DC Circuits Quiz

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Questions and Answers

What is Resistance, Inductance, and Capacitance?

Resistance is the opposition to current flow in a circuit. Inductance is the property of a circuit that opposes changes in current. Capacitance is the ability of a circuit to store electrical energy.

State Ohm's Law. Where is it not applicable?

Ohm's Law states that the current through a conductor is directly proportional to the voltage across it, provided the temperature remains constant. It is not applicable to devices with non-linear resistance, such as diodes and transistors.

Define voltage and current.

Voltage is the electrical potential difference between two points in a circuit, representing the energy per unit charge. Current is the rate of flow of electrical charge through a conductor.

What are dependent and independent sources?

Independent sources provide a fixed voltage or current regardless of the circuit conditions. Dependent sources, however, have their voltage or current determined by another variable in the circuit, such as a voltage or current elsewhere. Signup and view all the answers

What is source transformation?

Source transformation involves converting a voltage source in series with a resistor into an equivalent current source in parallel with the same resistor, or vice versa. Signup and view all the answers

State Kirchhoff's Laws.

Kirchhoff's Current Law (KCL) states that the sum of currents entering a node in a circuit equals the sum of currents leaving that node. Kirchhoff's Voltage Law (KVL) states that the sum of voltage drops around any closed loop in a circuit equals zero. Signup and view all the answers

Identify series and parallel circuits.

In a series circuit, components are connected end-to-end, so current flows through each component in turn. In a parallel circuit, components are connected across each other, so current splits into multiple paths. Signup and view all the answers

What is a star and delta connection? State formulas for transformations.

A star connection is a three-phase circuit configuration where three terminals are connected at a common point, forming a star shape. A delta connection is another three-phase configuration where three components are connected in a triangle. Transformations between these configurations are possible using specific formulas to calculate equivalent resistances. Signup and view all the answers

Identify star and delta networks and do conversion.

To identify star and delta networks, look for the specific connection patterns of the components. Conversion can be done using the appropriate formulas for transformation. Signup and view all the answers

State the Superposition theorem, thevenins, nortons, and max power theorem.

The Superposition theorem allows for the calculation of the response in a linear circuit with multiple sources by considering each source separately and summing the results. Thevenin's theorem allows for simplifying a complex circuit into a single voltage source and a resistor. Norton's theorem is similar to Thevenin's but uses a current source and a resistor. The maximum power transfer theorem states that a load receives maximum power when its resistance equals the internal resistance of the source. Signup and view all the answers

What are the internal resistances of an ideal voltage source and current source?

An ideal voltage source has zero internal resistance, meaning it can supply any amount of current without its voltage dropping. An ideal current source has infinite internal resistance, meaning it can maintain a constant current regardless of the voltage across it. Signup and view all the answers

What is the condition for maximum power transfer from source to load in any circuit?

Maximum power transfer occurs when the load resistance is equal to the internal resistance of the source. Signup and view all the answers

What is a DC and AC supply? What is the difference between them?

DC, or Direct Current, is a constant unidirectional flow of electrical charge. AC, or Alternating Current, is a flow of electrical charge that periodically reverses direction. Signup and view all the answers

Explain the generation of DC and AC voltages with their emf equations.

DC voltage is generated by a DC generator using electromagnetic induction by rotating a coil in a magnetic field. AC voltage is generated by an AC generator, also through electromagnetic induction, where the coil rotates in a magnetic field, resulting in sinusoidal voltage variation. Signup and view all the answers

What is period, frequency, average, RMS, peak amplitude, phase difference, leading, lagging waveform?

Period is the time taken for one complete cycle of an AC waveform. Frequency is the number of cycles per second. Average value is the average of the instantaneous values over a period. RMS (Root Mean Square) value is the effective value of the AC waveform. Peak amplitude is the maximum value of the waveform. Phase difference is the time delay between two waveforms. A leading waveform is ahead in time, and a lagging waveform is behind in time. Signup and view all the answers

Define amplitude factor and form factor.

Amplitude factor is the ratio of peak value to RMS value of an AC waveform. Form factor is the ratio of RMS value to average value of an AC waveform. Signup and view all the answers

What is the RMS and average value of a pure sinusoidal waveform?

The RMS value of a pure sinusoidal waveform is equal to the peak value divided by the square root of 2. The average value is zero over a complete cycle because the positive and negative halves cancel each other out. Signup and view all the answers

What is power factor? Should it be high or low? Explain.

Power factor is the ratio of real power (consumed by the load) to apparent power (total power supplied). A higher power factor is desirable because it indicates a more efficient use of power and less energy loss in the circuit. A low power factor indicates a large difference between real and apparent power, leading to higher energy consumption and losses. Signup and view all the answers

Add and subtract the following signals and find the resultant: V1 = 10 sin(wt + 30) , V2 = 20 cos(wt + 70)

The resultant waveform can be found by adding the individual waveforms, keeping in mind that the cosine function can be expressed as a sine function by shifting the phase by 90 degrees. Therefore, V2 = 20 sin(wt + 70 + 90) = 20 sin(wt + 160). Adding V1 and V2 can be done by combining the amplitude and phase using phasor addition. The resultant waveform will be of the form V = A sin(wt + φ), where A and φ can be calculated using trigonometry. Signup and view all the answers

If an AC supply is given to a resistor, draw a phasor diagram. What will the power factor be?

In a purely resistive circuit, the voltage and current are in phase. The phasor diagram will show both voltage and current phasors aligned, with an angle of 0 degrees between them. The power factor in this case will be unity (1). Signup and view all the answers

If the power factor is unity, what component does the circuit contain?

A circuit with a unity power factor primarily contains resistors. Signup and view all the answers

If the total impedance angle is negative, is the power factor lagging or leading?

A negative total impedance angle indicates a lagging power factor. This means the current lags behind the voltage. Signup and view all the answers

If an AC supply is given to a pure inductor, draw a phasor diagram. What will the power factor be?

In a purely inductive circuit, the current lags behind the voltage by 90 degrees. The phasor diagram will show the voltage phasor leading the current phasor by 90 degrees. The power factor in this case will be zero (0). Signup and view all the answers

If an AC supply is given to a choke coil, draw a phasor diagram. What will the power factor be?

A choke coil (inductor) causes the current to lag behind the voltage. The phasor diagram will show the voltage phasor leading the current phasor, and the power factor will be lagging and less than unity (1). The exact value will depend on the inductance and resistance of the choke coil. Signup and view all the answers

If an AC supply is given to a resistor-capacitor circuit, draw a phasor diagram. What will the power factor be?

In an RC circuit, the current leads the voltage due to the capacitor. The phasor diagram will show the voltage phasor lagging behind the current phasor. The power factor will be leading and less than unity (1). The exact value depends on the resistance and capacitance values. Signup and view all the answers

Draw impedance, power, and voltage triangle for RL and RC circuits.

Impedance triangle: In an RL circuit, impedance (Z) is the vector sum of resistance (R) and inductive reactance (XL). In an RC circuit, impedance (Z) is the vector sum of resistance (R) and capacitive reactance (XC). Power triangle: In both RL and RC circuits, apparent power (S) is the vector sum of real power (P) and reactive power (Q). Voltage triangle: In an RL circuit, voltage (V) is the vector sum of voltage across resistance (VR) and voltage across inductance (VL). In an RC circuit, voltage (V) is the vector sum of voltage across resistance (VR) and voltage across capacitance (VC). Signup and view all the answers

How many powers are involved in AC circuits? Which are they? Explain each one.

In AC circuits, three main types of power are important: real power (P), reactive power (Q), and apparent power (S). Real power is the actual power consumed by the load and is measured in watts (W). Reactive power is the power exchanged by the load due to its inductive or capacitive reactance and is measured in volt-amperes reactive (VAR). Apparent power is the total power supplied to the load and is measured in volt-amperes (VA). Signup and view all the answers

What are the concepts of susceptance, conductance, and admittance for parallel circuits?

Susceptance (B) is the reciprocal of reactance and represents the ease with which a circuit passes reactive power. Conductance (G) is the reciprocal of resistance and represents the ease with which a circuit passes real power. Admittance (Y) is the reciprocal of impedance and represents the overall ease with which a circuit passes current. These concepts provide insights into the behavior of parallel circuits and facilitate calculations related to current and power flows. Signup and view all the answers

What is resonance? What is the difference between series and parallel resonance?

Resonance occurs in a circuit when the capacitive and inductive reactances cancel each other out, resulting in the maximum current flow (in series resonance) or the maximum impedance (in parallel resonance). In series resonance, the impedance is at its minimum, resulting in maximum current flowing through the circuit at the resonant frequency. In parallel resonance, the impedance is at its maximum, resulting in minimum current flowing through the circuit at the resonant frequency. Signup and view all the answers

What is dynamic impedance?

Dynamic impedance is the impedance of a circuit at a specific frequency, typically around the resonant frequency. It is usually presented as a complex number, representing the magnitude and phase of the impedance at that frequency. Signup and view all the answers

What is the concept of 3 dB bandwidth in a resonance graph?

3 dB bandwidth is the range of frequencies in a resonance graph where the power is at least half of its maximum value. It is defined as the frequency range between two points on the resonance curve where the power is 3 dB lower than the maximum power. Signup and view all the answers

What is the quality factor? Justify whether it should be high or low.

Quality factor (Q) is a measure of a resonant circuit's selectivity or sharpness. It represents the ratio of energy stored in the circuit to the energy dissipated per cycle. A high Q factor indicates a sharp resonance curve, meaning the circuit efficiently amplifies a narrow band of frequencies. A low Q factor indicates a broad resonance curve, meaning the circuit amplifies a wider range of frequencies. The desired Q factor depends on the application. For example, a high Q factor is desirable for a radio receiver to selectively tune into a specific station, while a low Q factor might be preferable for a wideband amplifier. Signup and view all the answers

What is the difference between a single-phase and a polyphase system?

A single-phase system uses a single alternating current source, providing power to devices using one phase. Polyphase systems use multiple alternating current sources, each with a specific phase difference, resulting in a more balanced and efficient power delivery. Signup and view all the answers

What is the phase sequence for a three-phase system? What is the phase difference between each phase? And why?

The phase sequence in a three-phase system refers to the order in which the phases reach their peak values. It is typically designated as ABC, where phase A reaches its peak first, followed by phase B and then phase C. The phase difference between each phase is 120 degrees. This phase difference ensures a balanced power distribution and smooth operation of motors due to a constant torque. Signup and view all the answers

Explain the three-phase supply generation.

Three-phase AC supply is generated by rotating a three-phase alternator, where three coils are spaced 120 degrees apart, inducing voltages in each coil with a 120-degree phase difference. The three coils are connected in a wye (star) or delta configuration to provide balanced three-phase power output. Signup and view all the answers

Draw a phasor diagram and waveform for a three-phase supply.

A phasor diagram for a three-phase supply shows three phasors representing the voltages of each phase with a 120-degree angle between them. The waveform shows three sine waves, each shifted by 120 degrees, resulting in a continuous power flow over a cycle. Signup and view all the answers

What are the advantages and applications of a three-phase system?

Advantages of a three-phase system include: increased power efficiency, smoother operation of electric motors, reduced wiring costs, and improved torque characteristics. Three-phase systems are widely used in power transmission and distribution, industrial applications, and electric motor drives. Signup and view all the answers

What is the concept of phase voltage, phase current, line voltage, and line current?

Phase voltage is the voltage across each phase winding of a generator or load. Phase current is the current flowing through each phase winding. Line voltage is the voltage between any two line conductors in a three-phase system. Line current is the current flowing through each line conductor. Signup and view all the answers

Explain a balanced load and a balanced supply system.

A balanced load in a three-phase system has equal impedance in each phase, ensuring that each phase carries an equal amount of current. A balanced supply system provides equal voltages and currents to each phase, maintaining a stable and efficient power distribution. Signup and view all the answers

What is the relation between phase voltage and line voltage for star and delta connections?

In a star connection, line voltage is the square root of 3 times the phase voltage. In a delta connection, line voltage is equal to the phase voltage. Signup and view all the answers

What is the relation between phase current and line current for star and delta connections?

In a star connection, line current is equal to phase current. In a delta connection, line current is the square root of 3 times the phase current. Signup and view all the answers

What is the impedance and power relation between star and delta connections?

In a star connection, the impedance is the same between line and phase connections, while the power is the sum of the power in each phase. In a delta connection, the impedance between line and phase connections is different, and the total power is 3 times the power in each phase. Signup and view all the answers

Draw a phasor diagram for star and delta connections.

A phasor diagram for a star connection will show phase voltages with a 120-degree phase difference, with line voltages calculated as the vector sum of two phase voltages. A phasor diagram for a delta connection will show line voltages equal to phase voltages, with line currents calculated as the vector sum of two phase currents. Signup and view all the answers

What are the methods of power measurement in a three-phase system? Which method is more preferable and why?

Common power measurement methods include the two-wattmeter method, the three-wattmeter method, and the single-wattmeter method. The two-wattmeter method is generally preferred for balanced three-phase loads. This method is simpler in implementation and more accurate than the other methods, especially in practical situations. Signup and view all the answers

Explain the advantages of a three-phase system.

Three-phase systems offer several advantages over single-phase systems, including increased efficiency, smoother operation of motors, reduced wiring costs, and improved power quality. Three-phase systems are more efficient in power transmission and distribution, deliver more uniform torque to motors, require less wiring compared to single-phase systems, and provide a more stable and reliable power supply. Signup and view all the answers

Explain the working of a transformer with its emf equation.

A transformer works on the principle of electromagnetic induction. A changing magnetic field induced by an alternating current in the primary winding induces a voltage in the secondary winding. The EMF equation for a transformer is: E = 4.44 f N Φm, where E is the induced voltage, f is the frequency, N is the number of turns, and Φm is the maximum flux. Signup and view all the answers

What is the transformation ratio?

The transformation ratio (K) is the ratio of the number of turns on the secondary winding (Ns) to the number of turns on the primary winding (Np), i.e., K = Ns/Np. It determines the voltage transformation by the transformer. Signup and view all the answers

Explain the constructional details and types of transformers.

A typical transformer consists of a core, windings, and an insulating material. The core is made of laminated steel sheets to minimize eddy current losses. Windings are copper or aluminum coils wound around the core. Transformers are classified based on their core type (core-type, shell-type), cooling method (oil-cooled, air-cooled), and application (power transformer, distribution transformer, instrument transformer). Signup and view all the answers

What is the KVA rating of a transformer? What is full load current? No-load current?

KVA rating represents the apparent power that a transformer can handle without overheating. Full-load current is the current flowing in the winding under full load conditions. No-load current is the current flowing in the primary winding when the secondary winding is open-circuited. Signup and view all the answers

What are the losses in a transformer?

Losses in a transformer include copper losses (due to resistance in the windings), core losses (hysteresis and eddy current losses), and stray losses. Copper losses are proportional to the square of the current flowing in the windings, while core losses are dependent on frequency and magnetic flux density. Signup and view all the answers

What is a practical transformer? Draw and explain.

A practical transformer is a real-world transformer that incorporates losses and imperfections in its operation. It can be represented by an equivalent circuit consisting of an ideal transformer, a resistance representing the winding resistance, and a reactance representing the leakage reactance. Signup and view all the answers

Draw a phasor diagram for an ideal transformer with no load.

Under no-load conditions, the primary current is very small and is primarily magnetizing current. The phasor diagram will show the primary voltage (V1) leading the primary current (I1) by 90 degrees. The secondary voltage (V2) will be in phase with the primary voltage (V1) but scaled by the transformation ratio K. Signup and view all the answers

Draw a phasor diagram for a practical transformer with no load.

In a practical transformer, the no-load current (Io) includes a magnetizing current (Im) and core loss current (Ic). The phasor diagram shows the primary voltage (V1) leading the no-load current (Io) by an angle greater than 90 degrees. The secondary voltage (V2) will be in phase with the primary voltage (V1) but scaled by the transformation ratio K, accounting for losses. Signup and view all the answers

What are the two components of no-load current?

The two components of no-load current are the magnetizing current (Im) and the core loss current (Ic). Magnetizing current is responsible for establishing the magnetic flux in the core, while core loss current (Ic) represents the losses due to hysteresis and eddy currents in the core. Signup and view all the answers

Define voltage regulation with a formula

The equivalent circuit of a transformer includes an ideal transformer, a resistance representing the winding resistance (R), and a reactance representing the leakage reactance (X). The core loss is represented by a shunt resistance (Rc), and the magnetizing current is represented by a shunt reactance (Xm). Signup and view all the answers

Define efficiency with a formula.

Efficiency (η) is the ratio of output power to input power, expressed as a percentage. η = (Output power) / (Input power) x 100%. Signup and view all the answers

What are OC test and SC test and where are they used?

Open-circuit (OC) test measures the core losses of a transformer by applying voltage to the primary winding with the secondary winding open. Short-circuit (SC) test measures the copper losses by short-circuiting the secondary winding and applying a reduced voltage to the primary winding. These tests are used to determine transformer efficiency, voltage regulation, and equivalent circuit parameters. Signup and view all the answers

In a transformer, primary and secondary voltages are 180 degrees out of phase.

False (B) Signup and view all the answers

Explain the principle of operation of a three-phase induction motor.

A three-phase induction motor operates based on the principle of electromagnetic induction and rotating magnetic fields. A three-phase stator winding creates a rotating magnetic field when supplied with three-phase power. This rotating magnetic field induces a voltage in the rotor winding, which then carries current. The interaction between the rotor current and the rotating magnetic field creates a torque that drives the rotor. Signup and view all the answers

What are the different parts used in a three-phase induction machine?

A three-phase induction machine consists of a stator, rotor, and an air gap separating the stator and rotor. The stator houses the three-phase windings that create the rotating magnetic field. The rotor can be either a squirrel cage type or a wound rotor type. The air gap allows for the interaction between the rotating magnetic field and the rotor currents. Signup and view all the answers

What is the internal resistance of an ideal voltage source and a current source?

An ideal voltage source has zero internal resistance, meaning it maintains a constant voltage regardless of the current flowing through it. An ideal current source has infinite internal resistance, meaning it provides a constant current regardless of the voltage across it. Signup and view all the answers

Explain the generation of DC and AC voltages, with their respective EMF equations.

DC voltage is generated by converting AC to DC using various methods like rectifiers and filters. The output is a unidirectional voltage. AC voltage is generated by rotating a coil in a magnetic field. The EMF equation for AC is: * E = -N(dΦ/dt), where: <ul> <li>E is the induced EMF</li> <li>N is the number of turns in the coil</li> <li>dΦ/dt is the rate of change of magnetic flux through the coil.</li> </ul> The DC voltage can be generated from AC using various methods like rectifiers and filters; thus, an EMF equation is not defined for DC. It is the output of a DC generator, but the EMF equation derived is similar to AC EMF equation. Signup and view all the answers

If an AC supply is given to a Resistor, draw a phasor diagram. What will be the power factor?

In a purely resistive circuit, the current and voltage are in phase. The phasor diagram will show the voltage and current phasors aligned, with an angle of 0° between them. The power factor in this case is unity (1) because the current and voltage are in phase, meaning all the power drawn from the source is used as real power. Signup and view all the answers

If the power factor is unity, which component is present in the circuit?

If the power factor is unity, the circuit contains only resistance. This is because a unity power factor signifies that the current and voltage are in phase, which is only possible in a purely resistive circuit. Signup and view all the answers

If an AC supply is given to a pure Inductor, draw a phasor diagram. What will be the power factor?

In a purely inductive circuit, the current lags the voltage by 90 degrees. The phasor diagram will show the voltage phasor leading the current phasor by 90°. The power factor in this case is zero (0) because the current and voltage are 90° out of phase, meaning all the power drawn from the source is reactive power and no real power is consumed. Signup and view all the answers

If an AC supply is given to a choke coil, draw a phasor diagram. What will be the power factor?

A choke coil is a coil that has both inductance and resistance. The phasor diagram will show the voltage phasor leading the current phasor by an angle less than 90°. The power factor is less than 1 and leading because the current leads the voltage slightly due to the capacitance effect. The power factor of a choke coil will be lagging and less than 1. Signup and view all the answers

If an AC supply is given to a Resistor-Capacitor (RC) circuit, draw a phasor diagram. What will be the power factor?

In an RC circuit, the impedance is the combination of resistance R and capacitive reactance Xc. The phasor diagram will show the voltage phasor leading the current phasor by an angle less than 90 degrees. The power factor will be leading and less than 1 because the current leads the voltage due to the capacitive effect. Signup and view all the answers

Draw an impedance, power, and voltage triangle for both RL and RC circuits.

RL Circuit: <ul> <li>Impedance Triangle: R - horizontal leg, XL - vertical leg, Z - hypotenuse</li> <li>Power Triangle: P - horizontal leg, Q - vertical leg, S - hypotenuse</li> <li>Voltage Triangle: VR - horizontal leg, VL - vertical leg, V - hypotenuse</li> </ul> RC Circuit: <ul> <li>Impedance Triangle: R - horizontal leg, XC - vertical leg, Z - hypotenuse</li> <li>Power Triangle: P - horizontal leg, Q - vertical leg, S - hypotenuse</li> <li>Voltage Triangle: VR - horizontal leg, VC - vertical leg, V - hypotenuse</li> </ul> Signup and view all the answers

What is the Quality factor? Justify if it should be high or low.

The quality factor (Q) is a dimensionless parameter that describes how sharply tuned a resonant circuit is. It is defined as the ratio of the resonant frequency to the 3 dB bandwidth. A high Q means that the circuit is sharply tuned, meaning it will respond strongly to a narrow range of frequencies around the resonant frequency. This is desirable in applications like filters where precise frequency selection is essential. A low Q means that the circuit is broadly tuned, meaning it will respond to a wider range of frequencies around the resonant frequency. This might be desirable in applications where a wider band of frequencies needs to be allowed to pass through, such as in amplifiers. Signup and view all the answers

Draw an equivalent circuit of a transformer.

The equivalent circuit of a transformer includes: <ul> <li>Primary winding resistance (R1): Represents the copper losses in the primary winding.</li> <li>Secondary winding resistance (R2): Represents the copper losses in the secondary winding.</li> <li>Magnetizing reactance (Xm): Represents the energy required to magnetize the core.</li> <li>Core loss resistance (Rc): Represents hysteresis and eddy current losses in the core.</li> <li>Leakage reactance (Xl): Represents magnetic flux leakage from the core, reducing the efficiency of the transformer.</li> </ul> Signup and view all the answers

Flashcards

What is resistance?

Resistance is the opposition to the flow of electric current in a material. It is measured in ohms (Ω).

What is inductance?

Inductance is the property of a circuit that opposes changes in current. It is measured in henries (H).

What is capacitance?

Capacitance is the ability of a circuit to store electric charge. It is measured in farads (F).

State Ohm's Law.

Ohm's Law states that the current through a conductor is directly proportional to the voltage applied across it, provided the temperature remains constant. Mathematically, V=IR, where V is voltage, I is current, and R is resistance.