Difference Between AC and DC Important Questions

Difference between AC and DC depends on the direction where the electron flows. Alternating Current (AC) can be defined as a form of electric current that moves both forward and backward direction. It also sometimes changes direction as well. Direct Current (DC) can be defined as an electric current that moves in one direction alone. 

Discover about the Chapter video:

Current Electricity Detailed Video Explanation:

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Very Short Answer Questions [1 Mark Questions]

Ques. The instantaneous current and voltage of an AC circuit are mentioned as, i = 10 sin 300 tA and V = 200 sin 300 tV. Find power dissipation in the circuit. (All India 2008)

Ans. As given by the formula, P = I_rmsV_rmscos\(\phi\)

Since, cos\(\phi\) = 1 (\(\phi\) = 0°)

Thus, P = ½ I₀V₀

Hence, P = ½ \(\times\) 10 \(\times\)200 = 1000 W

Ques. Define watt less current?

Ans. Wattless current can be expressed as AC circuit’s current wherein the average power consumption is zero.

Ques. Instantaneous current and voltage of an AC circuit are mentioned as, i = 10 sin 314 tA and v = 50 sin 314 tV. Determine power dissipation in the circuit? (All India 2008)

Ans. P = ½ I₀V_{0 =} \(\frac{10 \times 50}{2}\)

= 250 W

Ques. Determine value of power factor in the circuit when it is at resonance.

Ans. Value of the power factor in the circuit when at resonance is going to be 1.

Ques. Justify why the core of a transformer is laminated? (Comptt. Delhi 2013)

Ans. The reason why the core of a transformer is laminated is to minimize eddy currents in the iron core.

Ques. What is capacitor reactance? Write its S.I. units. (Delhi 2015)
Ans. ‘Capacitor reactance’ can be expressed as the opposition to current flow in an AC circuit offered by a capacitor.

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Short Answer Questions [2 Marks Questions]

Ques. Why is AC voltage more widely used than DC voltage? Give two reasons. (All India 2013)

Ans. AC voltage is more widely used than DC voltage because of the following reasons :

1. AC voltage, unlike a DC voltage, can be stepped-up or stepped-down by a transformer.
2. The carrying losses in AC voltage is lesser than that of a DC voltage.

Ques. Justify the loss in energy in a transformer.

Ans.The reasons why there is a loss in energy in the transformer are:

• Hysteresis
• Loss of Eddy current
• Leakage of flux

Ques. Find the common base DC current gain.

Ans. The common base of current gain are:

1. Common Emitter current gain is often denoted by the greek letter, β.
2. Current gain can be defined as the relationship between output current and input current.

Ques. What is the ‘quality factor’ of resonance in a series LCR circuit? Also mention its SI unit. (Delhi 2016)

Ans. Quality factor (Q) can be expressed as, Q = ω₀LR
It offers the sharpness to resonance circuit and also has no SI unit.

Ques. List two properties of the material suitable for constructing core of a transformer. (All India 2012)

Ans. Some properties of material suitable for the core of a transformer includes:

• It must have high permeability.
• It must have low hysteresis loss.

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Long Answer Questions [3 Marks Questions]

Ques. The instantaneous current and voltage of an AC circuit are both expressed as, i = 10 sin 314 tA and v = 50 sin (314t+\(\frac{\pi}{2}\))V. Determine power disspation. (All India 2008)

Ans. As given by the formula, P = I_rmsV_rmscos\(\phi\)

Hence, \(\frac{\pi}{2}, \phi = \frac{\pi}{2}\)

Thus, 

P = I_rmsV_rmscos\(\frac{\pi}{2} \therefore \ P = 0 \quad [\because cos \frac{\pi}{2} = 0]\)

Ques. When AC source is joined across an ideal inductor, plot a graph to show the nature of voltage variation and the current over one complete cycle. (Comptt. Delhi 2012)

Ans. The graph to show the nature of voltage variation and current over one cycle is:

Graph of Voltage Variation and Current

Ques. One heating element is labelled 210 V, 630 W. Find the resistance of the element when attached to a 210 V DC source.​

Ans. The resistance of the element when connected to 210 V DC source: 

Using the formula,

Resistace = V²/P

(Once after replacing the values)

=  \(\frac{(210)^2}{630}\)

= \(\frac{210 \times 210}{630}\)

= 70 \(\Omega\)

Ques. The image demonstrates an arrangement where the flow of current via the bulb (X) connected with coil B, when a.c. is passed through coil A.

1. Name the involved phenomenon.
2. If one copper sheet is inserted in the gap between the coils, then elaborate upon how the brightness of the bulb would vary. (All India 2008)

Ans. (a) The phenomenon that is involved is called mutual induction.

(b) When a copper sheet is inserted, the eddy currents are established in it that opposes magnetic flux’s passage. Thus, the induced emf in the coil B decreases, causing the brightness of the bulb decreases.

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Very Long Questions [5 Marks Questions]

Ques. Show the energy losses in a transformer. (All India 2009)

Ans. A transformer can be defined as an electrical device that  is known to help in the conversion of alternating current at low voltages into high voltage or vice versa.
Assuming that it increases the input voltage, it is known as step-up transformer.

Principle: The above image works on the very principle of mutual induction, which is, when a changing current passes via one of the two inductively coupled coils, an induced emf is established in the other coil.

Working: As alternating current passes through the primary, it develops an alternating magnetic flux in the core that further moves via the secondary. This changing flux usually forms an induced emf in the secondary, alonsgide a self-induced emf in the primary. Assuming that there is no leakage of magnetic flux, the flux that is attached with each turn of the primary will be equivalent to the linked turns of the secondary.

Here,

• N_p and N_s = Number of turns (primary and secondary respectively)
• V_p and V_s =  Voltages

Therefore, Input power = output power
\(\because\) V_pl_p = V_SI_S

Ques. Derive an expression to determine the impedance of an AC circuit which contains an inductor and a resistor. (Delhi 2008)

Ans. We can get R,

Thus,

Therefore, the dissipated average power per cycle in a capacitor is zero.

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