Practice: Brightness, Switches and More Power
Solutions
Level 1
1.
When any four resistors are connected in parallel, the _______ each resistor is the same.
(A) charge on (B) current through (C) power from (D) resistance of (E) voltage across
by definition of a parallel circuit
2.
A certain coffeepot draws 4.0 A of current when it is operated on 120 V household lines. If electrical energy
costs 10 cents per kilowatt–hour, how much does it cost to operate the coffeepot for 2 hours?
(A) 2.4 cents (B) 4.8 cents (C) 8.0 cents (D) 9.6 cents (E) 16 cents.
Power = IV = 480 W = 0.48 kW. Energy = Pt = (0.48 kW)(2 hours) = 0.96 kW-h
Level 2
1. A circuit is connected as shown. All light bulbs are identical. When the switch in the circuit is closed
illuminating bulb #4, which other bulb(s) also become brighter?
(A) Bulb #1 only (B) Bulb #2 only (C) Bulbs #2 and #3 only (D) Bulbs #1, #2, and #3
(E) None of the bulbs.
Closing the switch reduces the total resistance of the circuit, increasing the current in the main
branch containing bulb 1
2.
Answer: B
Closing the switch short circuits Bulb 2 causing no current to flow to it. Since the bulbs were
originally in series, this decreases the total resistance and increases the total current, making bulb
1 brighter.
3. What is the current I1?
(A) 0.8 mA (B) 1.0 mA (C) 2.0 mA (D) 3.0 mA (E) 6.0 mA
Resistance of the 2000 Ω and 6000 Ω in parallel = 1500 Ω, adding the 2500 Ω in series gives a
total circuit resistance of 4000 Ω.
4. How do the currents I1, I2, and 13 compare?
(A) I1 > I2 > I3 (B) I1 > I3 > I2 (C) I2 > I1 > I3 (D) I3 > I1 > I2 (E) I3 > I2 > I1
I1 is the main branch current and is the largest. It will split into I2 and I3and since I2 moves
through the smaller resistor, it will be larger than I3
5. The circuit shown has an ideal ammeter with zero resistance and four identical resistance light bulbs which are
initially illuminated. A person removes the bulb R4 from its socket thereby permanently breaking the electrical
circuit at that point. Which statement is true of the circuit after removing the bulb?
(A) The voltage from B → C increases.
(B) The power supplied by the battery increases
(C) The voltage across R1
(D) The ammeter reading is unchanged.
increases.
(E) The bulb R2 maintains the same brightness.
Breaking the circuit in the lower branch lowers the total current in the circuit, decreasing the
voltage across R1. Looking at the upper loop, this means R2
now has a larger share of the battery
voltage and the voltage across AD is the same as the voltage across BC
Free Response Practice
2002B3B. Lightbulbs of fixed resistance 3.0 Ω and 6.0 Ω, a 9.0 V battery, and a switch S are connected as
shown in the schematic diagram above. The switch S is closed.
a. Calculate the current in bulb A.
b. Which lightbulb is brightest? Justify your answer.
c. Switch S is then opened. By checking the appropriate spaces below, indicate whether the brightness of
each lightbulb increases, decreases, or remains the same. Explain your reasoning for each lightbulb.
i. Bulb A: The brightness increases decreases remains the same
Explanation:
ii. Bulb B: The brightness increases decreases remains the same
Explanation:
iii. Bulb C: The brightness increases decreases remains the same
Explanation:
2002B3B: Solution
Jedi
1. Suppose you are given a constant voltage source V0 and three resistors R1, R2, and R3 with R1 > R2 > R3. If
you wish to heat water in a pail which of the following combinations of resistors will give the most rapid
heating?
Answer: E
Most rapid heating requires the largest power dissipation. This occurs with the resistors in
parallel.
2. Which of the following combinations of 4Ω resistors would dissipate 24 W when connected to a 12 Volt
battery?
To dissipate 24 W means R = V2/P = 6 Ω. The resistances, in order, are: 8 Ω, 4/3 Ω, 8/3 Ω, 12 Ωand 6 Ω