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# Massachusetts Comprehensive Assessment System

## 2011, High School Intro Physics

Question 12: Open-Response
An electrician places a variable resistor in series with an LED lamp and a 6 V battery, as shown below.
LED lamps are efficient sources of light that operate at a specified voltage for a wide range of currents. The LED lamp in this circuit operates at a constant 3 V for all settings of the variable resistor, while the remaining 3 V is always dropped across the variable resistor.
1. Calculate the current passing through the variable resistor when it is set to each of the values shown in the table below. Show your calculations and include units in your answers.
 Voltage (V) Resistance (O) Current (A) 3.0 4 3.0 6 3.0 10 3.0 15 3.0 20
1. On the grid in your Student Answer Booklet, make a graph using your answers to part (a). Current should be on the y-axis and resistance should be on the x-axis. Clearly label the axes and use a proper scale.
2. Based on the graph, describe what happens to the LED lamp as the resistance decreases.
The maximum recommended current for the LED lamp is 0.3 A.
1. If the LED lamp is operated for a long time with the variable resistor set at 4 O, describe the most likely effect on the LED lamp.

### Scoring Guide and Sample Student WorkSelect a score point in the table below to view the sample student response.

ScoreDescription
4 Response demonstrates a thorough understanding of the relationships between current, voltage, and resistance. The response correctly calculates the current values for each resistor value and correctly graphs current and resistance. The response also correctly describes what happens to the LED lamp as the resistance decreases and if the resistance is set at 4 Ω for a long time.
4
3 Response demonstrates a general understanding of the relationships between current, voltage, and resistance.
2 Response demonstrates a limited understanding of the relationships between current, voltage, and resistance.
1 Response demonstrates a minimal understanding of the relationships between current, voltage, and resistance.
0 Response is incorrect or contains some correct work that is irrelevant to the skill or concept being measured.

Note: There are 2 sample student responses for Score Point 4.

None found...