PHYS 5320 DPT2 Physics: Electricity and Magnetism
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Free PHYS 5320 DPT2 Physics: Electricity and Magnetism Questions
A single resistor is connected across the terminals of a battery. Which one or more of the following changes in voltage and current leaves unchanged the electric power dissipated in the resistor?
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Doubling the voltage and reducing the current by a factor of two.
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Doubling the voltage and increasing the resistance by a factor of four.
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Doubling the current and reducing the resistance by a factor of four.
Explanation
Explanation:
The electric power dissipated in a resistor is given by P = V × I = I²R = V²/R. To leave the power unchanged, any change in voltage, current, or resistance must satisfy one of these relationships so that the product V×I remains the same.
- Doubling the voltage and reducing the current by a factor of two does not satisfy Ohm’s law because for a single resistor, V = IR, so changing voltage and current independently is not physically consistent. Therefore, power would change.
- Doubling the voltage and increasing the resistance by a factor of four: Using P = V²/R, doubling V gives a factor of 4 in V², and increasing R by 4 divides by 4, resulting in the same power.
- Doubling the current and reducing the resistance by a factor of four: Using P = I²R, doubling I gives a factor of 4 in I², reducing R by 4 divides by 4, so power remains unchanged.
Hence, the second and third options correctly leave the power unchanged.
Correct Answer:
Doubling the voltage and increasing the resistance by a factor of four.
Doubling the current and reducing the resistance by a factor of four.
Two parallel wires are both carrying a current I but in opposite directions. the force between the wires is
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attractive
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repulsive
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zero since neither wire is charged
Explanation
Explanation:
Two parallel current-carrying wires exert a magnetic force on each other. The force is attractive if the currents flow in the same direction and repulsive if the currents flow in opposite directions. Since the wires carry currents in opposite directions, the magnetic fields produce a repulsive force between them.
Correct Answer:
repulsive
When you double the voltage across a certain material or device, you observe that the current increases by a factor of 4. What can you conclude?
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The resistance is ohmic
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The resistance is non-ohmic
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The resistance is zero
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The resistance is infinite
Explanation
Explanation:
Ohm’s law states that V=IR, meaning that in an ohmic material, the current is directly proportional to the applied voltage, and the resistance remains constant. In this case, doubling the voltage causes the current to increase by a factor of 4, not 2. This indicates a nonlinear relationship between voltage and current, meaning the resistance depends on the applied voltage. Therefore, the material is non-ohmic.
Correct Answer:
The resistance is non-ohmic
Doubling the current, number of turns, the area, and the ambient magnetic field for a coil changes the maximum torque it experiences by what factor?
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2
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4
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8
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16
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32
Explanation
Explanation:
The maximum torque on a current-carrying coil in a magnetic field is given by τmax = NIAB, where N is the number of turns, I is the current, A is the area of the coil, and B is the magnetic field. If all four quantities are doubled, the new torque becomes:
τ′= (2N)(2I)(2A)(2B) = 16(NIAB).
Thus, the maximum torque increases by a factor of 16.
Correct Answer:
16
What would happen to the force between two charges if the distance between them were tripled?
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The force between them would be 1/3 as large.
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The force between them would be 9 times as large.
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The force between them would be 3 times as large.
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The force between them would be 1/9 as large.
Explanation
Explanation:
Coulomb’s law states that the electrostatic force between two point charges is
F=kq1q2r2. If the separation distance rr is tripled (multiplied by 3), the new force is
F=kq1q2(3r)2 = 19F. Thus the force becomes one-ninth of the original.
Correct Answer:
The force between them would be 1/9 as large.
When the frequency of the voltage applied to a series RL circuit is increased, the phase angle _______.
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does not change
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cannot be determined without values
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decreases
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increases
Explanation
Explanation:
In a series RL circuit, the phase angle θ between the applied voltage and the current is given by θ = arctan(ωL/R), where ω = 2πf. As the frequency f increases, the inductive reactance ωL grows, making the ratio ωL/R larger. Because arctan of a larger number is a larger angle, the phase angle increases with increasing frequency.
Correct Answer:
increases
In an RLC series circuit, if the AC frequency is increased to a very large value, what value does the phase angle between the current and voltage approach?
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90°
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0°
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-90°
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45°
Explanation
Explanation:
In a series RLC circuit, the phase angle φ between the current and voltage is determined by φ = arctan((X_L - X_C)/R), where X_L = 2πfL is the inductive reactance and X_C = 1/(2πfC) is the capacitive reactance. As the AC frequency increases to a very large value, the inductive reactance X_L becomes much larger than the capacitive reactance X_C, making X_L - X_C ≈ X_L. The phase angle then approaches +90°, meaning the voltage leads the current by nearly 90°, which is characteristic of a highly inductive circuit.
Correct Answer:
90°
When two parallel, conducting plates are charged with equal negative charge, what is the behavior of the electric field lines between the plates?
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They converge towards the positive plate.
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They diverge away from both plates.
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They run parallel and uniform between the plates.
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They form closed loops around the plates.
Explanation
Explanation:
Electric field lines originate from positive charges and terminate on negative charges. If both plates carry equal negative charge, there is no positive plate to attract field lines across the gap. The like charges repel, so field lines emanate outward from each plate’s surface and spread away from each plate individually rather than forming a uniform field between them.
Correct Answer:
They diverge away from both plates.
For a transformer with number of secondary windings than the number of primary windings, the secondary current will be _____________ ?
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More than the primary current
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Less than the primary current
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Equal to the primary current
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Zero
Explanation
Explanation:
In a transformer, the relationship between voltage and current is governed by the turns ratio: V_s / V_p = N_s / N_p and I_s / I_p = N_p / N_s, where N_s and N_p are the number of secondary and primary windings, respectively. If the secondary winding has more turns than the primary (N_s > N_p), the secondary voltage is higher (step-up transformer), and the secondary current is correspondingly lower than the primary current, as current varies inversely with voltage in an ideal transformer.
Correct Answer:
Less than the primary current
The drift velocity of the electrons in a wire is ___________ than the electron's average speed.
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very much smaller
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slightly smaller
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slightly larger
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very much larger
Explanation
Explanation:
In a conductor, electrons move randomly at high thermal speeds on the order of 10^5 m/s. However, the net drift velocity due to an applied electric field is extremely small, typically on the order of millimeters per second. Thus the drift velocity is vastly smaller than the electrons’ average random thermal speed.
Correct Answer:
very much smaller
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