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physics
physics scientists and engineers

Questions and Answers of Physics Scientists and Engineers

The circuit shown in FIGURE P28.48 is inside a 15-cm diameter balloon filled with helium that is kept at a constant pressure of 1.2 atm.How long will it take the balloon??s diameter to increase to 16
A string of holiday lights can be wired in series, but all the bulbs go out if one burns out because that breaks the circuit. Most lights today are wired in series, but each bulb has a special fuse
What is the current in the 2 Ω resistor in FIGURE P28.53? 20 9V 3 E 3 V FIGURE P28.53
Suppose you have resistors 2.5 kΩ, 3.5 kΩ, and 4.5 kΩ and a 100 V power supply. What is the ratio of the total power delivered to the resistors if they are connected in parallel to the total power
For an ideal battery (r = 0 Ω), closing the switch in FIGURE P28.54 does not affect the brightness of bulb A. In practice, bulb A dims just a little when the switch closes. To see why, assume that
a. Load resistor R is attached to a battery of emf Ԑ and internal resistance r. For what value of the resistance R, in terms of Ԑ and r, will the power dissipated by the load resistor be a
The ammeter in FIGURE P28.52 reads 3.0 A. Find I1, I2, and Ԑ. 9.0 V 3.0 Ω ww- 2.0 Ω ww A 3.0 A 1.0Ω FIGURE P28.52
What are the battery current Ibat and the potential difference Va - Vb between points a and b when the switch in FIGURE P28.55 is (a) Open (b) Closed? 24 V FIGURE P28.55
A battery is a voltage source, always providing the same potential difference regardless of the current. It is possible to make a current source that always provides the same current regardless of
A circuit you're building needs an ammeter that goes from 0 mA to a full-scale reading of 50 mA. Unfortunately, the only ammeter in the storeroom goes from 0μA to a full scale reading of only
For the circuit shown in FIGURE P28.58, find the current through and the potential difference across each resistor. Place your results in a table for ease of reading. 6Ω 50 24 V= 10 Ω
For the circuit shown in FIGURE P28.60, find the current through and the potential difference across each resistor. Place your results in a table for ease of reading. 30 160 ww 12 V Ut 48
For the circuit shown in FIGURE P28.62, find the current through and the potential difference across each resistor. Place your results in a table for ease of reading. 24 0 5Ω ww ww 12 V 12Ω 3
What is the current through the 10 Ω resistor in FIGURE P28.63?Is the current from left to right or right to left? ww 50 12 V ww 10 N 3 V ww 50 9 V FIGURE P28.63
How much current flows through the bottom wire in FIGURE P28.66, and in which direction? 10Ω ww 9 V E15 V 12Ω 24Ω Bottom wire FIGURE P28.66
A 12 V car battery dies not so much because its voltage drops but because chemical reactions increase its internal resistance.A good battery connected with jumper cables can both start the engine and
The capacitor in an RC circuit is discharged with a time constant of 10 ms. At what time after the discharge begins are(a) The charge on the capacitor reduced to half its initial value(b) The energy
A 150 μF defibrillator capacitor is charged to 1500 V. When fired through a patient’s chest, it loses 95% of its charge in 40 ms.What is the resistance of the patient’s chest?
A 50 μF capacitor that had been charged to 30 V is discharged through a resistor. FIGURE P28.70 shows the capacitor voltage as a function of time.What is the value of the resistance? AVc (V) 30-
A 70 μF capacitor is discharged through two parallel resistors, 15 kΩ and 25 kΩ. By what factor will the time constant of this circuit increase if the resistors are instead placed in series with
A 0.25 μF capacitor is charged to 50 V. It is then connected in series with a 25 Ω resistor and a 100 Ω resistor and allowed to discharge completely. How much energy is dissipated by the 25 Ω
The capacitors in FIGURE P28.74 are charged and the switch closes at t = 0 s. At what time has the current in the 8 Ω resistor decayed to half the value it had immediately after the switch was
The flash on a compact camera stores energy in a 120 μF capacitor that is charged to 220 V. When the flash is fired, the capacitor is quickly discharged through a lightbulb with 5.0 Ω of
Large capacitors can hold a potentially dangerous charge long after a circuit has been turned off, so it is important to make sure they are discharged before you touch them. Suppose a 120 μF
Digital circuits require actions to take place at precise times, so they are controlled by a clock that generates a steady sequence of rectangular voltage pulses. One of the most widely used
What power is dissipated by the 2 Ω resistor in FIGURE CP28.78? 15 VE 203 12 V FIGURE CP28.78
You’ve made the finals of the Science Olympics! As one of your tasks, you’re given 1.0 g of aluminum and asked to make a wire, using all the aluminum, that will dissipate 7.5 W when connected to
The switch in FIGURE CP28.80 has been closed for a very long time. a. What is the charge on the capacitor?b. The switch is opened at t = 0 s. At what time has the charge on the capacitor decreased
The capacitor in Figure 28.38a begins to charge after the switch closes at t = 0 s. Analyze this circuit and show that Q = Qmax(1 - e-t/Ï„), where Qmax= CÔ.Figure 28.38a Switch
The switch in Figure 28.38a closes at t = 0 s and, after a very long time, the capacitor is fully charged. Find expressions forFigure 28.38a(a) The total energy supplied by the battery as the
An oscillator circuit is important to many applications. A simple oscillator circuit can be built by adding a neon gas tube to an RC circuit, as shown in FIGURE CP28.83. Gas is normally a good
The tip of a flashlight bulb is touching the top of the 3 V battery in FIGURE Q28.2. Does the bulb light? Why or why not? 00, 3 V FIGURE Q28.2
Draw a circuit diagram for the circuit of FIGURE EX28.1. 75 0 T00n FIGURE EX28.1
Rank in order, from largest to smallest, the currents Iato Idthrough the four resistors in FIGURE Q28.1. + 2V - + 1V - 2Ω + 2V - 1Ω + 1V - 1Ω Ia FIGURE Q28.1
A thin metal cylinder of length L and radius R1 is coaxial with a thin metal cylinder of length L and a larger radius R2. The space between the two coaxial cylinders is filled with a material that
A 300 μF capacitor is charged to 9.0 V, then connected in parallel with a 5000 Ω resistor. The capacitor will discharge because the resistor provides a conducting pathway between the capacitor
Figure CP27.74 shows a wire that is made of two equaldiameter segments with conductivities Ïƒ1and Ïƒ2. When current I passes through the wire, a thin layer of charge appears at
A 5.0-mm-diameter proton beam carries a total current of 1.5 mA. The current density in the proton beam, which increases with distance from the center, is given by J = Jedge(r/R), where R is the
A long, round wire has resistance R. What will the wire’s resistance be if you stretch it to twice its initial length?
Two 10-cm-diameter metal plates 1.0 cm apart are charged to ±12.5 nC. They are suddenly connected together by a 0.224-mm-diameter copper wire stretched taut from the center of one plate to the
The batteries in Figure P27.68 are identical. Both resistors have equal currents. What is the resistance of the resistor on the right? R ? FIGURE P27.68 3.
A 20-cm-long hollow nichrome tube of inner diameter 2.8 mm, outer diameter 3.0 mm is connected, at its ends, to a 3.0 V battery. What is the current in the tube?
A 0.60-mm-diameter wire made from an alloy (a combination of different metals) has a conductivity that decreases linearly with distance from the center of the wire: σ(r) = σ0 - cr, with σ0 = 5.0
A wire of radius R has a current density that increases linearly with distance from the center of the wire: J(r) = kr, where k is a constant. Find an expression for k in terms of R and the total
An aluminum wire consists of the three segments shown in FIGURE P27.64.The current in the top segment is 10 A.For each of these three segments, find thea. Current I.b. Current density J.c. Electric
What diameter should the nichrome wire in FIGURE P27.63 be in order for the electric field strength to be the same in both wires? Nichrome Aluminum 1.0 mm diameter FIGURE P27.63
The two wires in FIGURE P27.62 are made of the same material.What are the current and the electron drift speed in the 2.0-mm-diameter segment of the wire? Va = 2.0 x 104 m/s I = 2.0 A 1.0 mm 2.0 mm
The current supplied by a battery slowly decreases as the battery runs down. Suppose that the current as a function of time is I = (0.75 A)e-t/(6 h). What is the total number of electrons transported
The current in a wire at time t is given by the expression I = (2.0 A)e-t/(2.0 μs), where t is in microseconds and t ≥ 0.a. Find an expression for the total amount of charge (in coulombs) that has
The total amount of charge that has entered a wire at time t is given by the expression Q = (20 C)(1 - e-t/(2.0 s)), where t is in seconds and t ≥ 0.a. Find an expression for the current in the
A hollow metal sphere has inner radius a, outer radius b, and conductivity σ. The current I is radially outward from the inner surface to the outer surface.a. Find an expression for the electric
A hollow metal cylinder has inner radius a, outer radius b, length L, and conductivity σ. The current I is radially outward from the inner surface to the outer surface.a. Find an expression for the
You need to design a 1.0 A fuse that “blows” if the current exceeds 1.0 A. The fuse material in your stockroom melts at a current density of 500 A/cm2. What diameter wire of this material will do
Electrical engineers sometimes use a wire’s conductance, G = σA/L, instead of its resistance.a. Write Ohm’s law in terms of conductance, starting with “ΔV = ”.b. What is the conductance of
The resistivity of a metal increases slightly with increased temperature. This can be expressed as ρ = ρ0 [1 + α(T - T0)], where T0 is a reference temperature, usually 20°C, and α is the
The conductive tissues of the upper leg can be modeled as a 40-cm-long, 12-cm-diameter cylinder of muscle and fat. The resistivities of muscle and fat are 13 Ω m and 25 Ω m, respectively. One
The conducting path between the right hand and the left hand can be modeled as a 10-cm-diameter, 160-cm-long cylinder. The average resistivity of the interior of the human body is 5.0 Ω m. Dry skin
Variations in the resistivity of blood can give valuable clues about changes in various properties of the blood. Suppose a medical device attaches two electrodes into a 1.5-mm-diameter vein at
The resistivity of a metal increases slightly with increased temperature. This can be expressed as ρ = ρ0[ 1 + α(T - T0)], where T0 is a reference temperature, usually 20°C, and α is the
A 1.5-m-long wire is made of a metal with the same electron density as copper. The wire is connected across the terminals of a 9.0 V battery. What conductivity would the metal need for the drift
The starter motor of a car engine draws a current of 150 A from the battery. The copper wire to the motor is 5.0 mm in diameter and 1.2 m long. The starter motor runs for 0.80 s until the car engine
The biochemistry that takes place inside cells depends on various elements, such as sodium, potassium, and calcium, that are dissolved in water as ions. These ions enter cells through narrow pores in
You’ve been asked to determine whether a new material your company has made is ohmic and, if so, to measure its electrical conductivity. Taking a 0.50 mm × 1.0 mm × 45 mm sample, you wire the
In a classic model of the hydrogen atom, the electron moves around the proton in a circular orbit of radius 0.053 nm.a. What is the electron’s orbital frequency?b. What is the effective current of
A sculptor has asked you to help electroplate gold onto a brass statue. You know that the charge carriers in the ionic solution are singly charged gold ions, and you’ve calculated that you must
Figure P27.42 shows a 4.0-cm-wide plastic film being wrapped onto a 2.0-cm-diameter roller that turns at 90 rpm. The plastic has a uniform surface charge density of -2.0 nC/cm2.a. What is the current
Energetic particles, such as protons, can be detected with a silicon detector. When a particle strikes a thin piece of silicon, it creates a large number of free electrons by ionizing silicon atoms.
The electron beam inside an old television picture tube is 0.40 mm in diameter and carries a current of 50 μA. This electron beam impinges on the inside of the picture tube screen.a. How many
For what electric field strength would the current in a 2.0 mm-diameter nichrome wire be the same as the current in a 1.0-mm-diameter aluminum wire in which the electric field strength is 0.0080 V/m?
An ideal battery would produce an extraordinarily large current if “shorted” by connecting the positive and negative terminals with a short wire of very low resistance. Real batteries do not. The
Household wiring often uses 2.0-mm-diameter copper wires. The wires can get rather long as they snake through the walls from the fuse box to the farthest corners of your house. What is the potential
A circuit calls for a 0.50-mm-diameter copper wire to be stretched between two points. You don’t have any copper wire, but you do have aluminum wire in a wide variety of diameters.What diameter
Figure EX27.35 is a current versus- potential-difference graph for a material. What is the material€™s resistance? I (A) 2- 04 AV (V) 100 50
The resistance of a very fine aluminum wire with a 10 μm × 10 μm square cross section is 1000 Ω. A 1000 Ω resistor is made by wrapping this wire in a spiral around a 3.0-mm diameter glass core.
Pencil “lead” is actually carbon. What is the current if a 9.0 V potential difference is applied between the ends of a 0.70 mm-diameter, 6.0-cm-long lead from a mechanical pencil?
The femoral artery is the large artery that carries blood to the leg. What is the resistance of a 20-cm-long column of blood in a 1.0-cm-diameter femoral artery? The conductivity of blood is 0.63
The terminals of a 0.70 V watch battery are connected by a 100-m-long gold wire with a diameter of 0.10 mm. What is the current in the wire?
a. How long must a 0.60-mm-diameter aluminum wire be to have a 0.50 A current when connected to the terminals of a 1.5 V flashlight battery?b. What is the current if the wire is half this length?
The electric field inside a 30-cm-long copper wire is 5.0 mV/m. What is the potential difference between the ends of the wire?
An engineer cuts a 1.0-m-long, 0.33-mm-diameter piece of wire, connects it across a 1.5 V battery, and finds that the current in the wire is 8.0 A. Of what material is the wire made?
What is the resistance ofa. A 2.0-m-long gold wire that is 0.20 mm in diameter?b. A 10-cm-long piece of carbon with a 1.0 mm × 1.0 mm square cross section?
Wires 1 and 2 are made of the same metal. Wire 2 has twice the length and twice the diameter of wire 1. What are the ratios(a) ρ2/ρ1 of the resistivities(b) R2/R1 of the resistances of the two
A 1.5 V battery provides 0.50 A of current.a. At what rate (C/s) is charge lifted by the charge escalator?b. How much work does the charge escalator do to lift 1.0 C of charge?c. What is the power
The two segments of the wire in Figure EX27.24 have equal diameters but different conductivities Ïƒ1and Ïƒ2. Current I passes through this wire. If the conductivities have the
A 0.50-mm-diameter silver wire carries a 20 mA current.What are(a) The electric field(b) The electron drift speed in the wire?
What is the mean time between collisions for electrons in(a) An aluminum wire(b) An iron wire?
A car battery is rated at 90 A h, meaning that it can supply a 90 A current for 1 h before being completely discharged. If you leave your headlights on until the battery is completely dead, how much
A hollow copper wire with an inner diameter of 1.0 mm and an outer diameter of 2.0 mm carries a current of 10 A. What is the current density in the wire?
The current in a 2.0 mm × 2.0 mm square aluminum wire is 2.5 A. What are(a) The current density(b) The electron drift speed?
When a nerve cell fires, charge is transferred across the cell membrane to change the cell’s potential from negative to positive.For a typical nerve cell, 9.0 pC of charge flows in a time of 0.50
The current in an electric hair dryer is 10.0 A. How many electrons flow through the hair dryer in 5.0 min?
Which, if any, of these statements are true? (More than one may be true.) Explain. Assume the batteries are ideal.a. A battery supplies the energy to a circuit.b. A battery is a source of potential
In an integrated circuit, the current density in a 2.5-μm-thick × 75-μm-wide gold film is 7.5 × 105 A/m2. How much charge flows through the film in 15 min?
The wires in Figure Q27.11 are all made of the same material.Rank in order, from largest to smallest, the resistances Rato Reof these wires. Explain. 2L 2r 4L 2L 2r 2r FIGURE Q27.11
The current in a 100 watt lightbulb is 0.85 A. The filament inside the bulb is 0.25 mm in diameter.a. What is the current density in the filament?b. What is the electron current in the filament?
The current in a wire is doubled. What happens to(a) The current density,(b) The conduction-electron density,(c) The mean time between collisions,(d) The electron drift speed? Are each of these
The wires leading to and from a 0.12-mm-diameter lightbulb filament are 1.5 mm in diameter. The wire to the filament carries a current with a current density of 4.5 × 105 A/m2. What are(a) The
The wire in Figure Q27.9 consists of two segments of different diameters but made from the same metal. The current in segment 1 is I1.a. Compare the currents in the two segments. That is, is I2
The mean time between collisions in iron is 4.2 × 10-15 s.What electron current is driven through a 1.8-mm-diameter iron wire by a 0.065 V/m electric field?
Both batteries in Figure Q27.8 are ideal and identical, and all lightbulbs are the same. Rank in order, from brightest to least bright, the brightness of bulbs a to c. Explain. 00 Q00 000 FIGURE Q27.8

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