Zubrzycki: DC Circuits 1
Go with the Flow?
1.1 Observe and explain You charge a Wimshurst generator by cranking
the handle and then hang a Ping Pong ball with a metal coating from a
thin thread between the charged spheres of the generator.
Spheres of the
generator
https://www.youtube.com/watch?v=6MOPxRUjPg0&feature=youtube_gdata_player
Ping Pong ball
a) Record what happens in the experiment and devise an explanation
for your observation.
b) Describe the ways in which this experiment is analogous to a battery
connected to a light bulb (see Activity 1.3) and ways in which it is
different. Indicate the corresponding parts of the two different systems.
Hint: Focus on energy conversions.
1.2 Observe and explain You have a charged Wimshurst generator and a neon bulb with two leads.
Experiment
Place the bulb between the spheres of the generator leads to connect a
charged electroscope and an uncharged electroscope.
What do you
observe?
Explain
Why does the flash last just a short time interval?
1.3 Observe and explain You have a battery, a wire, and an incandescent light bulb. Try different
arrangements of these three elements to make the light bulb glow.
a) Draw pictures of the arrangements where the bulb lights and several where it does not.
b) Explain why for some arrangements the light bulb glows and for some it does not.
c) Examine (or Google) a clear or broken incandescent light bulb – draw a picture of the parts.
Devise a mechanism that will explain why the filament glows for some arrangements in (b).
Hint: Think back of what you know about electric fields and the internal structure of the metals.
Did you know?
Electric current: The electric current I through a wire in an electric circuit is the physical quantity
equal to the ratio of the electric charge ∆Q that passes a cross section in the wire and the time interval ∆t
needed for the charge to pass:
I = ∆Q / ∆t
The unit of measure for current is coulomb per second (C/s), or ampere (A).
In metal conductors the electric charge that flows is carried by free electrons (negatively charged
particles). Historically, the direction of the electric current was assigned as the direction that positive
electric charges moved. Thus, the direction of the electric current in an electric conductor is opposite the
direction that the free electrons move.
Zubrzycki: DC Circuits 1
1.4 Why would electrons flow through a circuit creating a current? Think of all of the experiments you
have done so far to answer this question.
Here is an Idea!
Electric field accelerates electrically charged objects. Therefore there must be electric field present in
the circuit for the charged particles to flow in one direction. Think of what part of the circuit is
responsible for this field. For the conventional electric current (moving positive charge) to flow through
a wire in the direction from 1 to 2, the potential of point 1 V1 should be higher than the potential of point
2 V2. Potential difference is sometimes noted as voltage (V). Positively charged objects flow in the
direction of decreasing potential. Free electrons move in the opposite direction – in the direction of
increasing potential.
1.5 You have two parallel plates charged with an equal but opposite charge.
a) Draw the electric field that results from the separation of charges.
b) Describe what happens to an electron placed near the location
indicated.
c) Now instead of parallel plates, imagine you have a wire in an
external electric field.
d) Describe what will happen to the electron inside the conductor.
What assumptions did you make?
e) In a circuit with wires connecting a battery and a light bulb, explain what makes the electrons
move in a coordinated fashion.
f) Use the work and energy approach to explain (e). Include a bar chart.
Did you know?
Emf ε The electromotive force (emf ε) equals the work done by a battery per coulomb of electric charge
that is moved through the battery from one terminal to the other to maintain a potential difference at the
battery terminals:
The unit of emf is J/C = V, the same as the unit of electric potential difference.
Electromotive force (emf), despite its name, it is not a force. Emf is work done per coulomb of charge.
Back in the 19th century, the terms „force‟ and „energy‟ were used somewhat interchangeably. The
linguistic distinction between force and energy had not yet been made clear. For example, kinetic energy
was called “live force” and potential energy was called “dead force”.
E
Zubrzycki: DC Circuits 1
1.6 A long wire is connected to the terminals of a battery. In 5.0 s, 5.8 x 1020 electrons pass a cross
section along the wire.
a) Determine the current in the wire.
b) If the electrons flow from left to right, in which direction is the current?
1.7 A typical flashlight battery will produce a 0.5-A current for about 3 h before losing its charge.
Determine the total number of electrons that have moved past a cross section of wire connecting the
battery and light bulb.
1.8 You have a 9 V battery connected to a small motor. Assume the electric potential of the – terminal is
zero and of the + terminal is 9 V. Represent the changes in electric potential in the circuit using a V-vslocation graph.
V (V)
9
5
0
A
B
C
D
A
1.9 How would the graph differ if there were two identical small motors attached one after the other to
the battery?
V (V)
9
5
0
A
B
C
D
E
F
A
1.10 Use your textbook, or the power point online, to familiarize yourself with circuit symbols.