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physics
modern physics
Questions and Answers of
Modern Physics
Lightning As a crude model for lightning, consider the ground to be one plate of a parallel-plate capacitor and a cloud at an altitude of 550 m to be the other plate. Assume the surface area of the
A parallel-plate capacitor is made from two aluminum-foil sheets, each 3.00 cm wide and 10.0 m long. Between the sheets is a mica strip of the same width and length that is 0.0225 mm thick. What is
Calculate the work done by a 9.0-V battery as it charges a 7.2-(F capacitor in the flash unit of a camera?
An automatic external defibrillator (AED) delivers 135 J of energy at a voltage of 725 V. What is the capacitance of this device?
The membrane of a living cell can be approximated by a parallel-plate capacitor with plates of area 4.75 × 10-9 m2, a plate separation of 8.5 × 10-9 m, and a dielectric with a dielectric constant
A capacitor with plate area 0.0440 m2 and plate separation 85.0 (m is to be charged to 12.0 V and store 7.50 (J of energy. What should be the dielectric constant of the material between the plates?
Find the electric energy density between the plates of a 225-(F parallel-plate capacitor. The potential difference between the plates is 345 V, and the plate separation is 0.223 mm?
When an ion accelerates through a potential difference of -1880 V, its electric potential energy increases by 6.02 × 10-16 J. What is the charge on the ion?
What electric field strength would store 17.5 J of energy in every 1.00 mm3 of space?
An electronic flash unit for a camera contains a capacitor with a capacitance of 750 (F. When the unit is fully charged and ready for operation, the potential difference between the capacitor plates
A parallel-plate capacitor has plates with an area of 405 cm2 and an air-filled gap between the plates that is 2.25 mm thick. The capacitor is charged by a battery to 575 V and then is disconnected
A proton is released from rest in a region of space with a nonzero electric field.(a) As the proton moves, does the electric potential energy of the system increase, decrease, or stay the same?(b)
The plates of a parallel-plate capacitor have constant charges of +Q and -Q. Do the following quantities increase, decrease, or remain the same as the separation of the plates is increased?(a) The
A parallel-plate capacitor is connected to a battery that maintains a constant potential difference V between the plates. If the plates of the capacitor are pulled farther apart, do the following
The plates of a parallel-plate capacitor have constant charges of +Q and -Q. Do the following quantities increase, decrease, or remain the same as a dielectric is inserted between the plates?(a) The
A parallel-plate capacitor is connected to a battery that maintains a constant potential difference V between the plates. If a dielectric is inserted between the plates of the capacitor, do the
Find the difference in electric potential, ΔV = VB - VA, between the points A and B for the following cases:(a) The electric field does 0.052 J of work as you move a +5.7-(C charge from A to B.(b)
A 0.32-(F capacitor is charged by a 1.5-V battery. After being charged, the capacitor is connected to a small electric motor. Assuming 100% efficiency, (a) To what height can the motor lift a 7.5-g
The Earth has a vertical electric field with a magnitude of approximately 100 V/m near its surface. What is the magnitude of the potential difference between a point on the ground and a point on the
A charge of 22.5 (C is located at (4.40 m, 6.22 m), and a charge of -14.2 (C is located at (-4.50 m, 6.75 m). What charge must be located at (2.23 m, -3.31 m) if the electric potential is to be zero
In the Bohr model of the hydrogen atom what is the smallest amount of work that must be done on the electron to move it from its circular orbit, with a radius of 0.529 × 10-10 m, to an infinite
A +1.2-(C charge and a -1.2-(C charge are placed at (0.50 m, 0), and (-0.50 m, 0), respectively.(a) In FIGURE 20-37, at which of the points A, B, C, or D is the electric potential smallest in value?
How much work is required to bring three protons, initially infinitely far apart, to a configuration where each proton is 1.5 × 10-15 m from the other two? (This is a typical separation for protons
A point charge Q = +87.1 (C is held fixed at the origin. A second point charge, with mass m = 0.0576 kg and charge q = -2.87 (C, is placed at the location (0.323 m, 0).(a) Find the electric potential
An electron is at rest just above the surface of a sphere with a radius of 2.7 mm and a uniformly distributed positive charge of 1.8 × 10-15 C. Like a rocket blasting off from the Earth, the
According to the quark model of fundamental particles, neutrons-the neutral particles in an atom's nucleus-are composed of three quarks. Two of these quarks are "down" quarks, each with a charge of
A parallel-plate capacitor is charged to an electric potential of 305 V by moving 3.75 × 1016 electrons from one plate to the other. How much work is done in charging the capacitor?
The three charges shown in Figure 20-33 are held in place as a fourth charge, q, is brought from infinity to the point P. The charge q starts at rest at infinity and is also at rest when it is placed
(a) In Figure 20-36 we see that the electric potential increases by 10.0 V as one moves 4.00 cm in the positive x direction. Use this information to calculate the x component of the electric field.
A uniform electric field with a magnitude of 6860 N/C points in the positive x direction. Find the change in electric potential energy when a +14.5-(C charge is moved 5.75 cm in(a) The positive x
The electric catfish (Malapterurus electricus) is an aggressive fish, 1.0 m in length, found today in tropical Africa. The catfish is capable of generating jolts of electricity up to 350 V by
Many electric cars can capture the kinetic energy of the car and store it in a capacitor or a battery by means of an electric generator. If a 1380-kg electric car brakes from 11.0 m/s to rest and
Many computer keyboards operate on the principle of capacitance. As shown in Figure 20-18, each key forms a small parallel-plate capacitor whose separation is reduced when the key is depressed.Figure
A point charge of mass 0.081 kg and charge +6.77 (C is suspended by a thread between the vertical parallel plates of a parallel-plate capacitor, as shown in FIGURE 20-38.Figure 20-38,(a) If the
Many cells in the body have a cell membrane whose inner and outer surfaces carry opposite charges, just like the plates of a parallel-plate capacitor. Suppose a typical cell membrane has a thickness
Every cell in the body has organelles called mitochondria that can generate a voltage difference between their interior and exterior. (a) If the capacitance of a mitochondrion is 4.3 × 10-11 F and
Long, long ago, on a planet far, far away, a physics experiment was carried out. First, a 0.250-kg ball with zero net charge was dropped from rest at a height of 1.00 m. The ball landed 0.552 s
In 1911, Ernest Rutherford developed a planetary model of the atom, in which a small positively charged nucleus is orbited by electrons. The model was motivated by an experiment carried out by
(a) One of the -Q charges in Figure 20-34 is given an outward "kick" that sends it off with an initial speed (0 while the other three charges are held at rest. If the moving charge has a mass m, what
Figure 20-38 shows a charge q = + 6.77 (C with a mass m = 0.081 kg suspended by a thread of length L = 0.022 m between the plates of a capacitor.Figure 20-38,(a) Plot the electric potential energy of
Explain why birds can roost on high-voltage wires without being electrocuted?
Explain how electrical devices can begin operating almost immediately after you throw a switch, even though individual electrons in the wire may take hours to reach the device?
A flashlight bulb carries a current of 0.38 A for 98 s. How much charge flows through the bulb in this time? How many electrons?
When a potential difference of 12 V is applied to a given wire, it conducts 0.45 A of current. What is the resistance of the wire?
A 15.0-V battery is connected to terminals A and B in Figure 21-52.Figure 21-52,(a) Given that C = 15.0 (F, find the charge on each of the capacitors.(b) Find the total energy stored in this
When two resistors, R1 and R2, are connected in series across a 6.0-V battery, the potential difference across R1 is 4.0 V. When R1 and R2 are connected in parallel to the same battery, the current
The circuit shown in FIGURE 21-62 is known as a Wheatstone bridge. Find the value of the resistor R such that the current through the 85.0-Ω resistor is zero?Figure 21-62,
Suppose the voltmeter measures a potential difference of 3.70 V across the resistor. What is the current that flows through the person's body? a. 3.70 × 10-6 A b. 5.00 × 10-5 A c. 0.0740 A d. 3.70 A
What is the resistance of the person and footwear when the voltmeter reads 3.70 V?a. 1.25 × 107 b. 1.35 × 107c. 4.63 × 107d. 1.71 × 108
The resistance of a given person and footwear is 4.00 × 107. What is the reading on the voltmeter when this person is tested? a. 0.976 V b. 1.22 V c. 1.25 V d. 50.0
The standard specifies that footwear should be tested three times: with the person standing on only the left shoe; with the person standing on only the right shoe; and with the person standing on the
Suppose the three resistors in this circuit have the values R1 = 100.0(, R2 = 200.0(, and R3 = 300.0(, and the emf of the battery is 12.0 V. (The resistor numbers are given in the Interactive
Suppose R1 = R2 = 225( and R3 = R. The emf of the battery is 12.0 V. (The resistor numbers are given in the Interactive Figure.)(a) Find the value of R such that the current supplied by the battery
Suppose the resistance of the 126-( resistor is reduced by a factor of 2. The other resistor is 275 (, the capacitor is 182 (F, and the battery has an emf of 3.00 V.(a) Does the final value of the
The tungsten filament of a light-bulb has a resistance of 0.07 Ω. If the filament is 27 cm long, what is its diameter?
Suppose the capacitance of the 182-(F capacitor is reduced by a factor of 2. The two resistors are 126 and 275 , and the battery has an emf of 3.00 V. (a) Find the final value of the charge on the
A wire is made out of aluminum with a cross-sectional area of 5.25 × 10−4 m2 and a length of 65 km. What is its resistance?
In a tDCS treatment procedure, 0.85 mA of current flows through a patient's brain when there is a 7.4-V potential difference between the electrodes. What is the equivalent resistance between the
The four conducting cylinders shown in FIGURE 21-38 are all made of the same material, though they differ in length and/or diameter. They are connected to four different batteries, which supply the
A bird lands on a bare copper wire carrying a current of 32 A. The wire is 6 gauge, which means that its cross-sectional area is 0.13 cm2.(a) Find the difference in potential between the bird's feet,
A current of 0.86 A flows through a copper wire 0.74 mm in diameter when it is connected to a potential difference of 15 V. How long is the wire?
A typical cell membrane is 8.0 nm thick and has an electrical resistivity of 1.3 × 107 Ω ( m.(a) If the potential difference between the inner and outer surfaces of a cell membrane is 75 mV, how
When a potential difference of 12 V is applied to a wire 6.9 m long and 0.33 mm in diameter, the result is an electric current of 2.1 A. What is the resistivity of the wire?
(a) What is the resistance per meter of a gold wire with a cross-sectional area of 1.6 × 10-7 m2?(b) Would your answer to part (a) increase, decrease, or stay the same if the diameter of the wire
A car battery does 360 J of work on the charge passing through it as it starts an engine.(a) If the emf of the battery is 12 V, how much charge passes through the battery during the start?(b) If the
The interior of the human body has an electrical resistivity of 0.15 Ω ( m. (a) Estimate the resistance for current flowing the length of your index finger. (For this calculation, ignore the much
Consider a rectangular block of metal of height A, width B, and length C, as shown in FIGURE 21-39. If a potential difference V is maintained between the two A × B faces of the block, a current
Light A has four times the power rating of light B when operated at the same voltage.(a) Is the resistance of light A greater than, less than, or equal to the resistance of light B? Explain.(b) What
Two light-bulbs operate on the same potential difference. Bulb A has four times the power output of bulb B.(a) Which bulb has the greater current passing through it? Explain.(b) What is the ratio of
Two light-bulbs operate on the same current. Bulb A has four times the power output of bulb B.(a) Is the potential difference across bulb A greater than or less than the potential difference across
A 65-V generator supplies 4.8 kW of power. How much current does the generator produce?
A portable CD player operates with a current of 18 mA at a potential difference of 4.3 V. What is the power usage of the player?
Find the power dissipated in a 22-Ω electric heater connected to a 120-V outlet?
The current in a 120-V reading lamp is 2.6 A. If the cost of electrical energy is $0.075 per kilowatt-hour, how much does it cost to operate the light for an hour?
Circuit A in a house has a voltage of 208 V and is limited by a 40.0-A circuit breaker. Circuit B is at 120.0 V and has a 20.0-A circuit breaker. What is the ratio of the maximum power delivered by
Highly sensitive ammeters can measure currents as small as 10.0 fA. How many electrons per second flow through a wire with a 10.0-fA current?
A 65-W light-bulb operates on a potential difference of 95 V. Find (a) The current in the bulb and (b) The resistance of the bulb. (c) If this bulb is replaced with one whose resistance is half the
Car batteries are rated by the following two numbers: (1) Cranking amps = current the battery can produce for 30.0 seconds while maintaining a terminal voltage of at least 7.2 V and (2) Reserve
A dozen identical light-bulbs are connected to a given emf.(a) Will the lights be brighter if they are connected in series or in parallel?(b) Choose the best explanation from among the following:I.
A circuit consists of three resistors, R1 < R2 < R3, connected in series to a battery. Rank these resistors in order of increasing(a) Current through them and(b) Potential difference across
Two resistors are connected in parallel. (a) If a third resistor is now connected in parallel with the original two, does the equivalent resistance of the circuit increase, decrease, or remain the
What is the minimum number of 88-Ω resistors that must be connected in parallel to produce an equivalent resistance of 12 Ω or less?
Find the equivalent resistance between points A and B for the group of resistors shown in FIGURE 21-40.
A 9.00-V battery is connected across the terminals A and B for the group of resistors shown in Figure 21-40. What is the potential difference across each resistor?
In a string of holiday lights, 50 lights-with a resistance of 13.5 Ω each-are connected in series. (a) If the string of lights is plugged into a 120.0-V outlet, what current flows through it? (b)
Your toaster has a power cord with a resistance of 0.020 Ω connected in series with a 9.6-Ω nichrome heating element. If the potential difference between the terminals of the toaster is 120 V, how
A television set connected to a 120-V outlet consumes 21 W of power.(a) How much current flows through the television?(b) How much time does it take for 10 million electrons to pass through the TV?
A circuit consists of a 12.0-V battery connected to three resistors (37 Ω, 22 Ω, and 141 Ω) in series. Find(a) The current that flows through the battery and(b) The potential difference across
Three resistors, 11 Ω, 53 Ω, and R, are connected in series with a 24.0-V battery. The total current flowing through the battery is 0.16 A.(a) Find the value of resistance R.(b) Find the potential
A circuit consists of a battery connected to three resistors (65 Ω, 25 Ω, and 170 Ω) in parallel. The total current through the resistors is 2.8 A. Find(a) The emf of the battery and(b) The
Three resistors, 22 Ω, 67 Ω, and R, are connected in parallel with a 12.0-V battery. The total current flowing through the battery is 0.88 A.(a) Find the value of resistance R.(b) Find the current
A 99-Ω resistor has a current of 0.82 A and is connected in series with a 110-Ω resistor. What is the emf of the battery to which the resistors are connected?
The equivalent resistance between points A and B of the resistors shown in FIGURE 21-41 is 33 Ω. Find the value of resistance R.
Find the equivalent resistance between points A and B shown in FIGURE 21-42.
How many 23-W light-bulbs can be connected in parallel across a potential difference of 92 V before the total current in the circuit exceeds 3.0 A?
The circuit in FIGURE 21-43 includes a battery with a finite internal resistance, r = 0.50 Ω.Figure 21-43,(a) Find the current flowing through the 7.1-Ω and the 3.2-Ω resistors.(b) How much
A 12-V battery is connected to terminals A and B in Figure 21-41.Figure 21-41,(a) Given that R = 85 Ω, find the current in each resistor.(b) Suppose the value of R is increased. For each resistor
Pacemakers designed for long-term use commonly employ a lithium-iodine battery capable of supplying 0.42 A ( h of charge.(a) How many coulombs of charge can such a battery supply?(b) If the average
The terminals A and B in Figure 21-42 are connected to a 9.0-V battery.Figure 21-42,(a) Find the current flowing through each resistor.(b) Is the potential difference across the 6.3-Ω resistor
Suppose the battery in Figure 21-43 has an internal resistance r = 0.25 Ω.Figure 21-43,(a) How much current flows through the battery?(b) What is the potential difference between the terminals of
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