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
• 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?
• ______
______