Fundamentals of Circuits I: Current Models, Batteries & Bulbs

Fundamentals of Circuits I: Current Models, Batteries & Bulbs v 0.1
Question 1.15 How can you determine which model, if any, correctly describes the current
through the bulb What would you measure, and where would you measure it Discuss
what measurements you might make to eliminate at least three of the four competing
models for current. Talk things over with your partners, and explain your reasoning.
In Activity 1.5 you will use the computer based laboratory system and two current probes
in your circuit to measure current and test the four models above. In addition to the
battery, bulb and wires you used above, you will need:
• Computer with Logger-Pro software
• Lab Pro interface
• two current probes
The current probe is a device that measures current. Your computer will take measurements
from the current probe and display them, as a function of time, on the computer
screen. The current proble will allow us to measure the current at different locations and
under different conditions in electric circuits.
− +
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Figure 7: Measuring current in a circuit with a battery, bulb and current probe.
In order to measure a current, the current must pass through the current probe. For
example, to measure the current in the bottom wire of the circuit in Fig. 5, the current
probe should be connected as shown in Fig. 7. To connect the current probe at this
location we must disconnect the circuit at the point where you want to measure the
current (the bottom wire), and insert the current probe. That is, disconnect the circuit,
put in the current probe, and reconnect with the probe in place.
Keep in mind that the current probe measures both the magnitude and direction of the
current. Just like we did for velocities, forces, and other vector quantities in Physics I,
we distinguish opposite directions for current flow by positive and negative. A current
in through the + terminal and out through the - terminal (in the direction of the arrow)
will be displayed as a positive current. By contrast, when the current probe measures a
negative current, the current is directed into the − terminal and out of the + terminal of
the probe.
Take a close look at Fig. 8. If the current in the loop is clockwise both ammeters will
measure currents which are positive.
PHYS-204: Physics II Laboratory 9