# 2 DC Electricity PHET

• Name ____________________                                                          TechnologyPhysics

Batteries, Resistance and Current (Giancoli CH 18)

Prelab:

1.  Batteries are made by the difference in the electronegativity of different metals.  Electrons can flow spontaneously between metals that give off electrons easier to metals that are less likely to give off electrons.  There is a substance between these metals that allow charge to flow (salt bridge) so net charge doesn’t build up and “turn off” the battery.  Draw a diagram (use your text) of an alkaline battery showing the + terminal, - terminal (and the type of metals used), the direction of current (+) flow and what’s inside.

2.  What is the resistivity (r) of a resistor?  What characteristics of a resistor affect the resistivity?  Write a formula for this relationship, label each variable and indicate the units used to measure each.  Would resistivity be constant for a specific resistor?

3.  What is the difference between resistivity (r) and resistance (R)?  What are the units of each?

Lab Activity:  Log on and go to the PhET website (PhET.colorado.edu) Go to simulations, then “electricity” then to the following:

“Battery-Resistor”:

Check “show battery” and “show cores”, watch what happens, adjust some variables

1.  Why do electrons (blue dots) move?  Draw a diagram of the battery, label the flow of electrons.  The flow of current (+) is opposite; draw this and note if toward or away from + terminal of the battery.

2.  What does the Ammeter (on the left) measure?  How is this shown in the sim?

3.  What role do the “green dots” in the resistor play in the sim?  What do you think they represent?  What does this tell you about the effect of resistors in a circuit?

4.  Increase the resistance (# green dots).  What affect does this have on temperature?  WHY?

5.  When the circuit gets hotter, what affect does this have on current?  Explain using kinetic-molecular theory.

6.  To make the circuit “cold”, what do you need to do?  WHY?

7.  Describe the relationship between voltage and temperature.

“Resistance in a Wire”:

1.  In this sim, what variables are you seeing the relationship of.  Write the formula below, and indicate the units used to measure each one.

2.  Try increasing the resistivity of the resistor, r.  How does this change the “look” of the resistor?  Describe how that relates to the formula you just wrote (direct, indirect relationships, etc.).  What happens to the value of “R” (Resistance)?  Is this something that can be changed in a resistor that you would buy in a store to use in a circuit?

3.  Now increase the length (L) (you would essentially be designing a different resistor, since you can’t do this to one you will be using in a circuit).What happens to “R”?  WHY?

4.  Now increase the area (A) of the resistor. What changes?  WHY?

5.  Write a summary about the different relationships you looked at in the properties and measurements of a resistor.

“Ohm’s Law”:

In this sim, vary the values of Resistance (R), Current (I) and Voltage (V).

1.  What are three different combinations of I and R which get a voltage of 6.0 Volts?

2.  Describe the relationship between I and R.  Why is this?

3.  What would happen if you could decrease the resistance to 0?  Explain.