R must be short-circuited in order for the circuit to become an LC oscillator. The capacitor will continue to drain, and all of the energy will be transferred to L and back. Energy will oscillate...
In the LCR circuit the ac driving voltage is v = vm sin ωt.
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 , 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})...
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...
For an LCR circuit driven at frequency ω, the equation reads L di/dt + Ri + q/C = vi = vm sin ꞷ t
(i) Multiply the equation by i and simplify where possible.
(ii) Interpret each term physically.
Given equation is, $L d i / d t+R i+q / C=v i=v m \sin \omega t$ i) On multiplying the above equation with I, we get, $\mathrm{d}\left(1 / 2 \mathrm{Li}^{2}\right) / \mathrm{dt}+1 / 2 \mathrm{C}...
Consider the LCR circuit such that the net current i and the phase of i. Show that i = V/Z . Find the impedance Z for this circuit.
The impedance $Z$ for the given circuit is represented as, $1 / \mathrm{Z}=\left[1 / \mathrm{R}^{2}+1 /(1 / \omega \mathrm{C}-\omega \mathrm{L})^{2}\right]^{1 / 2}$
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 $\mathrm{Cu}$...
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) ,
and (iii) . Another device has twice the values for , and . 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...
Explain why the reactance offered by an inductor increases with increasing frequency of an alternating voltage.
The reactance offered by an inductor increases with the increasing frequency of an alternating voltage because the induced emf is proportional to the rate of change of current.
Explain why the reactance provided by a capacitor to an alternating current decreases with increasing frequency.
Because capacitance reactance is inversely proportional to frequency, and capacitors have a tendency to pass high-frequency current while blocking low-frequency currents, the reactance delivered by...
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.
$P_{s}$ is given as $60W$ $I_{s}$ is given as $0.54A$ Primary voltage is given as $220V$ $V_{s}=60/0.54=110V$ The transformer is a step-down transformer because the secondary voltage is lower than...
A coil of 0.01-henry inductance and 1-ohm resistance is connected to 200 volt, 50 Hz ac supply. Find the impedance of the circuit and time lag between max. alternating voltage and current.
Inductance L is given as 0.01 H Resistance R is given as 1 ohm Voltage is given as 200 V Frequency is given as 50 Hz Impedance is given Z = 3.3 ohms Phase difference will be 72 × π/180 rad 0.004 sec...
Both alternating current and direct current are measured in amperes. But how is the ampere defined for an alternating current?
The phenomenon called Joule's law of heating is used to determine the ampere for an alternating current. According to Joule's law of heating in a one-ohm resistance, AC is the current produced in a...
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.
Identify the device ‘X’.
Solution: X could be an inductor, a capacitor, or a hybrid of the two.
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...
How does the sign of the phase angle φ, by which the supply voltage leads the current in an LCR series circuit, change as the supply frequency is gradually increased from very low to very high values.
$tan \phi = \frac{X_{l}-X_{c}}{R}$ $X_{l}<X_{c}$ At resonate frequency we have, $X_{l}=X_{c}$ $tan \phi = 0$
The alternating current in a circuit is described by the graph shown in the figure. Show rms current in this graph.
Solution: $I_{rms}=1.6A$
In series LCR circuit, the plot of I max vs ω is shown in the figure. Find the bandwidth and mark in the figure.
Frequency 1 is 0.8 rad/s Frequency 2 is 1.2 rad/s The bandwidth is 0.4 rad/s
Can the instantaneous power output of an ac source ever be negative? Can the average power output be negative?
a) When P < 0, then AC source's instantaneous power output can be negative. b) The average power output cannot be negative as $P_{av}>0$.
Study the circuits (a) and (b) shown in the figure and answer the following questions.
(a) Under which conditions would the rms currents in the two circuits be the same? (b) Can the rms current in the circuit (b) be larger than that in (a)? Solution: a) $(I_{rms})a=(I_{rms})b$...
Draw the effective equivalent circuit of the circuit shown in the figure, at very high frequencies and find the effective impedance.
Solution: The effective impedance will be, $Z_{eq}=R_{1}+R_{3}$
If an LC circuit is considered analogous to a harmonically oscillating spring block system, which energy of the LC circuit would be analogous to potential energy and which one analogous to kinetic energy?
The magnetic energy of the LC circuit is equivalent to kinetic energy, while electrostatic energy due to a capacitor's charge change is analogous to potential energy.
The line that draws power supply to your house from street has
(a) zero average current.
(b) 220 V average voltage.
(c) voltage and current out of phase by 90°.
(d) voltage and current possibly differing in phase φ such that |φ| < π/2.
The correct options are: (a) zero average current. (d) voltage and current possibly differing in phase φ such that |φ| < π/2.
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) power delivered to the capacitor is zero.
The correct options are: (c) the charge on the plates is in phase with the applied voltage. (d) power delivered to the capacitor is zero.
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...
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...
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.
The correct options are: (c) Resistor and a capacitor. (d) Only a capacitor.
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.
The correct options are: (a) Inductor and capacitor. (d) Resistor, inductor and capacitor.
The output of a step-down transformer is measured to be 24 V when connected to a 12 watt light bulb. The value of the peak current is
(a) 1/ √2 A.
(b) √2 A.
(c) 2 A.
(d) 2 √2 A
(a) 1/ √2 A.
An inductor of reactance 1 Ω and a resistor of 2 Ω are connected in series to the terminals of a 6 V (rms) a.c. source. The power dissipated in the circuit is
(a) 8 W.
(b) 12 W.
(c) 14.4 W.
(d) 18 W.
(c) 14.4 W.
Which of the following combinations should be selected for better tuning of an LCR circuit used for communication?
(a) R = 20 Ω, L = 1.5 H, C = 35µF.
(b) R = 25 Ω, L = 2.5 H, C = 45µF.
(c) R = 15 Ω, L = 3.5 H, C = 30µF.
(d) R = 25 Ω, L = 1.5 H, C = 45µF.
(c) R = 15 Ω, L = 3.5 H, C = 30µF.
To reduce the resonant frequency in an LCR series circuit with a generator
(a) the generator frequency should be reduced.
(b) another capacitor should be added in parallel to the first.
(c) the iron core of the inductor should be removed.
(d) dielectric in the capacitor should be removed.
(b) another capacitor should be added in parallel to the first
When a voltage measuring device is connected to AC mains, the meter shows the steady input voltage of 220V. This means
(a) input voltage cannot be AC voltage, but a DC voltage.
(b) maximum input voltage is 220V.
(c) the meter reads not v but and is calibrated to read .
(d) the pointer of the meter is stuck by some mechanical defect.
(c) the meter reads not v but $<v_{2}>$ and is calibrated to read $\sqrt <v_{2}>$
An alternating current generator has an internal resistance and an internal reactance . It is used to supply power to a passive load consisting of a resistance and a reactance . For maximum power to be delivered from the generator to the load, the value of is equal to
(a) zero.
(b) .
(c) .
(d) .
(c) $-X_{g}$.
If the rms current in a 50 Hz ac circuit is 5 A, the value of the current 1/300 seconds after its value becomes zero is
(a) 5 √2 A
(b) 5 √3/2 A
(c) 5/6 A
(d) 5/ √2 A
(b) 5 √3/2 A
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, $\mathrm{V}=110 \mathrm{~V}$...
A 100 µF capacitor in series with a 40 Ω resistance is connected to a 110 V, 60 Hz supply. (a) What is the maximum current in the circuit? (b) What is the time lag between the current maximum and the voltage maximum?
Capacitance of the capacitor, $\mathrm{C}=100 \mu \mathrm{F}=100 \times 10^{-6} \mathrm{~F}$ The resistance of the resistor $=40 \Omega$ Supply voltage, $\mathrm{V}=110 \mathrm{~V}$ Frequency, $v=60...
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)...
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...
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...
A power transmission line feeds input power at 2300 V to a step-down transformer with its primary windings having 4000 turns. What should be the number of turns in the secondary in order to get output power at 230 V?
The transformer's input voltage, $\mathrm{V}_{1}=2300 \mathrm{~V}$ The step-down transformer's primary windings, $\mathrm{n}_{1}=4000$ turns Output voltage, $V_{2}=230 \mathrm{~V}$ Let be the...
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...
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...
An LC circuit contains a 20 mH inductor and a 50 µF capacitor with an initial charge of 10 mC. The resistance of the circuit is negligible. If a resistor is inserted in the circuit, how much energy is eventually dissipated as heat?
The LC oscillations are eventually dampened by the resistor. The entire original energy (= 1.0 J) is dissipated as heat in the end.
A radio can tune over the frequency range of a portion of MW broadcast band: (800 kHz to 1200 kHz). If its LC circuit has an effective inductance of 200 µH, what must be the range of its variable capacitor?
ATQ, the frequency range of (ν) of radio is $800to1200kHz$ ATQ, the Lower tuning frequency of the circuit would be, ${{v}_{1}}=800kHz=800\times {{10}^{3}}Hz$ ATQ, the Upper tuning frequency of the...
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...
Suppose the initial charge on the capacitor is 6 mC. What is the total energy stored in the circuit initially? What is the total energy at a later time?
According to the question, Capacitance value of the capacitor will be, $C=30\mu F=30\times {{10}^{-6}}F$ Inductance of the inductor will be, $L=27mH=27\times {{10}^{-3}}H$ Charge on the...
A charged 30 µF capacitor is connected to a 27 mH inductor. What is the angular frequency of free oscillations of the circuit?
Given Capacitance value of the capacitor is , $C=30\mu F=30\times {{10}^{-6}}F$ Given Inductance value of the charged inductor will be , $L=27mH=27\times {{10}^{-3}}H$ Angular frequency according to...
Obtain the resonant frequency ωr of a series LCR circuit with L = 2.0H, C = 32 µF and R = 10 Ω. What is the Q-value of this circuit?
The inductance of the inductor can be considered as $L=2.0H$ The capacitance of the capacitor will be , $C=32\mu F=32\times {{10}^{-6}}F$ The resistance of the resistor as per question is...
A 60 µF capacitor is connected to a 110 V, 60 Hz ac supply. Determine the rms value of the current in the circuit.
The capacitance of the capacitor in the circuit is given as $C=60\mu For60\times {{10}^{-6}}F$ The source voltage is considered to be $V=110V$ The frequency of the source will be $v=60Hz$ The...
A 44 mH inductor is connected to 220 V, 50 Hz ac supply. Determine the rms value of the current in the circuit.
As per given : The inductor connected to the given AC supply has an inductance $L=44mH=44\times {{10}^{-3}}H$ The magnitude of the source voltage V is considered to be $220V$ The frequency of the...
The peak voltage of an ac supply is 300 V. What is the rms voltage? And the rms value of current in an ac circuit is 10 A. What is the peak current?
a)The peak voltage of the AC supply will be ${{V}_{o}}=300V$ And we also know that, ${{V}_{RMS}}=V=\frac{{{V}_{o}}}{\sqrt{2}}$ Substituting the values, we get to know that...
A 100 Ω resistor is connected to a 220 V and 50 Hz ac supply.
(a) What is the rms value of current in the given circuit? (b) What will be the net power consumed over a full cycle? Assume: The resistor's resistance R is 100. 220 V is the source voltage. The...