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Electrical Power

- From Power Plant to Toaster

Electrical Power

- Electrical power is transferred by electrons

through a circuit - The total power consumed by an appliance depends

on how much voltage and current it uses - In equation form
- P I x V
- P is power in watts
- I is current in amps
- V is voltage in volts

Power Loss in a Resistor

- When electric current passes through a resistor,

it converts power to heat - Using Ohms law, the power loss in a resistor is
- P I2 x R
- where
- P is power in Watts
- I is current in Amps
- R is resistance in ohms

Appliance Ratings

- Most appliances are labeled so that one can

calculate their power consumption - Two types of labeling
- Power and voltage
- Voltage and current
- Power, voltage, current and resistance can all be

calculated from the appliance label!

Example 1

- Here, voltage (120 V) and power (18 W) are

labeled - What current does this VCR use and what is its

resistance? - Current from P IV we know I P/V so
- I (120 V)/(18 W) 6.7 A
- Resistance from V IR we know R V/I so
- R (120 V)/(6.7 A) 18 ?

Example 2

- Here, voltage (120 V) and current (300 mA) are

labeled - How much power does this VCR use and what is its

resistance? - Power from P IV
- P (120 V)/(0.3 A) 36 W
- Resistance from V IR we know R V/I so
- R (120 V)/(0.3 A) 400 ?

Basic Laws of Electricity

Ohms Law V IR

Electric Power P VI

Power Loss P I2 R

- In many appliances, lost power is actually

converted into useful energy - Example A stereo converts most of the power

loss to sound

AC and DC

- In a dc (Direct Current) electrical system, the

voltage remains constant - In an ac (Alternating Current) electrical system,

the voltage fluctuates rapidly from positive to

negative values - In a household electrical outlet (which is AC),

the voltage fluctuates from -120 V to 120 V

sixty times every second

Why Two Types of Current?

- dc is better when
- an appliance needs to run on batteries because

batteries produce dc currents - an appliance uses integrated circuits (e.g.

computer chips) because most chips are designed

to run with 5 V or 12 V dc - ac is better when
- high voltages are required because the voltage

can be changed using a simple device called a

transformer - the appliance needs to run a motor because ac

motors are easier to build

A Brief History of Electricity

- Electricity was first studied in detail in the

18th century - Many practical uses for electricity (e.g. the

light bulb) were invented by Thomas Edison in the

late 19th century - Edison (an inventor) co-founded Edison Electric

(now General Electric), which used dc

- Nikola Tesla (a physicist) helped to found

Westinghouse, which used ac - To prove that ac is dangerous, Edison built an

electric chair (which was used) - Eventually, ac became the industry standard

because ac can be efficiently transferred over

large distances

How to Transfer Electrical Power

- Wires used to transfer electrical power have a

small resistance R - Power loss (P I2R) is greater when the current

I is large - High voltage is better because more power can be

transferred with less current

Example 1

- A power plant want to transfer 1 MW through a

line with a resistance of 1 ? - Case 1 The voltage is 1200 V, what is the power

loss? - Answer First find the current using P IV
- which is the same as I P/V
- so I (1,000,000 W)/(1200 V) 833 A
- the power loss is P I2R
- so P (833 A)2(1 ?) 693,000 W !!!!

Example 2

- A power plant want to transfer 1 MW through a

line with a resistance of 1 ? - Case 2 The voltage is 120,000 V, what is the

power loss? - Answer First find the current using P IV
- which is the same as I P/V
- so I (1,000,000 W)/(120,000 V) 8.33 A
- the power loss is P I2R
- so P (833 A)2(0.1 ?) 69.4 W !!!!

Transformers

- Transformers are simple, reliable devices that

change the voltage - When voltage goes up, current goes down and visa

versa--power remains the same - Transformers only work for ac circuits
- Transformers are constructed by simply winding

wire around two ends of a rectangular piece of

metal

Voltage In

Voltage Out

Types of Transformers

- Step-up transformers convert low voltages to high

voltages used between power plant and power

lines - Step-down transformers convert high voltages to

low voltages used just outside your home and in

small appliances

Step-up

Step-down

How Electricity Gets to Your House

- A power plant generates 24,000 V
- A power station steps up the voltage to 230,000 V

(so it can be transferred through power lines) - A substation steps the voltage down to 100,000 V
- A distributing substation steps the voltage down

to 20,000 V - A step down transformer outside your house steps

the voltage down to 120 V and 240 V (240 V is for

large appliances like your washer and dryer)

Why not Transfer Electricity at a Billion Volts?

- For one thing, air is not a perfect insulator
- At very high voltages, arcs of lightning would

shoot between power lines and the ground - Safety is also an issue
- (Note birds can land on a bare wire because they

do not create a closed circuit, i.e. a path to

the ground)

Do Electromagnetic Waves from Power Lines Cause

Cancer?

- No because
- low frequency electromagnetic waves (i.e. 60 Hz)

cannot, even in theory, interfere with chemical

reactions - the intensity of the radiation is much less than

the intensity of radiation already present in the

body - Note cell phones (which operate at millions of

Hertz) cant cause cancer either

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