Alternating Current quiz

The rms value of an AC voltage with peak value V0 is:

In a pure inductor, the current:

A capacitor in a DC circuit, after charging, acts as:

At resonance in a series LCR circuit, the impedance is:

The power factor of a pure inductor is:

A transformer works on the principle of:

Inductive reactance is:

The average power consumed by a pure capacitor in an AC circuit is:

The core of a transformer is laminated to reduce:

The resonant frequency of a series LCR circuit is:

Why is AC preferred over DC for long distance transmission? (2 marks)

Why does a capacitor allow AC but block DC? (2 marks)

What is the quality factor of a series LCR circuit? What does a high Q mean? (2 marks)

Why is the average value of AC over a complete cycle zero? (2 marks)

Can the voltage across the inductor in a series LCR circuit be greater than the supply voltage? Explain. (2 marks)

Write the relation between turns ratio and voltages in a transformer. (2 marks)

Why is copper used for transformer windings and soft iron for the core? (2 marks)

What happens to the brightness of a bulb in series with a capacitor if the frequency of the AC supply is increased? (2 marks)

State two conditions for resonance in a series LCR circuit. (2 marks)

Write the expression for average power in an AC circuit and explain each term. (2 marks)

The peak voltage of an AC supply is 300 V. Find the rms voltage. If the rms current is 10 A, find the peak current. (3 marks)

A 0.5 H inductor is connected to a 220 V, 50 Hz supply. Find its reactance and the rms current. (3 marks)

Find the resonant angular frequency and frequency of a series LCR circuit with L = 2 mH and C = 20 microF. (3 marks)

A series LCR circuit has R = 30 ohm, XL = 80 ohm and XC = 40 ohm, and is connected to a 200 V rms supply. Find the impedance, rms current, power factor and average power. (3 marks)

A transformer steps down 240 V to 12 V. The primary has 1,200 turns. Find the number of secondary turns. If the secondary current is 2 A, find the primary current, assuming an ideal transformer. (3 marks)

Read the passage and answer the questions. A power station supplies 800 kW to a town 15 km away through two-wire lines of resistance 0.5 ohm per km each. Power is sent at 4,000 V and stepped down to 220 V at the town. (i) Find the current in the line wires. (ii) Find the power lost in the lines. (iii) Why is power transmitted at high voltage? (5 marks)

Read the passage and answer the questions. The tuning circuit of a radio uses a coil of inductance 2.5 microH and a variable capacitor. The listener wants to tune to a station broadcasting at 1 MHz. (i) Which phenomenon is used in tuning? (ii) Find the capacitance required. (iii) Why should the circuit have a high quality factor? (5 marks)

Read the passage and answer the questions. Old fluorescent tube lights use a choke coil in series with the tube. The choke limits the current without wasting much power. (i) Why does the choke consume very little power? (ii) What is the power factor of an ideal choke? (iii) Why is a resistor not used instead? (5 marks)

Read the passage and answer the questions. A student connects a bulb in series with a capacitor first to a 12 V battery and then to a 12 V AC source. The bulb glows only with the AC source. (i) Explain the observation. (ii) What happens if the capacitance is increased? (iii) What happens if a dielectric slab is inserted in the capacitor? (5 marks)

Read the passage and answer the questions. A factory with many induction motors has a low power factor of 0.6. The electricity board charges a penalty and advises installing capacitor banks. (i) Why do motors lower the power factor? (ii) How do capacitors improve it? (iii) Why is a low power factor costly? (5 marks)

Derive the expression for impedance and phase angle in a series LCR circuit using a phasor diagram. Explain the condition for resonance. (6 marks)

Explain the construction, principle and working of a transformer with a labelled diagram. Derive the turns ratio relation and state the energy losses. (6 marks)

Derive the expression for average power in an AC circuit and discuss the special cases of pure resistor, pure inductor, pure capacitor and LCR circuit at resonance. (6 marks)

Show that the current through a capacitor connected to an AC source leads the voltage by pi/2. Derive the expression for capacitive reactance and draw the graphs. (6 marks)

Draw resonance curves of current against angular frequency for two values of R in a series LCR circuit. Explain sharpness of resonance and the quality factor. (6 marks)