Why is magnetic flux important

Glossary Definition for Figure 1: A flat loop of area A is in a region where the magnetic field is B shown with red field lines in a.

The component of B directed normally through the area is shown by the vertical dotted line in b. Magnetic flux is an important quantity that allows us to calculate the emf generated in a coil of wire when the flux through the coil changes, as happens in a dynamo or certain types of microphone.

The mechanism for the former typically involves the rotation of a magnet around a stationary coil of wire; the moving magnet creates a time-varying magnetic flux through the coil and hence generates an emf. Hence the magnetic flux through this coil is around 0.

For 33 Resources. The field around a permanent magnet should be familiar to your students. Conducting Plate Passing Between the Poles of a Magnet : A more detailed look at the conducting plate passing between the poles of a magnet.

As it enters and leaves the field, the change in flux produces an eddy current. Magnetic force on the current loop opposes the motion. There is no current and no magnetic drag when the plate is completely inside the uniform field. When a slotted metal plate enters the field, as shown in, an EMF is induced by the change in flux, but it is less effective because the slots limit the size of the current loops. Moreover, adjacent loops have currents in opposite directions, and their effects cancel.

When an insulating material is used, the eddy current is extremely small, and so magnetic damping on insulators is negligible. If eddy currents are to be avoided in conductors, then they can be slotted or constructed of thin layers of conducting material separated by insulating sheets. Eddy Currents Induced in a Slotted Metal Plate : Eddy currents induced in a slotted metal plate entering a magnetic field form small loops, and the forces on them tend to cancel, thereby making magnetic drag almost zero.

We learned the relationship between induced electromotive force EMF and magnetic flux. The number of turns of coil is included can be incorporated in the magnetic flux, so the factor is optional. In this Atom, we will learn about an alternative mathematical expression of the law. 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. 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.