Lab 5 – DC circuits II
Note: Any lettered section can be graded. Items which include a letter and a number in parentheses
refer to a rubric ability. For example, (O3) refers to the observation experiment rubric ability 3 and (T4)
refers to the testing experiment rubric ability 4. Items without such designation are given as additional
guidance.
I. Basic Circuit Elements (Observation Experiment)
Given only one piece of wire, a light bulb, and a battery, determine a way to make the bulb light up.
a. Draw a clearly labeled circuit diagram of the experimental set-up. Make sure to represent the
important aspects of the experiment.
Perform the experiment.
b. (O5) Describe what is observed without trying to explain.
c. (O8) Using the observations, construct an explanation for what must occur in order for the bulb to
light up, i.e. in order for the circuit to be complete.
II. Capacitors in a circuit
Equipment: wires, batteries, capacitors, resistors, and an ammeter.
Part A (Observation Experiment):
Devise an experiment to qualitatively determine the direction and strength of current flow in a circuit
while a capacitor is charging and discharging.
Follow these steps:
a. Connect a capacitor in series with a battery and a resistor (>600Ω). To discharge, you can simply
remove the battery from the circuit and close the circuit.
Hint: Where does the ammeter need to be placed to test current flow in each part of the circuit?
b. Draw a clearly labeled circuit diagram of the experimental set-up. Be sure to indicate any differences
in the diagram between when the capacitor is charging and when it is discharging.
Perform the experiment.
c. (O5) Describe what is observed without trying to explain.
•
How long does it take for the current to drop to one tenth of its original value?
•
Explain specifically how the direction of the current is being determined from the data.
d. (O7) Using your observations, construct a relationship that describes the direction of the current
flow.
e. (O8) Devise an explanation for the observed relationship.
Part B (Testing and observation experiment):
Qualitatively test the direction and strength of current flow through all wires of circuits with more than
one capacitor, as well as observe how the current flow changes. Do this for both charging and
discharging capacitors.
Follow these steps:
a. (T1) Identify a hypothesis to explain the direction of current flow in all wires of the circuit when the
capacitors are charging and discharging.
b. Replace the capacitor in part II with two capacitors in series.
c. (T4) What direction do you predict the current will flow in all parts of the circuit when the capacitors
are charging/discharging?
d. (T2) Design a reliable experiment that tests your prediction, and measures the strength and time it
will take for the current to drop to one tenth of its original value.
e. Draw a labeled sketch of the experimental set-up including a circuit diagram.
f. What are experimental uncertainties in this experiment?
g. Perform the experiment, and record its outcome.
h. (T7) Decide whether the prediction and the measurements agree or disagree.
i. (O5) Describe what you observed:
•
How long does it take for the current to drop to one tenth of its original value?
•
What the strength of the current higher or lower than in part A?
j. (T8) Make a reasonable judgment about the hypothesis based on the measurements and the estimated
uncertainty.
k. (O8) Devise an explanation for why the current flow did or did not change.
➢
If you have time, replace the capacitor in part II with two capacitors in parallel, and redo parts
c) to k).
III. Conceptual Question
Use what you know about potential
difference and current to rate the brightness
of the bulbs in the circuit shown, from
highest to lowest brightness. Do indicate of
any have equal brightness. You may assume
that the brightness of a bulb is directly
related to the power through that bulb, where
P=IΔV. All the bulbs in the given circuit are
identical. Explain your reasoning.
IV. Kirchoff's Laws (Testing Experiment)
Design an experiment to test whether Kirchoff‘s junction rule and loop rule work. The junction rule
states that the sum of the currents entering any junction must be equal to the sum of the currents
leaving that junction. The loop rule states that the algebraic sum of the changes in potential encountered
in a complete traversal of any loop of a circuit must be zero.
Equipment: Batteries, resistors, light bulbs of different ratings, an ammeter, and a voltmeter.
Follow these steps:
a. Start by creating a circuit that satisfies the following requirements:
•
There must be more than one junction in the circuit. (At least two loops.)
•
There must be more than one battery in the circuit. The batteries can be connected in
series/parrallel/some other way.
•
The must be at least 4 other circuit elements besides batteries and wires.
Check with your TA that your circuit satisfies the above requirements before starting to make
measurements.
b. Draw a labeled sketch of the experimental set-up including a circuit diagram.
c. Pick one junction and one loop in your circuit and predict what you expect to observe in each.
d. (T1) Identify the hypotheses (rules) to be tested.
e. (T2) Design reliable experiments that test each hypothesis on the junction and loop from c) including
a brief description of the procedure.
f. Write mathematical statements describing the relationship between the data that you will measure
and record in your experiment.
g. (T4) Make a prediction about the outcome of the experiment for the junction and loop in part c)
based on f).
h. What are experimental uncertainties in this experiment?
i. Clearly record the data and outcomes from your experiments.
j. (T7) Decide whether the prediction and the measurements agree or disagree.
k. (T8) Do the experimental uncertainties account for any discrepancy between the predicted and
measured values? Make a reasonable judgment about the hypothesis based on the measurements and
the estimated uncertainty.
➢
If you have time, verify that other junctions and loops in your circuit satisfy Kirchoffs's laws.