Chapter 7 Electricity

Lesson 2 What Are Static and Current Electricity?

Static Electricity

• Most objects have no charge= the atoms are neutral.

• They have equal numbers of protons and electrons.

• When objects rub against another, electrons move from the atoms of one to atoms of the

other object.

• The numbers of protons and electrons in the atoms are no longer equal: they are either

positively or negatively charged.

• The buildup of charges on an object is called static electricity.

• Opposite charges attract each other.

• Charged objects can also attract neutral objects.

• When items of clothing rub together in a dryer, they can pick up a static charge.

• Because some items are positive and some are negative, they stick together.

• When objects with opposite charges get close, electrons sometimes jump from the negative

object to the positive object.

• This evens out the charges, and the objects become neutral.

• The shocks you can feel are called static discharge. • The crackling noises you hear are the sounds of the sparks.

• Lightning is also a static discharge.

• Where does the charge come from?

• Scientists HYPOTHESIZE that collisions between water droplets in a cloud cause the drops

to become charged.

• Negative charges collect at the bottom of the cloud.

• Positive charges collect at the top of the cloud.

• When electrons jump from one cloud to another, or from a cloud to the ground, you see

lightning.

• The lightning heats the air, causing it to expand.

• As cooler air rushes in to fill the empty space, you hear thunder.

• Earth can absorb lightning’s powerful stream of electrons without being damaged.

• But lightning that strikes a tree or a house can start a fire.

• If lightning strikes a beach, it can melt grains of sand into pieces of glass!

Static Electricity CAUSE AND EFFECT

What causes an object to build up a static charge?

• ______

______

Current Electricity

• Static electricity is a kind of potential energy.

• Energy is stored when electrons move from one object to another and a static charge builds

up.

• The potential energy can change to kinetic energy.

• This is what happens when electrons move in a static discharge.

• The kinetic energy of a static discharge can change to other forms of energy: heat, light,

and sound.

• Because a static discharge is a short burst of kinetic energy, it isn’t very useful as an

energy source.

• For electricity to be a useful source of energy, it must be a steady flow of charges.

• If electrons have a path to follow, they will move in a steady flow instead of building up a

static charge.

• This flow of charges is called an electric current.

• Electric Current is the steady flow of negative charges.

• Electricity that flows in this way is a kind of kinetic energy called current electricity.

• To keep the charges flowing, a constant supply of electrons is needed.

• Cells supply the flow of charges to flashlights and other small devices. • Energy stations produce a much larger flow of charges to supply electric current to whole

cities.

• Electrical pressure is measured in volts.

• The rate at which electric current flows is measured in amps.

• There are many more amps in power lines than you need to operate the appliances in your

home.

• The combination of volts and amps can be dangerous, which is why many objects that use

electricity have warning labels.

• The amount of electrical energy a device uses each second is measured in watts.

• Electric energy companies bill people for the amount of electricity they use.

• A watt is a very small unit, so electrical use is measured in kilowatts.

• One kilowatt is equal to 1,000 watts.

• Current Electricity

CAUSE AND EFFECT

What causes an electric current to keep moving?

• ______

______

______

Conductors and Insulators

• Electricity moves more easily through some kinds of matter than others.

• A material through which electricity moves well is a conductor.

• Most metals are conductors.

• The electrons of metals are held loosely by the atoms.

• This makes it easy for the electrons to move between atoms, causing current to flow.

• Copper is a very good conductor.

• It’s used for most electrical wiring in homes.

• The inside of the cord you use to plug in a lamp is made of copper wire.

• If you look at a lamp cord, you won’t see the copper wire.

• The copper is covered with a layer of plastic.

• Plastic doesn’t conduct electricity well.

• Its electrons are not free to move between atoms. A material that conducts electricity poorly

is an insulator.

• Wood, glass, and rubber are also insulators.

• Insulators are important because they protect you from the electric current in the wire.

• If the layer of plastic on a wire peels off or cracks, the wire should be replaced.

• If you touch a bare wire that is conducting current, the current will flow through you and

could hurt you. • Also, wires get warm when they carry electricity.

• A bare wire that touches paper or cloth could start a fire.

• Conductors and Insulators

CAUSE AND EFFECT

What causes a metal to be a good conductor?

• ______

______