Tag: physics

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Questions Related to physics

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

A neon lamp is connected to a voltage a.c. source. The voltage is gradually increased from zero volt. It is observed that the neon flashes at 50 V. The a.c. source is now replaced by a variable dc source and the experiment is repeated. The neon bulb will flash at

  1. 50V

  2. 70V

  3. 100V

  4. 35V
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Answer is B.

The RMS (Root mean square) value tells us what equivalent DC voltage we would need to get the same power, for the neon bulb to glow.
In this case, the neon bulb glows at 50 V ac voltage. Therefore, for the bulb glow with dc voltage, $50\times \sqrt { 2 } \quad =\quad 50\times 1.414\quad =\quad 70\quad V$ dc voltage should be applied.
Hence, the neon bulb will flash at 70 V dc.

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

Assertion: Ohm's law cannot be applied to a.c circuit. 
Reason: Resistance offered by capacitor for a.c source depends upon the frequency of the source. 

  1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

  2. If both Assertion and Reason are true but the Reason is not the correct explanation of the Assertion.

  3. If Assertion is true statement but Reason is false.

  4. If both Assertion and Reason are false statements.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation
Assertion statement is false.
Ohm's can also be applied to a.c circuit.
Reason statement is correct.
Capacitor resistance, $R _C =\dfrac{1}{wC} = \dfrac{1}{(2\pi \nu)C}$
Multiple choice ac and dc ac vs dc electro-magnetism effects of electric current physics

A low frequency AC current is passed through a galvanometer. What will be the behavior of the pointer needle?

  1. The needle remains stationary at zero reading

  2. The needle points to the rms value of ac

  3. The needle fluctuates about zero reading with a frequency equal to current frequency

  4. The needle fluctuates about zero reading with a frequency half of current frequency
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Needle of galvanometer shows magnitude and direction of current through it. In AC, both are continuously changing periodically and this behavior is observed in the needle. 

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 zero
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Alternating currents are:

1. Periodic with a constant time period.
2. Have average value 0.
3. Magnitude of positive part of current is same as magnitude of negative part of current. Hence option C is correct.

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

which is the correct option?

  1. Alternating current always flows in one direction but the direct current reverses its direction periodically

  2. Direct current always flows in one direction but the alternating current reverses its direction periodically

  3. Direct and alternating current always flows in one direction

  4. above all
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Direct current never changes its direction it  have magnitude only but the alternating current is of the form: $I _0Sin \omega t$ which have magnitude and as well as direction.

So that it changes sign alternatively with time.

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

AC is not suitable for electrolysis because

  1. It has large current

  2. It has high voltage

  3. Polarity of the current changes

  4. Polarity of the current remains the same
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

In electrolysis the ionization of the electrolyte takes place where the positive and negative charges move towards opposite electrodes. For this the electrodes are required to be maintained at opposite polarity as long as the process continuous which is possible while supplying a dc current. But if we pass an ac current the polarity of the electrodes will be continuously changing and the ions will not be attracted towards any particular electrode resulting in no ionization. Hence electrolysis cannot take place with ac current.

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

Which of the statements is false?
AC: Alternating current
DC: Direct Current 

  1. AC generation is done by a generator

  2. DC generation is done by electrochemical cells

  3. AC generation can be done by electrochemical cells

  4. None
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Electrochemical cells produce DC.

AC is generated by a generator.

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.