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Transcript
Electricity Electric Charge Electrons have negative charge Protons have positive charge Just like magnets where like poles repel, and unlike attract In Electricity: •Like charges repel (negative to negative) •Unlike charges attract (negative to positive Electric charges are different than magnets Remember that magnetic poles can not exist alone, But, Electric charges CAN exist by themselves. Electric Force Electric force is another force just like gravity or magnetism Electric Force is the attraction or repulsion between electric charges An electric field extends around any electric charge. •Invisible field just like a magnetic field •Field lines can be drawn to show the direction of the field •Electric field lines point outward from a positive charge •Electric field lines point inward towards a negative charge Electric Fields The strength of an electric field is related to distance • This is just like gravity and magnetism!! • As the distance between the charges increases, the force between the charges decreases • Distance is inversely proportional to strength of the field •Where field lines are close together, the strength of the field is strong •Strongest force is felt near the charge •Farther from the charge the weaker the force Electricity Electricity occurs in 2 different forms • Static • Is stationary E.g. Brush your hair Wool socks in tumble drier • Current • Flows around circuit E.g. turn on light Static Electricity In static electricity, charges build up on an object, but they do not flow…they are stationary! • • • • Occurs with materials which are insulators Rubbing adds or removes electrons Object becomes charged Like objects repel, unlike attract Transferring Charge Law of Conservation of Charge: Charges are never created or destroyed, they can be transferred from one object to another 3 Ways to transfer charge: A. Charging by Friction B. Charging by Conduction C. Charging by Induction Transferring Charge Charging by Friction: Transfer of electrons from one uncharged object to another by rubbing •Caused by two objects rubbing against each other •Object that gains electrons becomes negatively charged •Object that loses electrons becomes positively charged Example: Walking on a carpet with socks, the socks pick up electrons and become negatively charged The carpet loses electrons and becomes positively charged Transferring Charge Charging by Conduction: Transfer of electrons from a charged object to an uncharged (or neutrally charged) object by direct contact •Electrons are transferred from the object with greater negative charge to the one with more positive charge Example: When a negatively charged sock touches your skin the electrons are transferred by direct contact. Transferring Charge Charging by Induction: The movement of electrons to one part of an object that is caused by the electric field of a second object •The electric field around a charged object attracts or repels electrons in the second object Example: •The negative charges in the your fingertip produce an electric field that repels the electrons on the surface of the doorknob. •The electrons on the doorknob move away from your finger •This movement of charge produces an induced positive charge on the doorknob nearest your finger Static Discharge These stationary charges don’t stay there forever as you’ve experienced by touching a doorknob! ZAP!!! When a negatively charged object and a positively charged object are brought together, electrons transfer until both objects have the same charge This is called a static discharge •Can produce a spark •Pet your cat at night •Pull a blanket away from another one •Lightning Detecting Charge Electric charge is invisible Can be detected with an instrument called an electroscope •Metal rod •Knob on top •Two metal leaves When a charged object is brought close to the knob, the leaves repel each other and spread apart Current Electricity To understand electricity and circuits you need to understand Current Voltage Resistance Conductors and Insulators And how they are all related! Current Electricity • Current is electrons flowing continuously through a material • Flow is from negative to positive • Circuit = continuous loop for electrons to flow • A circuit needs an energy supply to allow electrons to flow Current Electricity •Electric charges flow because of a difference in electric potential energy (EPE) •EPE is related to the amount of energy in the source (battery or generator) •Voltage causes a current in an electric circuit…it’s a force pushing the electric current through the circuit •Voltage (V) – Difference in electrical potential energy between two places in a circuit (can be a gain or loss of energy) Current Electricity In other words, Voltage is the short name for the electrical force need to drive an electric current between two points! Current Electricity Resistance: The measure of how difficult it is for charges to flow through a material •The greater the resistance, the less current there is for given voltage •Resistance is measured in Ohms (Ω) Current Electricity 4 Factors that determine resistance •What the resistance material is made of •Conductors have less resistance than insulators •Length of the resistor •Greater lengths give more resistance •Diameter of the material •Small diameters have more resistance •Temperature of the resistance •Higher temperatures cause greater Current Electricity Path of least Resistance…. Electric charges act just like you…they take the easy way! This is why a bird is not harmed when sitting on a electric fence or wire. The bird’s body has more resistance than the wire, so the electricity picks the “path of least resistance”….the wire, not the bird! Resistance (R) The amount that a component slows the current •As the electrons are slowed by a resistor, energy is lost in the form of heat. •This means that current, resistance and voltage must be linked. •This is Ohms law •The unit of resistance is the ohm, symbol V I × R Current Electricity In the 1800’s Georg Ohm developed a law that demonstrated how current, voltage, and resistance are related… Called…..Ohm’s Law (how original!) Ohm says… Resistance in a circuit is equal to the voltage in the circuit divided by the current Resistance = Voltage ÷ Current in units this is: Resistance (Ω) = Voltage (V) ÷ Amps (A) Current Electricity Another way to look at this law: Voltage = Current X Resistance Or Current = Voltage ÷ Resistance Current Electricity So looking at the basic equation Resistance (Ω) = Voltage (V) ÷ Current (I) How does changing the 3 factors effect each other? What happens to the current, if voltage is doubled? ….If nothing with the What happens tochanges the current , ifthe theresistance, resistancethen is increased? current is doubled Remember….C = Vtoo ÷R So, the current will decrease! What happens to the Voltage when the resistance is increased? Remember ----V = C ÷ R So, Voltage also decreases! Electrical Circuits Features of a circuit 1.Circuits have devices that are run by electricity 2.A circuit has a source of electrical energy 3.Electric circuits are connected by conducting wires Electrical components …for electric circuits! • Wire • Ammeter • Battery • Fuse • Connected wires • Bulb • Switch • Voltmeter • diode • Resistor • Rheostat Electric circuits Electric Circuits Conductors: Material that allows electrons to flow easily •Metals (copper, silver, aluminum, iron) •Water (both fresh and salt) •Humans (yikes!!!) Insulators: Material that DOES NOT allow electrons to flow easily •Rubber •Plastic •Glass •Wood •Resistors can be made of conductors or insulators! CIRCUIT TYPES The simplest type of circuit involves electricity going around with no “choices” (electrons don’t really choose). This is called a Series circuit. Draw the path the electrons travel. The other main type of circuit has two or more branches. This is called a Parallel circuit. Draw on the electron flow. What sort of circuit is this? A parallel… but, more importantly… A short circuit. (we’ll talk more of this later!) Series circuit • Has a single loop for electrons to travel round • Components are connected one after another...in series • Current has to travel through all components in turn • Current is the same at all points • Voltage is shared between components Series circuit Disadvantages to Series Circuits •If you have two or more lights hooked up in a series circuit, when one goes out, they ALL go out •As you add lights the lights become dimmer as more lights are added Parallel circuit • Has two or more paths for electrons to flow down • Current is shared between the branches • Sum of the current in each branch = total current • Voltage loss is the same across all components Parallel circuit Advantages of Parallel Circuits •Unlike in a series circuit, when on light burns out in a parallel circuit, the other lights connected in circuit stay on. •Switches can be added so that components can be turned on and off without affecting the other components •Resistance decreases in the circuit when adding more branches or loops to the circuit •Remember Ohm’s law, if resistance decreases then current increases! Current (I) Current is the flow of electrons around a circuit Two types of current: • DC = direct current like battery • Electrons flow in one direction • AC = Alternating current like mains • Electron flow changes direction 50x per second Ammeter • • • • Measures CURRENT(I) Unit = Amp (A) Current is flow of electrons Connect in series at the point you wish to measure • RED to RED and BLACK to BLACK Voltage (V) Gain or loss of energy as it passes through a component • Voltage lost = voltage gained • In series voltage loss is shared between components • In parallel voltage loss is the same across all components Voltmeter • Measures voltage • Unit = Volt (V) • Voltage increases as it passes through energy suppliers • Voltage is decreased as it passes through users • Connect in parallel around a component SUMMARY In Series Current Voltage Always the same Voltage from source = voltage used Voltage is shared between power users In Parallel The branches share electrons and add to the total Voltage is the same in all branches Electric Power The rate transformation of electrical energy into another form is power Electric Power = Voltage X Current P=VI The unit of Power is watts Power Energy used by component per second • Unit of power is the Watt, symbol is W • One watt means that 1 joule of electrical energy is being used up per second. • Current, voltage and power are linked P I × V PROBLEMS 1. Fill in the gaps in the table. a b c d e g i k j f h l Electricity Generation Fossil Fuels Hydro Dam Hydroelectric power Nuclear power Wind Power Tidal Power Geothermal Solar • Solar water heating • Photovoltaic cells • Solar furnace Electrical Safety Electricity in the wrong place can cause damage and injury! Potential Dangers •Short Circuit •Electric Shock •Ungrounded Wires Electrical Safety Short Circuit A connection that allows current to take the path of least resistance…usually a unintended path. Example: A downed electric wire on the ground and someone touching it, causes a short circuit. Their body becomes the path of least resistance and the electricity flows through them instead of the wire! Ouch! Electrical Safety Electric Shocks The severity of an electric shock depends on the current •A current of less than .001 A is almost unnoticeable •A current greater than .2 A can be dangerous, causing burns, or even stopping the heart •Household voltage: 120 V, if only 1Ω resistance, then the current would be 120 A!!! Yikes!