Journal · 2/19/2013 9 Section 20.2- Coulomb’s Law • Coulomb discovered that charges exert forces on other charges at a distance and that he force is stronger when the charges

Journal #

1. Identify the parts of the atom

and their charges.

2. When an atom loses electrons,

it becomes positive/negative

(CIRCLE ONE).

3. When an atom gains electrons,

it becomes positive/negative

(CIRCLE ONE).

4. An object that is positively

charged has more/less

(CIRCLE ONE) electrons than

protons.

5. An object that is negatively

charged has more/less

(CIRCLE ONE) electrons than

protons.

electron (-) proton (+)

neutron (0)

Journal # 1. Draw sketches of a charged electroscope and a

discharged electroscope.

2. What is the purpose of an electroscope?

3. Can an electroscope determine the charge of an

object (whether positive or negative)?

1. See diagram

2. The purpose of an

electroscope is to detect

charge.

3. No, an electroscope can

only detect charge. It

does not determine

whether the charge is

positive or negative.

2/19/2013 3

• In 1752, Benjamin Franklin flew a kite with

a key attached to the string. As a

thunderstorm approached, the loose

threads of the kite string began to stand up

and repel one another, and when Franklin

brought his knuckle close to the key, he

experienced a spark.

• Electrostatics is the study of electric

charges that can be collected and held in

one place.

Introduction to Electricity

2/19/2013 4

Section 20.1: Electric Charges

• With the addition of energy, the outer

electrons can be removed from

atoms.

• An atom who loses electrons has an

overall positive charge, and

consequently, any matter made of

these electron-deficient atoms is

positively charged.

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• A material that allows charges to

move about easily is called an electric

conductor. – Most metals, salt water, and acidic solutions are

good conductors.

• A material through which a charge will

not move easily is called an electric

insulator. – Glass, dry wood, most plastics, cloth, and dry air are

all good insulators and poor conductors.

Conductors versus Insulators

2/19/2013 6

Section 20.2: Electric Charges

• There are two kinds of electric charges:

positive and negative.

• Like charges repel; opposite charges

attract.

2/19/2013 7

• An electroscope consists of a metal

knob connected by a metal stem to

two thin, lightweight pieces of metal

foil, called leaves, and it detects

charge. It CANNOT determine what

type of charge.

The Electroscope

2/19/2013 8

• Charging a neutral body by touching

it with a charged body is called

charging by conduction.

• Charging without actually touching

the body is called charging by

induction.

Conduction versus Induction

2/19/2013 9

Section 20.2- Coulomb’s Law

• Coulomb discovered that charges exert forces

on other charges at a distance and that he force

is stronger when the charges are closer

together.

• The SI standard unit of charge is called the

coulomb (C).

• The charge on a single electron is 1.60×10−19 C.

The magnitude of the charge of a single electron

is called the elementary charge. There is no

charge less than this and all charges are

multiples of this number.

2/19/2013 10

Section 21.1-Electric Fields • The electric field extends around any charged

particle and can produce a force capable of

doing work.

• The field is invisible to the eye, so to represent it

on paper we use electric field lines.

– The field is strong where the lines are close together.

It is weaker where the lines are spaced farther apart.

– Force lines in an electric field are drawn away from a

positive charge and toward a negative charge.

2/19/2013 11

Journal #

Complete the table below.

The charge of an electron is 1.60 x 10-19 C.

Object # of Electrons Charge

A 1.5 x 107 deficient electrons

B 4.3 x 108 excess electrons

C 2.7 x 1010 deficient electrons

D 3.1 x 103 deficient electrons

E 7.8 x 1028 excess electrons

2/19/2013 12

Journal #

Object # of Electrons Charge

A 1.5 x 107 deficient electrons +2.4 x 10-12 C

B 4.3 x 108 excess electrons -6.9 x 10-11 C

C 2.7 x 1010 deficient electrons +4.3 x 10-9 C

D 3.1 x 103 deficient electrons +5.0 x 10-16 C

E 7.8 x 1028 excess electrons -1.2 x 1010 C


The charge of an electron is 1.60 x 10-19 C.

2/19/2013 13

• A person touching the

terminal of a Van de

Graaff machine is

charged electrically.

• The charges on the

person’s hairs repel each

other, causing the hairs

to follow the field lines as

shown in the animation.

Van De Graaff Generator

2/19/2013 14

Van de Graaff Generator Demo

• What happens when a conductor is

brought near to the dome?

• What effect does the generator have on a

fluorescent light bulb? Why?

Charging by CONDUCTION Charging by INDUCTION

Picture

To Touch or Not to Touch

What happens to the initial charge of the

charged object?

What is the final charge of the neutral object being charged

compared to the original charged

object?

Charging by CONDUCTION Charging by INDUCTION

Picture

To Touch or Not to Touch Touch No Touch

What happens to the initial charge of the

charged object? Loses a little of the initial charge Keeps all of the initial charge

What is the final charge of the neutral object being charged

compared to the original charged

object?

Same as the initial charge Opposite of the initial charge

2/19/2013 17

Chapter 21 Continued

• Electric potential difference, ΔV, is the change

in potential energy from one place to another.

Sometimes, the electric potential difference is

simply called the voltage because it is measured

in the unit of volts (V).

• The term volt (and voltage) comes from the

name of the inventor of the battery, Alessandro

Volta, an Italian physicist.

2/19/2013 18

• A battery converts

chemical potential energy to

electric energy.

• A common battery is made

from multiple layers of

different metals surrounded

by an electrolyte (either

acidic or alkaline) solution.

• FYI… there are many types

of batteries that use

different chemistry to

produce a potential

difference.

Batteries

2/19/2013 19

• The rate of flow of electric charge, called

electric current, is measured in amperes, A.

• Power, which is measured in watts, W,

measures the rate at which energy is

transferred.

• The property determining how much current

will flow is called resistance. The resistance

of the conductor, R, is measured in ohms (Ω).

Current, Power, and Resistance

2/19/2013 20

Current and Circuits

• Electric circuits provide the means to transfer

large quantities of energy over great distances

with little loss.

• When two conductors touch, charges flow from

the conductor at a higher potential to the one at

a lower potential.

• The flow continues until there is no potential

difference between the two spheres.

2/19/2013 21

P = IV V=IR (Ohm’s Law)

• P = power

• I = current

• V = potential difference aka voltage

• R = resistance

Important Formulas

Resistance

• A resistor is a device designed to

have a specific resistance.

• In wires, resistance increases

under the following conditions:

– Longer wires

– Thinner wires

– Higher temperatures

2/19/2013 Template copyright 2005 www.brainybetty.com 23

Diagram Symbols


Practice Question

A 30.0-V battery is connected to a

10.0 Ω resistor.

A. Draw the circuit.

B. What is the current in the circuit?


Practice Question

• V = 30.0 V

• R = 10 Ω

• I = ?

A

30.0V

10.0Ω


Practice Question

R

V I

10.0

V 30.0 I

I = 3.00 A

Journal #


Quantity Definition Symbol Unit

Current

Resistance

Potential Difference

Power

Journal #


Quantity Definition Symbol Unit

Current the flow rate of electrons I amperes (A)

Resistance a property that determines how

much current will flow R ohms (Ω)

Potential Difference the change in potential energy

required for charge to flow V volts (V)

Power the rate at which energy is

transferred P watts (W)

Types of Circuits

• Series circuit (single path)

• Parallel circuit (poly path)

2/19/2013

Series Circuit • A circuit in which all current travels

through each device, is called a

series circuit.

2/19/2013

Series Connection

A connection with

only one current path

is called a series

connection. An

ammeter is

connected to a circuit

in series.

2/19/2013

Parallel Circuit • A circuit in which there are several

current paths is called a parallel

circuit.

2/19/2013

Parallel Connection

When a voltmeter is

connected across

another component, it is

called a parallel

connection because the

circuit component and

the voltmeter are

aligned parallel to each

other in the circuit, as

diagrammed in the

figure.

2/19/2013 34

Series: • Current is the same at all

point in the circuit.

• Voltage drops each time

the current passes through

a resistor until it reaches

zero volts.

• same current and different

voltage

• If one bulb goes out, all

others in series with it will

go out.

Parallel: • Current depends on the

resistance and will always

flow down the path of least

resistance. The total current

prior to the branches is the

sum of the current in each

of the branches.

• Voltage is the same in each

branch.

• same voltage and different

current

• If one bulb goes out, the

others remain on.

2/19/2013

Series:

• Resistance can be

found 2 ways:

Parallel:

• Equivalence

resistance must be

found for the parallel

branches by using

the following

method:

R V

I

Rtotal R1 R2 ...

1

Rtotal

1

R1

1

R2

...

Useful when you don’t have a

calculator… I’ll show you a faster

way with a calculator.

Practice Question #1

1. What type of circuit does the diagram represent?

2. What is the equivalence resistance for the circuit?

3. What is the total current in the circuit?

4. What is the voltage drop across each resistor?

2. Finding Equivalence

Resistance Use the formula

Rtotal = R1 + R2 + R3

Rtotal = 2Ω + 3Ω + 4Ω

Rtotal = 9Ω

3. Finding Total Current

Use the formula

V = IR so I = VTotal/R

I = 12V/9Ω

I = 1.3A

4. Finding Voltage Drops

Use the formula

V = IR and use each individual resistance and total current so

V1 = IR1 = (1.3A)(2Ω) = 2.7V

V2 = IR2 = (1.3A)(3Ω) = 4V

V3 = IR3 = (1.3A)(4Ω) = 5.3V Check:

V1 + V2 + V3 = 12V

2.7V+ 4V + 5.3V = 12V

Practice Question #2

1. What type of circuit does the diagram represent?

2. What is the equivalence resistance for the circuit?

3. What is the total current leaving the generator?

4. What is the current through each resistor?

Use the formula,

1

Rtotal

1

R1

1

R2

1

R3

in a calculator... same as

Rtotal (R1

1 R2

1 R3

1)1

Rtotal (241 91 61)1

Rtotal 3.13

2. Finding Equivalence

Resistance

Use the formula,

?

V I Rtotal

V

Rtotal I

120V

3.13 38.3A

Notice how we

have used the

equivalence

resistance from

the previous

question

3. Finding Total Current

Use the formula three times, once for each resistor,

?

?

?

I V

R1

120V

24 5.0A

4. Finding Current in Each

Branch

I V

R2

120V

913.3A

I V

R3

120V

6 20A

The sum

of the

three is

equal to

the total

found

earlier

SERIES PARALLEL

Total Current

stays constant

Itotal = I1 = I2

adds up

Itotal = I1 + I2

Total Voltage

adds up

Vtotal = V1 + V2

stays constant

Vtotal = V1 = V2

Total Resistance

increases

Rtotal = R1 + R2

decreases

Rtotal = (R1-1 + R2

-1)-1

2/19/2013

Journal #

1. What type of circuit does the diagram below represent?

2. If R1 = 3.0Ω, R2 = 4.0Ω, and R3 = 5.0Ω, what is the

equivalent resistance of the circuit?

3. If the electric potential difference of the battery is 12 V,

what is the voltage across each resistor?

4. What is the current flowing in each branch?

R1 = 6.0 Ω, R2 = 4.0 Ω, R3 = 3.0 Ω, and V = 12V

Journal #

1. In which circuit is the current constant?

2. In which circuit is the voltage drop across

each resistor the same?

3. In which circuit is the current the greatest?

A B

2/19/2013

Journal #

1. How many batteries are drawn in the circuit?

2. What is the equivalent resistance in this circuit?

3. What is the voltage drop across each resistor?

4. What is the current in each resistor in the

circuit?

5. What is the power dissipated by each resistor?

R1 = R2 = R3 = 4.0 Ω

V = 12V

Documents

Journal · 2/19/2013 9 Section 20.2- Coulomb’s Law • Coulomb discovered that charges exert forces on other charges at a distance and that he force is stronger when the charges