Electricity Notes Electricity & Electric Current -We use electricity every day. (lights, ipods, etc.) -Where does this come from? -Electricity comes from positive and negative charges. -All substances are made of + and – particles (protons and electrons). -In neutral atoms, the protons = the electrons, so the charge is balanced. -But what happens when this balance is upset? -Electricity is the transfer of charged particles from one substance to another. -This flow – usually of electrons results in the electric current we think of. -Some substances have more “loosely” held electrons and thus can transfer charge easier. -Ex. When you walk around with socks on and carpet, many electrons are transferred to your socks/you from the carpet. When you touch someone, you transfer all these electrons at once and “shock” them. -This is called static electricity. -What causes the shock? -Electric charges are able to exert forces. -Much like magnets attract or repel, electric forces attract or repel. -Also like magnets, like charges (+ and +) repel while opposite charges (+ and -) attract. -If something positively charged comes close to a negatively charged object, they will exert forces on each other – the shock you see/feel. -Ex. Clothes in a dryer. -So where do the electric charges exert their force? -An electric field is an area around an electric charge where the force acts. -These fields are represented by arrows showing direction of action. -Compare gravity to electricity. -Both are forces but electricity is much greater. -You feel a small shock but you don’t feel gravity. -Atoms are held together by electric forces (called electrostatic forces) -However, gravity is able to act over longer distances. -Some things are better able to transfer electrons than others. -Conductors are items that are able to easily transfer electric charge. -Ex. Metal, wires, etc. -Insulators are things that are not able to transfer electric charge easily. -Ex. Plastic, rubber, etc. -Examples of electricity: -Lightning – Clouds build up static electricity and then transfer to the ground. -Thunder – Transferred electricity causes change in heat of air which causes sound waves. -Ground wires/rods – If charge is transferred to the earth, it accepts the energy and force easily. Thus we ground electricity so it gets transferred to the earth instead of us or appliances that can overheat, etc. Electricity Notes -Devices called electroscopes allow us to detect electricity. Electric Current -How come some shocks sting very little while others are potentially deadly? -The amount and rate of electricity transferred is called the electric current. -Some things have more current than others. -Ex. Lightning transfers large amounts of energy. -Static electricity in laundry transfers relatively little. -Relate to a river…the amount of water flowing equals the current. Stronger current results from more water. -We measure electric current in a unit called amperes. -The amount of electricity transferred also depends on the difference in charges. -For example, there must be a difference between positives and negatives for charge to be transferred. -We call this difference voltage difference or electric potential. -We measure the difference in volts. -Ex. A 6 volt battery has the ability to transfer 6 volts of electricity to something. -The difference in voltage is 6 volts. -When electric current travels, it follows a circuit. This means it follows a specific path. -An electric circuit is a pathway for current to travel. It must have a start and end point and the current must travel back to the source. -Ex. When you flip a switch, you put a break in the circuit which stops flow of electricity. -Batteries are examples of substances that use a direct current. -The electric current flows in one path, based on the potential difference. -Electric current loses energy as it flows through a circuit. -Some substances also act to slow the flow of the current down. -These substances are called resistors. -Resistance is the tendency for a material to slow the flow of electrons. -Resistance is measured in Ohms (Ω). -We use resistors to control the amount of current flowing to different household devices. -The flow of electric current is affected by the pathway it must take, such as material, temperature, length, and thickness. -Like a wide river is able to transport more water, a wide wire is able to transfer more electricity. -Thus, the current, the voltage and the resistance are all related in an electric circuit. -We combine these different aspects and come up with the formula: I = V/R or Current (Amps) = Voltage/Potential Difference (V) / Resistance (Ω). -This relationship is called Ohm’s Law and it states the current in a circuit equals the voltage difference divided by the resistance. -We configure all electrical devices according to this principle. Electricity Notes Electrical Energy -As current flows, it transfers energy and force. -It also loses some energy to the surroundings. -Dependent on the pathway it takes, electric current changes. -We have two major types of pathways or circuits: Parallel and series. -In series circuits, the current only has one pathway to follow. So if multiple devices occur along the circuit, energy is lost to each one so the amount of energy flowing through the circuit gradually decreases. -If something inhibits the flow in a series circuit, the flow of electrons is stopped. Ex. Christmas lights (one goes out, they all go out) -Parallel Circuits have multiple pathways to follow. -In this circuit, electricity flows in multiple paths, thus supplying energy in equal amounts to each device. -Because there are multiple paths, if the flow in a parallel circuit is stopped, the energy can still flow in one of the other pathways. -Ex. Most electrical circuits -Household items use various devices to control the flow of energy. -Fuses are pieces of metal that “blow” if the current gets too high, thus cutting off the flow of energy. -Circuit Breakers help to prevent the flow of too much energy and causing potential fires. -In our homes we convert electricity to power (measured in Watts). -The rate at which this is done is calculated by multiplying the current (amps) by the voltage (volts). -thus, P = IV (Power = Current x voltage) -We measure the amount of power we use with a unit called the kilowatt-hour. This is how many kilowatts (1000 watts) we use in a one hour period. The electric company then charges based on the watts used.