Concept Questions with Answers 8.02 W15D2

Week 15,Day 2 W15D2 Final Exam Review Class 36

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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