Slide 1 © 2005 Baylor University EGR 1301

Lecture 21Introduction to Engineering

Approximate Running Time - 23 minutesDistance Learning / Online Instructional Presentation

Presented byDepartment of Mechanical Engineering

Baylor University

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Slide 2 © 2005 Baylor University EGR 1301

Lecture 21:Electrical Engineering Topic 1

Voltage and Current

Professor Brian Thomas Speaking

Slide 3 © 2005 Baylor University EGR 1301

Outline

• Electrical Engineering: Information and Power

• Electricity Basics– Charge– Current, Voltage, – Power– AC, DC, grid power

Slide 4 © 2005 Baylor University EGR 1301

Information and Power

• Information– Radio– Television– Internet– Cellular telephone– Satellites– Embedded systems– Fiber optics, lasers

• Power– Lighting– Appliances– Motors– Heating– Welding &

manufacturing– On grid / off grid– EMP

Slide 5 © 2005 Baylor University EGR 1301

Electricity Basics

• Charge– The smallest bit of charge is the charge of an

electron– Charge flowing is called “Current”– Charge accumulated produces “Voltage”

Slide 6 © 2005 Baylor University EGR 1301

Electric Current

“Free” electron

Atom

• Electrons are free to flow in metals and other “conductors”

• Materials where charge may not flow are “insulators”

• An electron removed from a neutral atom leaves behind a “hole”

• By convention: electrons have negative charge, protons (and holes) have positive charge

Slide 7 © 2005 Baylor University EGR 1301

Electric Current

• 1 “Coulomb” of charge is 6,241,506,360,000,000,000 electrons

• If 1 Coulomb flows through a wire in 1 second, we say the current is 1 ampere or 1 “amp”

• Current always “flows through”

Slide 8 © 2005 Baylor University EGR 1301

Current Magnitudes

• Brain cell synaptic currents (10-13 A)

• Integrated circuits currents (10-4 A)

• Threshold of human sensation (10-3 A)

• Causes Ventricular fibrillation (10-1 A)

• Household appliances (10 A)

• Large Industrial Equipment (100 A)

• Lightning Bolt (1000 A)

Slide 9 © 2005 Baylor University EGR 1301

Electrostatic Forces

• Electrostatic force gives rise to stored energy

• Stored energy mental images– Stretched rubber bands– Compressed springs– Water pressure (or height

of column of water)

2

212

0

221

41

m-N10*854.8

)(0

C

r

qqF

Slide 10 © 2005 Baylor University EGR 1301

Electrostatic Force Example

f

2

m-N

12

2

20

21

lbbillion 2.02 Nbillion 99.8

)m1(10*854.8*4

1

)1(4

)1)(1(

1

1

if electrons, of

handfuleach on force theCalculate :Example

2

2

F

C

m

CCF

mr

Cqq

C

1q 2q

F F

r

Slide 11 © 2005 Baylor University EGR 1301

Voltage

• Voltage arises from accumulated charge being separated

• Opposite Charges Attract– Positive likes negative and visa versa – Energy is required to keep them apart

• Like the ends of a rubber band• Rubber bands can be stretched to different lengths

• “Voltage” is a measure of how much energy each coulomb of charge contains

Slide 12 © 2005 Baylor University EGR 1301

• 1 “volt” means each coulomb of charge can release 1 Joule of energy

• A 12 Volt battery will produce 12 Joules of energy for every coulomb of electrons delivered

Voltage

JoulesCqjoulesVolts

CoulombscoulombsAmps

000,180 1500*)/( 120

1500sec)/(15sec*100

dt

dqi

120 VAC

Slide 13 © 2005 Baylor University EGR 1301

• Voltage exists between two points, not at an individual point

• The earth is a common reference point– “Ground”– Voltages with respect ground are like rubber

bands with one end on the ground

Voltage

Slide 14 © 2005 Baylor University EGR 1301

Power

• Power is the rate at which energy is transferred

• Electric Power is found by multiplying voltage and current

Watt1second

Joule

time

Energytime

Coulomb*

Coulomb

Energy

Current*Voltage

Slide 15 © 2005 Baylor University EGR 1301

Power

• Question: How much power is supplied to a belt sander that draws 15 amps using “wall” voltage?

• Answer:– 120 Volts x 15 Amps = 1800 Watts

• 1800 W = 1.8 kW

Slide 16 © 2005 Baylor University EGR 1301

AC, DC, and all that

• DC stands for “direct current” and means the voltage and current are constant– All batteries supply DC power– “Cigarette Lighter Power” is DC

• AC stands for “alternating current” and means that the voltage and current “alternate” back and forth

• Grid-supplied electric power is AC, cycling 60 times every second, or at 60 Hz.

Slide 17 © 2005 Baylor University EGR 1301

This Concludes Lecture 20