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Transcript
Inside the Atom • Protons Unit 7: Electrical Circuits and Systems – Positive charge – Found in nucleus – Little freedom to move • Neutrons •How is electricity measured and described? •What is the relationship between current and voltage in a circuit? •What are the different types of circuits? •How much energy is carried by electricity? – No charge – Found in nucleus Inside the Atom Electric Current • Electric Current (I) • Electrons – Negative charge – Found in electron cloud outside nucleus – Freedom to move – The flow of electric charge • Usually electrons – Able to do work Electric Circuits • If you touch a light bulb to a battery will it light up? – No, it needs a complete circuit • Electric Circuit – Complete path through which electricity travels – Requires a source of energy – Ex: Battery or outlet Electric Circuits • Closed Circuit – Complete loop of electricity – Current flows through • Open Circuit – Circuit with a break in it – Current does not flow through – Ex: Light bulb burns out • Switch – Used to open or close a circuit 1 Conductors and Insulators Conductors and Insulators • Semiconductors • Conductors – Devices that allow current to flow – Ex: Metals • Insulators – Do not easily allow current to flow – Ex: Rubber, glass and wool – Carry a medium amount of current – Conductivity is usually variable – Ex: computer chips • Wires have insulators outside and conductors inside Batteries Batteries • Cathode • Batteries – Use chemical energy to create voltage difference – Adds potential energy to charges and moves them through the circuit – Voltage adds from all batteries in circuit Resistors • Resistors – Convert electrical energy into other energies – Ex: Light bulb, Heater, Speaker, Motor • Potentiometer (Pot) – Charge flows out – Usually the negative side • Anode – Charge is collected – Usually the positive side Electric Circuits • Circuit Diagram – Uses symbols to show components of circuit – Shorthand for describing a working circuit – Variable resistor – Ex: dimmer switch, volume control 2 Electric Circuits Voltage • Voltage (V) • Current flows from positive to negative • In diagrams, long line is positive, short line is negative • Arrow shows direction of movement – Electric potential energy – Measured in Volts (V) – 1 V = 1 J /(A ‧ s) = 1 W/A – Circuit must have a voltage difference across it for energy to flow Current Resistance • Current (I) • Resistance (R) – Measured in Amperes (A) – Current in equals current out – Flows from positive end to negative end Andre-Marie Ampere – Opposition to flow of current – High resistance means current flows slowly – Circuit wires usually have little to no resistance – Measured in Ohms (Ω) – 1 Ω = 1 V/A Resistance • Human skin usually has a resistance of ~100,000 Ω or more • Wet skin – less resistance – more dangerous • You feel current of 0.0005 A or more • A 9 V battery will cause 0.00009 A, so you won’t feel it • A wall socket will cause 0.0012 A, which is dangerous Ohm’s Law • Ohm’s Law – Used to calculate the current in a system V = IR – V – Voltage (V) – I – Current (A) – R – Resistance (Ω) Georg Ohm 3 Current and Resistance • The amount of resistance determines the current • More resistance means less current Series Circuits • Series Circuits – Only one path for current flow – Current is same at all points – Break in circuit means current stops everywhere in the circuit – Ex: Old Christmas Lights Series Circuits • Series Circuits – Resistance is sum of all resistances – Use Ohm’s Law to find current Voltage Drop • Kirchhoff’s Voltage Law – Voltage drops must add up to battery voltage – Energy in circuit is conserved Voltage Drop • Voltage Drop – Voltage difference across a resistor – Ex: 3 light bulbs with 3 volts Voltage Drop • Kirchhoff’s Voltage Law – If resistors are identical: • Divide the total voltage by number of devices – Ex: A series circuit contains a 9-volt battery and three identical bulbs. What is the voltage drop across each bulb? • 3V 4 Voltage Drop Parallel Circuits • Parallel Circuits • Kirchhoff’s Voltage Law – If resistors are not identical: 1. Calculate the total resistance 2. Use Ohm’s Law to find the current 3. Use Ohm’s Law at each resistor to find the voltage drop Parallel Circuits • Parallel Circuits – Voltage is always the same across each branch • Even when branches have different resistances – Each branch has voltage drop equal to total voltage Parallel Circuits • Each branch has the same voltage, but a different current • Use Ohm’s law to find the current in each branch (I = V/R) • Add up all branches to find total current – Multiple current paths – Each path is a Branch – Branch Current is the amount of current through each branch Parallel Circuits • Kirchhoff’s Current Law – All current in each branch must come out again – Total current is equal to sum of branch currents Parallel Circuits • More resistors lower the resistance in parallel circuits • To find the total resistance: • Draw the diagram • Add all the branch currents • Use Ohm’s law to find the total resistance ( R = V/I) 5 Parallel Circuits • Are houses wired in series or parallel? • Parallel, so you can turn lights on and off without affecting other rooms • Also so you can use the blender and microwave at the same time with full power Electrical Power Electrical Power • Electrical Power (P) – Measure of the rate of work done by a circuit – Rate at which electricity is converted to sound, heat, light, etc. Buying Electricity • Electrical Power (P) – Measured in Watts (W) P = IV • Kilowatt-hour (kWh) – Unit of energy • 1,000 W of power used for 1 hour • 3,600,000 J – Used to bill customers from power company Buying Electricity AC vs. DC • Kilowatt-hour (kWh) – Calculations • Convert W to kW (1,000 W = 1 kW) • Multiply kW by time in hours • kWh to $ using dimensional analysis • Direct Current – Always from positive to negative – Produced by batteries • Alternating Current – Switches constantly – In US, 60 hz or 60 times/second – You need adapters for other countries – Provided by power lines 6 AC vs. DC Winner of War of Currents Nikola Tesla – Westinghouse AC Current Advocate Circuit Safety • Wires Thomas Edison – GE DC Current Advocate – Can handle different amounts of power – Can absorb current • Usually make it into heat – Larger diameter can handle more current – Longer wires have more resistance Circuit Safety • Short Circuits – Occur when there is little or no resistance in a circuit – Usually if two wires at different points on the circuit touch – Produces heat and can start a fire Circuit Safety • Outlets – 3 connections – Hot wire – 120 V – Neutral Wire – 0 V – Ground wire – 0 V • Sends power to ground in short circuit Circuit Safety • Circuit Breaker – Automatic switch – Can be reset – Opens when current builds up too high • Fuse – Low-resistance resistor – Burns up when current is too high – Must be replaced after Circuit Safety • Ground Fault Interrupt (GFI) – Breaks circuit if current in does not match current out – Required near water or outdoors 7 Circuit Safety • Transformers – Change the voltage from the power plant to home • Higher voltage through wires because of increased resistance – Charging devices change AC to DC to power batteries • Ex: cell phone charger 8