Tag: electro-magnetism

Quizzes (12) Questions (113)

Questions Related to electro-magnetism

Multiple choice ac and dc ac vs dc electro-magnetism effects of electric current physics

What is not a characteristic of alternating current?

  1. Periodic in nature

  2. Time period does not change

  3. Maximum positive amplitude is more than maximum negative amplitude

  4. Average value is 0
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Characteristics of alternating current :

1. Periodic in nature
2. Time period does not change
3. Average value is zero.
Multiple choice ac and dc ac vs dc electro-magnetism effects of electric current physics

Why is AC preferred over DC for transmission of electrical power

  1. AC to DC conversion and vice versa is very easy

  2. Transmission losses in AC are lower

  3. AC transmission is unaffected by random phase change

  4. Frequency of AC do not change
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Transmission losses are given as $I^2R$. In AC, voltage transformation to higher(and also lower) values is easier at constant power. So, voltage is increased before transmission reducing current and hence reducing transmission losses.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

In the transformer phase difference between input and output is

  1. $0$

  2. $\dfrac {\pi}{2}$

  3. $\dfrac {\pi}{6}$

  4. $\pi$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

In an ideal transformer, the primary and secondary voltages are in phase if the secondary is open-circuited, but they are in anti-phase (180 degrees or pi radians) due to Lenz's law when considering the induced EMF relative to the primary input.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Transformer are used

  1. in $DC$ circuits only

  2. in $AC$ circuits only

  3. in both $DC$ and $AC$ circuits

  4. neither in $DC$ nor in $AC$ circuits.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Transformer is an electrical device that requires time varying current to link two or more electrical coils through electromagnetic induction and thus transfer energy between the magnetically coupled circuits. Therefore transformer are used only in AC circuits

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Why does the light of a car dim when the starter is operated?

  1. The starter motor draws a large current resulting in a large potential drop in the battery, which reduced the thermal voltage of the battery.

  2. When starter motor is switched on, it increases the resistance of the filament of the bulb in the car light.

  3. When the starter motor is switched on, it reduces the emf of the battery.

  4. The starter motor produces fumes, which absorb a part of the light of the car lamp.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Since, the starter motor draws a large current resulting in a large potential drop in the battery, which reduced the thermal voltage of the battery hence, the light of a car are dim when the starter is operated.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Why is the iron core of a transformer  laminated (thin sheet) instead of being in one solid piece?

  1. To increase the energy losses due to eddy current

  2. To reduce the energy losses due to eddy current

  3. No reason

  4. None of the above
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The iron core of a transformer is made of Laminated silicon steel with special insulated coating to limit Eddy current loss. Electrical steel, also called lamination steel is specialty steel tailored to produce certain magnetic properties, such as a small hysteresis area (small energy dissipation per cycle, or low core loss) and high permeability.
An eddy current is a swirling current set up in a conductor in response to a changing magnetic field. By Lenz's law, the current swirls in such a way as to create a magnetic field opposing the change. To do this in a conductor, electrons swirl in a plane perpendicular to the magnetic field. Because of the tendency of eddy currents to oppose, eddy currents cause energy to be lost. More accurately, eddy currents transform more useful forms of energy, such as kinetic energy, into heat, which is generally much less useful. 

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Name the device used to transform 12 V a.c. to 200 V a.c. Name the principle on which it works.

  1. Step down transformer, magnetic induction

  2. Step down transformer, electromagnetic induction

  3. Step up transformer, magnetic induction

  4. Step up transformer, electromagnetic induction
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Transformers are found everywhere alternating current is used. This includes both large power stations to the power cord for our laptop computer. A transformer is an electrical device that trades voltage for current in a circuit, while not affecting the total electrical power. This means it takes high-voltage electricity with a small current and changes it into low-voltage electricity with a large current, or vice versa. That is, step up or step down the voltage in a circuit.
A step-up transformer turns low-voltage electricity into high-voltage electricity while dropping the current. A step-down transformer changes high-voltage electricity into low-voltage electricity.
Therefore, to convert 12V a.c. to 200V a.c., a step up transformer is used.
This transformer works in the principle of Electromagnetic induction which states that rate of change of flux linkage with respect to time is directly proportional to the induced EMF in a conductor or coil.
Say we have one winding which is supplied by an alternating electrical source. The alternating current through the winding produces a continually changing flux or alternating flux that surrounds the winding. If any other winding is brought nearer to the previous one, obviously some portion of this flux will link with the second. As this flux is continually changing in its amplitude and direction, there must be a change in flux linkage in the second winding or coil. According to Faraday's law of electromagnetic induction, there must be an EMF induced in the second. If the circuit of the later winding is closed, there must be an current flowing through it. This is the simplest form of electrical power transformer and this is the most basic of working principle of transformer.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A step-up transformer increases the AC voltage. The statement is 

  1. true

  2. false

  3. ambiguous

  4. data insufficient 
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A step up transformer increases the voltage This transformer has more windings on the "secondary" side of the transformer, with fewer windings of thicker wire on the primary or input coil. 

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

State whether true or false:

Transformers can change high voltage to low voltage.

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A transformer takes in electricity at a higher voltage and lets it run through lots of coils wounded around an iron core. Because the current is alternating, the magnetism in the core is also alternating. Also around the core, there is an output wire with fewer coils. The magnetism changing back and forth creates a current in the wire. Having fewer coils means less voltage. So the voltage is "stepped-down."