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Grades 7-8 | Analysis | Source-Based

Source Lexile®: 920L-1080L

Learning Standards



Prompt: Today you will research electricity and consider some of the methods used in science texts to support different purposes. First, you will read a passage that explains some general principles of electricity. Then you will read an article about what causes a short circuit. Finally, you will read an article that explains how different materials conduct electricity.


Then, you will write an essay analyzing how each source uses explanations, examples, and/or descriptions to help accomplish its purpose. Support your response with evidence from each source.



Source 1

Energy Story



Electricity figures everywhere in our lives. Electricity lights up our homes, cooks our food, powers our computers, television sets, and other electronic devices. Electricity from batteries keeps our cars running and makes our flashlights shine in the dark.


Here's something you can do to see the importance of electricity. Take a walk through your school, house or apartment and write down all the different appliances, devices and machines that use electricity. You'll be amazed at how many things we use each and every day that depend on electricity.


But what is electricity? Where does it come from? How does it work? Before we understand all that, we need to know a little bit about atoms and their structure.


All matter is made up of atoms, and atoms are made up of smaller particles. The three main particles making up an atom are the proton, the neutron and the electron.


Electrons spin around the center, or nucleus, of atoms in the same way the moon spins around the earth. The nucleus is made up of neutrons and protons.


Electrons contain a negative charge, protons a positive charge. Neutrons are neutral — they have neither a positive nor a negative charge.


There are many different kinds of atoms, one for each type of element. An atom is a single part that makes up an element.  There are 118 different known elements that make up everything!  Some elements like the oxygen we breathe are essential to life.


Each atom has a specific number of electrons, protons and neutrons. But no matter how many particles an atom has, the number of electrons usually needs to be the same as the number of protons. If the numbers are the same, the atom is called balanced, and it is very stable.


So, if an atom had six protons, it should also have six electrons. The element with six protons and six electrons is called carbon. Carbon is found in abundance in the sun, stars, comets, atmospheres of most planets, and the food we eat. Coal is made of carbon; so are diamonds.


Some kinds of atoms have loosely attached electrons. An atom that loses electrons has more protons than electrons and is positively charged. An atom that gains electrons has more negative particles and is negatively charged. A "charged" atom is called an "ion."


Electrons can be made to move from one atom to another. When those electrons move between the atoms, a current of electricity is created. The electrons move from one atom to another in a "flow." One electron is attached and another electron is lost.


This chain is similar to the fire fighter's bucket brigades in olden times. But instead of passing one bucket from the start of the line of people to the other end, each person would have a bucket of water to pour from one bucket to another. The result was a lot of spilled water and not enough water to douse the fire. It is a situation that's very similar to electricity passing along a wire and a circuit. The charge is passed from atom to atom when electricity is "passed."


Scientists and engineers have learned many ways to move electrons off of atoms. That means that when you add up the electrons and protons, you would wind up with one more proton instead of being balanced.


Since all atoms want to be balanced, the atom that has been "unbalanced" will look for a free electron to fill the place of the missing one. We say that this unbalanced atom has a "positive charge" (+) because it has too many protons.


Since it got kicked off, the free electron moves around waiting for an unbalanced atom to give it a home. The free electron charge is negative, and has no proton to balance it out, so we say that it has a "negative charge" (-).


So what do positive and negative charges have to do with electricity?


Scientists and engineers have found several ways to create large numbers of positive atoms and free negative electrons. Since positive atoms want negative electrons so they can be balanced, they have a strong attraction for the electrons. The electrons also want to be part of a balanced atom, so they have a strong attraction to the positive atoms. So, the positive attracts the negative to balance out.


The more positive atoms or negative electrons you have, the stronger the attraction for the other. Since we have both positive and negative charged groups attracted to each other, we call the total attraction "charge."


Energy also can be measured in joules. Joules sounds exactly like the word jewels, as in diamonds and emeralds. A thousand joules is equal to a British thermal unit.image2.jpg


When electrons move among the atoms of matter, a current of electricity is created. This is what happens in a piece of wire. The electrons are passed from atom to atom, creating an electrical current from one end to other, just like in the picture.


Electricity is conducted through some things better than others. Its resistance measures how well something conducts electricity. Some things hold their electrons very tightly. Electrons do not move through them very well. These things are called insulators. Rubber, plastic, cloth, glass and dry air are good insulators and have very high resistance.


Other materials have some loosely held electrons, which move through them very easily. These are called conductors. Most metals — like copper, aluminum or steel — are good conductors.



Source 2

Short Circuit



What happens when you blow a fuse?


Current flowing through a wire heats the wire. The length of a wire affects its resistance, which determines how much current flows in the wire and how hot the wire gets.




  • A length of copper wire with alligator clips attached to each end (or a test lead) from any electronics supply store
  • A fresh 6-volt or 12-volt lantern battery
  • A 5- to 6-inch (13- to 15-cm) length of very fine steel wire, obtained by separating one strand from ordinary braided galvanized picture-hanging wire



1. Attach one end of the clip lead to one of the battery terminals.
2. Attach one end of the fine steel wire to the other terminal.
3. Attach the other end of the clip lead to the other end of the steel wire, placing the clip as far from the terminal as possible.




To Do and Notice


Observe what happens to the steel wire after you connect the clip. Move the clip on the steel wire a little closer to the battery and watch what happens. Keep moving the lead closer until you see the final dramatic result. (Be careful! The steel wire will get very hot!)


What's Going On?


The thin steel wire is a good conductor of electricity, but not as good as the copper wire, which is deliberately chosen to have very low resistance. Thus most of the resistance of the circuit is in the steel wire.

When you connect the clip to the steel wire, the voltage of the battery pushes electrons through the circuit against the resistance of the steel wire, causing the steel wire to heat up. As you move the clip closer to the battery, the resistance of the steel wire decreases. Because the same voltage is applied across a lower resistance, more current flows, and the wire heats up more. Eventually, when you make the steel wire short enough, so much current flows that it melts the wire. Even the copper wire becomes warm.

In a normal electric circuit, an electric current powers an appliance, such as a refrigerator or TV. Every such appliance has a certain amount of resistance to the current flow, which keeps the current from reaching very large values. A short circuit occurs when the current finds a way to bypass the appliance on a path that has little or no resistance — for example, where frayed insulation bares a wire and allows it to touch the frame of the appliance, so the current can flow straight to the ground. In this situation, a very large current can occur, producing a lot of heat and a fire hazard.

Although houses today often have circuit breakers instead of fuses, fuses are still around. A fuse contains a thin strip of wire, somewhat like the thin steel wire in our experiment. The current that goes to appliances must also pass through this strip of wire. If a short circuit occurs — or even if too many appliances get hooked up to one wire so that too much current flows — the wire in the fuse heats up quickly and melts, breaking the circuit and preventing a fire from starting.



Source 3

Conducting Solutions



An electric current is a flow of electrical charge. When a metal conducts electricity, the charge is carried by electrons moving through the metal. Electrons are subatomic particles with a negative electrical charge. When a solution conducts electricity, the charge is carried by ions moving through the solution. Ions are atoms or small groups of atoms that have an electrical charge. Some ions have a negative charge and some have a positive charge.


Pure water contains very few ions, so it does not conduct electricity very well. When table salt is dissolved in water, the solution conducts very well, because the solution contains ions. The ions come from the table salt, whose chemical name is sodium chloride. Sodium chloride contains sodium ions, which have a positive charge, and chloride ions, which have a negative charge. Because sodium chloride is made up of ions, it is called an ionic substance.


Not all substances are made up of ions. Some are made of uncharged particles called molecules.  Sugar is such a substance. When sugar is dissolved in water, the solution does not conduct electricity, because there are no ions in the solution.


Some substances that are made of molecules form solutions that do conduct electricity. Ammonia is such a substance. When ammonia dissolves in water, it reacts with the water and forms a few ions. This is why laundry ammonia, which is a solution of ammonia in water, conducts electricity, but not very well.


Sometimes, when two different solutions are mixed, the substances they contain react with each other and form ions. This is what happens when ammonia and vinegar are mixed. An ammonia solution contains only a few ions, and it conducts electricity only poorly. A vinegar solution also contains only a few ions and conducts only a little electricity. But when these solutions are mixed, the ammonia reacts with the acid in vinegar (acetic acid), and they form a lot of ions. This is why the mixture of ammonia and vinegar conducts electricity very well.






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