conductor is moving perpendicular to the magnetic field and thus 90 Generators, such as those in the chapv ter opening image, work in a similar fashion Potential energy from water stored behind a dam is converted to kinetic energy, which spins the turbines B The turbines, in turn, turn coils of cond ductors in a magnetic field, thereby inducing an EMF Generators and motors a are almost identical in construction, but I (induced)
25 Electromagnetic Induction
Current
I max I eff
Slip rings
Time
I max Pmax Power Pave 0
Figure 25-7 An alternatingcurrent generator transmits current to an external circuit by way of a brush-slip-ring arrangement (a) The alternating current produced varies with time (b) The resulting power is always positive and also is sinusoidal (c)
Brushes
Time
they convert energy in opposite directions A generator converts mechanical energy to electrical energy, while a motor converts electrical energy to mechanical energy
Alternating-Current Generators
An energy source turns the armature of a generator in a magnetic field at a fixed number of revolutions per second In the United States, electric utilities use a 60-Hz frequency, in which the current goes from one direction to the other and back to the first 60 times per second Figure 25-7a shows how an alternating current, AC, in an armature is transmitted to the rest of the circuit The brush-slip-ring arrangement permits the armature to turn freely while still allowing the current to pass into the external circuit As the armature turns, the alternating current varies between some maximum value and zero, as shown in the graph in Figure 25-7b Average power The power produced by a generator is the product of the current and the voltage Because both current and voltage vary, the power associated with an alternating current varies Figure 25-7c shows a graph of the power produced by an AC generator Note that power is always positive because I and V are either both positive or both negative Average power, PAC, is half the maximum power; thus, PAC 1 PAC max 2 Effective voltage and current It is common to describe alternating current and voltage in terms of effective current and voltage, rather than referring to their maximum values Recall from 22 that P I2R Thus, you can express effective current, Ieff , in terms of the average AC power as PAC Ieff2R To determine Ieff in terms of maximum current, Imax , 1 start with the power relationship, PAC P , and substitute in I2R 2 AC max Then solve for Ieff Effective Current Ieff
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