Download Prelab06

Physics Pre-lab 212P-6
DC Circuits
Name:__________________________
Section:_____
Date:__________
(Read this & answer the questions before coming to lab. Note that this prelab
requires the use of your experimental kit.)
Summary of relevant concepts:
A. EMF devices:
 An ideal EMF device maintains a constant potential difference across its terminals; we
denote the EMF E by an arrow pointing from the negative to positive terminals;
 Real (non-ideal) EMF devices are modeled as an ideal EMF with an internal resistance in
series; the potential difference across the terminals of such a device depends on the
resistance of the external circuit (or "load");
B. DC circuits:
 LOOP RULE: sum up the changes in potential that you encounter as you go around the
loop; this sum should be zero.
 RESISTANCE RULE: if you go thru' a resistance in the direction of current flow, the
change in potential is -iR
 EMF RULE: if you go thru' an EMF device from the - to + terminal i.e. in the direction
of the EMF arrow, the change in potential is +E.
 JUNCTION RULE: the sum of the currents entering a junction must equal the sum of
the currents leaving the junction
C. Resistances in series:
 Resistors are said to be in series with each other when the total potential difference across
the combination = sum of potential differences across individual resistances; the resistors
all carry the same current.
 The combination of resistances in series = sum of resistances
D. Resistances in parallel:
 Resistors are said to be in parallel when the total current through the combination = sum
of currents through each resistor; the resistors all have the same potential difference
across their ends.
 The reciprocal of the equivalent parallel resistance = sum of individual reciprocal
resistances.
Pre-lab Questions:
These prelab questions require you to carry out some simple experiments. It may be convenient
to enlist the help of a friend for an extra pair of hands.
Use the components in your lab kit to set up each of the circuits shown below. Observe the
brightness of the bulbs in each case, and also measure the current in the circuit using your
compass as an ammeter. Then, answer Q1.
-+
+
Round
Bulb
-+
+
2 Round
Bulbs in
series
Q1. Assume that each battery is ideal with an EMF of 1.5 V and that each bulb has a resistance
of 50 when it lights. (Note that -- in actuality -- the resistance of each bulb will change,
depending on how "bright" it is. Why?) Use these assumptions to explain the differences that you
observed in the two circuits shown above.
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
Use the components in your lab kit to set up the circuit shown below.
 Observe the brightness of each bulb, and also measure the current in the branch AB using
your compass as an ammeter.
 Unscrew one of the bulbs from its socket. Carefully note any changes in the brightness of
the remaining bulb, and also any changes in the current in the branch AB using your
compass as an ammeter.
 Then, answer Q2-Q5 assuming that each battery is IDEAL and has an EMF of 1.5 V.
Also, assume that each bulb has a resistance of 50  when it is lit.
B
A
-+
- +
Round
Bulb
Round
Bulb
Q2. When both bulbs are in parallel, what is the current in each bulb? What is the total current
delivered by the battery?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Q3. If one bulb is disconnected, what is the current through the remaining bulb? How does it
compare with the case when both bulbs were in parallel? (Remember that the battery is IDEAL.)
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Q4. If one bulb is disconnected, what is the current delivered by the battery? How does it
compare with the case when both bulbs were in parallel?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Q5. Given your answers to Q3-Q4, try to explain your experimental observations regarding the
change in bulb brightness and the change in total current when one of the bulbs is unscrewed?
Are any of your experimental observations inconsistent with your theoretical analysis? Which
one(s)?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________