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Electric Charges and Current Spring 2008 CHARGE!!!! • Protons (+) will repel other protons • Electrons (-) will repel other electrons • Protons and Electrons are attracted to each other. • SO…charges that are the same REPEL…charges that are opposite ATTRACT! • One BIG difference though…electric charges can exist on their own… Electric Fields… • An electric charge exerts a force through the electric field that surrounds the charge. • The electric field extends outward from every charged particle. • When a charged particle is placed in the electric field – it is either pushed or pulled…depends on the charge. More on Electric Fields… • Electric fields are strongest at the source of the charge. • The field decreases as you move away from the charge. • When you add electrical charges, the electric field is increased. STATIC…OUCH! • In most objects – the charge is canceled out – depending on the atomic structure of the object…this is why you are not shocked by EVERYTHING! • The build up of charges is called static electricity. • Static electricity behaves differently than electric current. • In an electric current – charges move CONTINUOUSLY…in static electricity the charges build up , but they do not flow. Transferring Charges… • 3 ways charges can build up: friction, conduction, and induction. • Friction – rubbing an object to build up static (balloon on hair) • Conduction – direct contact • Induction – caused by an electric field of another object. • Charges are not created or destroyed – they are only transferred…LAW OF CONSERVATION OF CHARGE… STATIC CLING… • Static electricity is responsible for static cling. The clothing in a dryer rub against each other and create the electricity. • Fabric softener creates a barrier around the clothing preventing static electricity. • Static electricity is also responsible for “cling wrap” and what draws the toner into a piece of paper in a photocopier. Static Discharge… • When a negatively charged object and a positively charged object are brought together, electrons move until both objects have the same charge. • The loss of static electricity is called static discharge. • This is what you feel when someone “shocks” you. Factors of Static Discharge… • Humidity can prevent static charges from building up. • Lightning is a dramatic example of static discharge. • During a storm particles move to areas of negative to positive – the resulting factor is a bolt of lightning. • Most lightning charges are caused by induction. Detecting Charges… • Electroscopes can detect electric charge since they are invisible. • The electroscope reacts only if a charge is present. Electrical Potential… • Electrons in a circuit have potential energy. • The potential energy per unit of electric charge is called electrical potential. VOLTAGE!!! • Electrical charges, just like energy, move from areas of high concentration to areas of low concentration. • The difference in electrical potential between 2 places is called potential difference. • This is what provides the force to push the charge through the circuit. • We measure potential difference in volts, so potential difference is commonly called VOLTAGE! More on VOLTAGE! • Voltage causes current to flow through an electric current. • An electric current requires a device to maintain a potential difference or voltage. • A voltage source creates a potential difference in an electric circuit – batteries and generators are examples of voltage sources. • A voltage source has 2 terminals (positive and negative). A Little More… • Some voltage sources are stronger than others… • An increase in voltage causes a greater flow of electric current. Resistance… • Current depends on more than just voltage…it depends on resistance… • Electrical resistance is the opposition to the flow of charge. • The greater the resistance, the less current there is for a given voltage. • Resistance depends on the thickness and length of the wire…long wires have more resistance…short wires less… More on Resistance… • Thin wires have more resistance than thick wires… • Resistance is also dependent upon the type of conductor used. • Temperature is another factor of resistance…electrical resistance decreases as temperature decreases – the opposite is true as well… Path of Least Resistance… • When given the option – a current will flow through the path with the least amount of resistance. Ohm’s LAW… • Due to extensive research by Georg Ohm in the 1820’s – the law governing resistance bears his name…Ohm’s Law… • The unit of measure for resistance is called the ohm and it looks like this…Ω. • In order to measure the resistance of a wire – you need to measure 2 points of voltage… Ohm…continued… • Voltage is measured with a voltmeter – measured in amps. • Current is measured in amps with a device called a ammeter. • These 2 devices are commonly combined into a “multimeter” that can determine both. One More Time… • Ohm found that resistance does not solely depend on voltage. • Conductors will obey Ohm’s Law. • Ohm’s Law states that the resistance is equal to the voltage divided by the current or Ohms = volts / amps. • It is commonly written: R = V / I…where R = Resistance; V = voltage; I = current. • The formula can also be rewritten to show: I = V / R or V = IR…if any 2 values are known, you can solve for the third… Key Notes about Ohm’s Law… • If voltage is doubled – then current is doubled. • So, the greater the voltage – the greater the current. • If resistance is doubled, then the current will be cut in half. • So, for greater resistance, the current is less. Light Bulbs… • Some resistors do not obey Ohm’s Law… • For example, the resistance of a light bulb increases when the bulb is turned on and the filament heats up. • A filament has the lowest resistance before it heats up, so a cold filament conducts the most current. • This is why a bulb typically burns out the instant you switch it on! Series Circuits… • If all the parts of an electric circuit are connected one after the other, the circuit is a series circuit. • In a series circuit, there is only one path for the current to take. • For example, a switch and the device it controls are connected in series with each other. Series Circuits…again… • So what happens??? • If one light in a series circuit goes out…they all go out! • Also, in a series circuit, as more lights are added the lights become dimmer. The resistance continues to increase. Parallel Circuits… • In a parallel circuit, the different parts of the circuit are on separate branches. • In a parallel circuit, there are several paths for circuits to take. • If a light burns out – the rest will still stay lit. The current still moves! • As more paths are added, the electric current more paths to follow, so resistance decreases. Hey, who turned out the… • The circuits in your home are parallel circuits. • Electricity is fed into your house by heavy wires – called lines. • These lines have VERY LOW resistance. They branch out to the wall sockets, etc. • The voltage in your home is 120 volts. • All switches are connected to a circuit breaker box in your home. Hey…don’t touch… • Downed wires – chances are they are still “live”. • If you touch a “live” wire – you become part of the circuit and we cannot withstand that many volts at one time. • A short circuit is a connection that allows current to take an unintentional path. No touchy! • The soles of your shoes will provide some protection between you and the Earth. • The current would not be enough to kill you, but could definitely do harm… • Grounding wires protect people from being shocked as well – the charge goes directly into the Earth. More No Touchy! • Third prongs are actually grounding prongs. • Lightning rods are metal rods mounted on t he roof of a building in order to protect it. The rod is connected to a grounding wire. When lightning strikes the rod, charges flow through the rod, into the wire, and then down to the Earth. Keep it Safe! • If you are caught out in an electrical storm… • Do not stand under a big tree – it acts light a lightning rod WITHOUT a grounding wire! • Do not stand around holding a metal handled umbrella – again it acts light a lightning rod and YOU are the grounding wire! • Stay LOW and DRY! Fuses and Circuit Breakers… • In order to prevent circuits from overheating, devices called fuses and circuit breakers are added to circuits. • A fuse is a device that contains a thin strip of metal that will melt if too much current flows through it. When the fuse melts or “blows” the circuit stops the current. • The overload can be reset using the circuit breaker. • Circuit Breaker is a safety device that uses electromagnets to shut off the circuits when the current gets too high. You can reset the circuit breaker by flipping a switch. Electric Shocks… • The severity if an electric shock depends on the current. • A current of .01 amp is almost undetectable, but .1amp and .2 amp can be deadly! • These types of current can cause an irregular heart beat or even stop your heart! Electric Shock – take 2! • The human body does have a certain degree of resistance from electric shock. • One factor is body tissue – living cells have a low resistance to current cause by the fluid in the cells that act like a conductor. • Another factor would be if your skin is wet or dry. You are much more likely to suffer a shock if your skin is wet! THE END!!