Oops - quick last minute practice

Sorry to not post this earlier!

Quiz next class

Be able to:

- solve a series circuit
- solve a parallel circuit
- find resistance using the new formula:  (1/Rtotal = 1/R1 + 1/R2 + 1/R3)

Practice:

1.  Resolve this circuit:  4 resistors (1, 2, 3, 4 ohms) in series with 12 volt battery.

2.  Resolve this circuit:  2 resistors (3 and 6 ohms) in parallel with a 24 volt battery.

3.  Check the value of the resistance in #2 above, using the new resistance formula.

Parallel circuit practice

Series circuit practice problem

Lab homework

For next class, make 2 graphs:

current vs. resistance

voltage vs. resistance

Start to think about what these graphs suggest, maybe even writing down your thoughts.

There will be a few lab questions that you might want to start thinking about as well.  Here they are:

1.  What is Ohm's law?

2.  Are there things that do not "obey" Ohm's law?

3.  What are sources of error in this experiment?

4.  Would you expect to have the same readings if you left things running for a few minutes?  How about after an hour?

5.  What exactly is "internal resistance" and how is it relevant in this experiment?

6.  Somewhere, maybe in your conclusion, be sure to address why the graphs look as they do.

Don't forget that you'll also need these things in your lab:

Purpose
Hypothesis (copied from the earlier homework - don't change it, and include the graph you predicted)
Data table with correct units
Graphs with correct units
Questions
Conclusion

Thanks everybody!  You'll have some time in the next class to work on the lab.

Circuit lab

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 a graph of current (I) vs. resistance (R).

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.