Download Section 4

Circuits Lab – Week 4
4.1 – Comparing currents in unlike bulbs
The pressure difference (voltage) drives the charge flow through a bulb.
The bulb’s resistance hinders the charge flow. These two things work
together to determine the charge flow through a bulb.
For the same current, a round bulb will have a smaller pressure difference
than a long bulb. You saw this when you connected the two in series – the
long bulb lit while the round bulb did not.
In more scientific terms:
What do your observations indicate about the charge flow through the top
and bottom bulbs before and after you add the fourth bulb?
What does this indicate about the overall resistance of the circuit after a
bulb is added in parallel with an original bulb compared to the original
overall resistance?
Using your observations about bulb brightness, color code the circuit below
after the fourth bulb has been added.
For things in series that have the same current – more resistance means
more voltage.
Likewise, for things in parallel that have the same voltage – more resistance
means less current.
4.2 – Parallel Pair in a Series Circuit
Set up the three bulb circuit like the one on the left, leaving a gap to add a
fourth bulb. Then add the fourth bulb to complete the circuit on the right.
4.3 – How does adding branches lower resistance?
Charge only flows one way in a circuit – from an
area of high pressure to an area of low pressure. It
does not turn around or repeat a path. When
charge comes to a parallel path, some of the
charge goes down each path. If the resistance of
each path is identical, the charge will split and half
will go down each path.
Write down what you observe about bulb brightness before and after you
add the fourth bulb.
Knowing that charge will always choose the path of least resistance, what
do you think happens to the charge when it reaches a parallel path where
each branch has a different resistance?
4.4 – Measuring Voltages and Currents
Read all instructions BEFORE trying to measure anything!!!
Always connect the black wire to the COM spot on your multimeter.
If you are measuring voltage, connect the red wire to the bottom red slot
(VΩmA).
If you are measuring current, connect the red wire to the top red slot
(10ADC)
*To measure voltage, the meter must be connected in parallel with the
object you would like to measure.
*To measure current, the meter must be connected in series with the
object.
To measure voltage, turn the dial to DCV and 20. If you get an error, you
may adjust the dial to anywhere inside the DCV area. You cannot select an
option outside of that area!
To measure current, turn the dial to 10A.
Once everything is connected, you may then turn your switch to the on
position.
4.5 – Actually Measuring Voltages
PREDICT: What values do you think you will see in the circuit below?
The symbol for voltage is “V”. The symbol ∆V is used for the difference
between two voltages. Because it’s the difference in electric pressure
(voltage) that determines the charge flow through a resistor, ∆V is the
symbol of importance to us. In a D battery, the positive end is designed to
maintain a value 1.5 V higher than the negative end.
Build the circuit below, but do not turn on the multimeter. (You only have
one multimeter so you will have to measure one voltage difference at a
time.)
Use your multimeter to measure the voltages for each location. Write the
values in on the above circuit.
PREDICT: What values do you think you will see in the circuit below?
Color code the circuit, including the wires that go to the multimeters.
PREDICT: What values do you expect to see in each reading above?
Use your multimeter to measure the voltages for each location. Write the
values in on the above circuit.
What did you notice about the voltages about the two long bulbs compared
to the total voltage of the battery?
Use your multimeter to measure the voltages for each location. Write the
values in on the above circuit.
4.6 – Voltmeter Resistance
Now replace one of the bulbs with a long bulb and repeat.
Connect the multimeter as shown below.
Set up the circuit as shown below. Use the multimeter to measure the
current in each wire as shown by the diagram.
What do you observe about the bulb and the voltmeter?
Based on your previous labs, do you think the voltmeter has a high or a low
resistance?
Why don’t we connect voltmeters in series? (Other than the fact that I told
you we don’t)
4.7 – Actually Measuring Currents
Current is measured in “amps” and the letter that represents current is “I”
Build the circuits below (using round bulbs instead of long) and measure the
current in each wire.
What did you notice about I2 and I3 compared to I1 and I4?
SUMMARY:
Fill in the diagrams below.
3V
5A