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physics
modern physics

Questions and Answers of Modern Physics

1. Which of the following can a transformer accomplish? (a) Changing voltage but not current. (b) Changing current but not voltage. (c) Changing power. (d) Changing both current and voltage. 2. A
1. Which of the following statements about transformers is false? (a) Transformers work using ac current or dc current. (b) If the current in the secondary is higher, the voltage is lower. (c) If the
1. The alternating electric current at a wall outlet is most commonly produced by (a) A connection to rechargeable batteries. (b) A rotating coil that is immersed in a magnetic field. (c)
Which of the following is true about all series ac circuits? (a) The voltage across any circuit element is a maximum when the current is a maximum in that circuit element. (b) The current at any
The magnetic flux through a coil of wire containing two loops changes at a constant rate from-58Wb to + 38 Wb in 0.34 s. What is the emf induced in the coil?
A circular loop in the plane of the paper lies in a 0.65-T magnetic field pointing into the paper. The loop's diameter changes from 20.0 cm to 6.0 cm in 0.50 s. What is (a) The direction of the
What is the direction of the induced current in the circular loop due to the current shown in each part of Fig. 21-61? Explain why.(a)(b) (c) (d)
A 600-turn solenoid, 25 cm long, has a diameter of 2.5 cm. A 14-turn coil is wound tightly around the center of the solenoid. If the current in the solenoid increases uniformly from 0 to 5.0A in 0.60
When a car drives through the Earth's magnetic field, an emf is induced in its vertical 55-cm-long radio antenna. If the Earth's field (5.0 x 10-5 T) points north with a dip angle of 38°, what is
Part of a single rectangular loop of wire with dimensions shown in Fig. 21-62 is situated inside a region of uniform magnetic field of 0.550 T. The total resistance of the loop is 0.230Î©.
In order to make the rod of Fig. 21-11a move to the right at speed v, you need to apply an external force on the rod to the right.(a) Explain and determine the magnitude of the required force.(b)
In Fig. 21-11, the rod moves with a speed of 1.6 m/s on rails 30.0 cm apart. The rod has a resistance of 2.5Î©. The magnetic field is 0.35 T, and the resistance of the conductor 21.0
A 22.0-cm-diameter coil consists of 30 turns of circular copper wire 2.6 mm in diameter. A uniform magnetic field, perpendicular to the plane of the coil, changes at a rate of 8.65 x 10-3 T/s.
The magnetic field perpendicular to a single 13.2-cmdiameter circular loop of copper wire decreases uniformly from 0.670 T to zero. If the wire is 2.25 mm in diameter, how much charge moves past a
The generator of a car idling at 1100 rpm produces 12.7 V. What will the output be at a rotation speed of 2500 rpm, assuming nothing else changes?
A 550-loop circular armature coil with a diameter of 8.0 cm rotates at 120 rev / s in a uniform magnetic field of strength 0.55 T. (a) What is the rms voltage output of the generator? (b) What would
A generator rotates at 85 Hz in a magnetic field of 0.030 T. It has 950 turns and produces an rms voltage of 150V and an rms current of 70.0 A. (a) What is the peak current produced? (b) What is the
A simple generator has a square armature 6.0 cm on a side. The armature has 85 turns of 0.59-mm-diameter copper wire and rotates in a 0.65-T magnetic field. The generator is used to power a light
A motor has an armature resistance of 3.65Ω. If it draws 8.20 A when running at full speed and connected to a 120-V line, how large is the back emf?
The back emf in a motor is 72 V when operating at 1800 rpm. What would be the back emf at 2300 rpm if the magnetic field is unchanged?
A transformer is designed to change 117 V into 13,500 V, and there are 148 turns in the primary coil. How many turns are in the secondary coil?
A transformer has 360 turns in the primary coil and 120 in the secondary coil. What kind of transformer is this, and by what factor does it change the voltage? By what factor does it change the
The rectangular loop in Fig. 21-58 is being pushed to the right, where the magnetic field points inward. In what direction is the induced current? Explain your reasoning.
The output voltage of a 95-W transformer is 12 V, and the input current is 25 A. (a) Is this a step-up or a step-down transformer? (b) By what factor is the voltage multiplied?
A transformer has 330 primary turns and 1240 secondary turns. The input voltage is 120 V and the output current is 15.0 A. What are the output voltage and input current?
If 35 MW of power at 45 kV (rms) arrives at a town from a generator 4.6-Ω via transmission lines, calculate (a) The emf at the generator end of the lines, (b) The fraction of the power generated
For the transmission of electric power from power plant to home, as depicted in Fig. 21-25, where the electric power sent by the plant is 100 kW, about how far away could the house be from the power
For the electric power transmission system shown in Fig. 21-25, what is the ratio NS / NP for(a) The step-up transformer,(b) The step-down transformer next to the home?
Suppose 2.0 MW is to arrive at a large shopping mall over two 0.100-Ω lines. Estimate how much power is saved if the voltage is stepped up from 120 V to 1200 V and then down again, rather than
Design a dc transmission line that can transmit 925MW of electricity 185 km with only a 2.5% loss. The wires are to be made of aluminum and the voltage is 660 kV.
If the current in a 160-mH coil changes steadily from 25.0 A to 10.0 A in 350 ms, what is the magnitude of the induced emf?
What is the inductance of a coil if the coil produces an emf of 2.50 V when the current in it changes from -28.0 mA to +31.0 mA in 14.0 ms?
Determine the inductance L of a 0.60-m-long air-filled solenoid 2.9 cm in diameter containing 8500 loops.
An air-filled cylindrical inductor has 2600 turns, and it is 2.5 cm in diameter and 28.2 cm long. (a) What is its inductance? (b) How many turns would you need to generate the same inductance if
A coil has 2.25-Ω resistance and 112-mH inductance. If the current is 3.00A and is increasing at a rate of 3.80 A/s, what is the potential difference across the coil at this moment?
The magnetic field inside an air-filled solenoid 36 cm long and 2.0 cm in diameter is 0.72 T. approximately how much energy is stored in this field?
At t = 0, the current through a 45.0-mH inductor is 50.0 mA and is increasing at the rate of 115mA/s. What is the initial energy stored in the inductor, and how long does it take for the energy to
An 18.5-cm-diameter loop of wire is initially oriented perpendicular to a 1.5-T magnetic field. The loop is rotated so that its plane is parallel to the field direction in 0.20 s. What is the average
It takes 2.56 ms for the current in an LR circuit to increase from zero to 0.75 its maximum value. Determine(a) The time constant of the circuit,(b) The resistance of the circuit if L = 31.0 mH.
How many time constants does it take for the potential difference across the resistor in an LR circuit like that in Fig. 21-37 to drop to 2.5% of its original value, after the switch is moved to the
What is the reactance of a 6.20-μF capacitor at a frequency of? (a) 60.0 Hz, (b) 1.00 MHz
At what frequency will a 32.0-mH inductor have a reactance of 660Ω?
At what frequency will a 2.40-μF capacitor have a reactance of 6.10k Ω?
Calculate the reactance of, and rms current in, a 260-mH radio coil connected to a 240-V (rms) 10.0-kHz ac line. Ignore resistance.
(a) What is the reactance of a well-insulated 0.030-μF capacitor connected to a 2.0-kV (rms) 720-Hz line? (b)What will be the peak value of the current?
A fixed 10.8-cm-diameter wire coil is perpendicular to a magnetic field 0.48 T pointing up. In 0.16 s, the field is changed to 0.25 T pointing down. What is the average induced emf in the coil?
For a 120-V rms 60-Hz voltage, an rms current of 70mA passing through the human body for 1.0 s could be lethal. What must be the impedance of the body for this to occur?
A 36-kΩ resistor is in series with a 55-mH inductor and an ac source. Calculate the impedance of the circuit if the source frequency is (a) 50 Hz, and (b) 3.0x 104 Hz.
A 3.5-k Ω resistor and a 3.0-μF capacitor are connected in series to an ac source. Calculate the impedance of the circuit if the source frequency is (a) 60 Hz, and (b) 60,000 Hz.
Determine the total impedance, phase angle, and rms current in an LRC circuit connected to a 10.0-kHz, 725-V (rms) source if L = 38.0 mH, R = 8.70 k Ω, and C = 6250 pF.
(a) What is the rms current in an LR circuit when a 60.0-Hz 120-V rms ac voltage is applied, where R = 2.80k Ω and L = 350mH? (b) What is the phase angle between voltage and current? (c) How much
(a) What is the rms current in an RC circuit if R = 6.60k Ω, C = 1.80 μF, and the rms applied voltage is 120 V at 60.0 Hz? (b) What is the phase angle between voltage and current? (c) What are
Suppose circuit B in Fig. 21-42a consists of a resistance R = 520 Î©. The filter capacitor has capacitance C = 1.2 Î¼F. Will this capacitor act to eliminate 60-Hz ac but pass
A 3500-pF capacitor is connected in series to a 55.0-μH coil of resistance 4.00 Ω. What is the resonant frequency of this circuit?
A 16-cm-diameter circular loop of wire is placed in a 0.50-T magnetic field.(a) When the plane of the loop is perpendicular to the field lines, what is the magnetic flux through the loop?(b) The
An LRC circuit has L = 14.8 mH and R = 4.10 Ω. (a) What value must C have to produce resonance at 3600 Hz? (b) What will be the maximum current at resonance if the peak external voltage is 150 V?
A resonant circuit using a 260-nF capacitor is to resonate at 18.0 kHz. The air-core inductor is to be a solenoid with closely packed coils made from 12.0 m of insulated wire 1.1 mm in diameter. How
A 2200-pF capacitor is charged to 120 V and then quickly connected to an inductor. The frequency of oscillation is observed to be 19 kHz. Determine (a) The inductance, (b) The peak value of the
Suppose you are looking at two wire loops in the plane of the page as shown in Fig. 21-64. When switch S is closed in the left-hand coil,(a) What is the direction of the induced current in the other
A square loop 24.0 cm on a side has a resistance of 6.10 Ω. It is initially in a 0.665-T magnetic field, with its plane perpendicular B̅ to but is removed from the field in 40.0 ms. Calculate the
A high-intensity desk lamp is rated at 45 W but requires only 12 V. It contains a transformer that converts 120-V household voltage. (a) Is the transformer step-up or step down? (b) What is the
A flashlight can be made that is powered by the induced current from a magnet moving through a coil of wire. The coil and magnet are inside a plastic tube that can be shaken causing the magnet to
Power is generated at 24 kV at a generating plant located 56 km from a town that requires 55 MW of power at 12 kV. Two transmission lines from the plant to the town each have a resistance of 0.10 Ω
The primary windings of a transformer which has 88% efficiency are connected to 110-V ac. The secondary windings are connected across a 2.4-Ω, 75-W light bulb. (a) Calculate the current through the
A pair of power transmission lines each has a 0.95-Ω resistance and carry 740A over 9.0 km. If the rms input voltage is 42 kV, calculate(a) The voltage at the other end,(b) The power input,(c) Power
A coil with 190 turns, a radius of 5.0 cm, and a resistance of 12 Î© surrounds a solenoid with 230 turns / cm and a radius of 4.5 cm (Fig. 21-65). The current in the solenoid changes at a
A certain electronic device needs to be protected against sudden surges in current. In particular, after the power is turned on, the current should raise no more than 7.5 mA in the first 120μs. The
A 35-turn 12.5-cm-diameter coil is placed between the pole pieces of an electromagnet. When the electromagnet is turned on, the flux through the coil changes, inducing an emf at what rate (in T/s)
Calculate the peak output voltage of a simple generator whose square armature windings are 6.60 cm on a side; the armature contains 125 loops and rotates in a field of 0.200 T at a rate of 120 rev /
The moving rod in Fig. 21-11 is 12.0 cm long and is pulled at a speed of 18.0cm/s. If the magnetic field is 0.800 T, calculate(a) The emf developed, and(b) The electric field felt by electrons in the
A 130-mH coil whose resistance is 15.8 Ω is connected to a capacitor C and a 1360-Hz source voltage. If the current and voltage are to be in phase, what value must C have?
The Q factor of a resonant ac circuit (Section 21-15) can be defined as the ratio of the voltage across the capacitor (or inductor) to the voltage across the resistor, at resonance. The larger the Q
(a) Sections 19-7 and 21-9 discuss conditions when and where it is especially important to have ground fault circuit interrupters (GFCIs) installed. What is it about those places that makes "touching
A small electric car overcomes a 250-N friction force when traveling 35 km/h. The electric motor is powered by ten 12-V batteries connected in series and is coupled directly to the wheels whose
A ballistic galvanometer is a device that measures the total charge Q that passes through it in a short time. It is connected to a search coil that measures B (also called a flip coil) which is a
A lost person may signal by switching a flashlight on and off using Morse code. This is actually a modulated EM wave. Is it AM or FM? What is the frequency of the carrier, approximately?
1. In a vacuum, what is the difference between a radio wave and an X-ray? (a) Wavelength. (b) Frequency. (c) Speed. 2. The radius of an atom is on the order of 10-10. In comparison, the wavelength of
1. Which of the following travel at the same speed as light? (Choose all that apply.) (a) Radio waves. (b) Microwaves. (c) Radar. (d) Ultrasonic waves. (e) Infrared radiation. (f) Cell phone
1. In empty space, which quantity is always larger for X-ray radiation than for a radio wave? (a) Amplitude. (c) Frequency. (b) Wavelength. (d) Speed. 2. If electrons in a wire vibrate up and down
1. If the Earth-Sun distance were doubled, the intensity of radiation from the Sun that reaches the Earth's surface would (a) Quadruple. (b) Double. (c) Drop to 1/2 (d) Drop to 1/4 2. An
1. If the intensity of an electromagnetic wave doubles, (a) The electric field must also double. (b) The magnetic field must also double. (c) Both the magnetic field and the electric field must
Starting in 2009, TV stations in the U.S. switched to digital signals. To watch today's digital broadcast TV, could you use a pre-2009 TV antenna meant for analog? Explain. (a) No; analog antennas do
Determine the rate at which the electric field changes between the round plates of a capacitor, 8.0 cm in diameter, if the plates are spaced 1.1 mm apart and the voltage across them is changing at a
An EM wave has a wavelength of 720 nm. What is its frequency, and how would we classify it?
An EM wave has frequency 7.14 x 1014 Hz. What is its wavelength, and how would we classify it?
What is the frequency of a microwave whose wavelength is 1.50 cm?
Electromagnetic waves and sound waves can have the same frequency. (a) What is the wavelength of a 1.00-kHz electromagnetic wave? (b) What is the wavelength of a 1.00-kHz sound wave? (The speed of
(a) What is the wavelength of a radar 22.75 x 109 Hz signal? (b) What is the frequency of an X-ray with wavelength 0.12 nm?
How long would it take a message sent as radio waves from Earth to reach Mars when Mars Is?(a) Nearest Earth,(b) Farthest from Earth? Assume that Mars and Earth are in the same plane and that their
Pulsed lasers used for science and medicine produce very brief bursts of electromagnetic energy. If the laser light wavelength is 1062 nm (Neodymium-YAG laser), and the pulse lasts for 34
Calculate the displacement current ID between the square plates, 5.8 cm on a side, of a capacitor if the electric field is changing at a rate of 1.6 x 106 V / m . s.
The E̅ field in an EM wave has a peak of 22.5 m V / m. What is the average rate at which this wave carries energy across unit area per unit time?
The magnetic field in a traveling EM wave has an rms strength of 22.5 nT. How long does it take to deliver 365 J of energy to 1.00 cm2 of a wall that it hits perpendicularly?
How much energy is transported across a 1.00-cm2 area per hour by an EM wave whose E field has an rms strength of 30.8 m V / m?
A spherically spreading EM wave comes from an 1800-W source. At a distance of 5.0 m, what is the intensity, and what is the rms value of the electric field?
If the amplitude of the B field of an EM wave is 2.2 x 10-7 T. (a) What is the amplitude of the E field? (b) What is the average power transported across unit area by the EM wave?
What is the average energy contained in a 1.00-m3 volume near the Earth's surface due to radiant energy from the Sun? See Example 22-4. In Example 22-4 Radiation from the Sun reaches the Earth (above
A 15.8-mW laser puts out a narrow beam 2.40 mm in diameter. What are the rms values of E and B in the beam?
At a given instant, a 3.8-A current flows in the wires connected to a parallel-plate capacitor. What is the rate at which the electric field is changing between the plates if the square plates are
A high-energy pulsed laser emits a 1.0-ns-long pulse of average power 1.5 x 1011W. The beam is nearly a cylinder 2.2 x 10-3 m in radius. Determine (a) The energy delivered in each pulse, and (b)

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