PHY 1409 Schaub Final

In the circuit shown in the figure (with answer), R1 = R2 = 90.0 Ω, R3 = R4 = 20.0 Ω , V1 = 7.0 V, V2 = 8.0V, and the batteries are both ideal. What current does the ammeter read? A) 0.40 A B) 0.056 A C) 0.83 A D) 0.050 A

A) 0.40 A Voltage change across the loop = 0. V2 - R4*A = 0 8.0 - 20Ω * A = 0 A = 0.04

What is the velocity of a beam of electrons that goes undeflected when moving perpendicular to an electric and magnetic field. E and B are perpendicular to each other and have a magnitude of 7700 V/m and 7.5 mT respectively. A) 1.0 x 10^6 m/s B) 10 x 10^6 m/s C) 20 x 10^6 m/s D) 2.0 x 10^6 m/s

A) 1.0 x 10^6 m/s v = E/B v = 7700 / 7.5 * 10^-3 v = 1.02 * 10^6 m/s

Two 5.0-μC point charges are 12 cm apart. What is the electric potential of this combination at the point where the electric field due to these charges is zero? A) 1.5 MV B) 0.75 MV C) 25 MV D) 12.5 MV E) 0.0 MV

A) 1.5 MV v = kq/r for one charge (so must be doubled for both charges) k = 9x10^9 (constant) q = 5.0 x 10^-6 C (from problem) d = 0.06 m (electric field is zero at the point between the to charges, so 0.12m / 2)

Each plate of an ideal air-filled parallel-plate capacitor has an area of 0.0020 m^2, and the separation of the p lates is 0.090 mm. An electric field of 2.1 x 10^6 N/C is present between the plates. What is the surface charge density on the plates? A) 19 μC/m^2 B) 9.3 μC/m^2 C) 58 μC/m^2 D) 39 μC/m^2 E) 1.7 μC/m^2

A) 19 μC/m^2 charge density = Q/A Q = CV C = ((kEo*A)/d) v = Ed therefore, charge density = k*Eo*E

The magnitude of charge for an electri dipole is 1.7 uC with a charge separation of 630 um. Determine the maximum torque on the dipole when it is placed in an electric field with a field strength of 2400 V/m. A) 2.6 uN*m B) 0.22 uN*m C) 24 uN*m D) 1.2 uN*m

A) 2.6 uN*m Torque = qlEsin(theta) q = 1.7 *10^-6 C d = 630 * 10^-6 m E = 2400 V/m sin(theta) = 1 (for maximum torque)

Estimate the average power output of the Sun, given that about 1370 W/m^2 reaches the upper atmosphere of teh Earth. The mean Earth-Sun distance is 149.6 x 10^6 km. A) 3.85 x 10^26 W B) 2.40 x 10^24 W C) 1.60 x 10^26 W D) 3.90 x 10^20 W

A) 3.85 x 10^26 W P = I*A I = 1370 W/m^2 A = 4*pi*(1.5*10^11)^2 = 2.83 x 10^23 m^2 P = 3.87 x 10^26 W

The power of accommodation for a person with normal vision is A) 4 diopters B) 1 dipters C) 2 diopters D) 8 diopters E) 6 diopters

A) 4 diopters Pa = 1/Do(np) - 1/Do(fp) Normal vision has a far point at infinity and a near point of 25 cm. Pa = 1/(0.25) - 1/(infinity) = 1/0.25 - 0 Pa = 4 D

A 9 V battery is connected to a resistor with the following colored bands, reading from left to right: red/black/orange, with a single gold band on the right end. If the current in the circuit is measured, which of the following values would not be possible? A) 420 uA B) 440 uA C) 455 uA D) 470 uA

A) 420 uA I = V / R R must be determined from the color code: Blk = 0 Brn = 1 R = 2 O = 3 Y = 4 G = 5 B = 6 V = 7 Gry = 8 Wht = 9 Brn = +- 1% Red = +- 2% Gold = +- 5% Silver = +- 10% No band = +- 20% The band is red then black then orange, so the resistance is 20 x 10^3 Ω. The error band is gold, so this resistance is precise +- 5%. Therefore, the actual resistance ranges from 19 x 10^3 Ω to 21 x 10^3 Ω. I = V / R Using the resistances at each extreme of the error, current can be calculated. I = 9 V / 19 x 10^3 Ω = 474 uA I = 9 V / 21 x 10^3 Ω = 429 uA Therefore, the current will measure between 429 uA and 474 uA. The only answer choice not in this range is A (420 uA).

Determine the magnitude of the force on an electron traveling 7.75 x 10^5 m/s horizontally to the east in a vertically upward magnetic field of strength 0.45 T A) 5.6 x 10^-14 N B) 2.7 x 10^-14 N C) 1.1 x 10^-14 N D) 8.9 x 10^-14 N

A) 5.6 x 10^14 N Fm = qvBsin(theta) q = 1.6 x 10^-19 v = 7.75 x 10^5 B = 0.45 T theta = 90 degrees Fm = (1.6 x 10^-19) * (7.75 x 10^5) * (0.45) * (1) Fm = 5.58 x 10^-14 N

An LRC circuit is connected to a 10.0 kHz, 725 Vrms source, L = 28 mH, R = 8.7 kΩ, and C = 6.25 nF. What is the total impedance of the circuit? A) 8.7 kΩ B) 12 kΩ C) 25 kΩ D) 4.2 kΩ

A) 8.7 kΩ Z = sqrt ((R^2) + (XL - XC)^2) XL = 2*pi*f*L = 1800 XC = 1/ (2*pi*f*C) = 2546 Note: Since the difference between these values (746) is tiny compared to the resistance (8.7 x 10^3), you know that the total impedance will be roughly equal to the original resistance without doing the math. Z = sqrt ((87000^2) + (1800 - 2546)^2)) = 87003 = 8.7 x 10^3 Ω

Three equal-magnitude point charges of varying signs are placed at the corners of a square as shown in the figure (with answer). Which arrows represents the direction of the net electric field at the center of the square? A) A B) B C) C D) D

A) A D and B cancel each other out, leaving A as the only force.

Which one of the arrows shown in the figure best represents the direction of the electric field between the two uniformly charged metal plates? A) A B) B C) C D) D E) None of the above

A) A Electric field is the path of a proton due to the electric force. A proton would move away from the positive and towards the negative.

Which of the following best describes the difference between radio waves and x-rays? A) Both can be thought of as electromagnetic waves. They differ only in wavelength and frequency B) Radio waves come from electric currents in an antenna. X-rays are not related to electric charge. C) X-rays are radiation whereas radio waves are electromagnetic waves. D) X-rays are pure energy. Radio waves are made of fields, not energy.

A) Both can be thought of as electromagnetic waves. They differ only in wavelength and frequency

Four point charges of varying magnitude and sign are arranged on the corners of the square of side d. Which direction is the net force acting on the point charge +Q (top left)? A) Diagonally away from the center of the square B) Down towards the bottom left +2Q C) Diagonally towards the center of the square D) To the right towards the top right +2Q

A) Diagonally away from the center of the square Use vector addition.

In the circuit shown in the figure (w/ answer), the resistor R has a variable resistance. As R is decreased, what happens to the currents? A) I1 decreases and I2 increases. B) I1 decreases and I2 decreases. C) I1 increases and I2 decreases. D) I1 increses and I2 increases. E) I1 remains unchanged and I2 increases.

A) I1 decreases and I2 increases. As a resistor decreases, the current running through that part increases. If the current in one part of a junction increases, the current in another part decreases (junction rule)

Two point charges, Q1 and Q2, are separated by a distance R. If the magnitudes of both charges are halved and their separation is also halved, what happens to the electrical force that each charge exerts on the other one? A) It remains the same. B) It increases by a factor of 4. C) It increases by a factor of 2. D) It increases by a factor of 8. E) It increases by a factor of 16.

A) It remains the same. F = (k*Q1*Q2)/r^2 The numerator is 1/4th (1/2 * 1/2) of what it was, and the denominator is 1/4th ((1/2)^2) of what it was. Therefore, there is no change in the force.

If you were to cut a small permanent bar magnet in half A) each piece would become a smaller magnet with north and south poles B) one piece would be a north pole, one a south pole C) neither piece would be magnetic D) None of the above.

A) each piece would become a smaller magnet with north and south poles

Suppose you have two capacitors and want to use them to store the maximum amount of energy by connecting them across a voltage source. You should connect them A) in parallel across the source B) in series across the source. C) It doesn't matter; same energy either way

A) in parallel across the source. U = (1/2)CV^2 The higher the C is, the higher the energy is. Connecting capacitors in parallel yields a higher capacitance (Cnet = C1 + C2...) which yields a higher energy.

A pure capacitor is connected to an ac power supply. In this circuit, the current A) leads the voltage by 90 degrees. B) lags the voltage by 180 degrees. C) is in phase with the voltage. D) lags the voltage by 90 degrees. E) none of the above.

A) leads the voltage by 90 degrees. AC current with resistor = in phase AC current with inductor = I lags behind V by 90 degrees (ELI) AC current with capacitor = I leads V by 90 degrees ICE

The direction of the force on a current-carrying wire in a magnetic field is A) perpendicular to both the current and the magnetic field B) perpendicular only to the magnetic field C) perpendicular only to the current D) in the direction opposite to the current E) in the same direction as the current

A) perpendicular to both the current and the magnetic field Right hand rule

Kirchhoff's loop rule is a statement of A) the law of conservation of energy B) the law of conservation of momentum C) the law of conservation of charge D) Newton's second law E) the law of conservation of angular momentum

A) the law of conservation of energy Loop rule states that voltages must equate to 0, and voltages are Joules (energy) per C. Junction rule is conservation of charge

A typical home may require a total of 2000 kWh of electrical energy per month. Suppose you want to obtain this from sunlight (I = 1000 W/m^2 per day). If light is available 8 hours a day 25 days a month, determine the smallest collector area that will provide the needed energy, assuming a collector efficiency of 25%. A) 80 m^2 B) 40 m^2 C) 25 m^2 D) 10 m^2

B) 40 m^2 I = P/A A = P/I Total energy needed by the house (P) = 2000 x 10^3 Wh Intensity of sunlight over one month (I) = (1000 W/m^2 * 8.0 hours * 25 days) = 200000 W/m^2 Collector operates at 25% efficiency, so it only harvests 25% of this intensity. Therefore, I = 200000 Wh/m^2 * 0.25 = 50000 W/m^2 A = 2000 x 10^3 Wh / 50000 Wh/m^2 = 40 m^2

If the magnetic field in a traveling EM wave has had an amplitude of 18 nT, what is the amplitude of the electric field? A) 11 V/m B) 5.4 V/m C) 1.1 V/m D) 2.8 V/m

B) 5.4 V/m c = E/B E = cB E = (3x10^8)*(18x10^-9) E = 5.4 V/m

If the intensity of an electromagnetic wave doubles A) the electric field must also double. B) both the magnetic field and the electric field must increase by a factor of √2. C) the magnetic field must also double. D) any of the above.

B) Both the magnetic field and electric field must increase by a factor of √2. The intensity of EM waves is given by the following equations: (1/2)*c*Eo*E^2 and (1/2)*c*(Bo^2)/Uo In order for the intensity to double, the magnetic field (B) and the electric field (E) must increase by √2.

An ideal parallel-plate capacitor having circular plates of diameter D that are a distance d apart stores energy U when it is connected across a fixed potential difference. If you want to triple the amount of energy stroed in this capacitor by cahnging only the size of its plates, the diameter should be changed to A) D/√3 B) D√3 C) D/3 D) 9D E) 3D

B) D√3 U = (1/2)kEo(E^2)dA A = pi*r^2 For U to triple, A must triple. For A to triple, the radius must be increased by a factor of √3. For the radius to increase by a factor of √3, the diameter must increase by a factor of √3.

In a series RLC ac circuit, a second resistor is connected in series with the resistor previously in the circuit. As a result of this change, what happens to the impedance of the circuit? A) It does not change B) It increases C) It decreases for frequencies below resonance and increases for frequencies above resonance. D) It decreases E) It increases for frequencies below resonance and decreases for frequencies above resonance.

B) It increases Z = sqrt ((R^2) + (XL - XC)^2) Connecting another resistor in series increases the total resistance (R) (Rnet = R1 + R2...). Increasing R also increases Z.

A resistor, an uncharged capacitor, a dc voltage source, and an open switch are all connected in series. The switch is closed at time t = 0s. Which one of the following is a correct statement about this circuit? A) The voltage after 1T is 1/e the maximum value. B) The charge after 4T is 98% the maximum value. C) The charge after 1T is 1/e of its maximum value. D) The voltage after 1T is 100% its maximum value. E) The charge after 1T is 50% its maximum value.

B) The charge after 4T is 98% the maximum value 1T = 0.63 2T = 0.86 3T = 0.95 4T = 0.98

When a nearsighted person looks at a distant object through her glasses, the image produced by the glasses should be A) at the far point for a normal eye B) at her eye's far point C) at her eye's near point D) about 25 cm from her eye

B) at her eye's far point The goal of glasses is to get the image to a place where the eye can see it. Thus, for a nearsighted person, this would be her far point.

A coil lies flat on a level tabletop in a region where the magnetic field vector points straight up. The magnetic field suddenly grows stronger. When viewed from above, what is the direction of the induced current in this coil as the field increases? A) clockwise initially, then counterclockwise before stopping B) clockwise C) counterclockwise D) No induced current

B) clockwise The magnetic field is increasing, so the magnetic flux is also increasing (Φ = BAcos(theta)). Since the flux is increasing (positive), the generated magnetic field opposes the original (so the generated magnetic field points downwards). Using the RHR, if B points downwards, the induced current must be clockwise.

The electron-volt is a unit of A) electric force. B) energy. C) electric field. D) charge. E) electric potential.

B) energy 1 eV = 1.6 x 10^-19 J

Which of the following statements is false? The magnetic field of a current-carrying wire A) is directed circularly around the wire B) exists only if the current in the wire is changing C) depends on the magnitude of the current D) decreases inversely with the distance from the wire

B) exists only if the current in the wire is changing B = (Uo*I)/(2*pi*r), which is not dependent on a change of current.

Over ordinary temperature ranges, the resistance of most metals such as copper A) decreases as the metal gets hotter B) increases as the metal gets hotter C) is a maximum at 20 degrees C D) is independent of the temperature of the metal

B) increases as the metal gets hotter R = Ro[1 + p(dT)] As temperature increases, p*dT is positive, so Ro is multiplied by a bigger number

A conducting rod moves downward in an electric field directed into the paper. Which side has an excess of negative charge? A) right side B) left side

B) left side Use RHR to find the force of positive charges (fingers in direction of velocity and mag field, thumb points to the force on the positive charges). Therefore, the positive charges move towards the right. A negative charge would move in the opposite direction, leaving an excess of negative charges on the left side.

The plates of a parallel-plate capacitor are maintained with constant potential by a battery as they are pulled apart. During this process, the amount of charge on the plates A) must remain constant B) must decrease C) must increase D) could either increase or decrease.

B) must decrease C = kEoA/d C = Q / V If d increases, C decreases. If C decreases and V remains constant, then Q must also decrease.

Parallel light rays cross interfaces from medium 1 into medium 2 and then into medium 3 as shown in the figure (with the answer). What can we say about the relative sizes of the indices of refraction? A) n3 > n2 > n1 B) n2 > n1 > n3 C) n1 > n3 > n2 D) n2 > n3 > n1 E) n1 > n2 > n3

B) n2 > n1 > n3 The angle gets smaller from 1 to 2, so n2 must be greater than n1 (n2 > n1). The angle gets bigger from 2 to 3, so n3 must be smaller than n2 (n3 < n2). n2 > n1 n2 > n3 Therefore, n2 must be the greatest, followed by either n1 or n3. Since the angle is largest for 3, it is assumed that n3 is the smallest. Therefore, the ordering is n2 > n1 > n3.

In an electromagnetic wave in free space, the electric and magnetic fields are A) perpendicular to one another and parallel to the direction of the wave propagation B) perpendicular to one another and perpendicular to the direction of wave propagation C) parallel to one another and parallel to the direction of wave propagation D) parallel to one another and perpendicular to the direction of wave propagation

B) perpendicular to one another and perpendicular to the direction of wave propagation

In an RLC ac circuit, the values of the inductance and capacitance are both doubled. As a result of this change, the resonance frequency of the circuit is A) reduced by 1/4 B) reduced by 1/2 C) doubled D) the same as before E) quadrupled

B) reduced by 1/2 fr= 1/2π * √(1/LC) If L and C are doubled, the square root of 1/LC becomes the square root of 1/4, which is 1/2. Therefore, the fr will be reduced by a factor of 1/2.

Which of the following equations represents a Kirchhoff's voltage rule for the circuit shown above? A) +4 V - (8Ω*I2) - (12 Ω*I1) = 0V B) -( 8 Ω)I2 + 4V + 6V + (12VI1) = 0V C) +6 V - (12 ΩI1) - (8 Ω)I2) + 4V = 0 D) -6 V + 2 V - (6 Ω)I3 = 0V

C) +6 V - (12 ΩI1) - (8 Ω)I2) + 4V = 0

Some rearview mirrors produce images of cars to your rear that are smaller than they would be if the mirror were flat. What is the mirror's focal length if cars 16.0 m away appear 0.25 their normal size? A) +5.3 m B) +3.2 m C) - 5.3 m D) - 3.2 m

C) - 5.3 m Rearview mirrors are convex. Therefore, we can expect the focal length to be negative. In addition, the image will be virtual and thus the di will be negative. m = 0.25 do = 16.0 m m = -di/do di = -m*do di = -0.25*16 di = -4 m 1/f = 1/do + 1/di 1/f = 1/16 - 1/4 1/f = -3/16 f = -5.3

Calculate the current through a 10.0 m long 22-gauge nichrome wire with a radius of 0.321 mm if it is connected across a 12.0 V battery. The resistivity of nichrome is 1.00 x 10^-6 Ω*m A) 30.9 A B) 61.8 A C) 0.388 A D) 0.776 A

C) 0.388 A I = V/R R = pl/A p = 1.00 x 10^-6 l = 10.0 m A = pi * (0.321*10^-3)^2 = 3.24 * 10^-7 R = (1.00 x 10^-6)*(10.0) / (3.24 * 10^-7) = 30.9 Ω I = 12.0 V / 30.9 Ω = 0.388 A

A flat coil is in a uniform magnetic field. The magnetic flux through the coil is greatest when the plane of its area is A) parallel to the magnetic field B) at 45 degrees with the magnetic field C) perpendicular to the magnetic field

C) perpendicular to the magnetic field Φ = BAcos(theta) In order to get the maximum flux, the cos(theta) must equal 1. For this to be the case, the angle between the area vector and the magnetic field must be 0 degrees. The area vector points perpendicular to the plane of the vector, so if the plane of the area is perpendicular to the magnetic field, the area vector and magnetic field vector will have an angle of 0 degrees.

A stamp collector using a converging lens with focal length 30 cm to view a stamp 15 cm in front of the lens. What is the magnification of the lens? A) - 0.50 B) + 0.50 C) - 2.0 D) + 2.0

D) + 2.0 Converging lens, so the focal length will be positive. f = + 30 cm do = 15 cm 1/f = 1/15 + 1/do di = -30 cm m = -di/do m = -(-30)/15 m = + 2.0

A person has a far point of 18 cm. What power glasses would correct this vision if the glasses were placed 2.0 cm from the eye? A) - 5.6 D B) + 5.0 D C) + 2.5 D D) - 6.3 D

D) - 6.3 D The person is nearsighted, so they need a diverging lens. Because of this, the power will be negative. di = 18 cm - 2 cm = - 16 cm (negative because virtual image) do = infinity P = -1/0.16 + 1/infinity P = -1/0.16 P = - 6.25 D

The square plates of a 5000-pF parallel-plate capacitor measure 50 mm by 50 mm and are separated by a dielectric that is 0.23 mm thick and totally fills the region between the plates. The voltage rating of the capacitor is 400 V. What is the maximum energy that can be stored in this capacitor? A) 0.70 mJ B) 0.50 mJ C) 0.60 mJ D) 0.40 mJ E) 0.80 mJ

D) 0.40 mJ U = (1/2) CV^2 C = 500 x 10^-12 F V = 400 V

A single square loop of wire 20.0 cm on a side is placed with its face parallel to a uniform magnetic field. When I = 6.0 A flows in the coil, the torque on it is 0.36 m*N. What is the magnetic field strength? A) 0.6 T B) 2.8 T C) 11 T D) 1.5 T

D) 1.5 T T = NIABsin(theta) B = T / (A*I*sintheta) B = 0.36 / ((4 * 10^-2) * 6) B = 1.5 T

The armature of a 60 Hz generator rotates in a 0.15 T magnetic field. If the area of the coil is 2.0 x 10^-2 m^2, how many loops must the coil contain if the peak output is to be 170 V? A) 250 turns B) 100 turns C) 50 turns D) 150 turns

D) 150 turns E = BAN(2*pi*f) N = E / (BA*2*pi*f) N = 170 / (0.15 * 2.0 x 10^-2 * 2 * pi * 60) N = 150.3

Three charges are at the vertices of an equilateral triangle. If the charge Q is 6.7 nC, what is the magnitude of the net electric force on the charge Q due to the other two charges? A) 0.19 x 10^-3 N B) 1.2 x 10^-3 N C) 4.6 x 10^-3 N D) 2.1 x 10^-3 N

D) 2.1 x 10^-3 N F = k*Q1Q2/r^2 (between the top charge and ONE of the corners) k = 9*10^9 Q1 = 6.7*10^-9 C Q2 = 2*10^-9 C r = 0.01cm To get the net Force for BOTH of the corners on the top, Fnet = 2*F*cos(30).

A transmission wire is 13 m above the ground. If the lines operate at 240 kV and provide a maximum power of 46 MW, estimate the magnetic field strength at ground level. A) 5.8 uT B) 1.3 uT C) 30 uT D) 2.9 uT

D) 2.9 uT B = (Uo*I) / (2*pi*r) P = IV, I = P/V I = 46 x 10^6 W / 240 x 10^3 V I = 191.67 B = ((4pi x 10^-7) * (191.67 A) / (2*pi*13.0m) B = 2.94 uT

A fixed 8.0 cm diameter wire coil is perpendicular to a magnetic field 0.50 T pointing up. In 0.16 s, the field is changed to 0.30 T pointing down. What is the average induced emf in this coil? A) 6.3 mV B) 81 mV C) 40 mV D) 25 mV

D) 25 mV Φ = BAcos(theta) E = -N*(dΦ/dt) A = pi*(0.04^2) = 0.005 m^2 dB = 0.50 - -0.30 = 0.80 dΦ = (0.80 T) * (0.005 m^2) = 0.004 E = dΦ / dt = (0.004 / 0.16s) = 0.025 V = 25 mV

A charger for a cell phone contains a transformer that reduces 120 Vac to 5.0 Vac to charge the 3.7 V cell phone battery. If the secondary coil contains 30 turns and the charger supplies 700 mA to the phone, calculate the transformer primary current. A) 68 mA B) 110 mA C) 8.7 mA D) 29 mA

D) 29 mA Vs/Vp = Ns/Np Is/Ip = Np/Ns Vs = 5 Vp = 120 Ns = 30 Np = ? Is = 0.7 A Ip = ? Find Np using the first equation then Ip using the second. Np = (Vp*Ns)/Vs Np = (120*30)/5 Np = 720 coils Ip = (Is*Ns)/Np Ip = (0.7*30)/(720) = 0.029 A = 29 mA

An ideal solenoid has a self-inductance L. If you now double its radius and its length but do not change the number of coils, what will the new self-inductance be? A) L/4 B) 4L C) L/2 D) 2L E) L

D) 2L L = (Uo*N^2*A)/l If you double the radius, the area quadruples. Therefore, doubling the radius and the length multiplies the fraction by 4/2, meaning that L would be doubled (2L).

A 4.0 uC charge is situated at the origin of an xy-coordinate system. What is the magnitude and direction of the electric potential at x = 6.0 m? A) 1500 V along the +x-axis B) 4000 V along the +x-axis C) 6000 V along the +x-axis D) 6000 V with no direction, as it is a scalar quantity

D) 6000 V with no direction, as it is a scalar quantity v = kq/r k = 9 x 10^9 q = 4 x 10^-6 C r = 6.0 m Electric potential is scalar, so no direction. Force is not scalar.

A generator output voltage is measured to be 440 Vrms. Determine the generator's peak voltage. A) 310 V B) 220 V C) 622 V D) 880 V

D) 622 V Vrms = Vmax / √2 Vmax = Vrms * √2 Vmax = 440 * √2 = 622 V

A parallel-plate capacitor consists of a set of two parallel plates of area A separated by a distance d. This capacitor is connected to a battery that maintains a constant potential difference across the plates. A slab of dielectric material is inserted in the region between the plates and completely fills it. What changes would you observe as the dielectric is inserted? A) Only the charge on the plates of the capacitor would change. B) Only the capacitance would change. C) Nothing would change. D) Both the charge on the plates of the capacitor and its capacitance would change.

D) Both the charge on the plates of the capacitor and its capacitance would change. C = Q/v C = kEoA/d Capacitance changes (because k changes) and v stays constant (connected to battery) so the charge must change as well. Therefore both the capacitance and the charge change when the dielectric is inserted.

Four unequal resistors are connected in a parallel with each other. Which of the following statements is correct about this combination? A) The equivalent resistance is more than the largest resistance. B) The equivalent resistance is midway between the largest and smallest resistance. C) The equivalent resistance is equal to the average of the four resistances. D) The equivalent resistance is less than that of the smallest resistor. E) None of the above.

D) The equivalent resistance is less than that of the smallest resistor. Parallel resistors are related in the following way: 1/Rn = 1/R1 + 1/R2 + 1/R3... Therefore, only D can be true mathematically.

An electron is initially moving to the right when it enters a uniform electric field directed upwards as shown in the figure (with the answer). Which trajectory (X, Y, Z, or W) will the electron follow in the field? A) W B) X C) Y D) Z

D) Z Since it is an electron, it will travel in a direction opposite that of the electric field.

Which of the following will increase the capacitance of a parallel-plate capacitor? A) a decrease in the charge on the plates B) a decrease in the plate area and an increase in the plate separation C) an increase in the potential difference between the plates D) an increase in the plate area and a decrease in the plate separation

D) an increase in the plate area and a decrease in the plate separation C = kEoA/d C = Q / V

A wire of resistivity p must be replaced in a circuit by a wire of the same material but four times as long. If, however, the total resistance is to remain as before, the diameter of the new wire must A) be one-half the original diameter B) be one-fourth the original diameter C) be the same as the original diameter D) be two times the original diameter E) be four times the originnal diameter

D) be two times the original diameter R = pl/A 4l/(pi*r^2) = 1 r = 2

If the Earth-Sun distance were doubled, the intensity of the radiation from the Sun that reaches the Earth's surface would A) quadruple B) drop to 1/2 C) double D) drop to 1/4

D) drop to 1/4 I = P / (4*pi*r^2) If r is doubled, then r^2 is quadrupled. If the denominator is increased by a factor of 4, then the intensity would drop by 1/4.

Which of the following is the correct order for the electromagnetic spectrum, from the shortest to the longest wavelengths? A) x-rays, gamma rays, ultraviolet, visible, infrared, radio waves, microwaves B) radio waves, microwaves, infrared, visible, ultraviolet, gamma rays, x-rays C) radio waves, microwaves, infrared, visible, ultraviolet, x-rays, gamma rays D) gamma rays, x-rays, ultraviolet, visible, infrared, microwaves, radio waves E) microwaves, radio waves, infrared, visible, ultraviolet, x-rays, gamma rays.

D) gamma rays, x-rays, ultraviolet, visible, infrared, microwaves, radio waves in order from lowest to highest frequency, it goes radio waves (AM then FM), microwaves, infrared waves, visible light (red to violet), x-rays, and gamma rays. Invert this to get the ordering from shortest to longest wavelength.

A transformer is a device used to A) transform a direct current into an alternating current B) increase or decrease a dc voltage C) transform an alternating current into a direct current D) increase or decrease an ac voltage

D) increase or decrease an ac voltage Transformers only work for ac circuits

A capacitor C is connected in series with a resistor R across a battery and an open switch. If a second capacitor of capacitance 2C is connected in parallel with he first one, the time constant of the new RC circuit will be A) one-fourth as large as before B) the same as before C) one-half as large as before D) three times as large as before E) twice as large as before

D) three times as large as before T = RC Cn (parallel) = C1 + C2... Cn = C + 2C = 3C T = R*3C = 3T

Which of the following types of electromagnetic radiation travels the fastest? A) X-rays B) Gamma rays C) Radio waves D) Visible light waves E) All the above travel at the same speed

E) All the above travel at the same speed All EM waves travel at the same speed (3 x 10^8 m/s)

How do eyeglasses help a nearsighted person see more clearly? A) Lenses adjust the distance from the cornea to the back of the eye. B) Diverging lenses bend light entering the eye, so the image focuses closer to the front of the eye. C) Converging lenses bend light entering the eye, so the image focuses closer to the front of the eye. D) Converging lenses bend light entering the eye, so the image focuses farther from the front of the eye. E) Diverging lenses bend light entering the eye, so the image focuses farther from the front of the eye.

E) Diverging lenses bend light entering the eye, so the image focuses farther from the front of the eye. Nearsightedness is defined as the inability to see objects far away. This is caused by the lens focusing light too far in front of the retina. In order to correct this, a diverging lens is needed that focuses light farther from the front of the eye (on the retina).

The electric field at point P due to a point charge Q at distance R away from P has magnitude E. In order to double the magnitude of the field at P, you could... A) Double the charge to 2Q and at the same time reduce the distance to R/2. B) Double the distance to 2R. C) Reduce the distance to R/4. D) Reduce the distance to R/2. E) Double the charge to 2Q.

E) Double the charge to 2Q. E = kq/r^2 To double E, the charge must be doubled OR the radius must be decreased by a factor of √2.

Which of the following is NOT a correct expression for the speed of light? A) c = EB B) c = f/lambda C) c = Iu D) c = sqrt(EoUo) E) None of the expressions are correct

E) None of the expressions are correct The four correct expressions are as follows c = E/B c = 1/sqrt(EoUo) c = f*lambda c = I/u

A converging lens, such as a typical magnifying glass, A) always produces an upright image. B) always produces a magnified image (taller than object C) always produces an inverted image (upside down) D) always produces an image smaller than the object E) None of these statements are true

E) None of these statements are true None of these are true for a converging lens, as each of these criterion can be broken depending on the object distance. If the lens is a diverging lens, then A, B, and D are true.

When the current through a resistor is increased by a factor of 4, the power dissipated by the resistor A) decreases by a factor of 16 B) increases by a factor of 2 C) decreases by a factor of 4 D) increases by a factor of 4 E) increases by a factor of 16

E) increases by a factor of 16 P = (I^2)*R P = (4^2)*R P = 16*R Careful: if you just look at P = IV, it looks like the power would increase by a power of 4. However, this neglects the fact that the voltage would also increase by a factor of 4, causing the power to increase by a factor of 16.

A 400-W computer (including its monitor) is turned on for 8.0 hours per day. If electricity costs 10c per kWh, how much does it cost to run the computer for a 365-day year? A) $1200 B) $120 C) $15 D) $150

B) $120 400-W = 0.4 kW 0.4 kW * 8 hours * 365 days = 1168 kWh 1 kWh = 0.1 dollars, so 1168 kWH = $117

A particle with a charge of +4.0 μC has a mass of 5.0 g. What magnitude electric field directed upward will exactly balance the weight of the particle? A) 7.2 x 10^4 N/C B) 1.2 x 10^4 N/C C) 4.1 x 10^4 N/C D) 5.1 x 10^4 N/C E) 11.0 x 10^4 N/C

B) 1.2 x 10^4 N/C F = mg E = F / q E = mg / q m = 0.05kg g = 9.80 m/s^2 q = 4.0 * 10^-6 C

A piece of plastic has a net charge of +2.00 μC. How many more protons than electrons does this piece of plastic have? A) 2.50 x 10^19 B) 1.25 x 10^13 C) 3.01 x 10^23 D) 2.50 x 10^13 E) 1.25 x 10^19

B) 1.25 x 10^13 Q = + 2.00 x 10^-6 C Charge of a proton = 1.6 x 10^-19 C # of protons = (2.00x10^-6) / (1.6 x 10^-19)

The critical angle for a certain liquid-air surface is 46.9 degrees. What is the index of refraction of the liquid? A) 2.55 B) 1.37 C) 1.92 D) 1.05

B) 1.37 sin(critical angle) = n2/n1 sin(46.9) =1/n1 n1 = 1.37

An electric device delivers a current of 3.0 A for 10 seconds. How many electrons flow through this device? A) 20 B) 1.9 x 10^20 C) 2.0 D) 19 x 20^20

B) 1.9 x 10^20 1 A = 1 C/s 3 A for 10 s = 30 C 1 e = 1.6 x 10^-19 C e = 30 C / (1.6 x 10^-19C/e)

For a graph of current (y-axis) and potential (x-axis), what physical quantity does the slope of the graph represent for an ohmic material? A) 1/resistivity B) 1/resistance C) resistance D) resistivity E) power

B) 1/resistance Slope is rise/run, or y/x Since y = current and x = potential, y/x = I/V From Ohm's law, R = V/I Therefore, the graph represents I/V and resistance is V/I. Therefore, the graph represents 1/R

An LRC circuit has L = 24 mH and R = 4 Ω. What value must the capacitor have to produce resonance at 3200 Hz? A) 210 nF B) 100 nF C) 1.2 uF D) 70 nF

B) 100 nF fr= 1/2π * √(1/LC) c = 1 / (L * f^2 * 4 * pi^2)

Radiation from the Sun reaches the upper atmosphere of the Earth at a rate of 1370 W/m^2. Calculate the amplitude of the electric field in the wave when it reaches the Earth's atmosphere. A) 2100 V/m B) 1020 V/m C) 4500 V/m D) 200 V/m

B) 1020 V/m I = (1/2)*c*Eo*E^2 E = sqrt (2I/cEo) E = sqrt (2*(1370)/((3 x 10^8)*(8.85 x 10^-12)) E = 1034 V/m

A 2.0-μF capacitor is charged to 12 V and then discharged through a 4.0 MΩ resistor. How long will it take for the voltage across the capacitor to drop to 3.0 V? A) 8.0 s B) 11 s C) 4 s D) 22 s

B) 11 s The time constant T is R*C = 8 s The voltage is dropping from 12 to 3, so it is dropping to 33% of the maximum. 1T = 0.37 2T = 0.14 3T = 0.05 4T = 0.02 0.33 falls between 1T and 2T, so it will take between 8 and 16 seconds for the capacitor to discharge to 33% (closer to 8 than 16).

A carbon resistor has a resistance of 18 Ω at a temperature of 20 degrees C. What is its resistance at a temperature of 120 degrees C? a = -5.0 x 10^-4/C A) 8 Ω B) 17 Ω C) 12 Ω D) 14 Ω

B) 17 Ω R = Ro[1 + a(dT)] Ro = 18 Ω a = -5.0 x 10^-4/C dT = 100

A wire carrying a steady (dc) 30-A current has a length l = 12 cm between the pole faces. The wire is at an angle of 60 degrees to the field. The magnetic field is approximately uniform at 0.90 T. Determine the magnitude of force on the wire. A) 2.0 N B) 2.8 N C) 11 N D) 29 N

B) 2.8 N Fm = ILBsin(theta) I = 30 A L = 0.12 m B = 0.90 T theta = 60 degrees Fm = 30 * 0.12 * 0.90 * sin(60) Fm = 2.805 N

A parallel-plate capacitor with plate separation of 4.0 cm has a plate area of 6.0 x10^-2 m^2. What is the capacitance of this capacitor if a dielectric material with a dielectric constant of 2.4 is placed between the plates? A) 3.7 x 10^-12 F B) 32 x 10^-12 F C) 32 x 10^-14 F D) 3.7 x 10^-14 F E) 16 x 10^-14 F

B) 32 x 10^-12 F C = kEoA/d k = 2.4 d = 0.04 m A = 6.0 x 10^-2 m^2 Eo = 8.85 x 10^-12

Three particles travel through a region of space where the magnetic field is out of the page (figure with answer). What are the signs and charges of these three particles? A) 1 is positive, 2 is negative, 3 is neutral B) 1 is neutral, 2 is negative, 3 is positive C) 1 is negative, 2 is neutral, 3 is positive D) 1 is positive, 2 is neutral, 3 is negative E) 1 is neutral, 2 is positive, 3 is negative

C) 1 is negative, 2 is neutral, 3 is positive Use the RHR to find the force on a positive charge (fingers in the direction of the particle velocity then the magnetic field; thumb points in the direction of the force on a positive charge) 2 is obviously neutral because it does not veer. Using the RHR, a positive charge should veer downward; therefore, 3 is positive. A negative charge would veer upward, so 1 is negative.

The power rating of a 400-Ω resistor is 0.25 W. What is the maximum voltage you can safely connect across its ends? A) 30 V B) 20 V C) 10 V D) 50 V E) 40 V

C) 10 V P = (V^2)/r V = sqrt(PR)

At a distance d from a point charge Q, the energy density in its electric field is u. If we now go to a distance d/2 from the charge, what is the energy density at the new location? A) 2u B) 8u C) 16u D) 4u E) u√2

C) 16u u = (1/2)kEoE^2 E = kq/d^2 d = d/2 A distance of d/2 makes the electric field 4x as strong. If the electric field is 4x as strong, the energy density is 16 (4^2) times as strong.

A person has a far point of 80.0 cm. Calculate the person's power of accommodation. Assume the distance between the person's lens and retina is approximately 1.3 cm. A) 1.25 D B) 4.00 D C) 2.75 D D) 6.50 D

C) 2.75 D Pa = 1/Do(np) - 1/Do(fp) Pa = 1/(0.25m) - 1/(0.80m) = 2.75 D

Rays of the Sun are seen to make a 45 degree angle to the vertical beneath the water (n = 1.33). At what angle above the horizon is the Sun? A) 50 degrees B) 10 degrees C) 20 degrees D) 70 degrees

C) 20 degrees Snell's Law: n1*sin(theta1) = n2*sin(theta2) n1 = 1.00 n2 = 1.33 theta1 = ? theta2 = 45 degrees sin(theta1) = n2*sin(theta2)/n1 sin(theta1) = 1.33*sin(45)/1 sin(theta1) = 0.94 theta1 = 70 degrees Theta1 is the angle between the vertical and the sun. To get the angle between the horizontal and the sun, take the component angle (90 - theta1) 90 - 70 = 20 degrees above the horizon

The magnetic field inside an air-filled solenoid 30 cm long and 2.0 cm in diameter is 0.80 T. How much energy is stored in the magnetic field of the solenoid? A) 11 J B) 59 J C) 24 J D) 250 J

C) 24 J u = (1/2) * (AlB^2)/Uo A = pi * (1 * 10^-2) m l = 0.30 m B = 0.80 T Uo = 4pi x 10^-7

An ideal air-filled parallel-plate capacitor with horizontal plates has a plate separation of 5.0 cm. If the potential difference between the plates is 2000 V, with the top plate at the higher potential, what are the magnitude and direction of the electric field between the plates? A) 100 N/C downward B) 40000 N/C upward C) 40000 N/C downward D) 100 N/C upward

C) 40000 N/C downward v = Ed, so E = v/d Electric fields (the movement of positive charges) would go from positive to negative (or from higher potential to lower potential). Since the higher potential is at the top, the charge (thus the electric field) would move downward.

A number of resistors are connected across points A and B as shown in the figure (w/ answer). What is the equivalent resistance between points A and B? A) 6 Ω B) 12 Ω C) 8 Ω D) 10 Ω E) 4 Ω

C) 8 Ω R = R1 + R2... for series 1/R = 1/R1 + 1/R2... for parallel

Suppose you place an object in front of a concave mirror. Which of the following statements must be true? A) The image of the object will always be smaller than the object. B) The image of the object will always be inverted. C) If you position the object between the mirror and the focal point of the mirror, its image must be upright and virtual. D) No matter where you place the object, the image of the object will always be virtual and upright. E) No matter where you place the object, a real image of the object will be formed.

C) If you position the object between the mirror and the focal point of the mirror, its image must be upright and virtual. A is true for convex mirrors, B is true for concave mirrors when the object is beyond the focal point.

A conventional current passes through a resistor moving towards the right. Which of the following statements are correct? A) The electric potential is higher on the right of the resistor than on the left. B) The electric potential is the same at both sides of the resistor. C) The electric potential is lower on the right of the resistor than on the left. D) The charges are moving slower on the right than on the left.

C) The electric potential is lower on the right of the resistor than on the left. Resistors have higher potential on the side where the current goes in than on the side where the current goes out ("loss of potential as a current passes through a resistor")

Which one of the following statements is correct? A) Earth's geographic south pole is the south pole of Earth's magnetic field. B) The north pole of a magnet points towards Earth's geographic south pole. C) The north pole of a magnet points towards Earth's geographic north pole. D) Earth's geographic north pole is the north pole of Earth's magnetic field. E) None of the above

C) The north pole of a magnet points towards Earth's geographic north pole. He also accepted B He admitted this one was messed up so I doubt it shows up.

If you shine a light through an optical fiber, why does it come out the ends but not out the sides? A) The light flows along the length of the fiber, never touching the sides. B) The sides are mirrored, so the light reflects. C) Total internal reflection makes light reflect from the sides. D) It does come out the sides, but the effect is not obvious.

C) Total internal reflection makes light reflect from the sides.

Which one of the following statements is correct? A) When a current-carrying wire is in your left hand, with your thumb in the direction of the current, your fingers point in the direction of the magnetic field lines. B) When a current-carrying wire is in your right hand, with your thumb in the direction of the current, your fingers point in the opposite direction of the magnetic field lines. C) When a current-carrying wire is in your right hand, with your thumb in the direction of the current, your fingers point in the direction of the magnetic field lines.

C) When a current-carrying wire is in your right hand, with your thumb in the direction of the current, your fingers point in the direction of the magnetic field lines. Right hand rule

A lens can be characterized by its power, which A) depends on where the object is located B) tells how much light the lens can focus C) is the reciprocal of the focal length D) is the same as the magnification

C) is the reciprocal of the focal length P = 1/f