Tag Archives: Circuits

A2L Item 240

Goal: Reason regarding power in a circuit

Source: 283 circuit powers

Consider the circuit below. Which resistor has the greatest power
consumption?

  1. The 50Ω resistor
  2. The 10Ω resistor
  3. The 1000Ω resistor
  4. The 50Ω and 10Ω resistors
  5. The 50Ω and 1000Ω resistors
  6. The 10Ω and 1000Ω resistors
  7. All have the same power.

Commentary:

Answer

(1) The potential drop over the 10Ω and 1000Ω resistors is the same.
Since power goes as V^2^/R, more power is consumed in the 10Ω resistor
than the 1000Ω resistor. Further, since power also goes as I^2^R and only
a fraction of the current through the 50Ω resistor flows through the 10Ω
resistor, the 50Ω resistor must dissipate the most energy.

A2L Item 225

Goal: Reason about electric circuits

Source: 283-580 100-watt and 40-watt bulbs in series

Two light bulbs are connected to a wall outlet as shown below. Bulb #1
is 100W and Bulb #2 is 40W. Which statement is true?

  1. Both bulbs are at their normal brightness.
  2. The 100W bulb is brighter than the 40W bulb.
  3. The 40W bulb is brighter than the 100W bulb.
  4. Both bulbs are at equal brightness.
  5. Cannot determine their relative brightness.

Commentary:

Answer

(3) Since the resistance of the bulbs goes as the reciprocal of its
wattage, students can reason that the most power is dropped in the 40W
bulb. Another way to reason is this. Suppose the potential source was
such that the potential difference over the 40W bulb was 120V, i.e. its
normal operating condition. The potential difference over the 100W bulb
is then 48V, which means that it is operating at (48/120)2 of its normal
condition. The bulb is dissipating about 16W.

A2L Item 224

Goal: Hone understanding of series circuits.

Source: 283-575 Two bulbs in series

A light bulb is connected to a battery as shown in Figure A below. When
a second bulb is connected as shown in Figure B, what happens to the
brightness of the original bulb?

  1. Increases
  2. Decreases
  3. Stays the same
  4. Bulb goes out
  5. Can’t determine

Commentary:

Answer

(2) Depending upon the bulbs and battery, the bulbs could appear to go
out.

A2L Item 223

Goal: Hone understanding of parallel circuits.

Source: 283-570 lights in parallel

A light bulb is connected to a battery as shown in Figure A below. When
a second bulb is connected as shown in Figure B, what happens to the
brightness of the original bulb?

  1. Increases
  2. Decreases
  3. Stays the same
  4. Bulb goes out
  5. Can’t determine

Commentary:

Answer

(3) If the battery cannot sustain the potential difference the bulb will
dim somewhat. This can confuse students so any demonstration should not
use a marginal battery.

A2L Item 221

Goal: Reason regarding circuits

Source: 283-560 Circuit voltages

Consider the circuit below. How does the voltage difference between
points A and C compare to the voltage difference between points A and D?

  1. VAC < VAD

  2. VAC = VAD

  3. VAC > VAD

  4. Cannot be determined

Commentary:

Answer

(2) It is interesting to find out how many students recognized that the
comparison did not require the use of Ohm’s law. To gauge student
understanding it is interesting to ask questions such as; does any
current flow in the wire between C and D? If C and D are at the same
potential, why does any current flow?

A2L Item 222

Goal: Reasoning regarding circuits

Source: 283-565 circuit currents

Consider the circuit below. Which statement(s) is correct?

  1. IAB = IBD +
    IBC
  2. IBC < IBD
  3. IBC > IBD
  1. 1 only
  2. 2 only
  3. 3 only
  4. 1 and 2
  5. 1 and 3

Commentary:

Answer

(4) Students should be encouraged to reason about the currents and not
calculate the value of the currents in circuits. Interestingly, if a
problem similar to this is given with the potential specified as a
number, many students will immediately calculate the currents.