Physics

Alternating Current

Question:

Explain why the reactance offered by an inductor increases with increasing frequency of an alternating voltage.

Answer:

The inductive reactance is given by XL = 2Ï€fL, XL is proportional to the frequency and current is inversely proportional to the reactance. An inductor opposes the flow of current through it by developing a back emf according to Lenz's law. If the current is decreasing, the polarity of the induced emf will be so as to increase the current and vice-versa.
Since, the induced emf is proportional to the rate of change of current.

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Alternating Current

Q 2.

Both alternating current and direct current are measured in amperes. But how is the ampere defined for an alternating current?

Q 5.

Explain why the reactance provided by a capacitor to an alternating current decreases with increasing frequency.

Q 6.

Explain why the reactance offered by an inductor increases with increasing frequency of an alternating voltage.

Q 7.

Study the circuits (a) and (b) shown in figure and answer the following questions.
ncert-exemplar-problems-class-12-physics-alternating-current-29
(a) Under which conditions would the rms currents in the two circuits be the same?
(b) Can the rms current in circuit (b) be larger than that in (a)?

Q 8.

A 60 W load is connected to the secondary of a transformer whose primary draws line voltage. If a current of 0.54 A flows in the load, what is the current in the primary coil? Comment on the type of transformer being used.

Q 9.

Draw the effective equivalent circuit of the circuit shown in figure, at very high frequencies and find the effective impedance.
ncert-exemplar-problems-class-12-physics-alternating-current-26

Q 10.

For an L-C-R circuit, the power transferred from the driving source to the driven oscillator is P = I2 Z cos Ф.
(a) Here, the power factor cos Ф > 0, P > 0
(b) The driving force can give no energy to the oscillator (P = 0) in some cases
(c) The driving force cannot syphon out (P < 0) the energy out of oscillator
(d) The driving force can take away energy out of the oscillator

Q 11.

In a series LCR circuit, the plot of Imax versus co is shown in figure. Find the bandwidth and mark in the figure.
ncert-exemplar-problems-class-12-physics-alternating-current-35

Q 12.

Consider the L-C-R circuit shown in figure. Find the net current i and the phase of i. Show that i = V/Z. Find the-impedance Z for this circuit.
ncert-exemplar-problems-class-12-physics-alternating-current-51

Q 13.

ncert-exemplar-problems-class-12-physics-alternating-current-45

Q 14.

ncert-exemplar-problems-class-12-physics-alternating-current-59

Q 15.

When an AC voltage of 220 V is applied to the capacitor C
(a) the maximum voltage between plates is 220 V
(b) the current is in phase with the applied voltage
(c) the charge on the plates is in phase with the applied voltage
(d) the power delivered to the capacitor is zero

Q 16.

The output of a step-down transformer is measured to be 24 V when connected to a 12 W light bulb. The value of the peak current is
ncert-exemplar-problems-class-12-physics-alternating-current-12

Q 17.

Electrical energy is transmitted over large distances at high alternating voltages. Which of the following statements is (are) correct?
(a) For a given power level, there is a lower current
(b) Lower current implies less power loss
(c) Transmission lines can be made thinner
(d) It is easy to reduce the voltage at the receiving end using step-down transformers

Q 18.

If an L-C circuit is considered analogous to a harmonically oscillating spring- block system, which energy of the L-C circuit would be analogous to potential energy and which one analogous to kinetic energy?

Q 19.

As the frequency of an AC circuit increases, the current first increases and then decreases. What combination of circuit elements is most likely to comprise the circuit?
(a) Inductor and capacitor (b) Resistor and inductor
(c) Resistor and capacitor (d) Resistor, inductor and capacitor

Q 20.

The alternating current in a circuit is described by the graph shown in figure. Show rms current in this graph.
ncert-exemplar-problems-class-12-physics-alternating-current-37

Q 21.

A device X is connected to an AC source. The variation of voltage, current and power in one complete cycle is shown in figure.
(a) Which curve shows power consumption over a full cycle?
(b) What is the average power consumption over a cycle?
(c) Identify the device X.
ncert-exemplar-problems-class-12-physics-alternating-current-41

Q 22.

Can the instantaneous power output of an AC source ever be negative? Can the average power output be negative?

Q 23.

In an alternating current circuit  consisting of elements in series, the current increases on increasing the frequency of supply. Which of the following elements are likely to constitute the circuit?
(a) Only resistor (b) Resistor and an inductor
(c) Resistor and a capacitor (d) Only a capacitor

Q 24.

How does the sign of the phase angle Ф, by which the supply voltage leads the current in an L-C-R series circuit, change as the supply frequency is gradually increased from very low to very high values?

Q 25.

If the rms current in a 50 Hz AC circuit is 5 A, the value of the current 1/300 s after its value becomes zero is
ncert-exemplar-problems-class-12-physics-alternating-current-1

Q 26.

ncert-exemplar-problems-class-12-physics-alternating-current-20

Q 27.

A coil of 0.01 H inductance and 1 ω resistance is connected to 200 V, 50 Hz AC supply. Find the impedance of the circuit and time lag between maximum alternating voltage and current.