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. It is measured in units of volts, not newtons, and thus, is not actually a force. Learning Objectives Identify process that induces motional electromotive force. That a moving magnetic field produces an electric field and conversely that a moving electric field produces a magnetic field is part of the reason electric and magnetic forces are now considered as different manifestations of the same force.
Any change in magnetic flux induces an electromotive force EMF opposing that change—a process known as induction. Learning Objectives Explain the relationship between the motional electromotive force, eddy currents, and magnetic damping.
If motional EMF can cause a current loop in the conductor, the current is called an eddy current. Learning Objectives Describe the relationship between the changing magnetic field and an electric field. Key Terms vector area : A vector whose magnitude is the area under consideration and whose direction is perpendicular to the plane. Electric Generators Electric generators convert mechanical energy to electrical energy; they induce an EMF by rotating a coil in a magnetic field.
Learning Objectives Explain how an electromotive force is induced in electric generators. A motor becomes a generator when its shaft rotates. Electric Motors An electric motor is a device that converts electrical energy into mechanical energy. Learning Objectives Explain how force is generated into electric motors. Key Takeaways Key Points Most electric motors use the interaction of magnetic fields and current -carrying conductors to generate force. Key Terms Lorentz force : The force exerted on a charged particle in an electromagnetic field.
Mechanics of a Motor Both motors and generators can be explained in terms of a coil that rotates in a magnetic field. Inductance Inductance is the property of a device that tells how effectively it induces an emf in another device or on itself.
Learning Objectives Describe properties of an inductor, distinguishing mutual inductance and self-inductance. Key Takeaways Key Points Mutual inductance is the effect of two devices in inducing emfs in each other. Self-inductance is the effect of the device inducing emf in itself. Their main use is to transfer energy between different voltage levels, which allows choosing most appropriate voltage for power generation, transmission and distribution separately.
Learning Objectives Formulate two views that are applied to calculate the electromotive force. Equivalence of the two phenomena is what triggered Einstein to work on special relativity. The EMF can be calculated from two different points of view: 1 in terms of the magnetic force on moving electrons in a magnetic field, and 2 in terms of the rate of change in magnetic flux.
Both yield the same result. Key Terms special relativity : A theory that neglecting the effects of gravity reconciles the principle of relativity with the observation that the speed of light is constant in all frames of reference. Mechanical Work and Electrical Energy Mechanical work done by an external force to produce motional EMF is converted to heat energy; energy is conserved in the process.
If we then push a bar magnet through the loop, the needle in the galvanometer will move, indicating an induced current. However, once we stop the motion of the magnet, the current returns to zero. The field from the magnet will only induce a current when it is increasing or decreasing.
If we pull the magnet back out, it will again induce a current in the wire, but this time it will be in the opposite direction. If we were to put a light bulb in the circuit, it would dissipate electrical energy in the form of light and heat, and we would feel resistance to the motion of the magnet as we moved it in and out of the loop. In order to move the magnet, we have to do work that is equivalent to the energy being used by the light bulb.
In yet another experiment, we might construct two wire loops, connect the ends of one to a battery with a switch, and connect the ends of the other loop to a galvanometer. If we place the two loops close to each other in a face-to-face orientation, and we turn on the power to the first loop, the galvanometer connected to the second loop will indicate an induced current and then quickly return to zero.
What is happening here is that the current in the first loop produces a magnetic field, which in turn induces a current in the second loop, but only for an instant when the magnetic field is changing. When you turn off the switch, the meter will deflect momentarily in the opposite direction. This is further indication that it is the change in the intensity of the magnetic field, and not its strength or motion that induces the current. The explanation for this is that a magnetic field causes electrons in a conductor to move.
This motion is what we know as electric current. Eventually, though, the electrons reach a point where they are in equilibrium with the field, at which point they will stop moving. Then when the field is removed or turned off, the electrons will flow back to their original location, producing a current in the opposite direction.
Unlike a gravitational or electric field, a magnetic dipole field is a more complex 3-dimensional structure that varies in strength and direction according to the location where it is measured, so it requires calculus to describe it fully. It states that the induced voltage in a circuit is proportional to the rate of change over time of the magnetic flux through that circuit. In other words, the faster the magnetic field changes, the greater will be the voltage in the circuit.
The direction of the change in the magnetic field determines the direction of the current. We can increase the voltage by increasing the number of loops in the circuit.
No matter how the change is produced, the voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc. Faraday's law is a fundamental relationship which comes from Maxwell's equations. It serves as a succinct summary of the ways a voltage or emf may be generated by a changing magnetic environment.
The induced emf in a coil is equal to the negative of the rate of change of magnetic flux times the number of turns in the coil. Visualize the magnetic field of a bar magnet. How does the flux of this field through the wire loop change? The induced current produces a magnetic field, which opposes the change in the magnetic flux.
The magnitude of the induced emf can be calculated using Faraday's law. The induced emf is proportional to the rate of change of the current in the coil. It can be several times the power supply voltage.
When a switch in a circuit carrying a large current is opened, reducing the current to zero in a very short time interval, this can result in a spark. The self inductance L depends only on the geometry of the circuit. A coil has an self inductance of 3 mH, and a current through it changes from 0. Find the magnitude of the average induced emf in the coil during this time. A 25 turn circular coil of wire has a diameter of 1 m.
It is placed with its axis along the direction of the Earth's magnetic field magnitude 50 microT , and then, in 0. What is the average emf generated.
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