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Physic
Q:
In the capacitor discharge formula q= q0e-t/RCthe symbol trepresents:A) the time constantB) the time it takes for C to lose the fraction 1/eof its initial chargeC) the time it takes for C to lose the fraction (1 - 1/e) of its initial chargeD) the time it takes for C to lose essentially all of its initial chargeE) none of the above
Q:
A certain capacitor, in series with a resistor, is being charged. At the end of 10 ms its charge is half the final value. The time constant for the process is about:
A) 5.0 ms
B) 6.9 ms
C) 10 ms
D) 14 ms
E) 20 ms
Q:
Here is a loop equation: . What does this equation represent?
A) a charging capacitor
B) a discharging capacitor
C) a capacitor that has been disconnected
D) a charging resistor
E) an oscillating circuit
Q:
Suppose the current charging a capacitor is kept constant. Which graph below correctly gives the potential difference Vacross the capacitor as a function of time? A) I
B) II
C) III
D) IV
E) V
Q:
The time constant RChas units of:
A) second/farad
B) second/ohm
C) 1/second
D) second/watt
E) none of these
Q:
A certain voltmeter has an internal resistance of 10,000 and a range from 0 to 12 V. To extend its range to 120 V, use a series resistance of:A) 1,111 B) 90,000 C) 100,000 D) 108,000 E) 120,000
Q:
Compared to a conductor, a semiconductor
A) has larger conductivity
B) has smaller resistivity
C) has a larger charge carrier density
D) has a negative temperature coefficient of resistivity
E) has zero resistivity
Q:
A 1.5-V battery is connected to a circuit and puts out a current of 0.45 A. How much power is the battery putting out?
A) 0.30 W
B) 0.68 W
C) 3.3 W
D) 5.0 W
E) 7.4 W
Q:
A current of 0.3 A passes through a lamp for 2 minutes using a 6 V power supply. The energy dissipated by this lamp during the 2 minutes is:
A) 1.8 J
B) 12 J
C) 20 J
D) 36 J
E) 216 J
Q:
You buy a "75 W" light bulb. The label means that:
A) no matter how you use the bulb, the power will be 75 W
B) the bulb was filled with 75 W at the factory
C) the actual power dissipated will be much higher than 75 W since most of the power appears as heat
D) the bulb is expected to "burn out" after you use up its 75 watts
E) none of the above
Q:
A certain x-ray tube requires a current of 7 mA at a voltage of 80 kV. The rate of energy dissipation is:
A) 5600W
B) 560 W
C) 26 W
D) 11.4 W
E) 0.088W
Q:
It is better to send 10,000 kW of electric power long distances at 10,000 V rather than at 220 V because:
A) there is less heating in the transmission wires
B) the resistance of the wires is less at high voltages
C) more current is transmitted at high voltages
D) the insulation is more effective at high voltages
E) the iRdrop along the wires is greater at high voltage
Q:
The mechanical equivalent of heat is 1 cal = 4.18 J. The specific heat of water is 1 cal/gK, and its mass is 1 g/cm3. An electric immersion water heater, rated at 400 W, should heat a liter of water from 10ï‚°C to 30ï‚°C in about:
A) 12 s
B) 50 s
C) 3.5 min
D) 15 min
E) 45 min
Q:
An incandescent light bulb is marked "60 watt, 120 volt". Its resistance is:A) 0.5B) 120 C) 180 D) 240 E) 300
Q:
A certain resistor dissipates 0.5 W when connected to a 3 V potential difference. When connected to a 1 V potential difference, this resistor will dissipate:
A) 0.056 W
B) 0.167 W
C) 0.50 W
D) 1.5 W
E) none of these
Q:
An iron is marked "120 volt, 600 watt". In normal use, the current in it is:
A) 0.2 A
B) 2 A
C) 4 A
D) 5 A
E) 7.2 A
Q:
A student kept her 60-watt, 120-volt study lamp turned on from 2:00 PMuntil 2:00 AM. How much charge went through it?
A) 720C
B) 3,600 C
C) 7,200 C
D) 18,000 C
E) 21,600 C
Q:
You wish to triple the rate of energy dissipation in a heating device. To do this you could triple:
A) the potential difference keeping the resistance the same
B) the current keeping the resistance the same
C) the resistance keeping the potential difference the same
D) the resistance keeping the current the same
E) both the potential difference and current
Q:
Which one of the following quantities is correctly matched to its unit?
A) power kWh
B) energy kW
C) potential difference J/C
D) current A/s
E) resistance V/C
Q:
The mechanical equivalent of heat is 1 cal = 4.18 J. A heating coil, connected to a 120-V source, develops 60,000 calories in 10 minutes. The current in the coil is:
A) 0.83 A
B) 2.0 A
C) 3.5 A
D) 500 A
E) 2100 A
Q:
Suppose the electric company charges 10 cents per kWh. How much does it cost to use a 125 watt lamp 4 hours a day for 30 days?
A) $1.20
B) $1.50
C) $1.80
D) $7.20
E) none of these
Q:
Energy may be measured in:
A) kilowatt
B) joulesecond
C) watt
D) wattsecond
E) volt/ohm
Q:
The rate at which electrical energy is used may be measured in:
A) watt/second
B) wattsecond
C) watt
D) joulesecond
E) kilowatthour
Q:
Two substances are identical except that the electron mean free time for substance A is twice the electron mean free time for substance B. If the same electric field exists in both substances the electron drift speed in A is:
A) the same as in B
B) twice that in B
C) half that in B
D) four times that in B
E) one fourth that in B
Q:
For an ohmic substance, the resistivity depends on:
A) the electric field
B) the potential difference
C) the current density
D) the electron mean free time
E) the cross-sectionalarea of the sample
Q:
For an ohmic substance, the electron drift velocity is proportional to:
A) the cross-sectional area of the sample
B) the length of the sample
C) the mass of an electron
D) the electric field in the sample
E) none of the above
Q:
For an ohmic substance the resistivity is the proportionality constant for:
A) current and potential difference
B) current and electric field
C) current density and potential difference
D) current density and electric field
E) potential difference and electric field
Q:
For a cylindrical resistor made of ohmic material, the resistance does NOT depend on:
A) the current
B) the length
C) the cross-sectional area
D) the resistivity
E) the electron drift velocity
Q:
For an ohmic resistor, resistance is the proportionality constant for:
A) potential difference and electric field
B) current density and electric field
C) current and length
D) current and cross-sectional area
E) current and potential difference
Q:
A certain sample carries a current of 4 A when the potential difference is 2 V and a current of 10 A when the potential difference is 4 V. This sample:A) obeys Ohm's lawB) has a resistance of 0.5 at 2 VC) has a resistance of 2at 2 VD) has a resistance of 25 at 2 VE) does not have a resistance
Q:
Which of the following graphs best represents the current-voltage relationship for a device that obeys Ohm's law? A) I
B) II
C) III
D) IV
E) V
Q:
Which of the following graphs best represents the current-voltage relationship of an incandescent light bulb?A) IB) IIC) IIID) IVE) V
Q:
The resistance of a rod does NOT depend on:
A) its temperature
B) its material
C) its length
D) its conductivity
E) shape of its (fixed) cross-sectional area
Q:
Of the following the copper conductor that has the least resistance is:
A) thin, long and hot
B) thick, short and cool
C) thick, long and hot
D) thin, short and cool
E) thin, short and hot
Q:
Two conductors are made of the same material and have the same length. Conductor A is a solid wire of diameter 1 m. Conductor B is a hollow tube of inside diameter 1 m and outside diameter 2 m. The ratio of their resistance, RA/RB, is:
A) 1
B) C) 2
D) 3
E) 4
Q:
A wire is 1 m long and 1x10-06m2in cross-sectional area. When connected to a potential difference of 2 V, a current of 4 A exists in the wire. The resistivity of this wire is:
Q:
A certain wire has resistance R. Another wire, of the same material, has half the length and half the diameter of the first wire. The resistance of the second wire is:
A) R/4
B) R/2
C) R
D) 2R
E) 4R
Q:
A cylindrical copper rod has resistance R. It is reformed to twice its original length with no change of volume. Its new resistance is:
A) R/2
B) R
C) 2R
D) 4R
E) 8R
Q:
Five cylindrical wires are made of the same material. Their lengths and radii areRank the wires according to their resistances, least to greatest.A) 1, 2, 3, 4, 5B) 5, 4, 3, 2, 1C) 1 and 2 tie, then 5, 3, 4D) 1, 3, 4, 2, 5E) 1, 2, 4, 3, 5
Q:
Conductivity is:A) the same as resistivity, it is just more convenient to use for good conductorsB) expressed in -1C) equal to 1/resistanceD) expressed in (.m)-1E) not a meaningful quantity for an insulator
Q:
A wire has an electric field of 6.2 V/m and carries a current density of 2.4 x 108A/m2. What is its resistivity?
Q:
A current of 0.5 ampere exists in a 60-ohm lamp. The applied potential difference is:
A) 15 V
B) 30 V
C) 60 V
D) 120 V
E) none of these
Q:
If the potential difference across a resistor is doubled:
A) only the current is doubled
B) only the current is halved
C) only the resistance is doubled
D) only the resistance is halved
E) both the current and resistance are doubled
Q:
The units of resistivity are:
A) ohm
B) ohmmeter
C) ohm/meter
D) ohm/meter2
E) none of these
Q:
Copper contains 8.4 x1028free electrons per cubic meter. A copper wire of cross-sectional area 7.4 x10-7m2carries a current of 1 A. The electron drift speed is approximately:A) 3 x108m/sB) 103m/sC) 1 m/sD) 10-4m/sE) 10-23m/s
Q:
The current density is the same in two wires. Wire A has twice the free electron concentration of wire B. The drift speed of electrons in A is:
A) twice that of electrons in B
B) four times that of electrons in B
C) half that of electrons in B
D) one-fourth that of electrons in B
E) the same as that of electrons in B
Q:
The current is zero in a conductor when no potential difference is applied because:
A) the electrons are not moving
B) the electrons are not moving fast enough
C) for every electron with a given velocity there is another with a velocity of equal magnitude and opposite direction
D) equal numbers of electrons and protons are moving together
E) otherwise Ohm's law would not be valid
Q:
In a conductor carrying a current we expect the electron drift speed to be:
A) much greater than the average electron speed
B) much less than the average electron speed
C) about the same as the average electron speed
D) less than the electron speed at low temperature and greater than the electron speed at high temperature
E) less than the electron speed at high temperature and greater than the electron speed at low temperature
Q:
A wire with a length of 150 m and a radius of 0.15 mm carries a current with a uniform current density of 2.8 ï‚´ï€ 107 A/m2. The current is:
A) 0.63 A
B) 2.0 A
C) 5.9 A
D) 300 A
E) 26000 A
Q:
Two wires made of different materials have the same uniform current density. They carry the same current only if:
A) their lengths are the same
B) their cross-sectional areas are the same
C) both their lengths and cross-sectional areas are the same
D) the potential differences across them are the same
E) the electric fields in them are the same
Q:
If is the current density and is a vector element of area then the integral over an area represents:
A) the electric flux through the area
B) the average current density at the position of the area
C) the resistance of the area
D) the resistivity of the area
E) the current through the area
Q:
Conduction electrons move to the right in a certain wire. This indicates that:
A) the current density and electric field both point right
B) the current density and electric field both point left
C) the current density points right and the electric field points left
D) the current density points left and the electric field points right
E) the current density points left but the direction of the electric field is unknown
Q:
In schematic diagrams, currents are indicated using arrows. What do the arrows indicate?
A) the direction of motion of the electrons
B) the direction of the current vector
C) the direction of motion of the charge carriers
D) the direction that positive charge carriers would move
E) nothing; they are just a convenient drawing tool
Q:
This figure shows a junction. What is true of the currents? A) i1= i0+ i2
B) i2= i0+ i1
C) i0= i1" i2
D) i1= i0" i2
E) i0= i2" i1
Q:
Current in a conductor is normally due to:
A) the collective motion of all electrons in the conductor
B) the thermal motion of the conduction electrons
C) the drift motion of the conduction electrons due to electric fields
D) the drift motion of both electrons and positrons
E) the drift motion of both electrons and protons
Q:
A 10-ohm resistor has a constant current. If 1200 C of charge flow through it in 4 minutes what is the value of the current?
A) 3.0 A
B) 5.0 A
C) 20 A
D) 120 A
E) 300 A
Q:
A 60-watt light bulb carries a current of 0.5 ampere. The total charge passing through it in one hour is:
A) 3600 C
B) 3000 C
C) 2400 C
D) 1800 C
E) 120 C
Q:
Current is a measure of:
A) force that moves a charge past a point
B) resistance to the movement of a charge past a point
C) energy used to move a charge past a point
D) amount of charge that moves past a point per unit time
E) speed with which a charge moves past a point
Q:
Current has units:
A) kilowatthour
B) ampere
C) coulomb
D) volt
E) ohm
Q:
Current has units:
A) kilowatthour
B) coulomb/second
C) coulomb
D) volt
E) ohm
Q:
A car battery is rated at 80 A.h. An ampere-hour is a unit of:A) powerB) energyC) currentD) chargeE) force
Q:
Two capacitors are identical except that one is filled with air and the other with oil. Both capacitors carry the same charge. The ratio of the electric fields Eair/Eoilis:
A) between 0 and 1
B) 0
C) 1
D) between 1 and infinity
E) infinite
Q:
An air-filled capacitor is charged, and then a dielectric is inserted. As a result, there is an induced charge on the dielectric. What is the difference between induced charge and free charge?
A) There is no difference.
B) Induced charge does not result in a net charge on the dielectric.
C) Induced charge is smaller than free charge.
D) Free charge creates an electric field, but induced charge does not.
E) Free charge creates an electric potential, but induced charge does not.
Q:
What happens to the atoms in a dielectric when it is placed between the plates of a charged capacitor?
A) They begin to conduct electricity.
B) They create an induced electric field that is in the opposite direction of the field due to the charges on the plates.
C) They create an induced electric field that is in the same direction as the field due to the charges on the plates.
D) They completely cancel the electric field due to the charges on the plates.
E) They rotate so their positive ends are towards the positively charged plate.
Q:
What is the difference between a polar dielectric and a nonpolar dielectric?
A) A polar dielectric has a permanent electric field.
B) A nonpolar dielectric never has an internal electric field.
C) The molecules of a polar dielectric have a permanent electric dipole moment.
D) A nonpolar dielectric can have an induced electric field in any direction.
E) A polar dielectric is always aligned with the Earth's electric field.
Q:
A parallel-plate capacitor, with air dielectric, is charged by a battery, after which the battery is disconnected. A slab of glass dielectric is then slowly inserted between the plates. As it is being inserted:
A) a force repels the glass out of the capacitor
B) a force attracts the glass into the capacitor
C) no force acts on the glass
D) a net charge appears on the glass
E) the glass makes the plates repel each other
Q:
A dielectric slab is slowly inserted between the plates of a parallel plate capacitor while the capacitor is connected to a battery. As it is being inserted:
A) the capacitance, the potential difference between the plates, and the charge on the positive plate all increase
B) the capacitance, the potential difference between the plates, the charge on the positive plate all decrease
C) the potential difference between the plates increases, the charge on the positive plate decreases, and the capacitance remains the same
D) the capacitance and the charge on the positive plate decrease but the potential difference between the plates remains the same
E) the capacitance and the charge on the plate increase but the potential difference between the plates remains the same
Q:
Which of the following is not a dielectric?
A) silicon
B) germanium
C) polystyrene
D) titanium
E) paper
Q:
One of the materials listed below is to be placed between two identical metal sheets, with no air gap, to form a parallel-plate capacitor. Which produces the greatest capacitance?
A) material of thickness 0.1 mm and dielectric constant 2
B) material of thickness 0.2 mm and dielectric constant 3
C) material of thickness 0.3 mm and dielectric constant 2
D) material of thickness 0.4 mm and dielectric constant 8
E) material of thickness 0.5 mm and dielectric constant 11
Q:
An air-filled parallel-plate capacitor has a capacitance of 1 pF. The plate separation is then doubled and a wax dielectric is inserted, completely filling the space between the plates. As a result, the capacitance becomes 2 pF. The dielectric constant of the wax is:
A) 0.25
B) 0.50
C) 2.0
D) 4.0
E) 8.0
Q:
There are approximately 10 explosions in the US every year due to agricultural grain (corn and soybean) dust. Why is corn dust so dangerous?
A) Corn contains a lot of oil and is therefore very flammable.
B) Grain elevators use a lot of flammable material while processing the corn.
C) Corn dust has a lot of surface area and can be ignited by even a very small spark, which can be created simply by a person walking around.
D) Due to the presence of lots of agricultural material, grain elevators are especially rich in oxygen.
E) Workers are not careful about where they throw their cigarette butts, and sometimes set the corn on fire.
Q:
The quantity has the dimensions of:A) energy/faradB) energy/coulombC) energyD) energy/volumeE) energy/volt
Q:
A parallel-plate capacitor has a plate area of 0.30 m2and a plate separation of 0.10 mm. If the charge on each plate has a magnitude of 5.0x10-6C, what is the energy density in its electric field?A) 0.16 J/m3B) 3.5 J/m3C) 7.8 J/m3D) 16 J/m3E) 24 J/m3
Q:
A battery is used to charge a parallel-plate capacitor, after which it is disconnected. Then the plates are pulled apart to twice their original separation. This process will double the:
A) capacitance
B) surface charge density on each plate
C) stored energy
D) electric field between the two places
E) charge on each plate
Q:
A certain capacitor has a capacitance of 5.0. After it is charged to 5 and isolated, the plates are brought closer together so its capacitance becomes 10 . The work done by the agent is about:
Q:
To store a total of 0.040 J of energy in the two identical capacitors shown, each should have a capacitance of:
Q:
Capacitors A and B are identical. Capacitor A is charged so it stores 4 J of energy and capacitor B is uncharged. The capacitors are then connected in parallel. The total stored energy in the capacitors is now:
A) 16 J
B) 8 J
C) 4 J
D) 2 J
E) 1 J
Q:
A 2 and a 1 capacitor are connected in series and charged by a battery. They store energies P and Q, respectively. When disconnected and charged separately using the same battery, they have energies Rand S, respectively. Then:A) R>P>S>QB) P>Q>R>SC) R>P>Q>SD) P>R>S>QE) R>S>Q>P
Q:
A charged capacitor stores 10 C at 40 V. Its stored energy is:
A) 400 J
B) 200 J
C) 4 J
D) 2.5 J
E) 1.25 J
Q:
A 20-F capacitor is charged to 200 V. Its stored energy is:
A) 4000 J
B) 4 J
C) 0.4 J
D) 0.1 J
E) 0.004 J