理系の中３なら以下の英文を辞書を使わずにスラスラ読めるはずだ。

Atom

Parts of the Atom

Figure below represents a simple model of an atom. Models help scientists make sense of things. 

Perhaps they are either too big or too small. 

Maybe they are just too complicated to make sense of. This simple model helps scientists think about the atom.

Is this how the atom really looks? 

Not exactly! 

Remember, a model helps us make sense of things. 

They may not be an exact copy of the object. 

You will learn about more complex models of atoms in the coming years, but this model is a good place to start. 

This simple atomic model shows the particles inside the atom.

The Nucleus

At the center of an atom is the nucleus (plural, nuclei). 

The nucleus contains most of the atom’s mass. 

However, in size, it’s just a tiny part of the atom. 

The model in Figure above is not to scale. 

If an atom were the size of a football stadium, the nucleus would be only about the size of a pea.

The nucleus, in turn, consists of two types of particles, called protons and neutrons. These particles are tightly packed inside the nucleus. Constantly moving about the nucleus are other particles called electrons. 

Protons

A proton is a particle inside the nucleus of an atom. It has a positive electric charge. All protons are identical. It is all about the number of protons in the atoms. 

The number of protons is what gives the atoms of different elements their unique properties. 

Atoms of each type of element have a characteristic number of protons. For example, each atom of carbon has six protons . No two elements have atoms with the same number of protons.

Neutrons

A neutron is a particle inside the nucleus of an atom. It has no electric charge. Atoms of an element often have the same number of neutrons as protons. For example, most carbon atoms have six neutrons as well as six protons. 

This is also shown in Figure below.

This model shows the particles that make up a carbon atom.

Electrons

An electron is a particle outside the nucleus of an atom. It has a negative electric charge. The charge of an electron is opposite but equal to the charge of a proton. Atoms have the same number of electrons as protons. As a result, the negative and positive charges "cancel out." This makes atoms electrically neutral. For example, a carbon atom has six electrons that "cancel out" its six protons. 

Electricity

Introduction

Electricity is the flow of tiny particles called electrons and protons. 

It can also mean the energy you get when electrons flow from place to place. Electricity can be seen in nature in a bolt of lightning. 

Lightning is nothing but a large number of electrons flowing through air all at once, releasing a huge amount of energy. 

Scientists have also learned how to generate, or create, electricity. 

This is useful because electricity that is generated can be controlled and sent through wires. 

It can then power such things as heaters, light bulbs, and computers. 

Today, electricity provides most of the energy to run the modern world.

How electricity works

Everything in the universe is made of tiny objects called atoms. 

Each atom has even tinier particles called protons and electrons. 

These tiny particles swirl around each other continuously. 

An electron has what is called a negative charge. 

A proton has a positive charge. 

Positive and negative charges try to pull each other together. 

However, two positive charges, or two negative charges, will push each other away. 

Electricity results when electrons are pushed and pulled from atom to atom.

Static electricity

Most objects have a balance of positive and negative charges, so they are considered neutral. 

This means that they do not push or pull on each other electrically. 

However, sometimes electrons can build up in an object. 

Two such objects can push or pull on each other because they are no longer neutral. 

This push or pull from extra electrons is called static electricity. 

Static electricity can cause interesting effects, such as sparks or lightning bolts, when it is released. 

Sometimes the extra electrons build up by rubbing one object against another. 

For example, when one rubs a balloon against one’s hair, electrons move from the balloon to the hair. 

Because the hairs then all have extra electrons, which all have the same kind of charge, they try to fly away from each other and end up sticking into the air like spikes!

Generating electricity

Many moving electrons are called an electric current. 

A city’s power plant produces a powerful electric current and sends it through wires. The electricity used for lighting, heating, and running appliances is made by machines called generators. 

Generators cause a current to flow by moving a magnet past a coil of wire, which pushes electrons through the wires of the coil. 

Wires carrying the current travel to houses and other buildings. 

More wires connect to the power outlets in rooms. 

When a person plugs in an iron or another electric device, the current travels into the device. 

The current then makes the device work. 

A chemical reaction in a battery can also produce an electric current.

History

The ancient Greeks were the first to study electric forces. 

In the American colonies during the 1700s, Benjamin Franklin proved that lightning is a form of electricity. 

Scientists later learned that electricity is related to magnetism. 

They then learned how to generate electricity using magnets.

electric current

Direct and Alternating Current

To use an electric appliance, you have to plug it into an outlet unless it has batteries. This may be all you need to know in order to use electric current. But did you ever wonder what electric current is or how it flows through wires inside the walls of your home? Electric current is a continuous flow of electric charges. The charges may flow in just one direction, or they may keep reversing direction. 

Direct Current

When current flows in just one direction, it is called direct current (DC). The diagram below shows how direct current flows through a simple circuit. An example of direct current is the current that flows through a battery-powered flashlight. In addition to batteries, solar cells and fuel cells can also provide direct current.

Alternating Current

When current keeps reversing direction, it is called alternating current (AC). You can see how it works in the two diagrams below. The current that comes from a power plant and supplies electricity to homes and businesses is alternating current. The current changes direction 60 times per second. It happens so quickly that the light bulb doesn’t have a chance to stop glowing when the reversals occur.

Electric Charge and Electric Force

A lightning bolt is like the spark that gives you a shock when you touch a metal doorknob. Of course, the lightning bolt is on a much larger scale. But both the lightning bolt and spark are a sudden transfer of electric charge.

Introducing Electric Charge

Electric charge is a physical property of particles or objects that causes them to attract or repel each other without touching. All electric charge is based on the protons and electrons in atoms. A proton has a positive electric charge, and an electron has a negative electric charge. In the Figure below, you can see that positively charged protons (+) are located in the nucleus of the atom, while negatively charged electrons (-) move around the nucleus.

Electric Force

When it comes to electric charges, opposites attract, so positive and negative particles attract each other. You can see this in the Figure below. This attraction explains why negative electrons keep moving around the positive nucleus of the atom. Like charges, on the other hand, repel each other, so two positive or two negative charges push apart. This is also shown in the diagram. The attraction or repulsion between charged particles is called electric force. The strength of electric force depends on the amount of electric charge on the particles and the distance between them. Larger charges or shorter distances result in greater force. 

Electric Current and Matter

Electrical energy is transmitted by moving electrons in an electric current. In order to travel, electric current needs matter. It cannot pass through empty space. However, matter resists the flow of electric current. That’s because flowing electrons in current collide with particles of matter, which absorb their energy. Some types of matter offer more or less resistance to electric current than others.

Electric Conductors

Materials that have low resistance to electric current are called electric conductors. Many metals—including copper, aluminum, and steel—are good conductors of electricity. The outer electrons of metal atoms are loosely bound and free to move, allowing electric current to flow. Water that has even a tiny amount of impurities is an electric conductor as well.

Q: What do you think lightning rods are made of?

A: Lightning rods are made of metal, usually copper or aluminum, both of which are excellent conductors of electricity.

Electric Insulators

Materials that have high resistance to electric current are called electric insulators. Examples include most nonmetallic solids, such as wood, rubber, and plastic. Their atoms hold onto their electrons tightly, so electric current cannot flow freely through them. Dry air is also an electric insulator. 

Q: You may have heard that rubber-soled shoes will protect you if you are struck by lightning. Do you think this is true? Why or why not?

A: It isn’t true. Rubber is an electric insulator, but a half-inch layer on the bottom of a pair of shoes is insignificant when it comes to lightning. The average lightning bolt has 100 million volts and can burn through any insulator, even the insulators on high-voltage power lines.