Pre-Lab homework

For our next formal lab, which we will begin next week, you will ask the question:

How does resistance affect electrical current and voltage?

Do NOT do any formal research on this question.  Instead, think about it and put your ideas into a testable hypothesis.  And since you will be making graphs of current (y-axis) vs. resistance (x-axis) AND voltage vs. resistance in the lab, show (in your hypothesis) what you think the graphs will resemble (with sketches).  

Important info:

Current (I) is the rate at which charge "flows" in a circuit.

Resistance (R) is a measure of "push back" against current.

So bring your hypothesis and expected graph sketch to next class.

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The lab set-up:

Current and Resistance - a lab!

In this lab, you will determine the relationship (if there is one) between electrical current and resistance.

At this point, you should have a hypothesis - including the graph you expect will represent the relationship between current and resistance (I vs. R).

1.  Set up the circuit depicted in the pictures below:

Two batteries in series with the "resistance box" and a meter set to measure current (in A - use the 20A setting and socket for the red wire).  Connections are made with alligator wires - wires with "alligator" clips on each end.

Have a separate meter set up to measure voltage.  To do this, the meter needs to be "in parallel" with the resistance box.  See the picture below.

2.  Change the resistance in small increments, starting at around 4 ohms.  Write down the following data:  resistance (in ohms), current (in A), and voltage (in V).  Take at least 20 trials.  If you get to a point where the current is staying the same (or reading zero), try switching the dial to the mA setting (and move the red wire to the mA socket as well).

3.  For homework, plot 2 graphs:  current (I) vs. resistance (R), and voltage (V) vs. resistance.

4.  A few questions will be forthcoming.

  

In the last picture, the yellow meter is measuring current - it is IN SERIES with the batteries and resistance box.  The red meter is measuring voltage - it is IN PARALLEL with the resistance.  You will notice that a student is holding the leads from the meter - you may need to do the same.