We spent a few minutes at the beginning of class reviewing the charges of the three subatomic particles. Protons have a positive charge, electrons are negative, and neutrons are neutral. I gave each student two balloons. They inflated them and then rubbed the balloons on their hair. The students held the balloons and tried to push them together. They noticed the balloons pushed each other away.
This happened because both balloons had the same charge. In science, opposites charges attract and like charges repel. Similarly, when the students rubbed the balloon on their hair, their hair "jumped" up to stick to the balloon. The hair and balloon had opposite charges causing them to be attracted to each other.
Coulomb's Law - Coulomb's Law has to do with the attraction or repulsion of forces. Part of the Law states that like charges repel and opposite charges attract. Coulomb's Law can also be used to show that a difference in the amount of charge between two objects affects the strength of the repulsion or attraction. French scientist Charles Augustin de Coulomb showed that forces of attraction or repulsion become stronger as objects move closer together. The opposite is also true; the attraction or repulsion becomes weaker as objects are moved farther apart. So, as the charge increases, so does the electrical force created between the two objects.
As a side note, Coulomb was probably not the first to discover this law of physics. Henry Cavendish, an English scientist, came up with similar ideas but he did not publish his work and the credit was given to Coulomb. Cavendish's work was not discovered until decades after he died.
Here's an experiment (similar to the one above) that can help explain Coulomb's Law.
http://www.science4mykid.com/project_detail.asp?pid=51&offset=
You can also demonstrate this with magnets. In fact, we will revisit this in a couple of weeks when we look at magnetism.
Fun fact - The SI (International System) unit for measuring electrical charge is the coulomb.
Conductors and Insulators - I showed the class a couple of power chords and explained that the plastic coating is an insulator. I also showed them a piece of wire that I'd stripped the insulation from so they could see what's inside.
We discussed that metals are generally good conductors of electricity (the students knew copper and gold are excellent conductors). Copper is often used to make electrical wires since it's relatively inexpensive.
Conductors allow electrical charges to move freely since they can carry an electrical current. Insulators, on the other hand, do not transfer electrical currents easily. While the charges inside a power chord can move through the conducting metal wires, they cannot escape through the insulating plastic. This helps appliances run efficiently and protects people from electric shock. Other insulators include rubber, glass, and cardboard.
Conductors and Insulators lab - The students were each given a D-cell battery, a battery holder, a light bulb and bulb holder (wires were already connected), and some test materials (foil, eraser, quarter, pencil, plastic-coated paper clip, uncoated paper clip, wooden ruler, plastic ruler).
The students connected the light bulb to the battery and then tested each object to discover if it was a conductor or not. As predicted, the only conductors were the quarter and uncoated paper clip.
You can try this at home with other test materials. Use electrical tape in place of the battery holder. You can also use old Christmas tree lights for the light bulb. Just cut a section with a bulb and then strip away part of the insulation from the wire.
Transfer of Electrons - We reviewed what we'd already discussed about the three subatomic particles and their charges. I drew an atom on the board with the protons on neutrons on the inside and the electrons on the outside of the atom. I then drew diagrams to show how atoms can lose or gain electrons and that this can change charges.
Charging by Friction - Think of sliding along a cloth car seat. As you do this, some electrons are transferred between your clothes and the car seat. One material gains electrons while the other loses electrons. Charging by friction also explains why you get zapped if you rub your shoes along a carpet and then touch a metal doorknob on a dry day.
Static electricity is an example of charging by friction. Static means stationary so static electricity is a build-up of negative or positive stationary charges.
Charging by Contact - This occurs when you charge an object by touching it with a charged object. To demonstrate this, I drew a door knob on the board with a rubber rod touching the door knob. The rubber rod was negatively charged and the doorknob had a neutral charge (equal numbers of protons and electrons). When the negatively charged rubber rod touched the door knob, electrons were transferred from the rod to the door knob giving the knob a negative charge.
Charging by Induction/Induced Charges - The students had already learned that like charges repel and opposite charges attract and that electrons can move between atoms and objects to change charges. I used the same door knob diagram to demonstrate this concept. In this one, we also had a negatively-charged rubber rod. When the rubber rod became close to the door knob (without touching), it caused the protons and electrons within the door knob to move. Rather than transferring electrons from the rod, though, the charge of the door knob remained neutral. What did happen was the positively-charged protons moved to the handle of the door knob (they were attracted by the negatively-charged rubber rod) and the negatively-charged electrons in the door knob moved away from the area closer to the rubber rob (they were repelled by it).
This can be a bit tricky but some of the labs we completed below provided hands-on experiences with induced charges.
Static Electricity lab - I found a great science kit at the MPCS library back at the beginning of the school year and it included a plastic tube with tiny styrofoam balls. Perfect for demonstrating static electricity!
The students were given some test materials - a piece of felt, a piece of wool, a cotton ball, and a piece of foil. They also tested the tube with the carpet and their hands. They rubbed the tube with each of the test materials and observed what happened to the styrofoam balls inside the tube. As the students rubbed the tube with the various materials, they noticed the balls were sometimes attracted to one another and sometimes repelled. This is due to a transfer of electrons causing some atoms to gain electrons and some to lose electrons. The foil created the least amount of static electricity.
This showed charging by friction.
Streamers lab - For this lab, I cut pieces of tissue paper to give them long thin streamers. Each student was given a piece of the cut tissue paper and a comb. They combed through their hair a few times then held the comb close to (without touching) the tissue paper. The paper strips moved toward the comb.
The students charged the comb with static electricity. This was charging by friction since electrons were rubbed from their hair onto the comb. This also showed an induced charge since the negatively-charged comb attracted the positively-charged atoms in the paper.
Electroscope lab - An electroscope is an instrument used to measure the charge of static electricity. The electroscope has two thin pieces of metal (leaves) suspended from a metal hook. When a negatively-charged object is placed close to the hook, some electrons will move to the hook. They then travel down the hook to the leaves. If a positively-charged object is placed near the hook, some electrons will transfer from the hook to the positively-charged object. As the leaves are charged (in either case), they will repel each other. A stronger charge will cause the leaves to repel each other farther apart.
We built our own electroscopes using plastic cups, paper clips, and foil. I uncurled some paper clips leaving one curved end. Each student was also given two thin strips of foil (leaves) and another piece of foil. They poked a hole in the bottom of the cup with the paper clip and pushed the straight end of the paper clip through the cup so the curved part remained inside the cup. They then attached the two foil leaves to the curved end of the clip. The students put the cup upside down on the table and scrunched the remaining foil into a ball around the straight end of the paperclip.
They then charged a balloon by rubbing it on a piece of wool and then placed the ball near the foil ball. They watched how the leaves reacted to the charge.
This lab shows an induced charge.
Tinkle lab - The students were given a small piece of aluminum foil that they ripped into small pieces. They then charged a comb by combing it through their hair several times. The students placed the comb near the small foil pieces and observed. The foil pieces jumped to stick to the comb.
As we know, this is due to the negatively-charged particles in the comb attracting the positively-charged particles in the foil. The attraction here was so strong that the foil overcame gravity to jump up to the comb.
Bending Water with Static Electricity - This quick lab gave one last demonstration of an induced charge.
The students charged a comb and then held the comb close to a thin stream of running water. They watched as the water bent toward the comb. This is due to the positively-charged particles in the water being attracted to the negatively-charged particles in the comb.
Current - Current in electricity is the movement of electrons from one object to another. We saw a lot of that in the labs we completed! Current is measured in units called amperes.
There are two main types of current - Direct Current (DC) which always flows in one direction and Alternating Current (AC) which is a flow back and forth. AC can also change directions. Batteries are an example of direct current. The electricity in our homes and outlets is alternating current.
Batteries - All batteries have a positive and a negative terminal. If these two terminals are connected, the electrons in the material connecting the terminals will flow to create a current. Electrons move from areas with higher negative charge to those with a lower negative charge. So, electrons like to move from the negative terminal of a battery to the positive terminal. These electrons are the charge. Charge moves from negative to positive. Current (DC), however, moves from positive to negative.
Hot lab - We created a circuit had a little preview of next week's class during this lab.
Each student was given an AA battery and a piece of aluminum foil. They folded the foil to make a thin strip and then held one end of the foil strip against each battery terminal or pole.
**If you try this at home, do not hold the foil against the battery for more than 10 or 15 seconds!**
The foil "wire" created a path or circuit for the electrons to move through. As we learned earlier, the electrons in a DC current (like a battery) flow from the negative terminal to the positive terminal. This movement (current) of electrons caused the foil wire to become hot.
This is similar to what happens in an incandescent light bulb. Electrons flow into the bulb and this flow or movement causes the filament to heat up. The wire filament becomes so hot that it gives of light. Scientists say the filament becomes incandescent, hence, incandescent bulbs are the ones with the thin wire filament inside.
Fruit Battery - We attempted to create a fruit battery using a lemon but we didn't have much luck with it. The directions I used called for a copper nail or copper wire. I didn't have a nail but did find some copper wire so we tried that. Unfortunately the wire didn't quite cut it but one of the students suggested we try this with a penny. Good idea so I'll try to squeeze this in again next week with the penny! We're actually going to make a potato circuit next week that uses a penny so this will relate quite nicely if we have time to try again.
Here's a link to the directions so you can try this at home if you like:
Fruit Battery
Try this with other fruits. If you're able to find some, use some pH paper to test the acidity levels of the fruits and see if a fruit with a lower pH (higher acidity) produces a brighter light than one with a higher pH/lower acidity.
Next Week:
We'll continue our study of electricity by looking at circuits. We'll create simple, parallel, and series circuits and will also learn how switches work. We're also going to attempt to use a solar cooker to make S'mores. Hopefully the sun will be shining and there won't be the usual San Diego "May Gray" but I have a back-up plan so the students can enjoy S'mores even if our solar cooker doesn't quite work as planned. :)
References:
The labs "Hot," "Streamers," and "Tinkle" are all from Physics for Every Kid.
VanCleave, J. (1991). Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound. San Francisco: Jossey-Bass.
"Fruit Battery" was found here:
http://chemistry.about.com/od/chemistryhowtoguide/a/fruitbattery.htm
"Static Electricity," "Conductors and Insulators," and "Electroscope" were all from a Lakeshore Learning Materials kit on Electricity.
