Buggé: 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. https://www.youtube.com/watch?v=6MOPxRUjPg0&feature=youtube_gdata_player a) Record what happens in the experiment and devise an explanation for your observation. Focus on energy conversions. 1.2 Observe and explain ​
You have a charged Wimshurst generator and a neon bulb with two leads. Experiment What do you observe? Explain Place the bulb between the spheres of the generator leads to connect a charged electroscope and an uncharged electroscope. 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. d) Describe the ways in which the experiment in 1.1 is analogous to a battery connected to a light bulb and ways in which it is different. Indicate the corresponding parts of the two different systems. Hint: Focus on energy conversions. 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. Buggé: 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 ​
V​
should be higher than the potential of point 1​
2 ​
V2​
​
. ​
Potential difference​
is sometimes noted as voltage (Δ​
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: ε=
W
q
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”.
Buggé: DC Circuits 1 20​
1.6 ​
A long wire is connected to the terminals of a battery. In 5.0 s, 5.8 x 10​
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 0.5­A 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. 1.9​
How would the graph differ if there were two identical small motors attached one after the other to the battery? 1.10​
Use your textbook, or the PowerPoint online, to familiarize yourself with circuit symbols.