Mood Brightener: ...more from Stay Homas. (Confination IX)

 

Circuits:

Electricity and machines that use electricity to do work for us are everywhere. I suspect it would be hard for us to truly imagine how different our world would be without this ubiquitous tool. At the same time there are a host of conceptual misunderstandings about electricity that might be helpful to understand better. We will start this discussion by exploring a fictitious setting that will ultimately serve as an analogy for our understanding of circuits. Along the way we will learn to reason about circuits and perhaps understand a little better what is going on inside our houses and in our robot.

The (fictitious) Setting:

Imagine, if you will, a factory that makes a product that we can think of as pure energy. Voodoo Donuts in Portland is a possible example. To get the donuts to their customers they use a unusual system where they create a continuous chain of small delivery vehicles (Vespas) from Portland out to Sandy and back.Think about how this is different than what actually happens when there are only a few large delivery vehicles on the road as opposed to a long chain of smaller vehicles. The entire road is filled with the delivery Vespas.

At the factory each Vespa is stops at a filling station and picks up a box of donuts. If the factory is small each Vespa only gets one box. If the factory is bigger and has more production lines each Vespa might pick up 2 or 3 boxes as the pass through the factory. There is the usual understanding that along the route everybody must deliver all of the donuts before returning to the factory. Intially we are imagining all the donuts being delivered just to Sandy.

In this analogy what happens when a single Vespa exits from the factory? If the route is already a continous chain then everyone will have to move forward to make room for the folks at the back. It's also true that if there is a slow down somewhere on the highway then all the delivery vehicles will have to slow down, even the ones in front of the slow down. If the roads are smooth and wide open everyone can travel quickly and more product can be moved out of the factory. If Sandy is very efficient it can off load boxes quickly and keep those Vespas moving. If it is less efficient the whole chain slows down and fewer donuts are delivered to Sandy every day.

Some things to notice about this model. In Sandy all the Vespas must pass through a single line to be unloaded. The process of unloading the donuts limits the flow of Vespas everywhere on the route. The factory can't put anyone on the road until everyone moves forward along the route. In addition the same number of Vespas are entering the factory to get restocked as are leaving it. If conditions on the road change everyone is affected. If there is a bridge out then nobody moves and no donuts are delivered!

What happens if the only thing I change is that I put another stop, say Gresham, on the route?

 

Fewer donuts are delivered to Sandy because there are now two delivery points. Two delivery points will also seem likely to slow the overall traffic because everyone has to stop twice. The factory has to slow overall production because traffic is slower but it is still loading 1 box on each Vespa.

What happens if the factory creates two production lines so each Vespa can be given 2 boxes of donuts?

 

Connection to Circuits:

It is tempting to spend a long time exploring this analogy (sort of a model) but let's start building the connection to circuits and see what we need to discuss. In the analogy there is something (Vespa) which carries the energy (donuts) along a path (roadway). In circuits there are charges (usually electrons) that carry the energy (described in volts) along a wire (road). The wires are completely full of charges (Vespas) that are delivering the volts (donuts) to your load (Sandy). The number of Vespas that pass by any point on the road is what we call the current which is measured in amps.

carriers (electrons) <=> Vespas............

energy (volts) <=> donuts......

..battery <=> factory

...wires <=> roads

current (amps) <=> # of Vespas/s

loads (resistance) <=> Sandy.............

Using this analogy can you explain why a light at the other end of your house comes on instantly when you turn the switch on?

the carriers at the switch start moving but because the wire is full the carriers at the far end of the house also start moving and delivering their energy

It is interesting to note that the charges (carriers) in the wires in your house travel about the same speed you can walk (1 - 2 m/s).

What happens if Sandy gets more efficient at offloading the boxes of donuts - offers less resistance to the movement of the Vespas?

More energy arrives in Sandy because the carriers can move faster and Sandy 'glows' brighter.

Again, using the analogy, what are some things that affect how much total energy is delivered to Sandy each day?

how many boxes on each Vespa which depends on the factory, how many Vespas pass through Sandy and get unloaded eveyr hour, how efficient Sandy is at the unloading which affects the speed of the Vespas.

Thinking about it in this way how many carriers leave your house and go back to Pacfic Power?

The power line that comes down from the Dallas loads 500,000 volts (500 kV) of energy on each carrier. Do the carriers have to move very fast to deliver a lot of energy to Bend.

Visualization:

We will talk about how this analogy can be applied to our understanding of what's happening in this basic circuit.

Bruce's notes to self:

donuts?

switch open or closed?

Vespas (# and speed)?

Brighter bulb?

# into and out of battery?

Assignment Breadcrumb Reading: Bb Assignment

What would you miss?

Think of all the things that we depend on for electricity. If electricity were gone what would you miss most? Why? Can it be replaced by a machine?

Before Next Class:

Assignment HW: Bb Assignment

Brighter Bulb:

Describe two features of our basic circuit that change when we put in a 'brighter' bulb? Describe one feature of the circuit that doesn't change.

Assignment HW: Bb Assignment

In Your House:

Find an electric appliance in your house. Find the power requirement label which is usually on the back. If it has a sleep power or current provide that also. Provide the Voltage, Current, and power values that are typically on that label. These labels are on microwaves, TV's, electric heaters, and many other items of equipment. Later you will be asked to calculate the energy consumption and cost.

Looking Ahead:

Look ahead to the next Breadcrumb: Circuits II

Assignment Breadcrumb Reading: Bb Quiz

Circuit I Concepts:

In our reference circuit (the first Circuit I) consider two (2) questions. If 12 Amps of current enters the light bulb how much current exits the light bulb? If the charges are carrying 20 V of energy when they enter the bulb how much are they carrying when they exit the bulb?