DC, AC, Electromagnetics
A load of 20 + j35 ohms is connected across a 220 volts source. Determine the power factor and the VARS.
49.6%, 1042 VARS
Determine the phase angle in the series circuit which consists of R = 25 ohms, L = 0.2 H across a power supply of 200 volts, 30 Hz?
56.4°
A small 1-mm-diameter circular loop is at the center of a large 20-cm-diameter loop. Both loops lie in the same plane. The current in outer loop changes from 2 A to 10 A in 5 seconds. What is the induced current in the inner loop if its resistance is 1 ohm?
0.4 x 10¯¹¹ A
A capacitance is connected to a 115 V, 25 Hz mains and takes 5 A. What current will it take when the capacitance and the frequency are both doubled?
20 A
In the dc circuit as shown, the high resistance voltmeter gives a reading of +0.435 volt. What is the value of the resistance R?
3 ohms
In a sinusoidally driven series RLC circuit, the inductive reactance is X_L = 200 Ω, the capacitive reactance is Xc = 100 Ω, and the resistance is R = 50 Ω. The current and applied emf would be in phase if:
The capacitance is halved, with no other changes.
Two wires made of the same material have the same lengths but different diameters. They are connected in series to a battery. The quantity that is the same for the wires is:
The current
A battery is connected across a series combination of two identical resistors. If the potential difference across the terminals is V and the current in the battery is i, then:
The potential difference across each resistor is V/2 and the current in each resistor is i
A series circuit composed of 100-ohm resistor and a 20-microfarad capacitor connected across a 240 V, 60 Hz line. Which of the following answers is WRONG?
The resulting current is 0.732 ampere.
What is the unit of the power factor?
None of the choices
Two 110-V light bulbs, one "25 W" and the other "100W", are connected in series to a 110 V source. Then:
None of the choices.
In an RLC series circuit, which is connected to a source of emf ε_m cos(ωt), the current lags the voltage by 45° if:
R = ωL - 1/ωC
To decrease the phase angle below 45°, the following condition must exist:
R > Xc
A 220-Ω R₁, 2.2-kΩ R₂, and 200-Ω R₃ are connected across 15 V of applied voltage. What happens to R_EQ if the applied voltage is doubled to 30 V?
R_EQ does not change.
Lines of the magnetic field produced by a long straight wire carrying a current are:
circles that are concentric with the wire
Choose the correct statement concerning electric field lines:
field lines are close together where the field is large
A series RC circuits is connected to an emf source having angular frequency ω. The current:
leads the source emf by an tan¯¹ (1/ωCR)
An electron moves in the negative x direction, through a uniform magnetic field in the negative y direction. The magnetic force on the electric is in the ______.
negative z direction
Two long parallel straight wires carry equal currents in opposite directions. At a point midway between the wires, the magnetic field they produce is:
non-zero and perpendicular to the plane of the two wires
The direction of the magnetic field in a certain region of space is determined by firing a test charge into the region with its velocity in various directions in different trials. The field direction is:
one of the directions of the velocity when the magnetic force is zero
The current is from left to right in the conductor shown. The magnetic field is into the page and point S is at a higher potential than point T. The charge carriers are _____.
positive
A constant current is sent through a helical coil. The coil:
tends to get shorter
A "coulomb" is:
the amount of charge that flows past a point in one second when the current is 1 A
The magnetic field B inside a long ideal solenoid is independent of:
the cross-sectional area of the solenoid
An electric field is most directly related to:
the force acting on a test charge
The resistance of resistor 1 is twice the resistance of resistor 2. The two are connected in series and a potential difference is maintained across the combination. Then:
the potential difference across 1 is twice that across 2
The figure shows the motion of electrons in a wire that is near the N pole of a magnet. The wire will be pushed:
upwards
An electron is moving north in a region where the magnetic field is south. The magnetic force exerted on the electron is _____.
zero
A series circuit composed of a 0.2 H inductor and a 74-μF capacitor is connected to a 60 V variable frequency source. At what frequency will the current be 4 amperes with a lagging power factor?
48 Hz
Determine the power factor angle in the series circuit which consists of R = 25 ohms, L = 0.2 H across a power supply of 200 volts, 30 Hz?
56.4°
What is the main reason that alternating current replaced direct current for general use?
AC voltage may be conveniently transformed.
When solving for the Thevenin equivalent resistance, RTH:
All voltage sources must be short-circuited.
With an electromagnet,
Both more current and more coil turns mean a stronger magnetic field and if there is no current in the coil, there is no magnetic field.
Resistor 1 has twice the resistance of resistor 2. The two are connected in parallel and a potential difference is maintained across the combination. What is the rate thermal energy generation in 1?
Half that in 2.
When a small voltage is generated across the width of a conductor carrying current in an external magnetic field, the effect is called the ____ effect.
Hall
A constant-voltage source:
Has very low internal resistance.
Which of the following statements is true for the junction in the network shown below?
I₁ - I₂ - I₃ - I₄ + I₅ = 0
In a purely capacitive circuit, the current:
Leads the voltage by one-fourth of a cycle.
In a purely inductive circuit, the current lags the voltage by:
One-fourth of a cycle.
The rms value of an ac current is:
That steady current that produces the same rate of heating in a resistor as the actual current.
Kirchhoff's current law states that:
The algebraic sum of the currents entering and leaving any point in a circuit must equal zero.
Kirchhoff's voltage law states that:
The algebraic sum of the voltage sources and IR voltage drops in any closed path must total zero.
In a series circuit, the largest resistance has:
The largest voltage drop.
A series circuit consists of a battery with internal resistance r and an external resistor R. If these two resistances are equal (r = R) Then the thermal energy generated per unit time by the internal resistance r is:
The same as by R
In the context of the loop and junction rules for electrical circuits a junction is:
Where three or more wires are joined.
The figure below shows a rectangular coil of wire placed in a magnetic field and free to rotate about axis AB. If the current flows into the coil at C, the coil will:
commence to rotate clockwise
The magnetic dipole moment of a current-carrying loop of wire is in the positive z direction. If a uniform magnetic field is in the positive x direction the magnetic torque on the loop is:
in the positive y direction
For loop of a current-carrying wire in a uniform magnetic field the potential energy is a minimum if the magnetic dipole moment of the loop is:
in the same direction as the field
When the frequency is increased, the impedance of a parallel RL circuit ______.
increases
When the frequency of the voltage applied to a series RC circuit is decreased, the phase angle ______.
increases
An inductor and a resistor are in series with a sinusoidal voltage source. The frequency is set so that the inductive reactance is equal to the resistance. If the frequency is increased, then:
V_L > V_R
A power station supplies 60 kW to a load over 2,500 ft, 100 mm², two-conductor copper feeder, the resistance of which is 0.078 ohm per 1000 ft. The bus bar voltage is maintained constant at 600 V. Determine the load current.
108 A
A coil of 25.0 μH and capacitor of 100 pF are connected in series. The frequency is 5.00 MHz. What is the impedance vector, R + jX?
0 + j467
Two resistances of 10 and 15 ohms, each respectively are connected in parallel. The two are then connected in series with a 5-ohm resistance. It is then connected across a 12-V battery, what are the current and power?
1.09 A, 13.08 W
A closed magnetic circuit of cast steel contains a 6 cm long path of cross-sectional area 1 cm² and a 2 cm path of cross-sectional area 0.5 cm². A coil of 200 turns is wound around the 6 cm length of the circuit and a current of 0.4 A flows. Determine the flux density in the 2 cm path, if the relative permeability of the cast steel is 750.
1.51 T
The magnetic field outside a long straight current-carrying wire depends on the distance R from the wire axis according to:
1/R
If R is the distance from a magnetic dipole, then the magnetic field it produces is proportional to:
1/R³
A 50-microfarad is connected in series with a coil having 50 ohms resistance and 150 mH inductance. The source voltage is 100 sin (ωt - 30°) V. What is the maximum power?
199 W
Two long straight wires carrying the same current 15 A exert a force of 0.001 N per unit length on each other. What is the distance between the wires?
4.50 cm
What is the equivalent resistance between points 1 and 2 of the circuit shown below?
5 Ω
Two long straight wires carrying the same current I and separated by a distance r exert a force F on each other. The current is increased to 4I and the separation is reduced to r/6 . What will be the force between two wires? Let Fo = F old.
96Fo
A Thevenin equivalent circuit consist of:
A voltage source in series with a resistance.
In the circuit shown R₁ > R₂ > R₃. Rank the three resistors according to the current in them, least to greatest.
All are same
Capacitive reactance, Xc:
Applies only to sine waves.
The output of a certain voltage divider is 9 V with no load. When a load is connected, the output voltage:
Decreases
In a sinusoidally driven series RLC circuit the current lags the applied emf. The rate at which energy is dissipated in the resistor can be increased by:
Decreasing the capacitance and making no other changes.
If one of the resistors in a parallel circuit is removed, the total resistance:
Increases.
A complete short develops across one of five parallel resistors on a PC board. The most likely result is:
The fuse in the power supply will blow.
For a sine wave, one-half cycle is often called a/an ______.
alternation
An electron travels due north through a vacuum in a region of uniform magnetic field that is also directed due north. It will:
be unaffected by the field
In residential house wiring the hot wire is usually color-coded ______.
black or red
Nine identical wires, each of diameter d and length L, are connected in series. The combination has the same resistance as a single similar wire of length L but whose diameter is:
d/3
When an additional resistor is connected across existing parallel circuit, the total resistance _______.
decreases
When the frequency of the voltage applied to a series RC is increased, the impedance ________.
decreases
A bar magnet is moved at a steady speed of 1.0 m/s towards a coil of wire which is connected to a center-zero galvanometer. The magnet is now withdrawn along the same path of 0.5 m/s. The deflection of the galvanometer is in the:
opposite direction as previously, with the magnitude of the deflection halved
