MCQs on Electric Circuits

01․ A parallel plate capacitor with air as dielectric, force between plate is F. If capacitor is now immersed water of dielectric ε_r, force between plate will be –

02․ If c₁ = 8 μF, c₂ = 4 μF, c₃ = 7 μF c₁, c₂ are in series with c₃ in parallel; effective capacitance will be

When C₁ and C₂ are in series, then, This is parallel to the C₃, So then the equivalent capacitance will be C_eq + C₃.

03․ A capacitor having a mica dielectric ε_R = 6, a plate area of a 10 inches² and a separation 0.01 inch. Find the value of C

s = 10 × (0.0254)² d = 0.1 × (0.0254) c = ε_R s/d.

04․ Two point charges of 4μC are placed at a distance of 3 cm apart from each other in free space. Force acting on two charges will be

From Coulomb's Law,

05․ A parallel plate capacitor contain there dielectric layers of 1, 2 and 3 respectively and thickness of dielectric layers are 0.2 mm, 0.3 mm and 0.4 mm respectively. If area of plate is 20 cm². What is the value of capacitance?

06․ Under resonance condition the power factor of a system is

The elements present in electrical systems are resistor, inductor and capacitor. Inductor stores the energy in the form of magnetic field and capacitor stores the energy in the form of electric field. When these two energies are equal in a system, large oscillations will occur and this is called resonance. Consider a simple resistor, inductor and capacitor series circuit. Total voltage V = V_r + j(V_l-V_c) Where, V_r = Voltage across resistor V_l = Voltage across Inductor V_c = Voltage across capacitor Resonance will occur when V_l = V_c. Therefore the system voltage is only V_r and current is in phase with voltage across resistor. Therefore, under resonance condition the power factor of a system is unity.

07․ Reactive power drawn by a pure resistor is

Reactive power Q = VI sinsφ Where, V = voltage cross resistor I = current through resistor φ = phase angle between voltage and current When a pure resistor is connected across a voltage source, current is in phase with the voltage. Therefore, Q =VI sin0^o Q = 0.

08․ The time required for a pulse to change from 10 % to 90 % of its maximum value is known as

Rise time is the time required for a over damped system to move from 10% to 90% of its peak value, it is also known as maximum peak overshoot.

09․ Fall time is the time required for a pulse to decrease from 90 to ________% if its maximum values

For an under-damped system a 90% to 10% decay is known as the fall time or decay time.

10․ An electrical motor has rating of 10 MVA at 0.8 lag, represented as series RL circuit at a 50 MVA base. Find the impedance value?

When load MVA and power factor are given: If load is represented with equivalent RL circuit, Z_pu = R_pu + jX_pu = (S_b/S(cosφ + j sinφ))^* Where, Z_pu = Per unit impedance R_pu = Per unit resistance X_pu = Per unit reactance S_b = Base impedance S = Motor rating cosφ = Power factor Z_pu = (50/10(0.8 + j0.6)^* = (4 + j3) pu.