HPP Activity 70v1
DIRECT CURRENT CIRCUITS
Objectives

to understand qualitatively current flow through circuit elements connected in series and
parallel

to understand qualitatively the total resistance of networks of bulbs in series and parallel

to understand qualitatively the inverse relationship of current and resistance for networks
of bulbs in series and parallel for constant potential difference

to understand independent and dependent networks of bulbs connected in series or in
parallel

to understand the concept of open circuits

to understand the concept of short circuits
Exploration
Work must be done to move charges from one potential to another. Work is done on the charges
in a cell to polarize the cell membranes and work is done, by a battery, to move charges through
a light bulb to make the filament glow. We define the movement of charge, current. We use the
letter “I” to represent current and mathematically current is given as:
I = Q/t (i.e. the amount of charge passing by in a given amount of time)
Charges will naturally move in the direction that the electric field pushes them and they can, in
turn, do work on objects through which they pass. What, if anything, is used up when a battery
“runs down” or a charged plate is discharged?
When testing the following circuits only connect to the battery momentarily to observe the
behavior. This will preserve battery life and will help you avoid any unpleasant consequences of
a short circuit.
Equipment:
1 piece of nichrome wire
1 battery
GE 1
1. Connect a short length (20 cm) of nichrome wire across the terminals of the
battery. Feel the wire. Does the wire seem to be the same temperature across
the entire length or are some sections warmer than others? Do a blind test—
Activity Guide
 2002-2010 The Humanized Physics Project
Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
HPP Activity 70v1
2
have someone hold the battery while a second person with their eyes closed
touches the wire at different places.
2. What does this observation suggest about the amount of current (charge per
unit time flowing) in the wire at one point compared to another point. Why?
Discuss your observations with an instructor.
To light a bulb, you must heat the filament inside the bulb to a very high temperature (at least
390C but as high as 3000C!). How can this happen? Where does the energy come from? We
learned earlier that the work done by an electric field on a moving charge is:
W=qV
where q is the charge being moved and V is our shorthand for the potential difference across
which, the charge moves (this could be from the positive end to the negative end of a batter for
instance). Recall also that power is the work done or the energy transformed per unit time so,
dividing the equation for work by time we get:
W q

V
t
t
P  I V
So, the work done by the electric field (produced by the battery) on the charges is transformed
into thermal energy and the temperature of the filament increases to the point that we can see it!

What then determines the brightness of a light bulb?
Exploration
Equipment:
6 wires with alligator clips
3 bulbs
3 sockets
1 battery holder with batteries
GE 2
Activity Guide
 2002-2010 The Humanized Physics Project
HPP Activity 70v1
3
1. Connect the single bulb circuit and note the brightness of the bulb. Do not
connect the two-bulb circuit yet! First: Predict the brightness of the bulbs in
the two-bulb circuit shown below compared to the brightness of a bulb in a
single-bulb circuit. Will the bulbs in the two-bulb circuit have the same
brightness? Will the bulbs in the two-bulb circuit be brighter, dimmer or
equally bright compared to a bulb in a single bulb circuit? Explain why you
think so.
2. Set up a two-bulb circuit and test your predictions. Based on your
observations, what can you conclude about the amount of current through each
bulb in the two-bulb circuit?
3. Based on your observations, how does the amount of current through a twobulb circuit compare to the amount of current through a single-bulb circuit?
Remember that brightness is a measure of the amount of power = work/time
done by the charges on the filament of the bulb.
4. Do you think it is appropriate to think about charge flowing (current)
through the battery? Specifically, if current did not flow through the battery,
what would be the consequence? Explain your reasoning.
Discuss your answers with an instructor.
5. How does the current through the battery in the two-bulb circuit in part 1
compare to the current through the battery in a single-bulb circuit? Explain
your reasoning.
Discuss your reasoning with an instructor.
Invention
Activity Guide
 2002-2010 The Humanized Physics Project
HPP Activity 70v1
4
Two (or more) bulbs connected as shown in part 1 are said to be connected in series. Every
charge that passes through one bulb must eventually pass through the other bulb(s) in series with
it. If there is a path around any one of the bulbs, then at least some of the charges can and will
flow around that bulb and it won’t meet the criterion for a series circuit.
Equipment:
6 wires with alligator clips
3 bulbs
3 sockets
2 batteries
1 battery holder
GE 3
1. Predict the brightness of the bulbs in the two-bulb circuit shown below as
compared to the brightness of a bulb in a single-bulb circuit. Will the bulbs in
the two-bulb circuit have the same brightness? Will the bulbs in the two-bulb
circuit be brighter, dimmer or equally bright compared to a bulb in a single
bulb circuit?
2. Set up a two-bulb circuit and test your predictions. Based on your
observations, what can you conclude about the brightness of each bulb in the
two-bulb circuit (compared to the one-bulb circuit)?
3. How must the current through the battery in the two-bulb circuit compare to
the current through the battery in a one-bulb circuit? Explain your reasoning.
Discuss your answers with an instructor.
Activity Guide
 2002-2010 The Humanized Physics Project
HPP Activity 70v1
5
Invention
Two (or more) bulbs connected as shown in question 1 are said to be connected in parallel. The
charges will separate and some will flow in each of the bulbs in the parallel circuit. The charges
that flow through one bulb will not be the same as the ones that flow through the other bulbs in
the parallel circuit.
Exploration
Equipment:
9 wires with alligator clips
3 bulbs
3 sockets
1 battery holder with batteries
GE 4.1
1. Predict the brightness of the bulbs in a circuit with three bulbs in series
compared to a one-bulb circuit and compared to a two-bulb series circuit.
2. Set up the circuits in question 1, and test your predictions.
3. Suppose you have a closed box in a circuit as shown below. You observe a
brightness in bulb A. Imagine someone makes a change in the electrical
elements inside the box. You do not see what has been done. However, you
observe that the indicator bulb becomes brighter. What can you conclude
about the current through the battery? Explain.
4. Suppose you observe that the indicator bulb becomes dimmer. What can
you conclude about the current through the battery? Explain how bulb A is
serving as an indicator bulb in this circuit.
Activity Guide
 2002-2010 The Humanized Physics Project
HPP Activity 70v1
6
We can think of a bulb as presenting an obstacle or resistance to the current in
the circuit. As the resistance increases, the current decreases.
5. When bulbs are added to a circuit in series, as in part 1, does the total
resistance of the circuit increase, decrease, or remain the same? Does the
current through the circuit, increase, decrease, or remain the same? Based on
your observations to this point, how are the current and resistance related for
electrical elements in series? Explain.
GE 4.2
1. Set up a circuit with two bulbs in series, as shown in the diagram below.
Consider bulb A to be an indicator bulb. Consider another bulb added to the
circuit in parallel to bulb B, as shown in the diagram below. Predict whether
the current through bulb A, the indicator bulb, will increase, decrease or
remain the same.
2. Test your prediction. How does the brightness of bulb A change when a
bulb is added in parallel to bulb B? How does the current through bulb A
change when a bulb is added in parallel to bulb B? How does the current
through the battery change when a bulb is added in parallel to bulb B?
Explain.
3. How does the total resistance of the circuit change when a bulb is added in
parallel to bulb B? How does this change in the resistance of the circuit affect
the total current through the battery?
Activity Guide
 2002-2010 The Humanized Physics Project
HPP Activity 70v1
7
d. Summarize how the total resistance of a circuit changes
(i) when bulbs are added in series
(ii) when bulbs are added in parallel
Discuss your answers with an instructor.
Exploration
Equipment:
9 wires with alligator clips
3 bulbs
3 sockets
1 battery holder with batteries
1 switch
GE 5.1
1. Set up the following circuit.
What happens to the brightness of bulb A when the switch is closed? Explain.
2. What happens the brightness of bulb B when the switch is closed? Is it
possible that there is current through bulb B when the switch is closed?
Explain.
Invention
When a bulb is in parallel to a wire, the bulb is said to be shorted out by the
wire.
Activity Guide
 2002-2010 The Humanized Physics Project
HPP Activity 70v1
8
In this case, the wire is a low resistance path around the bulb and it lowers the overall resistance
of the circuit. In general, a wire connected in parallel with any element in a circuit is said to be a
short circuit. Some of you may have found that when the element is a battery a large amount of
heat is generated in the wire – in general this is NOT a good thing. A short circuit in house
wiring can, and frequently does, cause a fire
SUMMARY
You should understand current flow through circuit elements connected in series and parallel.
You should understand qualitatively the total resistance of networks of bulbs in series and
parallel. You should understand qualitatively the inverse relationship of current and resistance for
networks of bulbs in series and parallel. You should understand independent and dependent
networks of bulbs connected in series or in parallel. You should understand the concept of open
circuits and the concept of short circuits.
Activity Guide
 2002-2010 The Humanized Physics Project