As part of an interview process, I was recently asked to provide a demo lesson in physics. The class had just started its unit on electronics, so I decided to teach with an online interactive simple circuit to give a conceptual basis for current, voltage, and resistance. In my experience, these topics are often presented in abstract form right away with students drawing circuit diagrams and cranking out solutions to equations without getting an intuitive sense what these concepts are. This is exacerbated by the fact that while we can observe the end result of an electrical system, we cannot see the inner workings of one.
There are two analogies that can be used for an electrical circuit. One is a water system, the other is a roller coaster. I’ll go over both here. For the demo lesson, I used the roller coaster. The school was in New York City and my thinking was the students would have, for the most part, experience riding a roller coaster. There is Paterson Falls in New Jersey, but most people I talked to in the region were not aware of those falls. I became aware of it while watching the movie Paterson. Had the lesson been in Buffalo where I live, I would have used the water system example as Niagara Falls is such a prominent feature in local geography.
Current defines the flow of electricity in a circuit in the direction of positive charge. It’s actually the flow of loose negatively charged electrons that create a current, but this convention was defined before the nature of the atom was unveiled in the 20th Century. Electrical charge is conserved, that is, it cannot be created of destroyed. One unit of charge is a Coulomb (C), and a flow of 1 C/s is referred to as an amp. During my high school years, students would brag about how many amps their stereos had, which delighted our parents no end.
If a stream has a flow of 10 gallons per second, we could call that its current. If you are watching a roller coaster and observe 10 cars pass a point in one second, then 10 cars per second is its current. The same holds true for a circuit, a flow of 10 units of charge in a wire is 10 C/s or 10 amps. A circuit has to complete a loop for current to flow. A switch in the on position completes a loop and allows a current to flow through the system. The off position breaks the loop. However, it takes more than a switch to create a current, and that’s where voltage comes in.
If an object is on the ground, it has zero potential energy. If we lift the object above the ground it gains potential energy. That potential energy is converted to kinetic energy if we release the object. Go back to the roller coaster analogy. How much potential energy do the cars have while level on the ground? Zero. The coaster adds potential energy by lifting the cars up on a hill. Coney Island’s Cyclone is 85 feet tall whereas modern coasters can be 200-300 feet tall. The potential energy is converted to kinetic energy as you reach the top and begin to drop. Batteries do the same by adding potential to a circuit. This potential is measured in volts.
In the water analogy, think of a canal that is level. Current does not flow and in fact, this causes canals to be stagnant and a health hazard. The canals of Amsterdam are flushed each morning for this reason. It is also why the Buffalo segment of the Erie Canal was filled in during the 1920’s. It is this segment that I-190 was built upon. What happens when you add a height difference? Think of Niagara Falls. It adds a current and potential energy which is used to produce hydroelectric power. Water in the amount of 748,000 gallons per second drops 180 feet into 25 turbines producing 2.6 megawatts of energy.
The lines from a power plant can have voltage in the hundreds of thousands. Transformers drop that to 120 volts before entering a household. Voltage can also be thought of as pressure. Think of a pressure washer. Higher pressure can deliver water farther. Higher voltage can send a spark longer. So while voltage and current are proportional to each other, they are not the same thing. You need voltage to start a current.
The final piece of the puzzle is resistance. This is akin to friction on the roller coaster. Without friction, a roller coaster would never stop but would travel in a continuous loop. Friction between the cars and rails converts kinetic energy into heat and is dissipated into the surrounding air. Hence, an engine has to push the coaster up the hill again to start another trip around the loop. Resistance in a circuit does the same. Energy in the circuit is converted by resistance in the wire and dissipated as heat. This causes voltage to drop as current travels in the loop. The battery serves the same purpose as the hill in the coaster. It adds voltage or potential to restart the current around the loop.
Superconductivity represents a state of zero resistance. This requires a very cold temperature. During the 1980’s, a ceramic material was discovered that raised the known temperature of superconductivity from 30 K to 92 K. The media at the time presented this as hope of building practical superconductive systems that would bring about high efficiencies to electric generation. Since then, progress has been slow on this front, at least in terms of some expectations after that discovery. You can think of a superconductive circuit as a roller coaster that would not require energy to start each successive loop after the initial potential was added.
The PhET interactive above allows the class to build their own circuits and analyze the relationship between current, voltage, and resistance. For the sake of the demo lesson, I used the Physics Classroom interactive as it is a bit more easier to get it up and running given the limitations involved of a demo lesson. Over the long haul, the PhET interactive is more robust. Both will allow a student to adjust voltage and current to see how it affects the circuit.
The key points for the class to learn are:
A circuit must be a closed loop from one terminal of the battery to the other for a current to flow. A switch in the off position breaks the loop while the on position closes the loop. A car ignition key serves the same function.
A potential or voltage must be applied to the circuit to get the current flowing. Otherwise, it would be like trying to ride a flat roller coaster.
Voltage or potential will drop as the current travels through the loop. This is analogous to a roller coaster lowering in elevation (and potential energy) as it completes the ride, eventually to be grounded.
In increase in voltage will increase current and an increase in resistance will decrease current. This is the basis for Ohm’s Law or I = V/R.
Of all the concepts here, voltage or potential tends to be the most difficult. The roller coaster example is just one of several that can be used. I think it best for a teacher to be flexible and use whatever example is most effective for each student. Another example could be that as the battery being like a water pump. The pump applies pressure in the circuit and thus, starts current. A slingshot could be used as well. As a battery forces a positive current towards the positive terminal, the two like charges want to repel each other. Once the positive charge is released into the wire, it is as if the positive terminal slingshots that charge inducing a current.
The key to the lesson is to enable students to visualize and obtain an intuitive grasp of the concepts of current, voltage and resistance. Once accomplished, the class can move on to real circuits and will have a better understanding what a voltmeter or ammeter is telling them as well as what the variables to Ohm’s law signify.