Physics 2020 Final
How long does it take for the X-rays emitted by the Sun to reach the Earth?
(a.) 8 minutes 20 sec.
A magnetic field has a magnitude of 1.2 mT, and an electric field has a magnitude of 4,600 N/C. Both fields point in the +x direction. A positive 1.8 μC charge moves at a speed of 3,100 km/s in the +y-direction. Determine the magnitude of the electric force that acts on the charge.
(a.) 8.28 ×10-3 N F(e) = qE =(q const.)(E-field magnitude)
Suppose that potential difference across a resistor drops to one-half of its initial value. Based on Ohm's law, which one of the following statements is correct?
(A) current through the resistor gets decreased to one-half of its initial value. Note, I=(1/RV).
An object has 2.0× 10^20 excess electrons on its surface. What charge does it carry?
(a.) -32.0C -> Q = Nqe -> total charge = # of elec. x charge of electron
How much work is required to move a charge of 1.0 C from 75-V surface to 190-V surface AND back to 75-V surface?
(a.) 0 J = W = QΔV = (1C) (75V - 75V)
How much work is required to move a charge of 1.0 C along the 75-V equipotential surface?
(a.) 0 J, no work is required to move a charge along equipotential surface
Two equipotential surfaces surround a +1.5 × 10C point charge. One is a 190-V surface and another is a 75-Vsurface. How much work is required to move a charge of 1.0 C along the 75-V equipotential surface?
(a.) 0 J, no work is required to move a charge along equipotential surface because in such a move ΔV=0 so W=qΔV = 0J.
An electron enters a 0.2 T magnetic field in the direction 30 degrees to the field and follows a helical path. If the electron enters with the initial speed of 7.2×103 m/s, how much work does the MAGNETIC FORCE perform on the electron?
(a.) 0.0 J, because at every instant, electron's displacement and magnetic force are at 90°
A constant magnetic field of magnitude 0.40 T is directed along the +y-axis. A 1.3-m long conductive rod moves with a constant speed 2.7 m/s. The length of the rod is aligned with the x-axis. What is the magnitude of the Emf induced in the rod when the rod moves along +y-axis?
(a.) 0.0V, because the rod is oriented along the direction of magnetic field.
When two point charges are at 4.0-cm distance apart they experience a repulsive force of 16.0 N. What will the magnitude of the repulsion be hen they become separated by a 16.0-cm distance?
(a.) 1.0N A 16.0-cm distance is 4 times larger than a 4.0 cm distance. Coulomb's law is an inverse-square distance law.
The resistance of a bagel toaster is 12.0 Ω. To prepare a bagel, the toaster is operated from a 120-V DC outlet. What is the power rating of the toaster?
(a.) 1.2 kW -> P=IV = V^2/R = (120V)^2/12Ω = 1200W = 1.2kW
What is the angular speed when freq. = 2.38095 Hz?
(a.) 100pi/21 rad/s = omega= 2πf = 2π/(0.42s)
The secondary coil of a step-up transformer provides the voltage that operates an electrostatic air filter. The turns ratio of the transformer is 50:1. The primary coil is plugged to a standard 120-V outlet. What is the voltage that operates the air filter?
(a.) 6,000 V = 120V(50 turns) /(1 turn) V(primary)*# turns(primary) = V(secondary)*#turns(secondary)
How much power is delivered to this circuit at resonance? (RLC at resonance, 144-Ω resistor, a 1.5-μF capacitor, a 40.0-mH inductor, 36.0V)
(a.) 9.0W = IV = VxV/R, because at resonance Z=R & L=C, so ignore capacitor and inductor
On a highway there is a flashing light to mark the start of a section of the road where the work is being done. Who measures the proper time interval between two flashes of light?
(a.) A worker standing still on the road.
What is the direction of the electric field must be to cause the ball to float above the ground?
(a.) DOWN. Direction of the E-field is determined by the direction of force that it exerts on a positive test charge. Our charge is negative and in order for it to float the ground must be negatively charged. But when the ground is negatively charged, the positive test charge will be attracted to it, i.e., it will be pulled down. Hence, the direction of E-field is "down."
One kWh is a unit of BLANK?
(a.) Energy
In a linearly polarized electromagnetic wave, oscillations of the electric field occur only BLANK.
(a.) along one direction that is perpendicular to the direction of wave propagation.
Suppose that potential difference across a resistor drops to one-half of its initial value. Based on Ohm's law, which one of the following statements is correct?
(a.) current through the resistor gets decreased to one-half of its initial value, because the current I is directly proportional to the potential drop V: I=V/R.
In Young's double-slit experiment, a bright interference fringe on the screen is formed at location where the incoming light waves BLANK.
(a.) meet in phase.
A unit of Ω•F is equivalent to which one of the following units?
(a.) seconds, because Ω•F is the unit of τau =RC that is the time constant of an RC circuit.
Two point charges, +3.4 μC and -6.1μC, are separated by 1.2 m. What is the electric potential midway between them?
(b.) -40.45 kV V=V1+V2 = k(Q1/r1)+k(Q2/r2) = 2(k/r)(Q1+Q2): Q's are known, r is known and k is constant.
A positive 1.8 μC charge moves at a speed of 3,100 km/s in the +y-direction & the E-field and B-field point in the +x-direction, determine the direction of the magnetic force that acts on the charge.
(b.) -z-direction, use RHR, charge is moving +y (thumb,up) & E-field is +x (index, right) = -z (middle finger)
In an RLC series circuit the impedance is 50.0-Ω and the AC voltage has a peak value of 25.0 V. What is the AC current amplitude?
(b.) 0.5A = I = V/Z
What voltage is required to store 8.0 × 10-9 C of charge on the plates of a 6.0-nF capacitor?
(b.) 1.333V, C=Q/V -> V=Q/C
A 60-W lamp is placed in parallel with a toaster and with a 120-V DC source. What is the voltage across the toaster?
(b.) 120 V, circuits elements connected in parallel have the same potential drop: here, 120V source = potential drop across the toaster = potential drop across the lamp
A 60-W lamp is placed in parallel with a 2-Ω resistor and with a 120-V DC source. What is the voltage across the lamp?
(b.) 120V, circuit elements connected in parallel have the same potential drop across.
A conducting wire is shaped in a circular coil of one turn that encloses a 2.0-m2 area. There is a current of 4.0 A in the wire. In the presence of the 0.25 T magnetic field, what is the maximum torque that this coil may experience?
(b.) 2.0 Nm: TauMAX = AIB = 2.0Nm
A 2.0μ capacitor is connected in series with a 4.0 Ω resistor. What is the current through the capacitor at the instant when the current through the resistor is 2.0 mA?
(b.) 2.0 mA, one current through circuit elements in series.
Two resistors, 2.0 Ω and 3.0 Ω are connected in parallel, and then this parallel combination is connected in series with a 1.0 Ω resistor. What is the equivalent resistance of this entire resistor network?
(b.) 2.2 Ω, First, equivalent resistance for two resistors, 2.0 Ω and 3.0 Ω, connected in parallel is 1.2 Ω Then, when 1.2 Ω resistor is connected in series with1.0 Ω resistor, resistances add up (1.2+1)Ω
Two capacitors are connected in parallel. One has a capacitance of 2.0 μF and the other has a capacitance of 4.0 μF. The potential drop across the 2.0-μF capacitor is measured to be 4.0 V. What is the potential drop measured across the 4.0-μF capacitor of this arrangement?
(b.) 4.0 V, circuits elements connected in parallel have the same potential drop
Sunlight enters the top of the Earth atmosphere with the electric field whose rms value is 720 N/C. What is the average total density of sunlight?
(b.) 4.59x10^-6 J/m^3 = epsilonNAUGHT (constant) x E^2rms = total AVERAGE density, b/c RMS
A tiny ball (mass = 0.012 kg) carries a charge of -18μC. What magnitude of electric field is needed to cause the ball to float above the ground?
(b.) 6,533 V/m. Force that pulls the ball down is w = mg = (0.012kg)(9.8m/s) = 0.1176N, and the electrostatic force that must pull the ball up is F=qE. In order for the ball to "float" these two forces must balance each other out: F=w -> qE=w -> E =(w/q) = (0.1176N)/(18x10-6 C)=6,533(N/C). Note, unit V/m is equivalent to unit N/C.
The wavelength of violet light is 405 nm, which corresponds to the frequency of BLANK.
(b.) 7.4×1014 Hz f=c/λ = c/405nm
A 60-W lamp is placed in series with a resistor and then a 120-V DC source is connected across. If the voltage across the lamp is 25.0 V, what is the voltage across the resistor?
(b.) 95 V, potential drop measured across circuits elements connected in series is the sum of potential drops measured across individual elements: 25 V + 95 V = 120 V.
At a location near the equator, the Earth's magnetic field is horizontal and points North. An electron is moving vertically upward from the ground. What is the direction of the magnetic force that acts on the electron?
(b.) East, by right-hand rule.
What is the direction of the electric field due to source (q1 = +8.5 μC@ +3.0cm) at location x = 0 cm?
(c.) to the west, away from the source because the source charge is positive. E-fields radiate AWAY from (+) source and TOWARD (-) source.
Electromagnetic waves that are generated by a microwave oven have a frequency of about 2.45 GHz, and the waves that are generated by X-ray machine at a dentist office have frequency between 3×10^16 Hz and 3×10^19 Hz. Which statement about their energies is correct?
(b.) X-ray have a greater energy than microwaves. (Because, the greater the frequency the greater the energy)
One Farad is a unit of BLANK?
(b.) capacitance
A beam of protons enters a 0.3 T magnetic field in the direction perpendicular to the field. What trajectory do the protons follow in this magnetic field?
(b.) circular, in the plane perpendicular to the field
A flat screen is located some distance away from a single slit. Light with a wavelength of 510 nm (in vacuum) shines through the slit and produces a diffraction pattern on the screen, where BLANK.
(b.) the central fringe is bright.
The force of repulsion that two like charges exert on each other is 3.5 N. What will the force be if the distance between the charges is increased to five times its original value?
(c.) 0.14 N, inverse-square law: when distance gets larger 5 times the force gets smaller 5^2 times.
Inside a long solenoid the magnetic field is 0.2 T when the current in the solenoid is 10.0 A. If the current gets increased to 20.0 A, what will the new magnitude of the magnetic field be?
(c.) 0.4 T, because magnetic field inside a long solenoid is B=μ0nI. When the current increases from I = 10 A to I' = 20 A = 2I OR Current and B-field are DIRECTLY proportional in solenoid.
How much work is required to move a charge of 1.0 C from 75-V surface to 190-V surface?
(c.) 115 J, W=qΔV = (1C)(190-75)V = 115V
Figure 1 shows a plot of the output of a generator as a function of time. The coil of this device has a cross-sectional area per turn of 0.020 m2 and contains 150 turns. What is the frequency (in Hz) of the generator? (Period = .42s)
(c.) 2.38095 Hz = f = 1/T = 1/(0.42s)
A cigarette lighter in a car is a resistor that, when activated, is connected across the 12-V battery. Suppose that a lighter uses 33W of power. What is the current that the battery delivers to the lighter?
(c.) 2.75A, P=IV -> I = (P/V) = (33W/12V) = 2.75A
A beam of protons enters a 0.3 T magnetic field in the direction perpendicular to the field. If the protons enter with the initial speed of 7.2×103 m/s, what is the magnitude of the magnetic force that acts on them?
(c.) 3.456x10-16 N -> F=evBsin(90°) = 3.456x10-16N e is const., velocity, B-field, Sin(90° )=1
A 4.0μF capacitor is connected across the terminals of a12.0-V battery. What is the charge on the capacitor's plates?
(c.) 48.0μC, Q=CV
At one point in space the x-component of an electric field vector has value 3.0 N/C, and the y-component has value 4.0 N/C. What is the magnitude of this electric field in this point in space?
(c.) 5 N/C. Use the parallelogram rule for vector addition, i.e., the superposition principle for vector fields: E=√(E_x^2+E_y^2 )= √(3^2+4^2 ) N/C=√25 N/C
RLC series circuit: R=300.0 Ω, the inductance is L=60.0 mH, the capacitance is C = 0.5 μF, and the voltage has an amplitude of V0 = 50.0 V. What is the impedance of this circuit (Z)?
(c.) 500Ω = Z = sqrt(R^2 =(Xc-XL)^2) Calculate Xc and XL from given inductance and capacitance.
Two capacitors are connected in series. One has a capacitance of 2.0 μF and the other has a capacitance of 4.0 μF. The potential drop across the 2.0-μF capacitor is measured to be 4.0 V and the potential drop across the 4.0-μF capacitor is measured to be 2.0 V. What is the potential drop measured across this arrangement of two capacitors?
(c.) 6.0 V, potential drop measured across circuits elements connected in series is the sum of potential drops measured across individual elements
A 8.0-Ω resistor and a12.0-Ω resistor are connected in parallel. The current through the 8.0 Ω resistor is 1.0 A. What is the potential drop across the 12.0-Ω resistor?
(c.) 8.0 V, First, use Ohm's law to get potential drop across the 8.0-Ω resistor: V=IR=(1A)(8Ω)=8V. Then, the potential drop across the 12.0-Ω resistor must be the same as the potential drop across the 8.0-Ω resistor because they are connected in parallel.
An electromagnetic wave travels in a vacuum. The wavelength of the wave is tripled. How is it accomplished?
(c.) By reducing the frequency of the wave by a factor of three, because c=fλ.
One eV is a unit of BLANK?
(c.) Energy
One kWh is a unit of BLANK
(c.) Energy
In SI system of units, self-inductance is measured in the units of BLANK.
(c.) Henry
A vacuum parallel-plate capacitor has a capacitance of 2.0 nF. When a dielectric material with a dielectric constant of 3.3 is inserted between its plates, which one of the following statements is correct?
(c.) The capacitance gets increased.
In spherical electromagnetic waves emitted by antenna, the rays are BLANK.
(c.) are radial lines showing the direction of propagation of the wave.
The RLC series circuit contains a 144-Ω resistor, a 1.5-μF capacitor, a 40.0-mH inductor, and a generator whose rms voltage is 36.0 V. At what frequency of the generator is the power transferred to this circuit at its maximum?
(c.) at the resonance frequency
A capacitor is a device that stores electric BLANK?
(c.) charge
Resistivity is a property of BLANK?
(c.) material
In a vacuum two particles have charges of q1 and q2, where q1 = +3.5 μC. They are separated by a distance of 0.26 m, and particle 1 experiences an attractive force of 3.4 N. What is q2 (magnitude and sign)?
(c.) q2 = -7.3μC, must be neg. b/c it is attracted to a positive charge. F = kq1q2/r^2, solve for q2 (F, k, q1 and r are given)
In light-interference experiments, a bright fringe on a screen occurs at some point when BLANK.
(c.) the difference in distances that the waves travel from the coherent source to this point is equal to an integer number of wavelengths.
One object carries a charge -8 nC and another object carries a charge of +5 nC. How much charge in total must be transferred to both these objects in order to make them neutral?
(d.) + 3 nC. We must transfer +8 nC to the first object and -5 nC to the second object, which gives the total of (+8 -5) nC = +3 nC that must be transferred.
A magnetic field has a magnitude of 0.078 T and is uniform over a circular surface whose area is πm2. What is the magnetic flux through this surface when the magnetic field is oriented PARALLEL to the plane of the circular loop?
(d.) 0.000 Wb, When the magnetic field is oriented parallel to the plane of the loop, the angle between the normal direction to the loop and the direction of magnetic field is 90° & cos (90°) = 0 :: Mag Flux = (surface area)x(field)x(cos(90°)) = 0 Wb
Two wires are identical, except that one is aluminum and one is copper. The aluminum wire has a resistance of 0.20 Ω. What is the resistance of the copper wire? (resistivity of copper: ρCu =1.72 × 10 mΩ & resistivity of aluminum: ρAl = 2.82 × 10 mΩ)
(d.) 0.12 Ω -> Rcu/Ral = ρCu / ρAl
Magnetic flux that passes through one turn of a 12-turn coil of wire changes from 4.0 Wb to 9.0 Wb in a time of 0.05s. What is the average emf induced in the coil?
(d.) 1,200V, Change of the flux through one turn of a coil is (9 - 4)Wb = 5 Wb. It is change in flux of B-field = no. of turns(12)x(5 Wb) = 60 Wb through a 12-turn coil. So the induced Emf is: abs. (Change in B-field Flux/change in time) = (60Wb/0.05s) = 1,200V
Suppose that the current between the walls of a biological cell is 2.0× 10-8 A. If the current is composed of Na+ ions (i.e., q = +1.6× 10-19 C), how many such ions flow in 1.0 ms?
(d.) 1.25x10^8 I=ΔQ/Δt = Ne/Δt -> number of ions = [current(I)xΔt]/Charge(C) = IΔt/q
A constant magnetic field of magnitude 0.40 T is directed along the +y-axis. A 1.3-m long conductive rod moves with a constant speed 2.7 m/s. What is the magnitude of the Emf induced in the rod when the rod moves along +z-axis?
(d.) 1.404 V, Emf = LBv = (1.3m)(0.4T)(2.7 m/s) = 1.404V length, B-field & velocity determine Emf FLUX
Two resistors, 42.0 Ω and 64.0 Ω are connected in series. What is the equivalent resistance of this arrangement?
(d.) 106.0 Ω, in series connection the equivalent resistance is the sum of resistances
Two resistors, 42.0Ω and 64.0Ω are connected in series. What is the equivalent resistance of this arrangement?
(d.) 106.0Ω= 42.0 Ω + 64.0 Ω, for series connection.
A capacitor stores 12.0 nC of charge when connected to a 6.0 V battery. How much charge does the capacitor store when connected to a 9.0 V battery?
(d.) 18.0 nC -> C=Q1/V1=Q2/V2, solve for Q2
Figure 1 shows a plot of the output of a generator as a function of time. The coil of this device has a cross-sectional area per turn of 0.020 m2 and contains 150 turns. What is the rms voltage generated by this generator? (Peroid = 0.42s, peak Voltage = +28V/-28V)
(d.) 19.798V = 28.0V/√2 Vrms = Vpeak/√2
An electric field of 260 N/C points due west at a certain spot. What are the magnitude and direction of the force that acts on a charge of -1.0 C at this spot?
(d.) 260 N, to the east on a negative charge, magnitude: F = Q E.
Two capacitors are connected in parallel. One has a capacitance of 2.0 μF and the other has a capacitance of 4.0 μF. What is the equivalent capacitance of this arrangement?
(d.) 6.0 μF For connection in parallel, equivalent capacitance C is: C = C1+C2=(2+4)μF
In RLC series circuit the resistance is R=300.0 Ω, the inductance is L =60.0 mH, the capacitance is C = 0.5 μF, and the voltage has an amplitude of V0 = 50.0 V and oscillates with an angular frequency of ω = 10,000 rad/s. What is the inductive reactance of this circuit?
(d.) 600Ω = XL = ωL = (10,000 rad/s) x (60x10^-3 Henry)
Two charges are placed on the x-axis. One of the charges (q1 = +8.5 μC) is at x1 = +3.0 cm, and the other charge (q2 = - 21.0 μC) is at x2 = +9.0 cm. What is the magnitude of the electric field due to the source q1 at location x = 0 cm?
(d.) 8.490x10^7 N/C -> just solving for E-field caused by 1 point charge: E=k(Q/r^2) [k, Q1, r are given]
Energy density of electromagnetic wave is measured in the units of BLANK.
(d.) J/m^3
A coil of conducting wire has a resistance of 38.0 Ω at 25°C. What resistance is expected when the temperature of the wire decreases to 5° C?
(d.) Resistance will decrease, as resistance tends to increase with temperature.
A long straight wire carries a current of 48 A. The magnetic field produced by this current at a certain point is 0.8 μT. Where is this point located?
(d.) at a distance 12 m to the wire: B=(μ/2π)(I/R) -> R=(μ/2π)(I/B) = 12m.
One eV is a unit of BLANK
(d.) energy
Two equipotential surfaces surround a +1.5 × 10-8 C point charge. How far is the 190-V surface from 75-V surface?
(d.)1.088m Δr(distance) = kQ([1/V1]-[1/V2]), (constant 'k') x (point charge 'Q') x (Δ(inverse of voltages 1 & 2))
Special theory of relativity was proposed by BLANK.
(e.) A. Einstein in 1905.
At a location near the equator, the Earth's magnetic field is horizontal and points North. A neutron is moving vertically upward from the ground. What is the direction of the magnetic force that acts on the neutron?
(e.) The magnetic force is zero because the charge of neutron is 0 C.
At a location near the equator, the Earth's magnetic field is horizontal and points North. An electron is moving horizontally to the ground in the direction from North to South. What is the direction of the magnetic force that acts on the electron?
(e.) The magnetic force is zero, because its velocity is along the lines of B-field.
Consider a person along with a frame of reference in each of the following situations. In which one of the situations is the frame of reference an inertial frame of reference?
(e.) The person is on a train that moves with a constant speed along straight-line track.
In SI system of units, magnetic flux is measured in the units of BLANK.
(e.) Weber
Choose the correct answer: 'Motional electromotive force' (i.e., 'motional Emf') is BLANK.
(e.) a potential difference that is induced in a conducting rod when the rod moves in magnetic field.
A long straight wire carries a current of 48 A in the direction from West to East. The magnetic field produced by this current is located BLANK.
(e.) outside the wire, its lines form concentric circles around the wire.
Faraday's law of electromagnetic induction states that BLANK.
(e.) time-varying magnetic flux through a coil produces time-varying emf in a coil.
When polarizer and analyzer are crossed, i.e., when the angle between their transmission axis is 90 degrees, the average intensity of light leaving the analyzer is equal to BLANK.
(e.) zero
