Alternating Current

In the LCR circuit the ac driving voltage is v = vm sin ωt.
(i) Write down the equation of motion for q (t).
(ii) At t=t_{0}, the voltage source stops and R is short-circuited. Now write down how much energy is stored in each of L and C.

i) The equation for charge motion variation with respect to time is as follows: $\mathrm{L} d^{2} q(\mathrm{t}) / \mathrm{dt}+\mathrm{R} \mathrm{dq}(\mathrm{t}) / \mathrm{dt}+\mathrm{q}(\mathrm{t})...

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For an LCR circuit driven at frequency ω, the equation reads L di/dt + Ri + q/C = vi = vm sin ꞷ t
(i) Cast the equation in the form of a conservation of energy statement
(ii) Integrate the equation over one cycle to find that the phase difference between v and i must be acute.

i) The first equation is in the form of conservation of energy ii) We get dt as positive when we integrate the equation from $0$ to $T$, which is possible when the phase difference is constant and...

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1MW power is to be delivered from a power station to a town 10 km away. One uses a pair of Cu wires of radius 0.5 cm for this purpose. Calculate the fraction of ohmic losses to power transmitted if
(i) power is transmitted at 220V. Comment on the feasibility of doing this.
(ii) a step-up transformer is used to boost the voltage to 11000 V, power transmitted, then a step-down transformer is used to bring the voltage to 220 V.

i) It is given that the power station is $10 \mathrm{~km}$ away from the town Length of the $\mathrm{Cu}$ wire is given as $\mathrm{L}=20 \mathrm{~km}=20000 \mathrm{~m}$ Resistance of $\mathrm{Cu}$...

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An electrical device draws 2kW power from AC mains (voltage 223V (rms) = 50,000 V). The current differs (lags) in phase by φ (tan φ = -3/4) as compared to voltage.
Find (i) R,
(ii) X_{c}-X_{l},
and (iii) I_{M}. Another device has twice the values for R, X_{C} and X_{L}. How are the answers affected?

Impedance is given as Z = 25 ohms $635=25R^{2}/16$ a) Resistance can be calculated as, $R=\sqrt 25\times 16=\sqrt 400=20 ohms$ b) $X_{c}-X_{l}=-3R/4=-15ohms$ c) Main current will be...

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A device ‘X’ is connected to an a.c source. The variation of voltage, current and power in one complete cycle is shown in the figure.
(a) Which curve shows power consumption over a full cycle?
(b) What is the average power consumption over a cycle?

Solution: a) The greatest amplitude of the power curve is equal to the product of the voltage and current amplitudes. As a result, curve A denotes power. b) Because the whole cycle in the graph...

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For an LCR circuit, the power transferred from the driving source to the driven oscillator is P = I2Z cos φ.
(a) Here, the power factor cos 0, 0. φ ≥ ≥ P
(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.

The correct options are: (a) Here, the power factor cos 0, 0. φ ≥ ≥ P (b) The driving force can give no energy to the oscillator (P = 0) in some cases. (c) The driving force cannot syphon out (P...

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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.

The correct options are: (a) For a given power level, there is a lower current. (b) Lower current implies less power loss. (d) It is easy to reduce the voltage at the receiving end using step-down...

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Obtain the answers to (a) and (b) in Exercise 7.15 if the circuit is connected to a 110 V, 12 kHz supply? Hence, explain the statement that a capacitor is a conductor at very high frequencies. Compare this behaviour with that of a capacitor in a dc circuit after the steady-state.

Capacitance of the capacitor, $\mathrm{C}=100 \mu \mathrm{F}=100 \times 10^{-6} \mathrm{~F}$ The resistance of the resistor, $\mathrm{R}=40 \Omega$ Supply voltage, $\mathrm{V}=110 \mathrm{~V}$...

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Do the same exercise as above with the replacement of the earlier transformer by a 40,000-220 V step-down transformer (Neglect, as before, leakage losses though this may not be a good assumption any longer because of the very high voltage transmission involved). Hence, explain why high voltage transmission is preferred?

Therefore, RMS current in the line, $1=\left(800 \times 10^{3}\right) / 40,000$ $$\mathrm{I}=20 \mathrm{~A}$$ Total resistance of the two wire line, $\mathrm{R}=2 \times 15 \times 0.5=15 \Omega$ (a)...

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A small town with a demand of 800 kW of electric power at 220 V is situated 15 km away from an electric plant generating power at 440 V. The resistance of the two wirelines carrying power is 0.5 Ω per km. The town gets power from the line through a 4000-220 V step-down transformer at a sub-station in the town. (a) Estimate the line power loss in the form of heat. (b) How much power must the plant supply, assuming there is negligible power loss due to leakage? (c) Characterise the step-up transformer at the plant.

Power required, $P=800 \mathrm{~kW}=800 \times 10^{3} \mathrm{~W}$ Power = Voltage $x$ current $\Rightarrow 800 \times 10^{3}=4000 \times 1$ Therefore, RMS current in the line, $I=\left(800 \times...

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At a hydroelectric power plant, the water pressure head is at a height of 300 m and the water flow available is 100 m3s–1. If the turbine generator efficiency is 60%, estimate the electric power available from the plant (g = 9.8 ms–2).

$$\mathrm{h}=300 \mathrm{~m}$$ Flowing water volume $=100 \mathrm{~m}^{3} \mathrm{~s}^{-1}$ Density of water, $\rho=1000 \mathrm{~kg} / \mathrm{m}^{3}$ Electric power, $\mathrm{P}=\mathrm{h} \rho...

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Answer the following questions: (a) In any ac circuit, is the applied instantaneous voltage equal to the algebraic sum of the instantaneous voltages across the series elements of the circuit? Is the same true for rms voltage? (b) A capacitor is used in the primary circuit of an induction coil. (c) An applied voltage signal consists of a superposition of a dc voltage and an ac voltage of high frequency. The circuit consists of an inductor and a capacitor in series. Show that the dc signal will appear across C and the ac signal across L. (d) A choke coil in series with a lamp is connected to a dc line. The lamp is seen to shine brightly. Insertion of an iron core in the choke causes no change in the lamp’s brightness. Predict the corresponding observations if the connection is to an ac line. (e) Why is choke coil needed in the use of fluorescent tubes with ac mains? Why can we not use an ordinary resistor instead of the choke coil?

(a) Of course. The instantaneous voltage applied is equal to the algebraic sum of the instantaneous voltages across the circuit's series parts. The same cannot be said for rms voltage since voltages...

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Obtain the resonant frequency and Q-factor of a series LCR circuit with L = 3.0 H, C = 27 µF, and R = 7.4 Ω. It is desired to improve the sharpness of the resonance of the circuit by reducing its ‘full width at half maximum’ by a factor of 2. Suggest a suitable way.

Inductance, $L=3.0 \mathrm{H}$ Capacitance, $\mathrm{C}=27 \mu \mathrm{F}=27 \times 10^{-6} \mathrm{~F}$ Resistance, $\mathrm{R}=7.4 \Omega$ The resonant frequency of the source in the LCR series...

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A series LCR circuit with R = 20 Ω, L = 1.5 H and C = 35 µF is connected to a variable-frequency 200 V ac supply. When the frequency of the supply equals the natural frequency of the circuit, what is the average power transferred to the circuit in one complete cycle?

 At resonance in the circuit, the supply frequency and the natural frequency are equal. $R=20\Omega $ is the resistor's resistance. Given the inductor's inductance, $L=1.5H$ Given the...

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