chapter 32 electrostatics. electric charge and electric field static electricity – unmoving charge...

Chapter 32

Electrostatics

Electric Charge and Electric Field

Static Electricity – Unmoving charge Two types

Positive – lack of electrons Negative – excess electrons

Like charges - Repel Opposite Charges - Attract

Electric Charges

Charge can be induced by rubbing an object – View demonstrations

Charge is detected using an electroscope.

Charge can travel via a conductor. Poor conductors are insulators.

Force Exerted by Charges

Coulomb’s Law F = kQ1Q2/r2

k = 9 x 109 N•m2/C2

Positive solution – repulsion Negative solution - attraction

Sample Problem

Two charges, Q1 = +10 µC, and Q2 = -15 µC, are separated by 1.5 meters.

What is the electrostatic force acting between them?

SolutionF = kQ1Q2/r2 =

(9 x 109 N•m2/C2)(+10 x 10-6 C)(-15 x 10-6 C)/(1.5 m)2

= -0.6 N

Conductors and Insulators

A good conductor transfers charge easily.

A good insulator inhibits the transfer of charge.

A good conductor is a poor insulator and a good insulator is a poor conductor.

Creation of Static Charges

Two ways to create a static charge are: Charging by contact (friction) – when

electrons are transferred from one object to another by touching.

Induction – when a charge is transferred by bringing one object near another without actually touching

Chapter 33

Electric Fields and Potential

Electric Field

Field – Affect that acts at a distance, without contact Examples

Electric Field Gravitational Field

Electric Field Strength – E = F/q = kQ/r2

Sample Problem

Calculate the strength of an electric field at a point 30 cm

from a point charge Q = +3 µC

SolutionE = kQ/r2 =

(9 x 109 N•m2/C2)(+3 x 10-6 C)/(0.3 m)2

= 300000 N/C

Electrical Energy Electrical Energy is generated from other

forms of energy and transmitted over power lines and/or stored in batteries

Vocabulary Voltage (V)

Force in an electrical system; Volt = Work/Charge = W/q = Joule/Coloumb

Current (I) Rate in an electrical system = Charge/time = q/t

=Coloumb/sec = 1 Ampere

Energy in Electrical System

Volts =Work/charge = V =W/q Work is measured in joules (the same

as energy) Charge is measured in Coloumbs (C) The charge on an electron is 1.6 x 10-

19 C 1 V = 1 Joule/1 Coloumb

Work = Volts * Charge = Vq

Sample Problem

How much work is needed to move a 10 μC charge to a point where the potential is 70 V?

W = Vq = (70 V)(10 x 10-6 C) = 7 x 10-4 J

Electrical Energy Storage Electrical Energy can be stored in two

ways: Batteries

Long term storage, even flow of charge Storage ability measured in Volts

Capacitors Short term storage, releases charge all at once (boost

in charge) Storage capacity measured in Farads (F) 1 Farad = 1 Coloumb/Volt Mathematically Charge = Capacitance * Voltage = q

= CV

Chapter 34

Electric Current

Electric Current

Circuit – A continuous path connected between the terminals of a power source.

Current – Flow of Charge I = ΔQ/Δt Current is measured in

Coloumbs/Sec which is called an Ampere.

Electric Current

Electron Flow is from – terminal to + terminal.

Conventional Current is from + terminal to – terminal.

Sample Problem

A steady current of 2.5 Amps passes through a wire for 4 minutes. How much charge passed through any point in

the circuit?Solution

Q = IΔt (2.5 C/s)(240 s) = 600 C

Ohm’s Law

Resistance – how much the conductor slows down the flow of electrons through it.

Resistance is measured in Ohms (Ω)

Ohm’s law -In any Circuit:V = IR or R = V/I

Sample Problem

A small flashlight bulb draws a current of 300 mA from a 1.5 V battery. What is the resistance

of the bulb?SolutionR = V/I =

(1.5 V)/(0.3 A) = 5 Ω

Resistor Color Code Resistors are banded in order to

describe the amount of resistance they provide. Each resistor is banded with 4 stripes.

Band Represents

1 First Digit

2 Second Digit

3 Multiplier

4 Tolerance

Bright Black 0

Boys Brown 1

Remember Red 2

Our Orange 3

Young Yellow 4

Girls Green 5

Become Blue 6

Very Violet 7

Good Grey 8

Wives White 9

Gold 5%

Silver 10%

None 20%

Resistor Color Code

Sample Problem

Calculate the resistance of a resistor which is banded with the

following colors: Red, Green, Blue, Silver.Solution

Red = 2, Green = 5, Blue = 6 and Silver = 10% R = 25000000 ± 10%

OrR = 25 MΩ ± 10%

Electrical Power

Electrical Power is measured in Watts.

Power = current x voltageP = IV or P = I2R or P = V2/R

Since Energy is Power x Time electrical energy is often measured in Kilowatt•hours or power x time.

Chapter 35

DC Circuits

DC Circuits Batteries Connected in Series

Increase Voltage Et= E1 + E2 + E3. . .

Produce the Same Current It= I1 = I2 = I3. . .

Batteries Connected in Parallel Produce the Same Voltage

Et= E1 = E2 = E3. . . Increase Current

It= I1 + I2 + I3. . .

Sample Problem

Calculate the voltage and current when 3 batteries (1.5 V, 0.25 A are connected in

A) SeriesB) Parallel

Solutiona) Et= E1 + E2 + E3 =1.5 V + 1.5 V + 1.5 V = 4.5 V

It= I1 + I2 + I3= 0.25 A

b) Et= E1 = E2 = E3=1.5 V

It= I1 + I2 + I3=0.25 A + 0.25 A + 0.25 A = 0.75 A

DC Circuits

Resistance in SeriesRt=R1+R2+R3. . .

Resistance in Parallel

...1111

321 RRRRt

Sample Problem

Calculate the resistance when a 5 Ω, 6 Ω, and 3 Ω resistor are connected in

A) SeriesB) Parallel

Solution

a) Rt=R1+R2+R3 = 5 Ω+ 6 Ω+ 3 Ω = 14 Ωb)

Rt= 1.43 Ω

30

21

30

10

30

5

30

6

3

1

6

1

5

11111

321 RRRRt