Practice combination circuit

Also, FYI regarding quiz tomorrow

Topics are:

- series and parallel circuit
- how to find resistance using 1/R equation

There will be NO full combination circuit questions (unless it is a bonus).

OK?  OK!

SAT physics prep

Thank you to those who made it today.  If you didn't, but still want to prep for the test, see me asap and I'll give you the packet of information.  We will be meeting at 1 PM on Mondays.

Here's the homepage for the test itself:

https://collegereadiness.collegeboard.org/sat-subject-tests/subjects/science/physics

Note that we will NOT be covering thermodynamics or modern topics in class, so we will spend the first few sessions covering these ideas.

Combination circuit notes

The two examples from class today:

We will have a quiz next class, focusing on series and parallel circuits.  There will be no combo circuit problems, unless it is a bonus question.

Difference between series and parallel circuits

Practice

Practice series circuit with answers

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.

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.