1. Concept Questions with Answers 8.02 W15D2

2. Week 15,Day 2
W15D2
Final Exam Review
Class 36

3. Concept Question: LC Circuit
Consider the LC circuit at right. At the time shown the current has its maximum value. At this time:
the charge on the capacitor has its maximum value.
the magnetic field is zero.
the electric field has its maximum value.
the charge on the capacitor is zero.
Class 28

4. Concept Q. Answer: LC Circuit
Answer: 4. The current is maximum when the charge on the capacitor is zero
Current and charge are exactly 90 degrees out of phase in an ideal LC circuit (no resistance), so when the current is maximum the charge must be identically zero.
Class 28

5. Concept Question: LC Circuit
In the LC circuit at right the current is in the direction shown and the charges on the capacitor have the signs shown. At this time,
I is increasing and Q is increasing.
I is increasing and Q is decreasing.
I is decreasing and Q is increasing.
I is decreasing and Q is decreasing.
Class 28

6. Concept Q. Answer: LC Circuit
Answer: 2. I is increasing; Q is decreasing
With current in the direction shown, the capacitor is discharging (Q is decreasing).
But since Q on the right plate is positive, I must be increasing. The positive charge wants to flow, and the current will increase until the charge on the capacitor changes sign. That is, we are in the first quarter period of the discharge of the capacitor, when Q is decreasing and positive and I is increasing and positive.
Class 28

7. Week 12, Day 1
Concept: RLC Circuit
The plot shows the charge on a capacitor (black curve) and the current through it (red curve) after you turn off the power supply. If you put a core into the inductor what will happen to the time TLag?
It will increase.
It will decrease.
It will stay the same.
Class 28

8. Concept Answer: RLC Circuit
Week 12, Day 1
Concept Answer: RLC Circuit
Answer: 1. TLag will increase
Putting in a core increases the inductor’s inductance and hence decreases the natural frequency of the circuit. Lower frequency means longer period. The phase will remain at 90º (a quarter period) so TLag will increase.
Class 28

9. Week 12, Day 1
Concept: RLC Circuit
If you increase the resistance in the circuit what will happen to rate of decay of the pictured amplitudes?
It will increase (decay more rapidly).
It will decrease (decay less rapidly).
It will stay the same.
Class 28

10. Concept Answer: RLC Circuit
Week 12, Day 1
Concept Answer: RLC Circuit
Answer: 1. It will increase (decay more rapidly)
Resistance is what dissipates power in the circuit and causes the amplitude of oscillations to decrease. Increasing the resistance makes the energy (and hence amplitude) decay more rapidly.
Class 28

11. Concept Question: Direction of Propagation
The figure shows the E (yellow) and B (blue) fields of a plane wave. This wave is propagating in the
+x direction
–x direction
+z direction
–z direction
Class 30

12. Concept Question Answer: Propagation
Answer: 4. The wave is moving in the –z direction
The propagation direction is given by the
(Yellow x Blue)
Class 30

13. Concept Question: Traveling Wave
The B field of a plane EM wave is
The electric field of this wave is given by
Class 32

14. Concept Q. Ans.: Traveling Wave
Answer: 4.
From the argument of the , we know the wave propagates in the positive y-direction.
Class 32

15. Concept Question EM Wave
The electric field of a plane wave is:
The magnetic field of this wave is given by:
Class 30

16. Concept Q. Ans.: EM Wave Answer: 1.
Week 13, Day 2
Concept Q. Ans.: EM Wave
Answer: 1.
From the argument of the , we know the wave propagates in the negative z-direction.
Class 31 16

17. Concept Question: Capacitor
The figures above show a side and top view of a capacitor with charge Q and electric and magnetic fields E and B at time t. At this time the charge Q is:
Increasing in time
Constant in time.
Decreasing in time.
Not enough information given to determine how Q is changing.
Class 28

18. Concept Q. Answer: Capacitor
Answer: 3. The charge Q is decreasing in time
Use the Ampere-Maxwell Law. Choose positive unit normal out of plane. Because the magnetic field points clockwise line integral is negative hence positive electric flux (out of the plane of the figure on the right) must be decreasing. Hence E is decreasing. Thus Q must be decreasing, since E is proportional to Q.
Class 28

19. Concept Question: Capacitor
The figures above show a side and top view of a capacitor with charge Q and electric and magnetic fields E and B at time t. At this time the energy stored in the electric field is:
Increasing in
Constant in time.
Decreasing in time.
Class 28

20. Concept Q. Answer: Capacitor
Answer: 1. The the energy stored in the electric field is increasing in time
The direction of the Poynting Flux S (= E x B) inside the capacitor is inward. Therefore electromagnetic energy is flowing inward, and the energy in the electric field inside is increasing.
Class 28

21. Concept Question: Inductor
The figures above show a side and top view of a solenoid carrying current I with electric and magnetic fields E and B at time t. The current I is
increasing in time.
constant in time.
decreasing in time.
Class 28

22. Concept Question Answer: Inductor
Answer: 3. The current I is decreasing in time
Use Faraday’s law. Choose positive unit normal out of plane. Because the electric field points counterclockwise line integral is positive, therefore the positive magnetic flux must be decreasing (out of the plane of the figure on the right). Hence B is decreasing. Thus I must be decreasing, since B is proportional to I.
Class 28

23. Concept Question: Inductor
The figures above show a side and top view of a solenoid carrying current I with electric and magnetic fields E and B at time t. The energy stored in the magnetic field is
Increasing in time
Constant in time.
Decreasing in time.
Class 28

24. Concept Question Answer: Inductor
Answer: 3. The energy stored in the magnetic field is decreasing in time.
The Poynting Flux S (= E x B) inside the solenoid is directed outward from the center of the solenoid. Therefore EM energy is flowing outward, and the energy stored in the magnetic field inside is decreasing.
Class 28