Why is Choke Coil Needed in the Use of Fluorescent Tubes with Ac Mains? - Physics


Why is choke coil needed in the use of fluorescent tubes with ac mains?



A choke coil is an electrical appliance used for controlling current in an a.c. circuit. Therefore, if we use a resistance R for the same purpose, a lot of energy would be wasted in the form of heat etc

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2013-2014 (March) Delhi Set 1


 Derive the relationship between the peak and the rms value of current in an a.c. circuit.

In a series RL circuit, the resistance and inductive reactance are the same. Then the phase difference between the voltage and current in the circuit is 

An inductor 20 mH, a capacitor 50 μF and a resistor 40Ω are connected in series across a source of emf V = 10 sin 340 t. The power loss in AC circuit is

What are phasors?

Define electric resonance.

What do you mean by resonant frequency?

Define inductive reactance and give its units.

Define capacitive reactance. Give its units.

Obtain an expression for average power of AC over a cycle. Discuss its special cases.

Predict the polarity of the capacitor in a closed circular loop when two bar magnets are moved as shown in the figure.

When does power factor of a series RLC circuit become maximum?

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?

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?

The rms current Irms is related to the peak current Io as ______.

The alternating current can be measured with the help of ______.

The rms value of potential difference V shown in the figure is ______.

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?

In an a.c circuit, peak value of voltage is 423 volts, its effective voltage is ______.

Which of the following is independent of the frequency of a.c?

An inductor of inductance L, a capacitor of capacitance C and a resistor of resistance ‘R’ are connected in series to an ac source of potential difference ‘V’ volts as shown in figure. Potential difference across L, C and R is 40 V, 10 V and 40 V, respectively. The amplitude of current flowing through LCR series circuit is `10sqrt2` A. The impedance of the circuit is :

For a series LCR circuit, I vs ω curve is shown:

  1. To the left of ωr, the circuit is mainly capacitive.
  2. To the left of ωr, the circuit is mainly inductive.
  3. At ωr, impedance of the circuit is equal to the resistance of the circuit.
  4.  At ωr, impedance of the circuit is 0.

An inductor of 0.5 mH, a capacitor of 200 µF, and a resistor of 2Ω are connected in series with a 220V ac source. If the current is in phase with the emf, the frequency of ac source will be ______ × 102 Hz.

An inductor coil stores 64 J of magnetic field energy and dissipates energy at the rate of 640 W when a current of 8 A is passed through it. If this coil is joined across an ideal battery, find the time constant of the circuit in seconds ______.

In young's double slit experiment `"d"/"D"`= 10-4 D (d = distance between slits, D = distance of screen from the slits). At a point P on the screen resulting intensity is equal to the intensity due to individual slit l0. Then the distance of point P from the central maximum is (λ = 6000 `"A"^°`)

An alternating voltage v(t) = 220 sin 100πt volt is applied to a purely resistive load of 50 Ω. The time taken for the current to rise from half of the peak value to the peak value is ______.

The r.m.s. value of alternating current is 10 A, having frequency of 50 Hz. The time taken by the current to increase from zero to maximum and the maximum value of current will be ______. 

A capacitor and resistor are connected with an AC source as shown in figure. Reactance of capacitor is XC = 3 Ω and resistance of resistor is 4 Ω. Phase difference between current I and I1 is approx ______.

A 50 Hz AC source of 20 volts is connected across R and C as shown in figure. The voltage across R is 12 volt. The voltage across c is ______.

A direct current of 4 A and an alternating current of peak value 4 A flow through resistance of 3Ω and 2Ω respectively. The ratio of heat produced in the two resistances in same interval of time will be ______.

The alternating current is given by

i = `{sqrt42sin((2pi)/"T""t")+10}"A"`

The r.m.s. value of this current is ______ A.

A series CR circuit with R = 200 Ω and C = (50/π) µF is connected across an ac source of peak voltage ε0 = 100 V and frequency v = 50 Hz. Calculate (a) impedance of the circuit (Z), (b) phase angle (Φ), and (c) voltage across the resistor.

An alternating current I = 14 sin (100 πt) A passes through a series combination of a resistor of 30 Ω and an inductor of `(2/(5pi))` H. Taking `sqrt2` = 1.4 calculate the rms value of the voltage drops across the resistor and the inductor.

Draw graphs showing the variations of inductive reactance and capacitive reactance with the frequency of the applied ac source.


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