Tag: physics

Questions Related to 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

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

  1. 50V

  2. 70V

  3. 100V

  4. 35V

Show answer
Correct Option: B
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.

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. 

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

  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.

Show answer
Correct Option: D
Explanation:
Assertion statement is false.
Ohm's can also be applied to a.c circuit.
Reason statement is correct.
Capacitor resistance, $R _C =\dfrac{1}{wC} = \dfrac{1}{(2\pi \nu)C}$

Direct current is observed in :

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

  1. Dry cell

  2. Household Power supply

  3. AC Generator output

  4. None of the above

Show answer
Correct Option: A
Explanation:
Direct current such as the power from dry cells is characterized by a uniform direction of flow and amount of electricity. 

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

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

  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

Show answer
Correct Option: C
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. 

What is not a characteristic of alternating current ?

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

  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

Show answer
Correct Option: C
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.

which is the correct option?

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

  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

Show answer
Correct Option: B
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.

AC is not suitable for electrolysis because

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

  1. It has large current

  2. It has high voltage

  3. Polarity of the current changes

  4. Polarity of the current remains the same

Show answer
Correct Option: A
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.

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

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

  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

Show answer
Correct Option: C
Explanation:

Electrochemical cells produce DC.

AC is generated by a generator.

What is not a characteristic of alternating current?

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

  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

Show answer
Correct Option: C
Explanation:
Characteristics of alternating current :

1. Periodic in nature
2. Time period does not change
3. Average value is zero.

Why is AC preferred over DC for transmission of electrical power

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

  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

Show answer
Correct Option: B
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.