Chapter Chapter 2.3 Notes2.3 Notes

Work in Work in Electrical Electrical SystemsSystems

A force does work on an object A force does work on an object when it when it movesmoves the object. the object.

A collection of A collection of chargescharges creates creates an electric field.an electric field.

In an electrical system, when In an electrical system, when there is a difference between there is a difference between charges (positive and negative) charges (positive and negative) a force is exerted and causes a force is exerted and causes charges to move and does charges to move and does workwork..

There must be an electrical voltage There must be an electrical voltage between two points within a field between two points within a field so that a so that a forceforce can be exerted on a can be exerted on a charge placed in the field.charge placed in the field.

If you connect the two points, the If you connect the two points, the force causes the charge to force causes the charge to movemove from one point to the other.from one point to the other.

The presence of The presence of voltagevoltage is what is what causes the charge to move and to causes the charge to move and to do work.do work.

BatteriesBatteries are a source of voltage. are a source of voltage.

Electric charge is measure Electric charge is measure in in coulombscoulombs (C). (C).

A single electron has a A single electron has a charge of charge of 1.6 x 10 1.6 x 10 -19-19 C C

One Coulomb is equal to One Coulomb is equal to the charge of the charge of 6.25 billion 6.25 billion billionbillion electrons.electrons.

Electrical work Electrical work = = Change in Change in Voltage x Voltage x chargecharge

W = W = V x qV x q

• A battery is being charged. A battery is being charged. During the first 15 minutes of During the first 15 minutes of charging, 800 C of charge are charging, 800 C of charge are deposited in the battery by an deposited in the battery by an average potential difference average potential difference (voltage) of 8 V. How much (voltage) of 8 V. How much electrical work is done during this electrical work is done during this 15 minutes?15 minutes?

• W = W = V x qV x q• W = 8 V x 800 CW = 8 V x 800 C• W = 6400 VC = 6400 JoulesW = 6400 VC = 6400 Joules

The rate at which charges The rate at which charges flow is called flow is called electric electric currentcurrent

Electric current in measured Electric current in measured in coulombs/sec which is in coulombs/sec which is equal to an equal to an ampere.ampere.

A device measuring an A device measuring an ampere is called an ampere is called an ammeterammeter. .

• CurrentCurrent = = charge / charge / change in change in timetime

• I = q/I = q/tt

A 12 volt DC motor used a current of 4 A 12 volt DC motor used a current of 4 amperes. Find the electrical work done amperes. Find the electrical work done to operate this motor for 2 minutes.to operate this motor for 2 minutes.

q = I x change in tq = I x change in t Minutes to seconds; 2 minutes = 120 Minutes to seconds; 2 minutes = 120

secsec q = 4 A x 120 secq = 4 A x 120 sec q = 480 Cq = 480 C W = change in V x qW = change in V x q W = 12 V x 480 CW = 12 V x 480 C W = 5760 JW = 5760 J

List 4 things electrical List 4 things electrical work can do:work can do:

Produce MovementProduce Movement Create Thermal EnergyCreate Thermal Energy Create LightCreate Light Create SoundCreate Sound

Produce Produce MovementMovementMove fluids with Move fluids with

pumpspumpsElectric CarsElectric CarsElectric fansElectric fans

Create Thermal Create Thermal EnergyEnergyElectric HeatersElectric Heaters

Clothes DryersClothes DryersFurnacesFurnacesOvensOvensBlow DryersBlow Dryers

Create Create LightLight

Light bulbsLight bulbsLaserLaser

Produces Produces SoundSoundRadiosRadios

TV’sTV’sCD playersCD playersCell PhonesCell Phones

Not all electrical Not all electrical work is converted to work is converted to mechanical mechanical WorkWork..

Some of the Some of the electrical work is electrical work is lost to lost to thermal thermal energy (heat)energy (heat)

Efficiency = Efficiency = output output work / input workwork / input work

Percent Efficiency = Percent Efficiency = output work / output work / input workinput work x 100 % x 100 %

• The input work is 5760 Nm. The crane The input work is 5760 Nm. The crane lifts the bean that weighs 2000 N to a lifts the bean that weighs 2000 N to a height of 2.5 meters. What is the height of 2.5 meters. What is the output work? What is the efficiency? output work? What is the efficiency? What is the percent efficiency?What is the percent efficiency?

• Efficiency = output work / input workEfficiency = output work / input work• Output Work = Force x distanceOutput Work = Force x distance• Output Work = 2000 N x 2.5 mOutput Work = 2000 N x 2.5 m• Output Work = 5000 NmOutput Work = 5000 Nm• Efficiency = 5000 Nm / 5760 NmEfficiency = 5000 Nm / 5760 Nm• Efficiency = .87 or 87%Efficiency = .87 or 87%

Stop after Steam PlantStop after Steam Plant

Most electricity is produced at Most electricity is produced at electrical generating stations electrical generating stations by:by:

Burning coal, gas, or oil to Burning coal, gas, or oil to produce heatproduce heat

Heat converts water into steamHeat converts water into steam Steam turns a turbineSteam turns a turbine Turbine drives an electrical Turbine drives an electrical

generator to produce electricitygenerator to produce electricity

The overall efficiency of The overall efficiency of this process to produce this process to produce electricity is electricity is 60%60%..

Where would they lose the Where would they lose the majority of their efficiency?majority of their efficiency?

Lose the mechanical Lose the mechanical work to heatwork to heat