How is emf induced in a coil

When the coils are stationary, no current is induced. But when the small coil is moved in or out of the large coil B , the magnetic flux through the large coil changes, inducing a current which is detected by the galvanometer G. A device that can maintain a potential difference, despite the flow of current is a source of electromotive force. Electric generators convert mechanical energy to electrical energy; they induce an EMF by rotating a coil in a magnetic field. Electric generators are devices that convert mechanical energy to electrical energy.

They induce an electromotive force EMF by rotating a coil in a magnetic field. It is a device that converts mechanical energy to electrical energy. A generator forces electric charge usually carried by electrons to flow through an external electrical circuit. Possible sources of mechanical energy include: a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, compressed air, or any other source of mechanical energy.

Steam Turbine Generator : A modern steam turbine generator. Consider the setup shown in. Charges in the wires of the loop experience the magnetic force because they are moving in a magnetic field. Charges in the vertical wires experience forces parallel to the wire, causing currents.

However, those in the top and bottom segments feel a force perpendicular to the wire; this force does not cause a current. We can thus find the induced EMF by considering only the side wires. Diagram of an Electric Generator : 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. Note the generator is similar to a motor, except the shaft is rotated to produce a current rather than the other way around.

This expression is valid, but it does not give EMF as a function of time. Generators illustrated in this Atom look very much like the motors illustrated previously. This is not coincidental. In fact, a motor becomes a generator when its shaft rotates. The basic principles of operation for a motor are the same as those for a generator, except that a motor converts electrical energy into mechanical energy motion. Read our atom on electric generators first.

Most electric motors use the interaction of magnetic fields and current-carrying conductors to generate force. Electric motors are found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and disk drives.

If you were to place a moving charged particle in a magnetic field, it would experience a force called the Lorentz force:. Right-Hand Rule : Right-hand rule showing the direction of the Lorentz force. Current in a conductor consists of moving charges. Therefore, a current-carrying coil in a magnetic field will also feel the Lorentz force. For a straight current carrying wire that is not moving, the Lorentz force is:.

The direction of the Lorentz force is perpendicular to both the direction of the flow of current and the magnetic field and can be found using the right-hand rule, shown in. Using your right hand, point your thumb in the direction of the current, and point your first finger in the direction of the magnetic field. Your third finger will now be pointing in the direction of the force. Torque : The force on opposite sides of the coil will be in opposite directions because the charges are moving in opposite directions.

This means the coil will rotate. Both motors and generators can be explained in terms of a coil that rotates in a magnetic field. In a generator the coil is attached to an external circuit that is then turned.

This results in a changing flux, which induces an electromagnetic field. In a motor, a current-carrying coil in a magnetic field experiences a force on both sides of the coil, which creates a twisting force called a torque that makes it turn.

Any coil carrying current can feel a force in a magnetic field. This force is the Lorentz force on the moving charges in the conductor. The force on opposite sides of the coil will be in opposite directions because the charges are moving in opposite directions. Inductance is the property of a device that tells how effectively it induces an emf in another device or on itself.

Induction is the process in which an emf is induced by changing magnetic flux. The answer is yes, and that physical quantity is called inductance. See, where simple coils induce emfs in one another. Mutual Inductance in Coils : These coils can induce emfs in one another like an inefficient transformer. Their mutual inductance M indicates the effectiveness of the coupling between them.

Here a change in current in coil 1 is seen to induce an emf in coil 2. In the many cases where the geometry of the devices is fixed, flux is changed by varying current. A change in the current I 1 in one device, coil 1, induces an EMF 2 in the other. We express this in equation form as. The larger the mutual inductance M, the more effective the coupling. Nature is symmetric here. If we change the current I2 in coil 2, we induce an emf1 in coil 1, which is given by. Transformers run backward with the same effectiveness, or mutual inductance M.

Conversely, if the current is decreased, an emf is induced that opposes the decrease. The induced emf is related to the physical geometry of the device and the rate of change of current. It is given by. A device that exhibits significant self-inductance is called an inductor. In this Atom we see that they are indeed the same phenomenon, shown in different frame of reference. The current loop is moving into a stationary magnet. The direction of the magnetic field is into the screen.

Current loop is stationary, and the magnet is moving. From Eq. In fact, the equivalence of the two phenomena is what triggered Albert Einstein to examine special relativity. In his seminal paper on special relativity published in , Einstein begins by mentioning the equivalence of the two phenomena:. The observable phenomenon here depends only on the relative motion of the conductor and the magnet, whereas the customary view draws a sharp distinction between the two cases in which either the one or the other of these bodies is in motion.

For if the magnet is in motion and the conductor at rest, there arises in the neighbourhood of the magnet an electric field with a certain definite energy , producing a current at the places where parts of the conductor are situated.

But if the magnet is stationary and the conductor in motion, no electric field arises in the neighbourhood of the magnet. In the conductor, however, we find an electromotive force, to which in itself there is no corresponding energy, but which gives rise—assuming equality of relative motion in the two cases discussed—to electric currents of the same path and intensity as those produced by the electric forces in the former case. Mechanical work done by an external force to produce motional EMF is converted to heat energy; energy is conserved in the process.

Apply the law of conservation of energy to describe the production motional electromotive force with mechanical work. B , l , and v are all perpendicular to each other as shown in the image below. In this atom, we will consider the system from the energy perspective. As the rod moves and carries current i , it will feel the Lorentz force.

To keep the rod moving at a constant speed v , we must constantly apply an external force F ext equal to magnitude of F L and opposite in its direction to the rod along its motion.

Since the rod is moving at v , the power P delivered by the external force would be:. In the final step, we used the first equation we talked about. Therefore, we conclude that the mechanical work done by an external force to keep the rod moving at a constant speed is converted to heat energy in the loop.

More generally, mechanical work done by an external force to produce motional EMF is converted to heat energy. Energy is conserved in the process. If the induced EMF were in the same direction as the change in flux, there would be a positive feedback causing the rod to fly away from the slightest perturbation.

The apparatus used by Faraday to demonstrate that magnetic fields can create currents is illustrated in Figure 1. When the switch is closed, a magnetic field is produced in the coil on the top part of the iron ring and transmitted to the coil on the bottom part of the ring. The galvanometer is used to detect any current induced in the coil on the bottom. It was found that each time the switch is closed, the galvanometer detects a current in one direction in the coil on the bottom.

You can also observe this in a physics lab. Each time the switch is opened, the galvanometer detects a current in the opposite direction. Interestingly, if the switch remains closed or open for any length of time, there is no current through the galvanometer. Closing and opening the switch induces the current. It is the change in magnetic field that creates the current. More basic than the current that flows is the emfthat causes it. 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.

Conceptual Questions 1. Explain how magnetic flux can be zero when the magnetic field is not zero. Figure 5. A circular coil of wire is stretched in a magnetic field. What is the value of the magnetic flux at coil 2 in Figure 6 due to coil 1? What is the value of the magnetic flux through the coil in Figure 6 b due to the wire? As the magnet moves further away from the coil's magnetic field, work is done against the magnetic field and so this work done is converted into electrical energy once again, producing an emf.

I think this is correct but I don't understand why the coil has a magnetic field in the first place. Oh, I think I might understand whilst writing this. As the coil is a conductor, it has a high number density a lot of delocalised electrons and so this when the magnet is close to it, the magnet causes the coil to be magnetised? Then we the magnet moves away, the coil becomes demagnetised and so the coil no longer has a magnetic field?

The coil inherently does not have any magnetic flux. It has a flux linked with it due the presence of the magnet near by. The magnetic field of the magnet gives the coil flux since the coil has a finite area. Apart from that, when the magnet is moved towards the coil, the changing magnetic field creates an electric field which "curls" around the direction of change in magnetic field at the coil and around it.

This electric field is oriented in such a way that the current induced would create a magnetic field that opposes the change in the magnetic field. This begs the question of why the electric field is in this direction to which I don't know the answer.

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