| 单词 | emf |
| 例句 | The values of currents and electric potential differences in an electric circuit are determined by the properties and arrangement of the individual circuit elements such as sources of emf, resistors, and capacitors. College Physics for AP Courses 2015-08-12T00:00:00Z The same is true for every point of the object EMF, and thus a small image, emf, will be formed. Half-hours with the Telescope Being a Popular Guide to the Use of the Telescope as a Means of Amusement and Instruction. The Tethered Satellite was to be let out on a 20 km length of wire as shown in Figure 23.12, to create a 5 kV emf by moving at orbital speed through the Earth’s field. College Physics for AP Courses 2015-08-12T00:00:00Z This emf could be used to convert some of the shuttle’s kinetic and potential energy into electrical energy if a complete circuit could be made. College Physics for AP Courses 2015-08-12T00:00:00Z If motional emf can cause a current loop in the conductor, we refer to that current as an eddy current. College Physics for AP Courses 2015-08-12T00:00:00Z However, emf differs from the voltage output of the device when current flows. College Physics for AP Courses 2015-08-12T00:00:00Z On the other hand, every point of the image emf has received the whole of the light gathered up by the large glass AB. Half-hours with the Telescope Being a Popular Guide to the Use of the Telescope as a Means of Amusement and Instruction. Electric generators induce an emf by rotating a coil in a magnetic field, as briefly discussed in Induced Emf and Magnetic Flux. College Physics for AP Courses 2015-08-12T00:00:00Z What is the emf at any given instant? College Physics for AP Courses 2015-08-12T00:00:00Z Figure 23.21 A generator with a single rectangular coil rotated at constant angular velocity in a uniform magnetic field produces an emf that varies sinusoidally in time. College Physics for AP Courses 2015-08-12T00:00:00Z Since V = emf − Ir , terminal voltage equals emf only if there is no current flowing. College Physics for AP Courses 2015-08-12T00:00:00Z Or, if it did receive the whole of this pencil, it clearly could not receive the whole of the pencils proceeding from other parts of the image emf. Half-hours with the Telescope Being a Popular Guide to the Use of the Telescope as a Means of Amusement and Instruction. The back emf is represented as a variable emf that opposes the one driving the motor. College Physics for AP Courses 2015-08-12T00:00:00Z Back emf is zero when the motor is not turning, and it increases proportionally to the motor’s angular velocity. College Physics for AP Courses 2015-08-12T00:00:00Z As the motor turns faster and faster, the back emf grows, always opposing the driving emf, and reduces the voltage across the coil and the amount of current it draws. College Physics for AP Courses 2015-08-12T00:00:00Z Using the data given in the table and applying graphical analysis, determine the emf and internal resistance of the battery. College Physics for AP Courses 2015-08-12T00:00:00Z When a mechanical load is placed on the motor, like an electric wheelchair going up a hill, the motor slows, the back emf drops, more current flows, and more work can be done. College Physics for AP Courses 2015-08-12T00:00:00Z Assuming negligible coil resistance, Kirchhoff’s loop rule tells us that the induced emf exactly equals the input voltage. College Physics for AP Courses 2015-08-12T00:00:00Z Figure 23.35 AC currents can induce an emf on the case of an appliance. College Physics for AP Courses 2015-08-12T00:00:00Z When, for example, current through a coil is increased, the magnetic field and flux also increase, inducing a counter emf, as required by Lenz’s law. College Physics for AP Courses 2015-08-12T00:00:00Z The terminal voltage of a battery is equal to the emf of the battery minus the voltage drop across the internal resistance of the battery or V = emf – Ir. College Physics for AP Courses 2015-08-12T00:00:00Z The induced emf is related to the physical geometry of the device and the rate of change of current. College Physics for AP Courses 2015-08-12T00:00:00Z The opposition it poses is in the form of an induced emf, which decreases to zero as the current approaches its final value. College Physics for AP Courses 2015-08-12T00:00:00Z The opposing emf is proportional to the amount of change left. College Physics for AP Courses 2015-08-12T00:00:00Z If emf is induced in a coil, N is its number of turns. College Physics for AP Courses 2015-08-12T00:00:00Z Using this linear relationship and the plotted graph, the internal resistance and emf of the battery can be found. College Physics for AP Courses 2015-08-12T00:00:00Z The minus sign means that the emf creates a current I and magnetic field B that oppose the change in flux ΔΦ —this opposition is known as Lenz’s law. College Physics for AP Courses 2015-08-12T00:00:00Z Current changes in a device induce an emf in the device itself. College Physics for AP Courses 2015-08-12T00:00:00Z Self-inductance is the effect of the device inducing emf in itself. College Physics for AP Courses 2015-08-12T00:00:00Z Is an emf induced in the coil in Figure 23.54 when it is stretched? College Physics for AP Courses 2015-08-12T00:00:00Z The voltage output of the battery charger must be greater than the emf of the battery to reverse current through it. College Physics for AP Courses 2015-08-12T00:00:00Z Electric fish generate current with biological cells called electroplaques, which are physiological emf devices. College Physics for AP Courses 2015-08-12T00:00:00Z This separation of charge creates a voltage ε , known as the Hall emf, across the conductor. College Physics for AP Courses 2015-08-12T00:00:00Z To examine these quantitatively, we need an expression for the Hall emf, ε , across a conductor. College Physics for AP Courses 2015-08-12T00:00:00Z For a uniform electric field, the relationship between electric field and voltage is E = ε / l , where l is the width of the conductor and ε is the Hall emf. College Physics for AP Courses 2015-08-12T00:00:00Z This will cause the terminal voltage of the battery to be greater than the emf, since V = emf − Ir , and I is now negative. College Physics for AP Courses 2015-08-12T00:00:00Z The magnitude of the Hall emf is ε = Blv , where l is the pipe diameter, so that the average velocity v can be determined from ε providing the other factors are known. College Physics for AP Courses 2015-08-12T00:00:00Z The current is a result of an emf induced by a changing magnetic field, whether or not there is a path for current to flow. College Physics for AP Courses 2015-08-12T00:00:00Z A change in the field produced by the top coil induces an emf and, hence, a current in the bottom coil. College Physics for AP Courses 2015-08-12T00:00:00Z The faster the motion, the greater the emf, and there is no emf when the magnet is stationary relative to the coil. College Physics for AP Courses 2015-08-12T00:00:00Z Usually, the cells are in series in order to produce a larger total emf. College Physics for AP Courses 2015-08-12T00:00:00Z The method of inducing an emf used in most electric generators is shown in Figure 23.5. College Physics for AP Courses 2015-08-12T00:00:00Z A coil is rotated in a magnetic field, producing an alternating current emf, which depends on rotation rate and other factors that will be explored in later sections. College Physics for AP Courses 2015-08-12T00:00:00Z First, emf is directly proportional to the change in flux ΔΦ . College Physics for AP Courses 2015-08-12T00:00:00Z The minus means that the emf creates a current I and magnetic field B that oppose the change in flux ΔΦ —this is known as Lenz’s law. College Physics for AP Courses 2015-08-12T00:00:00Z But if the cells oppose one another, such as when one is put into an appliance backward, the total emf is less, since it is the algebraic sum of the individual emfs. College Physics for AP Courses 2015-08-12T00:00:00Z The different magnetic field orientations of the iron oxide molecules on the tape induces an emf in the coil of wire in the playback head. College Physics for AP Courses 2015-08-12T00:00:00Z As we have seen, any change in magnetic flux induces an emf opposing that change—a process known as induction. College Physics for AP Courses 2015-08-12T00:00:00Z In this section, we concentrate on motion in a magnetic field that is stationary relative to the Earth, producing what is loosely called motional emf. College Physics for AP Courses 2015-08-12T00:00:00Z One situation where motional emf occurs is known as the Hall effect and has already been examined. College Physics for AP Courses 2015-08-12T00:00:00Z This can yield an emf of approximately 600 V, and a current of 1 A—deadly. College Physics for AP Courses 2015-08-12T00:00:00Z We will now see that the Hall effect is one aspect of the broader phenomenon of induction, and we will find that motional emf can be used as a power source. College Physics for AP Courses 2015-08-12T00:00:00Z What average emf is induced, given the plane of the coil is originally perpendicular to the Earth’s field and is rotated to be parallel to the field in 10.0 ms? College Physics for AP Courses 2015-08-12T00:00:00Z How fast can the 150 A current through a 0.250 H inductor be shut off if the induced emf cannot exceed 75.0 V? College Physics for AP Courses 2015-08-12T00:00:00Z Any change in magnetic flux Φ induces an emf—the process is defined to be electromagnetic induction. College Physics for AP Courses 2015-08-12T00:00:00Z Recall that emf is the potential difference of a source when no current is flowing. College Physics for AP Courses 2015-08-12T00:00:00Z The minus sign indicates that emf opposes the change in current, as required by Lenz’s law. College Physics for AP Courses 2015-08-12T00:00:00Z What is the strength of the magnetic field if a 95.0 V emf is induced? College Physics for AP Courses 2015-08-12T00:00:00Z What is the back emf of a 120 V motor that draws 8.00 A at its normal speed and 20.0 A when first starting? College Physics for AP Courses 2015-08-12T00:00:00Z What is the self-inductance of the device if an induced 150 V emf opposes this? College Physics for AP Courses 2015-08-12T00:00:00Z If the current I could be made zero, then V = emf , and so emf could be directly measured. College Physics for AP Courses 2015-08-12T00:00:00Z We thus use the name electromotive force, abbreviated emf. College Physics for AP Courses 2015-08-12T00:00:00Z The voltage output of each depends on its construction and load, and equals emf only if there is no load. College Physics for AP Courses 2015-08-12T00:00:00Z The voltage across the terminals of a battery, for example, is less than the emf when the battery supplies current, and it declines further as the battery is depleted or loaded down. College Physics for AP Courses 2015-08-12T00:00:00Z Both are lead-acid batteries with identical emf, but, because of its size, the truck battery has a smaller internal resistance r . College Physics for AP Courses 2015-08-12T00:00:00Z Since the internal resistance of the battery is not known precisely, it is not possible to calculate the emf precisely. College Physics for AP Courses 2015-08-12T00:00:00Z The smaller the internal resistance for a given emf, the more current and the more power the source can supply. College Physics for AP Courses 2015-08-12T00:00:00Z Note also that since the chemical reactions involve substances with resistance, it is not possible to create the emf without an internal resistance. College Physics for AP Courses 2015-08-12T00:00:00Z Once the current is found, the terminal voltage can be calculated using the equation V = emf − Ir . College Physics for AP Courses 2015-08-12T00:00:00Z The charger must have a larger emf than the battery to reverse current through it. College Physics for AP Courses 2015-08-12T00:00:00Z Since no current flows through the galvanometer, none flows through the unknown emf, and so emfx is directly sensed. College Physics for AP Courses 2015-08-12T00:00:00Z In this simple case, the total emf is the same as the individual emfs. College Physics for AP Courses 2015-08-12T00:00:00Z The emf could be accurately calculated if r were very accurately known, but it is usually not. College Physics for AP Courses 2015-08-12T00:00:00Z Figure 21.37 An analog voltmeter attached to a battery draws a small but nonzero current and measures a terminal voltage that differs from the emf of the battery. College Physics for AP Courses 2015-08-12T00:00:00Z The voltage approaches emf asymptotically, since the closer it gets to emf the less current flows. College Physics for AP Courses 2015-08-12T00:00:00Z The unknown emf is thus proportional to the resistance of the wire segment. College Physics for AP Courses 2015-08-12T00:00:00Z The emf is the potential difference of a source when no current is flowing. College Physics for AP Courses 2015-08-12T00:00:00Z The numerical value of the emf depends on the source of potential difference. College Physics for AP Courses 2015-08-12T00:00:00Z The Hall emf ε is measured across the tube perpendicular to the applied magnetic field and is proportional to the average velocity v . College Physics for AP Courses 2015-08-12T00:00:00Z Changing magnetic flux induces an electric field that can establish an induced emf in a system. College Physics for AP Courses 2015-08-12T00:00:00Z Because a long uniform wire is used for R , the ratio of resistances Rx / Rs is the same as the ratio of the lengths of wire that zero the galvanometer for each emf. College Physics for AP Courses 2015-08-12T00:00:00Z An emf is induced in the coil when a bar magnet is pushed in and out of it. College Physics for AP Courses 2015-08-12T00:00:00Z The greater the speed, the greater the magnitude of the emf, and the emf is zero when there is no motion. College Physics for AP Courses 2015-08-12T00:00:00Z Figure 23.5 Rotation of a coil in a magnetic field produces an emf. College Physics for AP Courses 2015-08-12T00:00:00Z So we see that changing the magnitude or direction of a magnetic field produces an emf. College Physics for AP Courses 2015-08-12T00:00:00Z When there is no current, there is no IR drop, and so the voltage on the capacitor must then equal the emf of the voltage source. College Physics for AP Courses 2015-08-12T00:00:00Z Just how great an emf and what direction it takes depend on the change in Φ and how rapidly the change is made, as examined in the next section. College Physics for AP Courses 2015-08-12T00:00:00Z Finally, if a coil has N turns, an emf will be produced that is N times greater than for a single coil, so that emf is directly proportional to N . College Physics for AP Courses 2015-08-12T00:00:00Z The units for emf are volts, as is usual. College Physics for AP Courses 2015-08-12T00:00:00Z For example, a magnet moved toward a coil induces an emf, and a coil moved toward a magnet produces a similar emf. College Physics for AP Courses 2015-08-12T00:00:00Z The initial current is I0 = emf , because all of the IR drop is in the resistance. College Physics for AP Courses 2015-08-12T00:00:00Z When flux changes, an emf is induced according to Faraday’s law of induction. College Physics for AP Courses 2015-08-12T00:00:00Z Here and below, “emf” implies the magnitude of the emf. College Physics for AP Courses 2015-08-12T00:00:00Z We have seen an example of this in the situation where a moving magnet induces an emf in a stationary coil. College Physics for AP Courses 2015-08-12T00:00:00Z The emf calculated in Example 23.3 is the average over one-fourth of a revolution. College Physics for AP Courses 2015-08-12T00:00:00Z After two time constants, what percentage of the final voltage, emf, is on an initially uncharged capacitor C , charged through a resistance R ? College Physics for AP Courses 2015-08-12T00:00:00Z We can thus find the induced emf by considering only the side wires. College Physics for AP Courses 2015-08-12T00:00:00Z Motional emf is given to be emf = Bℓv , where the velocity v is perpendicular to the magnetic field B . College Physics for AP Courses 2015-08-12T00:00:00Z Thus in this case the emf induced on each side is emf = Bℓv sin θ , and they are in the same direction. College Physics for AP Courses 2015-08-12T00:00:00Z To find the time dependence of emf, we assume the coil rotates at a constant angular velocity ω . College Physics for AP Courses 2015-08-12T00:00:00Z Among the things to be considered are the emf and useful terminal voltage of a lithium cell and the current it should be able to supply to a camcorder. College Physics for AP Courses 2015-08-12T00:00:00Z Figure 23.22 shows a graph of emf as a function of time, and it now seems reasonable that AC voltage is sinusoidal. College Physics for AP Courses 2015-08-12T00:00:00Z Figure 23.22 The emf of a generator is sent to a light bulb with the system of rings and brushes shown. College Physics for AP Courses 2015-08-12T00:00:00Z The graph gives the emf of the generator as a function of time. emf0 is the peak emf. College Physics for AP Courses 2015-08-12T00:00:00Z Note that the script E stands for emf. College Physics for AP Courses 2015-08-12T00:00:00Z A magnetic field applied perpendicular to the flow direction produces a Hall emf ε as shown. College Physics for AP Courses 2015-08-12T00:00:00Z Figure 23.23 Split rings, called commutators, produce a pulsed DC emf output in this configuration. College Physics for AP Courses 2015-08-12T00:00:00Z Lenz’s law tells us the emf opposes any change, so that the input emf that powers the motor will be opposed by the motor’s self-generated emf, called the back emf of the motor. College Physics for AP Courses 2015-08-12T00:00:00Z Back emf is the generator output of a motor, and so it is proportional to the motor’s angular velocity ω . College Physics for AP Courses 2015-08-12T00:00:00Z On the other hand, if there is no mechanical load on the motor, it will increase its angular velocity ω until the back emf is nearly equal to the driving emf. College Physics for AP Courses 2015-08-12T00:00:00Z To find the direction of the induced field, the direction of the current, and the polarity of the induced emf, we apply Lenz’s law as explained in Faraday's Law of Induction: Lenz's Law. College Physics for AP Courses 2015-08-12T00:00:00Z The AC current in appliances can induce an emf on the case. College Physics for AP Courses 2015-08-12T00:00:00Z If the case is grounded, the induced emf is kept near zero. College Physics for AP Courses 2015-08-12T00:00:00Z Here a change in current in coil 1 is seen to induce an emf in coil 2. College Physics for AP Courses 2015-08-12T00:00:00Z But an appliance, such as an electric clothes dryer, can induce a dangerous emf on its case if the mutual inductance between its coils and the case is large. College Physics for AP Courses 2015-08-12T00:00:00Z A moving magnetic field produces an electric field through its induced emf. College Physics for AP Courses 2015-08-12T00:00:00Z Conversely, if the current is decreased, an emf is induced that opposes the decrease. College Physics for AP Courses 2015-08-12T00:00:00Z The minus sign is an expression of Lenz’s law, indicating that emf opposes the change in current. College Physics for AP Courses 2015-08-12T00:00:00Z |
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