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? • ___________________________________________________________ _____________________________________________________ .