The magnet moves into the coil of wire and the ammeter registers positive current flow. The magnet is stationary within the coil of wire. There is no current flow. The magnet moves out of the coil of wire and the ammeter registers negative current flow.
An induced potential difference or induced current will increase if:. Induced potential and the generator effect. By understanding and using induction, we have an electric power grid and many of the things we plug into it. Electric charge is a fundamental property of matter, according to the Rochester Institute of Technology. Although it is difficult to describe what it actually is, we are quite familiar with how it behaves and interacts with other charges and fields.
The electric field from a localized point charge is relatively simple, according to Serif Uran, a professor of physics at Pittsburg State University. When you move twice as far away, the field strength decreases to one-fourth, and when you move three times farther away, it decreases to one-ninth.
Protons have positive charge, while electrons have negative charge. However, protons are mostly immobilized inside atomic nuclei, so the job of carrying charge from one place to another is handled by electrons. Electrons in a conducting material such as a metal are largely free to move from one atom to another along their conduction bands, which are the highest electron orbits.
A sufficient electromotive force emf , or voltage, produces a charge imbalance that can cause electrons move through a conductor from a region of more negative charge to a region of more positive charge. This movement is what we recognize as an electric current. Compared to the electric field, the magnetic field is more complex. While positive and negative electric charges can exist separately, magnetic poles always come in pairs — one north and one south, according to San Jose State University.
Typically, magnets of all sizes — from sub-atomic particles to industrial-size magnets to planets and stars — are dipoles, meaning they each have two poles. We call these poles north and south after the direction in which compass needles point. Interestingly, since opposite poles attract, and like poles repel, the magnetic north pole of the Earth is actually a south magnetic pole because it attracts the north poles of compass needles. A magnetic field is often depicted as lines of magnetic flux.
In the case of a bar magnet, the flux lines exit from the north pole and curve around to reenter at the south pole.
In this model, the number of flux lines passing through a given surface in space represents the flux density, or the strength of the field. However, it should be noted that this is only a model. A magnetic field is smooth and continuous and does not actually consist of discrete lines.
Therefore, only a small amount of flux passes through a given area, resulting in a relatively weak field. By comparison, the flux from a refrigerator magnet is tiny compared to that of the Earth, but its field strength is many times stronger at close range where its flux lines are much more densely packed. However, the field quickly becomes much weaker as you move away.
If we run an electric current through a wire, it will produce a magnetic field around the wire. The direction of this magnetic field can be determined by the right-hand rule. According to the physics department at Buffalo State University of New York, if you extend your thumb and curl the fingers of your right hand, your thumb points in the positive direction of the current, and your fingers curl in the north direction of the magnetic field.
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. Magnetic field stores energy. Energy is needed to generate a magnetic field both to work against the electric field that a changing magnetic field creates and to change the magnetization of any material within the magnetic field.
For non-dispersive materials this same energy is released when the magnetic field is destroyed. Magnetic Field Created By A Solenoid : Magnetic field created by a solenoid cross-sectional view described using field lines. Energy density is the amount of energy stored in a given system or region of space per unit volume. The above equation cannot be used for nonlinear materials, though; a more general expression given below must be used.
Once the relationship between H and B is known this equation is used to determine the work needed to reach a given magnetic state. For hysteretic materials such as ferromagnets and superconductors, the work needed also depends on how the magnetic field is created.
For linear non-dispersive materials, though, the general equation leads directly to the simpler energy density equation given above. The energy stored by an inductor is equal to the amount of work required to establish the current through the inductor, and therefore the magnetic field. This is given by:. Proof: Power that should be supplied to an inductor with inductance L to run current I through it it given as. Transformers transform voltages from one value to another; its function is governed by the transformer equation.
Transformers change voltages from one value to another. For example, devices such as cell phones, laptops, video games, power tools and small appliances have a transformer built into their plug-in unit that changes V into the proper voltage for the device. Transformers are also used at several points in power distribution systems, as shown in.
Power is sent long distances at high voltages, as less current is required for a given amount of power this means less line loss. Transformer Setup : Transformers change voltages at several points in a power distribution system. Electric power is usually generated at greater than 10 kV, and transmitted long distances at voltages over kV—sometimes as great as kV—to limit energy losses.
Local power distribution to neighborhoods or industries goes through a substation and is sent short distances at voltages ranging from 5 to 13 kV. This is reduced to , , or V for safety at the individual user site. The two coils are called the primary and secondary coils. In normal use, the input voltage is placed on the primary, and the secondary produces the transformed output voltage.
Not only does the iron core trap the magnetic field created by the primary coil, its magnetization increases the field strength. Since the input voltage is AC, a time-varying magnetic flux is sent to the secondary, inducing its AC output voltage.
Simple Transformer : A typical construction of a simple transformer has two coils wound on a ferromagnetic core that is laminated to minimize eddy currents. The magnetic field created by the primary is mostly confined to and increased by the core, which transmits it to the secondary coil. Any change in current in the primary induces a current in the secondary. The figure shows a simple transformer with two coils wound on either sides of a laminated ferromagnetic core.
The set of coil on left side of the core is marked as the primary and there number is given as N p. The voltage across the primary is given by V p. The set of coil on right side of the core is marked as the secondary and there number is represented as N s. The voltage across the secondary is given by V s. A symbol of the transformer is also shown below the diagram. It consists of two inductor coils separated by two equal parallel lines representing the core.
For the simple transformer shown in, the output voltage V s depends almost entirely on the input voltage V p and the ratio of the number of loops in the primary and secondary coils. The input primary voltage V p is also related to changing flux by:.
This is known as the transformer equation , which simply states that the ratio of the secondary to primary voltages in a transformer equals the ratio of the number of loops in their coils. The output voltage of a transformer can be less than, greater than or equal to the input voltage, depending on the ratio of the number of loops in their coils.
Some transformers even provide a variable output by allowing connection to be made at different points on the secondary coil. A step-up transformer is one that increases voltage, whereas a step-down transformer decreases voltage. Assuming, as we have, that resistance is negligible, the electrical power output of a transformer equals its input. Equating the power input and output,. Privacy Policy. Skip to main content.
Search for:. Learning Objectives Explain the relationship between the magnetic field and the electromotive force. Key Takeaways Key Points It is a change in the magnetic field flux that results in an electromotive force or voltage.
It is the integral sum of all of the magnetic field passing through infinitesimal area elements dA. Key Terms vector area : A vector whose magnitude is the area under consideration, and whose direction is perpendicular to the surface area.
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