Chp 16: EMI Flashcards
define Faraday’s Law:
Faraday’s Law of electromagnetic induction states that the induced emf is proportional to the rate of change of magnetic flux linkage.
define Magnetic Flux:
The magnetic flux is the product of the magnetic flux density and the area normal to the magnetic flux density.
define Magnetic Flux Linkage:
The magnetic flux linkage of a coil is the product of the magnetic flux passing through the coil and the number of turns on the coil.
define Lenz’s Law:
Lenz’s law states that the direction of the induced emf is such that if an induced current flows, it must flow in such a way to produce an induced magnetic field to oppose the change in the magnetic flux linkage.
define The weber:
The weber is defined as the magnetic flux through an area of 1m2 when the magnetic flux density perpendicular to the area is 1 T.
State Lenz’s Law and explain how it is directly related to the law of conservation of energy.
Lenz’s law states that the direction of the induced current is such that it opposes the change in magnetic flux linkage experienced by the conductor. This implies that external agent has to do positive work done against the opposing induced magnetic force, hence the electrical energy associated with the induced EMF and induced current is derived from the positive work done by external agent.
By reference to the laws of electromagnetic induction, suggest why a transformer will operate for an alternating input voltage but not for a direct voltage.
An alternating input voltage generates an alternating input current which produces an alternating magnetic field hence the secondary coil experiences a changing magnetic flux linkage. By Faraday’s Law, since there is an alternating magnetic flux linkage, there can be and induced EMF. However, in a direct current voltage, the magnetic field produced is constant and there is no change in magnetic flux linkage experienced by the secondary coil. By Faraday’s Law, there is no induced EMF in the secondary coil.
Consider a metal disc oscillating within a uniform magnetic field generated by 2 opposing poles of a magnet. Describe and explain the subsequent motion of the metal disc.
As the metal disc moves in and out of the uniform magnetic field, it experiences a change in magnetic flux linkage, hence by Faraday’s Law, an EMF is induced in the metal disc. Since the metal is a conductor of electricity, it is a closed circuit, there is an induced current. By P = I2R since there is heat energy dissipated in the metal disc, by the conservation of energy the total energy of the oscillation must decrease and is converted to the electrical energy and hence the heat energy dissipated in the metal disc. Since the total energy of the oscillation is proportional to the square of the amplitude, the amplitude of oscillations decreases, the metal disc undergoes a damped oscillation.
By reference to the laws of electromagnetic induction, suggest why the input voltage and the output EMF of a transformer have the same frequency.
NBA proportional to B ,input I, input V. Frequency of NBA = input V.By Faraday’s law, the induced EMF (output voltage) is the rate of change of magnetic flux linkage experience by the secondary coil, the output voltage will have the same frequency as the input voltage.
Using Faraday’s law, explain why an alternating current in the primary coil gives rise to an alternating emf in the secondary coil of a transformer.
An alternating current in the primary coil produces an alternating magnetic field since magnetic flux density is proportional to current. The magnetic field lines hence magnetic flux cut the secondary coil due to the iron core, which causes an alternating magnetic flux linkage experienced by the secondary coil. By Faraday’s law, the induced EMF in the secondary coil is directly proportional to the rate of change of magnetic flux linkage experience by the secondary coil, hence the induced EMF is alternating as well.
Explain the variation of the EMF induced in a rectangular flat coil as rotates in a uniform magnetic field (the standard case, φ = φ0sin wt)
As the coil rotates in the uniform magnetic field, the component of the magnetic field perpendicular to the surface of the rectangular coil varies sinusoidally. Since the magnetic flux linkage = NBA, the magnetic flux linkage varies sinusoidally as well. By Faraday’s Law, there is an induced EMF in the coil and the induced EMF is directly proportional to the rate of change of magnetic flux linkage hence the induced EMF in the coil varies sinusoidally as well.
The diagram shows a vertical iron rod with a wire coil of many turns wrapped around its base. A copper ring slides over the rod and rests on the wire coil. Initially, the switch connecting the coil to a battery is open but when it is closed, the ring flies into the air.
When the switch is closed, a magnetic field is produced in the wire coil and the iron rod. This results in an increase in magnetic flux density. The iron core links the magnetic flux density from coil P to the ring, hence the ring experiences an increase in magnetic flux linkage. According to Faraday’s law, an EMF is induced in the metal ring. Since it is a closed circuit, there is an induced current which will produce its induced magnetic field that will be opposite in direction to the field in the iron rod. This magnetic repulsion propels the ring into the air.
Use Faraday’s Law to explain if the output (secondary coil) and the input (primary coil) potential differences are in phase for a transformer.
Since magnetic flux linkage is proportional to magnetic flux density which is proportional to current also proportional to input potential difference, the magnetic flux linkage cutting through the secondary is in phase with the input pd. According to Faraday’s Law, the induced EMF in the secondary coil (output pd) is directly proportional to the rate of change of magnetic flux linkage experienced by the secondary coil. Therefore, there will be a phase difference between the input pd and the output pd.
