ELECTROMAGNETISM – EN Transformers
ELECTROMAGNETISM – TRANSFORMERS
ELECTROMAGNETISM
Induced Electromotive Force – Transformers
01-(ENEM-MEC)
There are several types of treatments for brain diseases that require stimulating parts of the brain with electrical currents. Electrodes are inserted into the brain to generate small currents in specific areas. To eliminate the need for electrodes to be inserted into the brain, one alternative is to use coils that, placed outside the head, are capable of inducing electrical currents in brain tissue.
For the treatment of brain pathologies with coils to be carried out satisfactorily, it is necessary that
a) there is a large number of turns in the coils, which reduces the induced voltage.
b) the magnetic field created by the coils is constant, so that there is electromagnetic induction.
c) it is observed that the intensity of the induced currents depends on the intensity of the current in the coils.
d) the current in the coils is continuous, so that the magnetic field can be of great intensity.
e) the magnetic field directs the electric current from the coils into the patient’s brain.
Exercises with ENEM characteristics
02-(UNIFESP-SP)
The photo shows a battery-free flashlight that was recently launched on the market. It works by transforming the kinetic energy supplied to it by the user into electrical energy – to do this, the user must shake it vigorously in the direction of its length. Since the inside of this flashlight is visible, one can see how it works: by shaking it, the user makes a cylindrical magnet pass through a coil back and forth. The movement of the magnet through the coil causes an induced current to appear in it, which flows through and lights the lamp.
The physical principle on which this flashlight is based and the current induced in the coil are, respectively:
a) electromagnetic induction; alternating current.
b) electromagnetic induction; direct current.
c) Coulomb’s law; direct current.
d) Coulomb’s law; alternating current.
e) Ampère’s law; alternating or direct currents can be induced.
03-(UFRN)
At the end of his work day, Pedro Pedreiro calmly faces the darkness of the roads on his bicycle because, in order to travel at night more safely, he installed a dynamo on his bicycle that powers a 12V bulb. In a bicycle dynamo, the fixed part (stator) is made up of coils (coils), where the electric current is generated, and a moving part (rotor), where there is a permanent magnet, which rotates due to the contact of the rotor shaft with the bicycle tire. Based on the description above and with the help of knowledge of Physics, it can be stated that:
a) the energy per unit time emitted by the lamp shown in figure I does not depend on the speed of the bicycle.
b) at the instant represented in figure II, the correct direction of the induced electric current is from point Q to point P.
c) the conversion of mechanical energy into electrical energy occurs due to the temporal variation of the magnetic flux in the coils (figure II).
d) the angular speed of the rotor (figure II) must be equal to the angular speed of the bicycle tire (figure I) for the lamp to work.
04-(FUVEST-SP)
An aluminum ring, suspended by an insulating wire, oscillates between the poles of a magnet, initially remaining in the plane perpendicular to the N-S axis and equidistant from the polar faces. The ring oscillates, entering and leaving the region between the poles, with a certain amplitude.
Under these conditions, without taking into account air resistance and other forms of mechanical friction, it can be stated that, over time,
a) the amplitude of the ring’s oscillation decreases.
b) the amplitude of the ring’s oscillation increases.
c) the amplitude of the ring’s oscillation remains constant.
d) the ring is attracted to the North pole of the magnet and remains there.
e) the ring is attracted to the south pole of the magnet and remains there.
05-(Fazu-MG)
An airplane, flying at an altitude of 10,000 m, suffers a breakdown and crashes, cutting the Earth’s magnetic field lines.
Under these conditions, it is possible to measure, between the ends of the airplane’s wings:
a) an electric current
b) a magnetic field
c) a temperature difference
d) a mechanical compression
e) a potential difference
06-(UFSM-RS)
Electrical energy is transmitted at high voltages, but in homes, sockets provide low voltages.
Transformers are electromagnetic devices that, based on the law of ____________, change the value of electrical ____________ ____________.
Select the alternative that completes the gaps.
a) Faraday – voltage – alternating
b) Faraday – voltage – continuous
c) Ampere – voltage – alternating
d) Ampere – force – alternating
e) Ampere – force – continuous
07- (UFSM-RS)
Electric power generating plants produce _____ which allows, through a transformer, to increase the _____ and, thus, decrease the ______, in order to reduce energy losses due to the Joule effect in transmission lines.
Select the alternative that correctly fills in the gaps.
a) voltage – electric current – voltage
b) direct current – electric current – voltage
c) alternating current – voltage – electric current
d) direct current – voltage – electric current
e) alternating current – electric current – voltage
08- (UFSCAR-SP)
At the end of the 19th century, a technological dispute over which electric current was most suitable for the transmission and distribution of electrical energy generated in power plants made it clear that alternating current was more advantageous than direct current. One of the decisive factors in this choice was the possibility of using transformers in the electricity distribution network.
Transformers can increase or decrease the voltage supplied to them, allowing the adjustment of the values of the intensity of the transmitted current and reducing losses due to the Joule effect, BUT THEY ONLY WORK ON ALTERNATING CURRENT. The physical principle on which the operation of transformers is based and the characteristic of the alternating current that satisfies this principle are, respectively,
a) conservation of charge and the oscillating motion of electric charge carriers.
b) electrostatic induction and the continuous movement of electric charge carriers.
c) electrostatic induction and the oscillating movement of electric charge carriers.
d) electromagnetic induction and the continuous movement of electric charge carriers.
e) electromagnetic induction and the oscillating movement of electric charge carriers.
09- (UFRN-RN)
The residential landline telephone line is powered by direct current (DC) with a voltage of 45V and operates independently of the conventional electrical network, which is alternating current (AC) and has a voltage of 220V. Due to a frequent power outage on the conventional line in his home, Joãozinho, a high school student
medium, thought of making a voltage step-up transformer, to use in the lamp on his study table. His idea is to take power from the telephone socket (which is prohibited by law) and use it in an emergency situation.
It can be said that Joãozinho’s objective
a) will be achieved, but, according to Faraday’s law, the luminaire’s efficiency will drop slightly compared to that obtained when the luminaire is connected to the conventional network.
b) will only be achieved if the telephone line has a voltage of at least 110 V.
c) will not be achieved, due to the impossibility of increasing direct voltage to alternating voltage with just a transformer.
d) will not be achieved, because voltage can only be lowered, not raised.
10-(UFRGS-RS)
Select the alternative that correctly fills in the gaps in the text below.
Materials with special magnetic properties play a very important role in modern technology. Among their many applications, we can mention magnetic recording and reading, used in magnetic tapes and computer disks.
The basic idea behind magnetic reading is the following: variations in the intensity of fields ………., produced by the tape or disk in motion, induce ………. in a coil in the reading head, giving rise to signals that are then amplified.
a) magnetic – magnetization
b) magnetic – electric currents
c) electric – electric currents
d) electrical – magnetization
e) electric – electric charges
To drain the electrical energy produced by its turbines, the Itaipu hydroelectric plant raises the output voltage to approximately 700,000 V. In your home, the sockets have a voltage of 127 V and/or 220 V.
The equipment that performs this task of raising and lowering the voltage is the transformer. It is correct to state that
a) the operating principle of a transformer requires that the voltage/current be continuous.
b) the operating principle of a transformer requires that the voltage/current be alternating.
c) the transformer will work both in a network with alternating voltage/current and in one with direct voltage/current.
d) the transformer will work when, in the primary winding, there is a direct voltage/current and, in the secondary, an alternating voltage/current.