Electricity and Hydrogen: The energy carriers Energy and Power for Electricity Electrical Distribution Hydrogen not a source, a carrier Unit 07 Electricity - Slide 1
Quiz The wheels of a 5000kg truck place a 6000 newton force on the truck for a distance of 25 meters. a) How much work is done on the truck? b) If the truck starts from rest, what is its final velocity in meter/sec? c) Use the factor-label method to convert its velocity to miles/hour. Electricity Unit 07 - Slide 2
Quiz There are two ways to calculate the power output of a car when you know that (1) the car has a 180 horsepower engine, (2) the car gets 24 miles per gallon when traveling at 60 miles per hour and (3) there are 36 kwh of energy in each gallon of gas. (a) Use the factor-label method to convert the horsepower into watts (b) Use the factor-label method and the gasoline consumption rate to convert the power into watts Electricity Unit 07 - Slide 3
Electricity Electric Charge (Q) Units are Coulombs (C) Electron charge 1.6X10-19 C Current (I) tells you coulombs/sec Units are Ampere (A) Like gallons per minute in a pipe Electricity Unit 07 - Slide 4
Example 7.1 How many electrons pass through a wire in 5 minutes if the wire has a current of 2 amperes? Electricity Unit 07 - Slide 5
Electricity Resistance (R) tells you how hard it is to push electrons through the wire Units are Ohms (W) Like the size of the pipe Voltage(V) tells you how hard we re pushing on the electrons Units are Volts (V) Like pressure ( psi ) in a pipe Electricity Unit 07 - Slide 6
Ohm s Law V = IR I = V/R the bigger the pipe, the less pressure you have to exert to get lots of gallons per minute Electricity Unit 07 - Slide 7
Example 7.2 A 60 watt light bulb has a resistance of around 240W. How much current will flow the bulb on average? Your house has an average voltage of 120V. Unit 07 Electricity - Slide 8
Electrical Power Power = V*I = I 2 *R Electricity Unit 07 - Slide 9
Example 7.3 What is the current through a 100 Watt light bulb? What is the resistance of the bulb? Your house has an average voltage of 120V. Unit 07 Electricity - Slide 10
Question A. 0.025 ohms B. 3 ohms C. 40 ohms D. 120 ohms E. 360 ohms A toaster draws about 3 amperes of current when plugged into a 120V outlet. What is it s resistance? Unit 07 Electricity - Slide 11
Example 7.4 How much current is drawn by a 3.5HP electric motor if it is running at 120V? What if it s running at 240V? Unit 07 Electricity - Slide 12
Circuit #1 Hook generator up to one light bulb with a switch Unit 07 Electricity - Slide 13
Circuit #2 Add a second switch and light bulb Unit 07 Electricity - Slide 14
Thermal Losses I 2 *R Where does energy go if there is no work done? HEAT! (What your circuit breakers in your house are for.) Electricity Unit 07 - Slide
Example 7.5 Chicago uses around 8X10 9 Watts of electrical power on average. What is the power lost in getting the electricity to Chicago if it is transmitted over a distance of 100km with a 1.5W resistance each way at a voltage of 345kV? What if we increase the voltage to 765kV? (Similar to Ex 11.2 in text.) Unit 07 Electricity - Slide 16
Test Will be in room 3740. On chapters 1 through 5 2 or 3 calculation 10 or so multiple choice 2 or 3 short answer. Study: homework questions Big Ideas Terms to know Electricity Unit 07 - Slide
Avoiding Thermal Losses How to avoid these losses? Move electricity over long distances at high voltage. Decrease voltage right before your house. How do we change voltage? Unit 07 Electricity - Slide 18
Transformers Large banks increase voltage for transmission over long distances ( step-up ) Pole mounted to decrease voltage just before it enters your home ( step-down ) Small ones decrease voltage to enter your computer or cell phone Electricity Unit 07 - Slide 19
How Transformers works Changing electric fields Cause magnetic fields Changing magnetic fields Cause electric fields Electromagnetic Induction Changing electric fields here causes magnetic field here which causes a different electric field here. Unit 07 Electricity - Slide 20
How Transformers works Primary Side Secondary Side V sec V pri N N sec pri Unit 07 Electricity - Slide 21
Example 7.6: Transformers The distribution lines outside your house operate at 2000 Volts. We d like to decrease that voltage to 240 Volts for use inside your home. If the secondary side of the transformer has 400 turns, how many turns should be on the primary side? V sec V pri N N sec pri Unit 07 Electricity - Slide 22
Something for nothing? V sec V pri N N sec pri I sec I pri N N pri sec V * I V * I sec sec pri pri Power Power sec pri Unit 07 Electricity - Slide 23
Question: Doorbell Transformer The doorbell in your house wants around 6 volts, but the wires in your house normally deliver 120Volts so we need a transformer. If the primary side of the transformer has 2000 turns, how many should the secondary side have? a) 40,000 b) 12,000 c) 2000 d) 100 N V V sec sec pri e) 6 N pri Unit 07 Electricity - Slide 24
Electricity is carrier Electricity is a carrier not a source You can t mine electricity It takes roughly 3 Joules of thermal energy to produce 1 Joule of electrical energy (remember the second law of thermo?) Unit 07 Electricity - Slide 25
Immediacy of Electricity Electrical energy can t be stored in large amounts You want more, need to produce it right then You produce some, you need to use it right then Unit 07 Electricity - Slide 26
Immediacy of Electricity Electrical energy can t be stored in large amounts You want more, need to produce it right then You produce some, you need to use it right then Unit 07 Electricity - Slide 27
Renewable You can t control the rate for some electrical power sources Unit 07 Electricity - Slide 28
Storage Batteries Flywheels Capacitors Pumped Storage Hydrogen Unit 07 Electricity - Slide 29
Hydrogen is a carrier or storage Hydrogen is a carrier or storage not a source You can t mine hydrogen It takes more than 1 Joule of energy to store 1 Joule of energy in hydrogen Unit 07 Electricity - Slide 30
Making hydrogen Most common element in the universe Most on earth trapped in H 2 0 Electrolysis efficient but needs electricity Thermal splitting needs lots of heat. Waste heat from power plants? Unit 07 Electricity, Slide 31
Using hydrogen Burn directly need hydrogen gas stations Fuel Cells - electrolysis in reverse Unit 07 Electricity, Slide 32