23. Electricity#
23.1. Static electricity and textile, ballpoint pen and paper#
Let students tear tiny pieces of paper from their notebooks, rub their ballpoints on their sweaters, and approach the pieces of paper with the ballpoint pen. Both electrostatic attraction and repulsion (after touching the ballpoint, the pieces of paper will be repulsed).
23.2. Static electricity and balloons#
I always have a balloon in my bag. There are many uses in teaching physics. Just rub it on your clothing, stick it to the wall, or see how pieces of paper react, or the long hair of your students or the hair on their skin.
23.3. Separating salt from pepper with a balloon#
In school cafeterias or in the faculty room one can find salt and pepper for the soup. Mix some salt and pepper. Then rub a balloon with a piece of cloth, your sweater or other materials students wear, might work, and hold it above the mixture. The pepper will be attracted, the salt will not be. Instead of the balloon, one could also use a plastic ruler. The pepper will be attracted by induced separation of charges. Now why didn’t the salt get attracted? Are the grains too heavy or is salt is too conductive?
23.4. Plastic Straws of PVC pipe#
I always have some plastic straws in my bag, but usually they can be obtained from the faculty room of the school. Rub with a paper hand kerchief, then there is a possibility the straw will stick to the wall, or to a metal object such as the leg of a chair. If it does not work so well (humidity in a room full of students), then usually the straw will still attract the hair on the skin. But that can be done also with a ballpoint after rubbing with your sweater. A piece of PVC pipe gives better results.
23.5. Coulomb’s Law#
Ask how much charge there could be on a ballpoint or a straw. Because of the large constant in Coulomb’s Law (\(9\text{x}10^9\)), charges must be quite small, certainly smaller than \(10^{-8} \text{C}\). If \(q_1~ = q_2~ = 10^{-8}\) C, then \(F_{Coulomb}~= 9 \cdot 10^9 \cdot 10^{-8} \cdot 10^{-8}/10^{-4}\) N, assume a distance of 1 cm, and the result is 1/100 Newton.
23.6. Water molecules are dipoles#
Use a fork to position an aluminum coin or paperclip very carefully on top of the water surface in a glass of water. Then rub a plastic pen or straw or better PVC pipe on your jumper or other textile and let the tip of the pen or straw or pipe approach the end of the paperclip or the coin (vid_10). The paperclip will move away from the tip of the straw. How come? Start a discussion. Let students propose explanations. If it was electrostatic induction between coin and straw, then there should be attraction rather than repulsion. In the end, somebody might suggest that the water has a role. Water molecules are dipoles. The straw or pipe attracts the water creating a little slope and the coin or paperclip slides down that slope. Actually the slope could be shown by reflecting an almost horizontal beam of a laser pointer from the surface of the water. When the charged straw or pipe comes close to the surface, the reflected beam moves. Could an electrostatic generator be used as an outboard engine for a boat?
23.7. Flow of electricity compared to water#
Switch on the light, the effect is immediate. Switch on the faucet….it takes some time. From this you may want to get into field driven (electricity) versus particle driven (water) motion. The water is like the traffic when a red light turns green. Electricity is not. Fluorescent lamps may spoil your demo as they can be slow to light but they are on the way out anyway.
23.8. Conductors and insulators#
Bare classroom? Just bring a battery, a small bulb in a holder, and some wires. They do fit in your pocket. Then in interaction with the class try out materials to see whether they conduct electricity or not. Kids may be surprised with the core of a pencil.
Take away the bulb holder and just use loose wires, the battery, and a bulb. Who can make the bulb light? Challenge the students and let them first individually draw the connections first. Walk around and see the results. Some students may draw a circuit with one wire only, others may have trouble where to connect the wires with the bulb. They are in good company, graduating students of the Massachusetts Institute of Technology (MIT) had trouble as documented in the Annenberg Private Universe video’s. Then get the one student who wanted to show his skill right away, but you did not let him/her. Let the student show and explain and draw the connections on the board.
23.9. Circuits with PhET#
Nowadays even bare classrooms, often have a beamer and internet connection, so with PhET simulations you can show any circuit. If there is no beamer, students will have telephones and can log into PhET. So you can even do a simulation lab activity.
23.10. Role-play to distinguish between electric current, power, and voltage#
Students (= electrons) with bags of energy (energy per unit charge = voltage) move through light bulbs and deposit their energy (conversion from electric energy to light/heat) and return to the battery to get another load of energy. The electrons are conserved in the process as they are only carriers, the “trucks” that transport energy through circuits. It is the energy that gets transformed. The power increases when the current increases (more trucks that deliver energy) and when voltage increases (more load per truck and more current). One can play series and parallel circuits, etc. Complete roleplay instructions are available from the author, but also consider [] very valid criticism of carrier analogies for electricity and Muller’s Veritasium video. As any model, visualization, or analogy to facilitate learning, the roleplay is a scaffold for learning but in the last step of the process the scaffold should be taken away and the teacher will point out both the benefits and the flaws of the model.