Electric Charge and Electric field

ConcepTest 16.1a Electric Charge I Two charged balls are repelling each other as they hang from the ceiling. What can you say about their charges? 1) one is positive, the other is negative 2) both are positive 3) both are negative 4) both are positive or both are negative The fact that the balls repel each other only can tell you that they have the same charge, but you do not know the sign. So they can be either both positive or both negative. Follow-up: What does the picture look like if the two balls are oppositely charged? What about if both balls are neutral?

ConcepTest 16.3a Coulomb s Law I What is the magnitude of the force F 2? F 1 = 3N F 2 =? Q Q 1) 1.0 N 2) 1.5 N 3) 2.0 N 4) 3.0 N 5) 6.0 N

ConcepTest 16.3a Coulomb s Law I What is the magnitude of the force F 2? F 1 = 3N F 2 =? Q Q 1) 1.0 N 2) 1.5 N 3) 2.0 N 4) 3.0 N 5) 6.0 N The force F 2 must have the same magnitude as F 1. This is due to the fact that the form of Coulomb s Law is totally symmetric with respect to the two charges involved. The force of one on the other of a pair is the same as the reverse. Note that this sounds suspiciously like Newton s 3rd Law!!

Electric field lines is as shown below. What type of charges are present at A and B? 1. A Negative, B Positive A B 2. Both Negative 3. Both Positive 4. Cannot tell

Electric Potential and Capacitors

ConcepTest 17.6 Equipotential of Point Charge Which two points have the same potential? 1) A and C 2) B and E 3) B and D 4) C and E 5) no pair A C B E Q D

ConcepTest 17.6 Equipotential of Point Charge 1) A and C Which two points have the same potential? 2) B and E 3) B and D 4) C and E 5) no pair Since the potential of a point charge is: V = k Q r only points that are at the same distance from charge Q are at the same potential. This is true for points C and E. B E Q C D A They lie on an Equipotential Surface. Follow-up: Which point has the smallest potential?

ConcepTest 17.9a Varying Capacitance I What must be done to a capacitor in order to increase the amount of charge it can hold (for a constant voltage)? 1) increase the area of the plates 2) decrease separation between the plates 3) decrease the area of the plates 4) either (1) or (2) 5) either (2) or (3) Since Q = C V, in order to increase the charge that a capacitor can hold at constant voltage, one has to increase its capacitance. Since the capacitance is given by C = ε A, that can be 0 d done by either increasing A or decreasing d. +Q Q

Electric Current and Circuits

ConcepTest 18.1 Which is the correct way to light the lightbulb with the battery? Connect the Battery 4) all are correct 5) none are correct 1) 2) 3)

ConcepTest 18.1 Which is the correct way to light the lightbulb with the battery? Connect the Battery 4) all are correct 5) none are correct 1) 2) 3) Current can only flow if there is a continuous connection from the negative terminal through the bulb to the positive terminal. This is only the case for Fig. (3).

ConcepTest 19.2b Points P and Q are connected to a battery of fixed voltage. As more resistors R are added to the parallel circuit, what happens to the total current in the circuit? Parallel Resistors II 1) increases 2) remains the same 3) decreases 4) drops to zero As we add parallel resistors, the overall resistance of the circuit drops. Since V = IR, and V is held constant by the battery, when resistance decreases, the current must increase. Follow-up: What happens to the current through each resistor?

ConcepTest 19.1a Series Resistors I Assume that the voltage of the battery is 9 V and that the three resistors are identical. What is the potential difference across each resistor? Since the resistors are all equal, the voltage will drop evenly across the 3 resistors, with 1/3 of 9 V across each one. So we get a 3 V drop across each. 1) 12 V 2) zero 3) 3 V 4) 4 V 5) you need to know the actual value of R 9 V Follow-up: What would be the potential difference if R= 1 Ω, 2 Ω, 3 Ω