Electric Potential and Electric Circuits

Electric Potential Total electrical potential energy divided by the charge Electric potential = Electric potential energy Charge Charge

VOLT SI unit for electric potential – VOLT Named after Allesandro Volta ( ) Named after Allesandro Volta ( ) 1 volt = 1 joule/coulomb 1 volt = 1 joule/coulomb

Voltage Same as volts One volt takes 1 joule of energy per coulomb to add it to the conductor Rub a balloon in your hair It may take several thousand volts It may take several thousand volts HOWEVER – it is very few coulombs – less than one millionth HOWEVER – it is very few coulombs – less than one millionth SO – the energy it very low SO – the energy it very low

Capacitors Electrical storage Uses In computers to store “1” and “0” In computers to store “1” and “0” In keyboards In keyboards Photoflash bulb Photoflash bulb Giant lasers Giant lasers Electric fences Electric fences

Simple Capacitors Conducting plates separated by a small distance Connected to battery which moves electrons from battery to plate Complete when voltage on plates = battery voltage Advantage – Discharged in large quantity Can be dangerous – even when appliances are not powered.

Formula C= Q/V C – capacitance (Farads or Coulombs/Volt) C – capacitance (Farads or Coulombs/Volt) Q – charge (coulombs) Q – charge (coulombs) V – voltage difference (volts) V – voltage difference (volts)

Electric Terms Voltage – electric “pressure” Current – flow of charge Resistance – Restrains the electron flow DC (Direct current) – flows in one direction AC (Alternating current) – flows back and forth Power – Rate at which energy is transferred

Flow of Charge When ends of electric conductor (example-wire) have different potential energy the charge flows Flow continues until it reaches the same potential Examples Shock Shock Flashlight Flashlight

Electric Current Flow of electric charge Electrons carry the charge Electrons carry the charge Protons are bound to atoms Protons are bound to atoms In solutions, ions can carry the charge In solutions, ions can carry the charge Measured in amperes (amps) SI unit (A) SI unit (A) Amps = 1 coulomb/second Amps = 1 coulomb/second Example - 5 amps wire carries 5 coulombs of charge pass a point in 1 second When there is no current, the wire has no charge

Formula for Current I = ΔQ/Δt I – current (amperes or amps) I – current (amperes or amps) Q – charge (coulombs) Q – charge (coulombs) T – time (seconds) T – time (seconds) 1 amp = 1 coulomb/second

Voltage Sources Electron pump Needs to slowly release the electrons Not all at once – like in a shock Not all at once – like in a shock Steady flow Dry cells – chemical energy Dry cells – chemical energy Wet cells – chemical energy Wet cells – chemical energy Generator – converts mechanical energy Generator – converts mechanical energy Voltage provides “pressure” to move electrons between terminals

Generators Average home outlets are AC Pressure of 120 volts This means there is 120 joules of energy forcing the coulombs through the wire This means there is 120 joules of energy forcing the coulombs through the wire Voltage does not FLOW through the wire The electrons are pushed by the voltage The electrons are pushed by the voltage

Electric Resistance Current depends on Voltage Voltage Resistance Resistance Resistance – slows the flow Depends on Depends on Conductivity of material – silver best Thickness – more thick, less resistance Length of wire - longer, more resistance Temperature – hotter, more resistance

Drawing Currents

Ohm’s Law Current in a circuit is directly proportional to the voltage and inversely proportional to the resistance Current = voltage/resistance Current = voltage/resistance I = V/R Units  1 ampere = 1 volt/ohm Units  1 ampere = 1 volt/ohm Double the voltage – double the current Increase the resistance – decrease the current

Typical resistance Cord – less than 1 ohm 100 W light bulb – 100 ohms Iron – 15 ohms Electric toaster – ohms Low resistance = larger current which produces lots of heat Low resistance = larger current which produces lots of heat

Ohm’s Law Formula V=IR V – voltage (volts) V – voltage (volts) I – Current (amps) I – Current (amps) R – resistance (ohms – Ω) R – resistance (ohms – Ω)

Resistors Radios and TV electronics Current is regulated by resistors Ranges from few ohms to millions Ranges from few ohms to millions

Question What is the resistance of an electric frying pan that draws a 12 ampere current when connected to a 120-volt circuit? R = V/I = 120 V/12 A = 10 Ω How much current is drawn by a lamp that has a resistance of 100 Ω when a voltage of 50 V is impressed across it? I = V/R = 50 V/ 100 Ω = 0.5 amps

Electric Shock What effects us – current or voltage? Damage comes from current through the body Current depends on voltage and resistance in body Current depends on voltage and resistance in body Resistance varies from 100 Ω (covered in salt water) to 500,000 Ω (very dry skin) Resistance varies from 100 Ω (covered in salt water) to 500,000 Ω (very dry skin) Usually can’t feel 12 volts and 24 volts would tingle If wet – 24 volts could be very uncomfortable If wet – 24 volts could be very uncomfortable

Effect of Various Electric Current Current in Amps Effect Can be felt 0.005Painful 0.010Spasms Loss of muscle control Probably fatal (if through the heart) for more than one second

Questions If resistance of your body were 100,000 ohms, what would be the current in your body when you touched the terminals of a 12-volt battery? Current = V/R = 12 v/100,000 Ω = A If your skin were wet, your resistance would be 1000 Ω, what would you feel on a 24 volt battery? Current = 24 V/ 1000 Ω = A

Shocked 120 V in house normally our shoes provide resistance normally our shoes provide resistance Standing barefoot in bathtub – very little resistance to ground – don’t use appliances in bathtub!! Standing barefoot in bathtub – very little resistance to ground – don’t use appliances in bathtub!! Birds sitting on wire Same potential on both sides of body Same potential on both sides of body Only get it if they touch wires with different potential Only get it if they touch wires with different potential

Grounded Most appliances are grounded Helps prevent potential differences Helps prevent potential differences All ground wires are connected together All ground wires are connected together Provides a direct route to the ground Provides a direct route to the ground What causes shock – current or voltage? The initial cause is the voltage, but the current does the damage The initial cause is the voltage, but the current does the damage

Types of current DC – Direct current Flow of charge is always in one direction Flow of charge is always in one direction Source – batteries (wet or dry) Source – batteries (wet or dry) Move from negative terminal to positive Move from negative terminal to positive AC – Alternating current Charge moves back and forth in the line Charge moves back and forth in the line In North America – frequency is 60 cycles per second – 60 Hz In North America – frequency is 60 cycles per second – 60 Hz

Voltage North America Small appliances – volts Small appliances – volts Large appliances – 220 volts Large appliances – 220 volts Obtain by “combining voltage” Europe – 220 volts (more efficient) Electrons do NOT travel from power plants, they oscillate in place They sell energy, you provide the electrons in your wiring They sell energy, you provide the electrons in your wiring

Diode One way valve in a circuit Only allows electrons to move one way Can be used to convert AC to DC current Can be used to convert AC to DC current

Power Rate at which electric energy is converted to another energy form Electric power = current x voltage Units – 1 watt = 1 amp x 1 volt Units – 1 watt = 1 amp x 1 volt 1 kilowatt = 1000 watts 1 kilowatt = 1000 watts 1 kilowatt hour = amount of energy consumed in 1 hour at rate of 1 kilowatt 1 kilowatt hour = amount of energy consumed in 1 hour at rate of 1 kilowatt

Calculations How much power is used by a calculator that operates on 8 volts and 0.1 amps? If it is used for one hour, who much energy does it use? Power = A x V = 0.1 A x 8 V = 0.8 W Energy = Power x time = 0.8 W x 1 hour = 0.8 watt-hours or kilowatt-hours FYI – Speed of e- in wire = 0.01 cm/s

Lighting a bulb – which one works? Must have a closed loop!!

Electric Circuits Need Pathway Pathway Voltage source (battery or AC) Voltage source (battery or AC) Resistance (light, fan, bell, etc) Resistance (light, fan, bell, etc) Optional Optional switch – stops and starts current Capacitor – hold a large voltage for quick release Diode – one-way valve Transformer and capacitors– controls current flow

Types of Circuits Series – single pathway for all electrons to flow Parallel – branched pathway for electrons to flow

Series circuit When switch is closed, current runs through all lamps in same path Open – when one break occurs, all flow stops Break from switch, broken wire, burned out light-bulb Break from switch, broken wire, burned out light-bulb

Series Circuit Features Single pathway – same current Total resistance = sum of individual resistance Current = voltage/ total resistance (Ohms law) Voltage drop across each device depends on resistance of the device

Disadvantage of Series If one device fails, the whole circuit ceases

Parallel Circuits Devices are connected to the same points in the circuit Each resistor has its own path to the voltage source If one device fails, it does not interupt the other devices

Features of Parallel Circuits Each device connects directly to the voltage source. Therefore the voltage is the same across each device. Total current divides between devices. The one with the lowest resistance gets the most current (Ohm’s law) Total current = sum of current in the devices As number of devices increases, resistance decreases

Schematic Diagrams Resistance Battery (2)

Combined Compound Circuits

Calculating Current in Compound Circuits When resistors are in Series Series The resistance equal sum of devices -----^^^^^-----^^^^^^ = -----^^^^^ ^^^^^-----^^^^^^ = -----^^^^^ Ω 8Ω 16 Ω 8Ω 8Ω 16 Ω Parallel (2 devices with same resistance) Parallel (2 devices with same resistance) The resistance equal half of one device ----^^^^^ = ----^^^^ ^^^^^ = ----^^^^ Ω 4 Ω 8Ω 4 Ω ----^^^^^ ^^^^^ Ω 8Ω

Another Example ----^^^^^ ^^^^^ Ω ----^^^^^----  ----^^^^^-----^^^^^-- 8Ω 8Ω 4Ω 8Ω 8Ω 4Ω----^^^^^ Ω 8Ω  ---^^^^^-----  ---^^^^^ Ω 12Ω

Overloading The more devices on a line, the more current it draws as resistance lowers Overloaded – line carrying more than a safe amount of current Fuses – put in a current in SERIES Cuts the line if it overheats Cuts the line if it overheats Overheating caused by Overheating caused by Short circuit (cuts the resistance) Too many devices on a line