Environmental Physics Chapter 10: Electricity, Circuits and Superconductors Copyright © 2012 by DBS

Environmental Physics

Chapter 10:Electricity, Circuits and Superconductors

Copyright © 2012 by DBS

Introduction

• A

Introduction

• A 1999 Roper poll concluded that less than a third of consumers knew where their electricity comes from

• Electricity consumption has the largest growth rate of any major energy-use sector

• 50% of the energy produced in the US is used to make electricity

Dual-fired (oil and natural gas) 850-MWe electrical power plant across the Hudson River from Manhattan.

Introduction

Introduction

• 1950s, 60s and 70s increased demand for appliances, heating, shopping centers, arenas and conversions to electrical processes in industry caused consumption to increase 7 % per year

Figure 10.1a: U.S. production of electricity by type of generation, 1950–2003

Introduction

• Per capita consumption of electricity was six times higher in 1998 than 1948

• Rate has slowed since:

– Most households have acquired basic appliances

– In 2000: 99 % have TV, 65 % AC, 90 % have a microwave

– Rising electricity costs (cost of new plants increased)

U.S. electricity production versus GDP, 1950–2003. EI = production / GDP

Introduction

• Developing World: China: consumption is rising 9 % per year

• USA: 50 % from coal

• 1000 MW plant uses 9000 tons of coal per day (one trainload = 90 carts x 100 tons each)

Figure 10.2: Electric power industry generation, 2003, for utilities and independent power producers.

Restructuring of the Electricity Utility Industry

• PURPA (1978) – Public Utility Regulatory Policy Act

• Allowed for competition

• Required utilities to compare cost of adding new capacity with the cost of purchasing it from independent producers using renewable sources/cogeneration and chose the least expensive option

• By 1990’s independent producers accounted for 50 % of all new additions to generating capacity(natural gas, wind, geothermal, biomass, solar)

• Today they account for 1/3 production of all electricity in the US

• Major roadblock for PURPA was transmission of electrical power

• Federal Energy Policy Act of 1996 made transmission lines available for anyone to use


• From 1997 consumers could buy electricity from suppliers other than their local utility

• Energy choice, as the practice is called, is the product of deregulation of the electricity and natural gas markets

• The idea was that competition among electricity and gas providers would:

– Lower prices for consumers

– Expand use of “green power”, electricity produced from renewable sources

– Require your electric bill to disclose what sources are used

• We have transitioned from a highly regulated structure of 20 th century to more competitive 21st century model

• Reforms came about due to public unhappiness with prices and environmental concerns


• A


• In some cases electricity prices have increased!!!

• Why? Higher demand outstrips supply

• Blackouts in California (winter 2000), North-East (Aug 2003) (See Frontline: Blackout)

• Due to reluctance to build new plants due to high cost


LAT

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6150

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Electrical Charges and Currents

• Charges are both positive and negative

e.g. plastic rod rubbed with fur – rod becomes –ve

e.g. glass rod rubbed with nylon – rod becomes +ve

• An electric force exists between charged objects

• Force is repulsive if the net charge on both objects is the same and attractive if the objects have different charges

Like charges repel; unlike charges attract


• Unit of charge is the coulomb (C)

• Electron (e-) has negative charge whilst a proton (p+) has positive charge

• 1 C = 6.25 x 1018 e-

• Whilst charging, e- are transferred

• -vely charge object has excess e- whilst +vely charge object has e- deficit

• If you bring copper wire near a charged object the net charge will decrease

• If you bring a piece of glass into contact with the charged object the charge remains the same

• In a conductor the e- are free to move around and can more easily migrate through the material

• In an insulator the e- are more strongly bound to atoms


• Flow of e- is called electrical current

• Electrical current is expressed as the charge flowing past a point in a given time

1 amp = 1 Coulomb / 1 second

• Potential difference between two points A and B is defined as the work that must be done to move charge from point A to B

1 volt = 1 joule / 1 coulomb

• For there to be current, there must be a potential difference and a path between the points for charge to flow, called a circuit

[analogous to flow (amps) of water controlled by pressure (volts)]


• Flashlight with 1.5 V D-cell batteries

• What is total potential difference?

• What is required to complete the circuit?

Figure 10.4: Diagram of the inside of a flashlight with a plastic case. When the switch is on, the metal strip makes contact with the metal ring around the bulb. This makes a continuous circuit through the metal strip on the inside of the casing to the spring and then to the negative terminal of the battery. The positive end of the battery is in contact with the filament of the bulb.

p. 324

Observe electrical charge…

Wimhurst

Van Der Graff

Charge a balloon…

Demos

16-21 16-22

16-23 16-24

17-01

17-05

17-04

17-07

End

• Review

Batteries and Electric Vehicles

• Battery is an energy converter

• Converts chemical energy to electrical energy

• Cell: Consists of two electrodes submerged in an electrolyte

• A battery is a combination of cells

• Small amounts of compounds making up the electrodes go into solution as free ions, creating –ve and +ve terminals

• The electrolyte can be a liquid (e.g. sulfuric acid) or a paste (dry cell)

• The potential difference is maintained by continued chemical reaction at each electrode


• Alessandro Volta (1745-1827) invented first battery

• Small plates of zinc and copper, separated by salt soaked cardboard

• Zinc atom enters electrolyte as Zn2+,

Zn → Zn2+ + 2e-

• Two e- flow as current joining with a Cu2+ ion and plating out copper on the –ve electrode:

Cu2+ + 2e- → Cu


• Dry Cell batteries – do not contain water

Alkaline Dry Cell

• electrolyte is alkaline KOH paste

• anode = Zn oxidized

Zn(s) Zn2+(aq) + 2 e-

• cathode = graphite rod immersed in MnO2 - reduced

2MnO2(s) + 2H2O(l) + 2e- 2MnO(OH)(s) + 2OH-(aq)

• cell voltage = 1.54 v

• longer shelf life than acidic dry cells and rechargeable, little corrosion of zinc


Lead Acid Storage Batteries

• 6 cells in series

• electrolyte = 30% H2SO4

• anode = Pb

Pb(s) + SO42-(aq) PbSO4(s) + 2 e-

• cathode = Pb coated with PbO2

• PbO2 is reduced

PbO2(s) + 4 H+(aq) + SO42-(aq) + 2 e-

PbSO4(s) + 2 H2O(l)

• cell voltage = 2.09 v

• Rechargeable (PbSO4 back to Pb and PbO), heavy


NiCad Battery• electrolyte is concentrated KOH solution• anode = Cd

Cd(s) + 2 OH-(aq) Cd(OH)2(s) + 2 e-

• cathode = Ni coated with NiO2

• NiO2 is reduced

NiO2(s) + 2 H2O(l) + 2 e- Ni(OH)2(s) + 2OH-

• cell voltage = 1.30 v• rechargeable, long life, light – however recharging incorrectly can lead to battery breakdown


Nickel-Metal Hydride (NiMH) Battery• Rechargeable• Similar to NiCad but uses hydrogen-absorbing alloy for anode instead of cadmium


Lithium Ion Battery• electrolyte is concentrated KOH solution

• anode = graphite impregnated with Li ions

• cathode = Li - transition metal oxide– reduction of transition metal

• work on Li ion migration from anode to cathode causing a corresponding migration of electrons from anode to cathode

• rechargeable, long life, very light, more environmentally friendly, greater energy density


• Batteries are convenient, portable and reliable

• Parameters: Voltage and discharge capacity

• Discharge capacity = current (amps) x time (hours battery can supply current)

e.g. good car battery has discharge capacity of 60 amp-hours (3 amps for 20 hours, or 10 amps for 6 hrs)


• EVs – first speeding ticket was given to an EV!

• In 1914 there were 20,000 on the road

• Todays EV’s have 8 12-V lead-acid batteries (500 lbs) and take 6-8 hrs to charge

• Range: 60-160 mi

Basic circuit for an electric vehicle. The charger feeds the batteries, which can then supply power to the motor to move the car. The speed and power of the motor are regulated by the controller, which is in turn controlled by the accelerator.


• R&D to create lightweight, inexpensive and reliably charging batteries is very active

• Energy density of new batteries is higher but they have short lifetimes and are more expensive to produce

EVs have been found to have limited applications because they do not meet the multiple demands of driving

– range, power, longevity, cost


• Interest in EV’s has been displaced by hybrids and fuel cells

• Hybrids have been found to meet more of the demands for multiuse driving

Figure 10.5: The Lexus RX 400h is a hybrid SUV that gets 31 mpg city and 27 mpg highway driving. (Its battery pack has a 288 V nominal voltage, boosted to 650 V by the boost converter.)


• Hybrid - Gasoline/battery

• 1 kWh vs 30 kWh for a pure EV

• Battery used for speeds up to 25 mph – 144-V nickel metal hydride battery – does not need charging since electric motor acts as generator during deceleration and braking

• Fuel efficiency of 70 mpg for highway driving, 65 % less emissions than gasoline powered vehicle

• Cost of second engine system raises price by 20% (can be offset by lower gasoline consumption over time)


• Popular Science Nov 2008 Chevy Volt article

End

• Review

Ohms Law

• The current through a conductor between two points is directly proportional to the potential difference across the two points and inversely proportional to the resistance between them

I = V

R

• Where I = current (Amps), V = potential difference (Volts), and R = resistance (ohms)

• The reciprocal of R is electrical conductance (siemens)

Ohms Law

• A

Ohms Law

• Resistivity (ρ) is a measure of how strongly a material opposes the flow of an electric current

• Low resistivity indicates a material that readily allows the movement of electrical charge

ρ = R A

l

Where A = area (m2) and l = length (m)

Metals have ρ = 10-8 ohm.m

Insulators have ρ = 1016 ohm.m

R = ρ l

A

• Larger cross-sectional area, more e- from metal atoms are available to carry current – lower resistance

• Longer the conductor the more scattering events occur in each e - path - higher the resistance

Ohms Law

Figure 10.8: Household circuit with toaster.

Find the current, I

Superconductivity

• 1911 – Dutch physicist Heike Kamerlingh-Onnes discovered that the resistance of solid mercury disappeared at temperatures below -269 °C (4 K) (using liquid Helium)

• Current flows without a power source!

• Search was on for a superconductor with critical temperature = ambient temperature

• 1986 – Muller and Bednorz produced superconducting ceramics (35 K)

• 1987 – yttrium-barium-copper oxide superconducts at 100 K (using cheaper liquid nitrogen)(economically important – now we see real uses)

Superconductivity

• Superconducting material excludes magnetic fields from its interior• Electrical "screening currents" flow at the surface of the superconductor generate a magnetic

field • Cancels any externally applied field

• ‘Meissner Effect ‘ leads to levitation

Figure 10.7: Demonstration of magnetic levitation. A magnet “floats” above a superconductor, which is in a bath of liquid nitrogen at 77 K (−196°C).

Searching for New Superconductors

• Moving electricity – greater efficiency – if the grid were made of superconductors then there would no need to transform the

electricity to a higher voltage (this lowers the current, which reduces energy loss to heat) and then back down again

• Generating electricity– Superconducting magnets are more efficient in generating electricity -

superconducting generator is 99% efficient vs around 50% efficient for copper wire.

• Levitation– 'MagLev' trains - the train floats above the track using superconducting magnets;

this eliminates friction and energy loss as heat, allowing the train to reach such high speeds

• MRI – smaller and more efficient magnets

• Computers - faster electronic switches

Elementary Circuits

• A potential difference connected to a load converts electrical energy to heat, light and work

• Each device has an associated resistance, R

Figure 10.12a: A circuit containing resistors in series. As more devices are added, the total resistance increases, and so the current decreases; b: A circuit containing resistors in parallel. As more devices are added, the total resistance decreases. However, the current through each device (I1, I2, etc.) remains the same.

Elementary Circuits

Electrical Power

• Electrical energy is either converted into work (as in a motor) or heat (as in a resistor)

Figure 10.13: Electrical energy goes into work or heat.

Electrical Power

• Power (Watts) = voltage (V) x current (I)P = IV

e.g. toaster = 120 V x 8 A = 960 W

• The rate at which electricity is converted into heat is related to resistance of the device• Since Ohm’s law, V = IR

P = I 2R = V2/R

• For home circuits copper wiring has low resistivity, current flow primarily determined by devices

• For transmission lines R is a problem. I is kept low and V is increased

Electrical Power

• Home circuits – wired in parallel

• Maximum power on one circuit with a 20-amp fuse is 120 V x 20 A = 2400 W

• If a hair dryer is 1000 W and a fridge is 400 W this circuit is nearly full

• Require multiple circuits

Figure 10.14: Parallel connections in a household circuit. (Wiring connections are shown by a •.)

Pricing Electrical Energy Use

• Energy used (kWh) is the power (W) expended x period of use (h)

E = Pt

• Average home uses 500 kWh per month (excluding space heating)

• kWh more important unit than W

• e.g. microwave vs. conventional oven

Question

Given below are the electrical requirements for five household appliances. Determine the number of kWh of electrical energy consumed if all these appliances are running simultaneously for 2 hours in a house

Color TV: 145 W

Washing machine: 512 W

Furnace: 500 W

Clock: 2 W

Humidifier: 177 W

2.67 kWh

Question

A typical computer on the internet consumes 100 watts of electrical power.

If you use your computer for 24 hrs a day, how much energy do you use in (i) kW, (ii) MJ, (iii) BtU? If energy costs 10 cents per kilowatt-hour, how much does it cost?

E = Pt

= 100 W x 24 h = 2400 Wh = 2.4 kWh

= 2.4 kWh x 3.6 MJ/kWh = 8.6 MJ

= 8.6 x 106 J x 1 BtU / 1055 J = 8.2 x 103 BtU

Cost = (10 cents / KWh) x 2.4 kWh = 24 cents

Table 10-2a, p. 340


• Energy conservation using lifecycle costs (initial cost + maintenance cost + energy costs)

• Initial cost may be large but lifetime cost may be less on energy efficient devices

Figure 10.12: Sticker displaying energy costs for an appliance, a dishwasher in this case.


• CFL use less power to supply same amount of light

• LED’s use even less

Figure 10.16: Energy-efficient fluorescent light bulbs with standard bases.


• p343 Payback of CFL

• A 22-W fluorescent bulb costs $8 and emits the same amount of light as a regular 70-W bulb costs $1, and electricity costs $0.08 / kWh what is the payback time?

Fuel Cells

• Efficient and nonpolluting!

• Combines fuel (natural gas or hydrogen) with oxygen to produce electricity

• Designed to power vehicles, homes and even laptop computers

Pros Cons

High power to weight ratio High cost

Compact Unknown durability / lifespan

Reliable (no moving parts)

Higher efficiency than ICE

Nonpolluting, fewer GH gases

Fuel Cells

• like batteries in which reactants are constantly being added– so it never runs down!

• Anode and Cathode both Pt coated metal

• Anode Reaction (+ve electrode):

2H2 → 4H+ + 4 e-

• Cathode Reaction (-ve electrode):

O2 + 4H+ + 4 e- → 2H2O

Overall reaction?

Renewable Energy Energy Storage

Scientific American, Oct. 2002

Fuel Cells

• Electrons pass directly from the fuel through an external circuit to the oxidizer

• In a PEM fuel cell H+ ions move through Proton Exchange Membrane and combine with O and e- to form water and heat

• 5 types of fuel cell:

Alkaline fuel cell dates back to Gemini space program

Fuel Cells

• PEM systems are suitable for use in passenger vehicles

• Drawback is the need for heavy pressurized container for storing hydrogen

– Possible to use alcohol or natural gas and produce H2 gas onboard

– Store H2 gas in metal hydrides

• Fuel cells are arranged in stacks for higher voltage and currents needed

No more than 0.5 amps!

p. 348

The Mail Processing Center in Anchorage, Alaska, uses one of the nation’s largest fuel cell systems, 1 MW.

Summary

• Two types of electrical charge: positive and negative

• A potential difference (EMF) is needed for a current to flow

• Current (amps) is equal to potential difference across a device (volts) divided by resistance (ohms)

• Devices can be arranged in either series of parallel

• In a series circuit the current passing through each device is the same, while in a parallel circuit the voltage across each device is the same

• The rate at which electrical energy is converted into work or heat is the power (P = IV)

• The cost of running a device is equal to the power delivered multiplied by the time of use multiplied by the cost per unit energy

Fig. 10-15, p. 351

Figure 10.15: Three-way switch.

Fig. 10-16a, p. 354

Figure 10.16: The pith ball is at first attracted to the plastic rod; after making contact, it flies away in the opposite direction.

Fig. 10-17, p. 356

Figure 10.17: The comb has acquired a negative charge by being run through hair. It will attract a small piece of paper.

Fig. 10-18, p. 357

Figure 10.18: Personal electrification. Sliding on a car seat to get out can give you a net charge, which is discharged to the car door after you step out onto the ground. The shock results as the charge imbalance between you and the car is equalized.

Fig. 10-19, p. 357

Figure 10.19: Schematic diagram of an electrostatic precipitator for the removal of particulate matter from combustion gases. Note the large negative voltage V on the center wire.

Documents

Environmental Physics Chapter 10: Electricity, Circuits and Superconductors Copyright © 2012 by DBS