pre-lab preparation sheet for lab 4—batteries, bulbs, and current

Question 2-7: When you added the second bulb to the circuit did the voltage across the
battery change significantly (i.e., by more than a few percent)?
Question 2-8: Does the battery appear to be a source of constant current, constant
voltage, or neither when different elements are added to a circuit?
INVESTIGATION 3: AN ANALOGY TO
CURRENTAND RESISTANCE
You found in Activity 1-5 that current is not used up in flowing through a bulb, but this
may seem counterintuitive to you. Also, how can we explain that there is less current in
the circuit with two bulbs instead of one? Lots of physics teachers have invented
analogies to help explain these electric circuit concepts. One approach is to construct a
model of a gravitational system that is in some ways analogous to the electrical system
you are studying. This is shown in Figure 4-5.
It is believed that the electrons flowing through a conductor have frequent collisions
that slow them down and change their directions. Between collisions each electron
accelerates and finally staggers through the material with an average drift velocity,
drift
.
Figure 4-15: A simplified depiction of an electron in a uniform electric field staggering through a conductor
as a result of collisions. Despite the constant force to the right caused by the electric field, these collisions
cause the electron to move through the conductor with a constant average velocity,
velocity‖ (instead of accelerating).
drift
, the ―drift
As you saw in Investigation 2, we can talk about the resistance to flow of electrons
that materials offer. A wire has a low resistance. A light bulb has a much higher
resistance. Special electric elements that resist current are called resistors. You will
examine the behavior of these in electric circuits in future labs.
Lab 04 – Batteries and Current