Electricity

What is electricity?

Electricity is a natural phenomenon that occurs throughout nature and

takes many different forms.

To understand the fundamentals of electricity, we need to begin by

focusing in on atoms, one of the basic building blocks of life and

matter.

Structure of the atom

Atoms consist of a nucleus and electron orbital shells

Particles in the nucleus

Neutrons – neutral charge (green)

Protons – positive charge (orange)

Particles in the shells

Electron – negative charge (yellow)

Charge of particles

The fundamental electrical property to which the mutual

attractions or repulsions between electrons or protons is

attributed is called charge.

By convention, electrons are negatively charged and

protons positively charged.

Neutrons have no charge, and are neither attracted nor

repelled by charged particles.

Important facts about atoms

1. Every atom has a positively charged nucleus surrounded by negatively charged

electrons.

2. All electrons are identical.

3. The nucleus is composed of protons and neutrons. All protons are identical;

similarly, all neutrons are identical.

4. Atoms usually have as many electrons as protons, so the atom has zero net charge.

Important facts about atoms

Note: A proton has nearly 2000 times the mass of an

electron, but its positive charge is equal in magnitude to

the negative charge of the electron.

Fundamental Rule of Charge

1. Like charges repel

2. Opposite charges attract

Neutral Atoms

Electrons and protons have electric charge.

In a neutral atom, there are as many electrons as

protons, so there is no net charge.

2 electrons = 2 negative

2 protons = 2 positive

Net charge is zero = stable

Charged objects

An object that has unequal numbers of electrons and protons is

electrically charged.

If an electron is removed from an atom, the atom is no longer neutral. It has one more positive charge than negative charge.

There is an imbalance of charges.

A charged atom is called an ion.

A positive ion has a net positive charge; it has

lost one or more electrons.

A negative ion has a net negative charge; it

has gained one or more extra electrons.

How electrons are transferred

Electrons are being transferred by friction

when one material rubs against another.

Electrons can also be transferred from one

material to another by simply touching.

Phet simulation

https://phet.colorado.edu/sims/h

tml/balloons-and-static-

electricity/latest/balloons-and-

static-electricity_en.html

Force between charged objects

The electrical force between any two objects obeys a

similar inverse-square relationship with distance.

The relationship among electrical force, charges, and

distance—Coulomb’s law—was discovered by the

French physicist Charles Coulomb in the eighteenth

century.

Coulomb’s Law

For charged objects, the force between the charges varies

directly as the product of the charges and inversely as the square

of the distance between them.

Where:

d is the distance between the charged particles.

q1 represents the quantity of charge of one particle.

q2 is the quantity of charge of the other particle.

k is the proportionality constant (8.99 x 109 Nm2/C2)

Coulomb

The SI unit of charge is the coulomb, abbreviated C.

A charge of 1 C is the charge of 6.24 × 1018 electrons.

6,240,000,000,000,000,000 electrons

A coulomb represents the amount of charge that passes through a

common 100-W light bulb in about one second.

Conductor and Insulator

Materials through which electric charge can flow are

called conductors.

Outer electrons of the atoms in a metal are not anchored

to the nuclei of particular atoms, but are free to roam in the

material.

Metals are good conductors for the motion of electric

charges because their electrons are “loose.”


Insulators are materials that tightly bound their electrons

to the nucleus and are not free to wander.

Materials such as rubber or glass.

These materials are poor conductors of electricity.


Electrons move easily in good conductors and

poorly in good insulators.

Induction

If a charged object is brought near a conducting surface, even

without physical contact, electrons will move in the conducting

surface.

Induction

Charging by induction can be illustrated

using two insulated metal spheres.

Uncharged insulated metal spheres touching

each other, in effect, form a single

noncharged conductor.

Induction

When a negatively charged rod is held near one

sphere, electrons in the metal are repelled by

the rod.

Excess negative charge has moved to the other

sphere, leaving the first sphere with an excess

positive charge.

The charge on the spheres has been

redistributed, or induced.

Induction

When the spheres are separated and the

rod removed, the spheres are charged

equally and oppositely.

They have been charged by induction,

which is the charging of an object

without direct contact.

Electric Field

It is a region around a charged particle or object

within which a force would be exerted on other

charged particles or objects.

Electric Field

The space around a concentration of

electric charge is different from how it

would be if the charge were not there. If

you walk by the charged dome of an

electrostatic machine—a Van de Graaff

generator, for example—you can sense

the charge. Hair on your body stands

out—just a tiny bit if you’re more than a

meter away, and more if you’re closer. The

space is said to contain a force field.

Electric Field

An electric field has both magnitude and direction. The magnitude can be

measured by its effect on charges located in the field.

Imagine a small positive “test charge” placed in an electric field.

Where the force is greatest on the test charge, the field is strongest.

Where the force on the test charge is weak, the field is small.

Direction of the field

The direction of an electric field at any

point, by convention, is the direction

of the electrical force on a small

positive test charge.

If the charge that sets up the field is

positive, the field points away from

that charge.

If the charge that sets up the field is

negative, the field points toward that

charge.

https://phet.colorado.edu/sims/html/charges-

and-fields/latest/charges-and-fields_en.html

Calculating the Electric Field Strength

E = electric field strength

Q = charge

d = distance

k = 9.0 x 109 Nm2 / C2

(technically: 8.99 x 109)

Examples

The electric field strength in a region is 2,200 N/C. What is the force on an

object with a charge of 0.0040 C?

Example

The electric field strength in a region is 2,200 N/C. What is the force on

an object with a charge of 0.0040 C?

What equation are we going to use?

E = F / q

E = 2200 N/C

q = 0.0040 C

F = ?

Rearrange the equation and we get F = Eq

F = (2200)(0.0040) = 8.8 N

Example

If two charges (q1= 2.3mC & q2=1.0mC) are placed 0.50m apart what

force is experienced by q1? By q2?

Example

If two charges (q1= 2.3mC & q2=1.0mC) are placed 0.50m apart what

force is experienced by q1? By q2?

𝐹 = 𝑘𝑞1𝑞2𝑑2

𝑘 = 9.0 × 109𝑁𝑚2/𝐶2

𝑞1 = 2.3𝑚𝐶𝑞2 = 1.0𝑚𝐶𝑑 = 0.5𝑚

( )

NF

F

d

qqkF

82800

5.0

)0010.0)(0023.0(100.9

2

9

2

21

=

=

=

Example

What is the field strength 2.0m away from a -0.060C charge? Is the field directed

towards or away from the charge?

Example

What is the field strength 2.0m away from a -0.060C charge? Is the field

directed towards or away from the charge?

md

CQ

CNmk

d

kQE

2

060.0

/100.9 229

2

=

−=

=

=

CNE

E

d

kQE

/1035.1

2

)060.0)(100.9(

8

2

9

2

−=

−=

=

Since the charge is negative, the direction would be towards the charge.

Question

1. What is electricity?

2. What are the three particles contained in an atom?

3. What is an ion?

4. What are the two ways electrons can be transferred?

5. What is the equation for Coulomb’s Law?

6. What is the difference between a conductor and an insulator?

7. What is an electric field?

8. What are the equations for the strength of an electric field?

Electricity - MYP PHYSICS

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