Discussion 06/25/08 – Section 302
Prepared by Annie Tran
Conceptual Problems – Pg 485: 3, 6, 9
3. Suppose the energy transferred to a dead battery during charging is W. The recharged
battery is then used until fully discharged again. Is the total energy transferred out of the
battery during use also W?
There is no way to get back all the energy put into the battery, W, because all batteries
have an internal resistance. This internal resistance is a source of losing power, and it can be
shown with the equation P=I2R.
However, the law of conservation of energy states that there may be output of energy in
different forms, and not necessarily as an output of battery power; in that case, the output of
energy would equal the input of energy.
6.
(a) The equivalent resistance of a group of resistors connected in parallel is:
(ii) less than any of the resistors in the group
Equation: 1/Req = 1/R1 + 1/R2 + 1/R3. . .
1/Req = 1/3 + 1/5
=>
1/Req = 8/15 =>
Req = 1.875
Req is less than both the 3- and 5- ohm resistor
(b) The equivalent resistance of a group of resistors connected in series is:
(i) greater than any one of the resistors in the group
Equation: Req = R1 + R2 + R3 . . .
Req = 3 + 5
=>
Req = 8
Req is greater than both the 3- and 5- ohm resistor.
9. A series circuit consists of three identical lamps connected to a battery as in Figure
Q18.19. When the switch S is closed, what happens:
(a) to the intensities of lamps A and B?
Both of these intensities go up
(b) to the intensity of lamp C?
There will be no current flowing through bulb C; therefore, it will no longer light
up.
(c) to the current in the circuit?
Current is I=V/R. V stays the same and R decreases; therefore I increases.
(d) to the voltage drop across the three lamps?
It will be the same because that is dependent solely upon the battery
(e) Does the power dissipated in the circuit increase, decrease, or remain the same?
It increases and this can be seen with P=V2/R. V is constant, but R goes down,
causing P to increase.
Quantitative Problems – Pg 486: 2, 9
We did not have time to go through these problems. We will finish these problems off in
tomorrow’s discussion 6/26.