Tag: physics

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Questions Related to physics

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."

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

The transformer is used in ................. current circuits.

  1. alternating

  2. directing

  3. no

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

A transformer consists of two coils - the primary coil and the secondary coil both wound on a soft iron core. If an ac voltage is applied to the primary coil then this will produce a changing magnetic field in the iron core. This induces an alternating voltage in the secondary coil according to electromagnetic induction.
A voltage in the secondary coil occurs only if the magnetic field is changing. Therefore, transformers will only work with ac current only. The transformer is used in alternating current circuits.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Name the material of core in electric bell, electromagnet, d.c. motor, a.c. generator and transformer.

  1. soft iron

  2. hard iron

  3. soft steel

  4. hard steel
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A magnetic core is a piece of magnetic material with a high permeability used to confine and guide magnetic fields in electrical, electro-mechanical and magnetic devices such as electromagnets, transformers, electric motors, generators, inductors, magnetic recording heads, and magnetic assemblies. It is made of ferromagnetic metal such as iron, or ferrimagnetic compounds such as ferrites.
The magnetic field is often created by a coil of wire around the core that carries a current. The presence of the core can increase the magnetic field of a coil by a factor of several thousand over what it would be without the core. The use of a magnetic core can enormously concentrate the strength and increase the effect of magnetic fields produced by electric currents and permanent magnets. 
The properties of a device will depend on the following factors:

  • The geometry of the magnetic core.
  • The amount of air gap in the magnetic circuit.
  • The properties of the core material like permeability and hysteresis.
  • The operating temperature of the core.
  • Whether the core is laminated to reduce eddy currents.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Transformers are used

  1. in DC circuit 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

Transformers work on the principle of voltage induction by the linkage of changing magnetic flux between between the two cores. Changing magnetic flux requires changing/alternating current. 
Transformers are used only in AC circuits. In DC circuits, no alternating/changing magnetic flux is produced to induce voltage in secondary from the primary terminals of the transformers.