Misconceptions: Electrical Circuits 1

Students often have difficulty with electrical circuit problems. In my experience, most of these difficulties are due to misconceptions.

Here is one of the most commonly held misconception;

Electricity exits the battery at the positive side, travels around the circuit and ends up at the negative side.

There are a few other related misconceptions that come out of this statement;

  • The moving charge, called electricity, travels at the speed of light
  • The wires are like empty pipes that the electricity travels through
  • Multiple bulbs in series are of ever decreasing brightness, with the bulb nearest the positive terminal of the battery the brightest, followed by the bulb next to it and so on.
  • Resistors need to be on the positive side in order to do their job of limiting the electricity (or current) after it.

I’m sure there are more, but these are all linked to a single misconception; that the battery supplies the current, or charge, or electricity, to the circuit.

So, what is actually happening?

All of the components in the circuit are made up of atoms, which are themselves made up of charged particles. It is these charge particles that will become the electrical current when a voltage is applied to the circuit.

In other words, the circuit already has charges in it, but for these charges to move a battery needs to push them.

 

Analogy Time

***Think Bike Chain on a bicycle***

The links in the chain represent the charges in the circuit. They are always present and, in a way, ‘connected’ by the force of repulsion between them.

If you peddle on the bicycle (apply a voltage or connect a battery in the circuit) then ALL of the charges in the circuit move.

Not only that, but they all move at the same speed!

 

Still not sure?

Check out the following Java Applet from PHET;

Screen Shot 2014-10-22 at 21.11.55

http://phet.colorado.edu/en/simulation/circuit-construction-kit-ac

Set up a circuit and see how the charges are already in the wire. Connecting up a battery makes the charges move and this moving charge is an electrical current.

Connect three bulbs in series and exam their brightness.

Add a resistor to both sides of these three bulbs and exam what happens.

POST your findings in the comments below.

 

Resolving the Misconceptions

Now it’s time to answer the following questions to check your understanding. Post your answers and reasoning in the comments below.

How fast does electrical current flow?

  1. At the speed of light
  2. It doesn’t flow, it just stays in the same place
  3. It depends on the battery
  4. Very slowly

 

What are electrical wires?

  1. Mainly empty space, with metal atoms throughout
  2. Empty space so they can allow the electrons to flow easily through them
  3. Solid, but the electrons in the wires hit off each other to force the current around

 

If there are 3 bulbs in series, how bright are they relative to each other?

  1. The two bulbs on the ends are brightest. The one in the middle is less bright
  2. The two bulbs on the ends are less bright. The one in the middle is the brightest
  3. Equal brightness
  4. The bulb nearest the positive terminal is brightest, followed by the bulb in the middle. The bulb closest the negative terminal is the least brightest
  5. The bulb nearest the negative terminal is brightest, followed by the bulb in the middle. The bulb closest the positive terminal is the least brightest

 

Feeling Brave?

Try the tests on the following pdf document (starting on page 10)

http://www.ncsu.edu/per/Articles/Engelhardt&Beichner.pdf

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