Electricity & Megnetism

Electromagnetic induction

Electromagnetic induction occurs when the variation of a magnetic field in the vicinity of a conductor causes the appearance of an induced electromotive force . It is described qualitatively and mathematically by the Faraday-Lenz law . This phenomenon explains the emergence of the electromotive force induced in conductive coils inserted in regions endowed with a variable magnetic field , which may be this variation in modulus , direction or direction.

According to the Faraday-Lenz law , when a coil is inserted into a region of magnetic field, as when placed between the magnetic poles of a magnet, it is continuously traversed by a certain number of induction lines, also known as lines of induction. magnetic field . If the “ number” of induction lines changes, an electric current will appear in the coil, giving rise to a magnetic field that opposes the change in the number of lines that cross this coil.

magnetic flux

The amount of magnetic field lines (also known as induction lines) that cross the area of ​​a conductor is called the magnetic flux . It is a scalar quantity , measured in weber (Wb), which depends on factors such as the strength of the magnetic field, the area of ​​the conductor and the angle formed between the magnetic field lines and the direction normal to the area of the driver.

The formula used to calculate the magnetic flux is as follows:

Φ – magnetic flux (Wb or T/s)

B – magnetic field (T)

A – conductor area (m²)

θ – angle formed between the magnetic field and the direction normal to area A (º)

Also read: Know the risks we run when charging the cell phone battery

Induced electromotive force ( emf )

The induced electromotive force emf ) is actually a dynamic electric potential. This electric potential, measured in volts (V), arises to compensate for the variation of the magnetic field flux that passes through a conductor. Therefore, in conductors, the emergence of emf is accompanied by the establishment of an induced electric current.

The induced electromotive force is equal to the change in magnetic flux with respect to a certain time interval. Watch:

ε – induced electromotive force (V)

Δ Φ = Φ F – Φ i – variation of magnetic flux (Wb)

Direction of induced electric current

The direction of the induced electric current in a loop depends on the variation of the magnetic flux over it. If the north magnetic pole of a magnet is pointed towards the loop and that magnet approaches, for example, the loop will produce a north magnetic field to oppose such variation. In this case, the electric current will have its direction of circulation determined by the right hand rule: we point the thumb in the direction of the magnetic field and then close the fingers of the right hand . The direction of the closing of the fingers indicates the direction of the electric current. The following figure illustrates the described situation:

Exercises on electromagnetic induction

Question 1 – (UPF) Electromagnetic induction is a phenomenon that is present in various equipment that we use daily. It is used to generate electrical energy and its physical principle consists in the appearance of an electromotive force between the ends of a conducting wire. For this electromotive force to arise, there must be a change in

a) electric field.

b) electrical resistance.

c) electrical capacitance.

d) temperature.

e) magnetic flux.

Template: letter E. According to Faraday’s law, for an electromotive force to arise, a change in the magnetic field flux must occur.

Question 2 – (Enem) RFID tag communication technology (called smart tag) has been used for years to track cattle, train cars, air luggage and cars at tolls. The cheaper model of these tags can run without batteries and is made up of three components: a silicon microprocessor; a metal coil, made of copper or aluminum, which is wound in a circular pattern; and an encapsulator, which is a glass or polymer material surrounding the microprocessor and coil. In the presence of an RF field generated by the reader, the tag transmits signals. The reading distance is determined by the size of the coil and the power of the radio wave emitted by the reader. Available at: https://eleletronicos.hsw.uol.com.br. Accessed on: 27 Feb.2012 (adapted).

The tag works without batteries because the field

a) Electricity of the radio wave stirs electrons from the coil.

b) Electricity of the radio wave creates a voltage in the coil.

c) Magnetic radio wave induces current in the coil.

d) Magnetic radio wave heats the coil wires.

e) Magnetic radio wave reduces the resonance inside the coil.

Template: letter C. The variable magnetic field that hits the coil induces the emergence of an electromotive force, which, in turn, favors the establishment of an electric current.

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