The Strong Force The Weak Force. The Electromagnetic Force Gravity The strong and weak forces are...

The Strong Force

The Weak Force.

The Electromagnetic Force

Gravity

The strong and weak forces are short range forces, that is, they only act over very small distances (like inside the nucleus of an atom). Gravity and the electromagnetic forces are long range forces and can act over very large distances.

Science knows of four forces that exist in nature.

It is approximately 3x1047 times stronger than gravity.

Actually, it’s not that the electric force is so strong, it’s that gravity is so weak.

Perhaps the most startling thing about the electric force is how strong it is compared to gravity

Rules of Electrostatics

1. There are two kinds of charge that exist in nature (positive charge and negative charge)

• unlike charges attract one another

• like charges repel one another.

2. The force between charges varies as the inverse square of the distance, and directly with the charges. (Coulomb’s Law)

3. Charge is conserved.

4. Charge is quantized. (quantized means small discrete packets that can not be further subdivided. For example, you can have 1 or 2 electrons, but never 1.5 electrons)

The basic unit of positive charge is the proton. (Although protons are ultimately made up of quarks)

The basic unit of negative charge is the electron. It is almost always electrons that are moving when charge “flows”

The SI unit of charge is the Coulomb ( C).

Charge of 1e- = 1 proton = 1.6x10-19 Coulombs

1) Protons are removed from the rod

2) Electrons are added to the rod

3) The fur is also charged negatively

4) The fur is left neutral

Jeff rubs a piece of fur on glass rod, giving the rod a negative charge. What is the most likely thing that happens?

Jeff rubs a piece of fur on glass rod, giving the rod a negative charge. What is the most likely thing that happens?

1) Protons are removed from the rod

2) Electrons are added to the rod

3) The fur is also charged negatively

4) The fur is left neutral

Conductors are materials in which charges are free to move. Metals are a good example.

Insulators are materials in which charges can not move. Glass, plastics, and wood, are examples.

Q

Conductors & Insulators

1) Low mass density 2) High tensile strength

3) Poor heat conductors 4) charges move freely

5) All of the above

Which of the following best characterizes electrical conductors?

Which of the following best characterizes electrical conductors?

1) Low mass density 2) High tensile strength

3) Poor heat conductors 4) charges move freely

5) All of the above

Consider the “pith” ball…

++

+ ++

+

- -

-

-

-

-

Charges are balanced, so the ball is neutral


++

+++

+

-

-

-

-

-

-

-- -- -- --


++

+++

+

-

-

--

-

--- -- -

- --- ---

Consider the electroscope


+++++++++ + + +


+

++

++

+

++

++

+ + +

Charging by conduction—A physical transfer of charge

Explain what happens in pictures & words

+++++ + + + +

+++

++

-- -- ---- -

+

+

++

++

++ Polarized but Still neutral

+++++ + + + +

+++

++

-- -- ---- -

+

+

++

++

++

+++++ + + + +

+++

++

Electrons transfer from the scope to

the rod

+

++

++

+

++

++

+ + +Scope is left positive.

I can tell it is charged because the leaves repel each other.


++++++++ ++ ++

Charging by Induction - A transfer of charge, but only two neutral objects touch

Be ready to explain what happens in pictures & words

++++++++ ++ ++

Charging by Induction

-- -- ---- -

+

+

++

++

++ Still neutral

++++++++ ++ ++


-- -- ---- -

+

+

++

++

++

++++++++ ++ ++


-- -- ---- -

-

- --

-These

charges remain held in place

These charges flow

from the ground to the electroscope.

+

+

++

++

+

++++++++ ++ ++

Charging by Inductionhttp://regentsprep.org/Regents/physics/phys03/aeleclab/induct.htm

-- -- ---- - These charges

now spread

out.

-

--

- -

-

-

The electroscope is now charged.

Explain what happens in pictures & words

http://regentsprep.org/Regents/physics/phys03/aeleclab/induct.htm

http://regentsprep.org/Regents/physics/phys03/aeleclab/induct.htm

An uncharged conductor is supported by an insulating stand. I pass a positively charged rod near the left end of the conductor, but do not touch it. The right end of the conductor will be…

1) Negative 2) Positive 3) Neutral

4) Attracted 5) Depends on the materials.


1) Negative 2) Positive 3) Neutral 4) Attracted 5) Depends on the materials.


1) Negative 2) Positive 3) Neutral 4) Attracted 5) Depends on the materials.

++++ +++ ++

++ +-- ---

Force between charges….

Charles Augustin de Coulomb (1736-1806)

F kQ1Q

2

d 2

Where: k = Coulomb’s Constant

(9.0x109 N m2/C2)Q = Charge in Coulombs

d = Distance between charges Q

Also seen as “q”

To determine the direction of the

force, look at the charges.

Coulomb’s Law

The significance of Coulomb’s Law goes far beyond the description of the forces acting between charged balls or rods. This law correctly describes the forces that…

* Binds the electrons of an atom to its nucleus* Binds atoms together to form molecules

* Binds atoms or molecules together to form solids and liquids

Coulomb’s Law

Two charges, + 2.0x10-5 C and –3.0x10-5 C are 5.0 m apart. Calculate the force between them.

F kQ

1Q

2

d 2

9109 (210 5 )( 310 5 )

52

= .22 N

Example

5.0 nC

-3.0 nC

-6.0 nC

0.30m0.10m

Another Example (draw the picture)Find the net force on the 5.0 nC charge.

5.0 nC

-3.0 nC

-6.0 nC

0.30m

0.10m

Q

F 3 (9109 )(510 9 )(310 9 )

(.1)21.3510 5 N

F-3

Another Example (draw the picture)Find the net force on the 5.0 nC charge.

F6

= - 1.05x10-5 N

= 1.05x10-5 N

1) ½ 2) 2 3) ¼ 4) 4 5) depends on

the charges

Two point charges are 4 cm apart. They are moved to a new separation of 2 cm. By what factor does the resulting mutual force between them change?

Two point charges are 4 cm apart. They are moved to a new separation of 2 cm. By what factor does the resulting mutual force between them change?

1) ½ 2) 2 3) ¼ 4) 4 5) depends on

the charges

Q

1) 9.0 2) 3.0 3) 1/3 4) 6.0 5) 1/9

If the size of the charge value is tripled for both of two point charges maintained at a constant separation, the mutual force between them will be changed by what factor?

If the size of the charge value is tripled for both of two point charges maintained at a constant separation, the mutual force between them will be changed by what factor?

1) 9.0 2) 3.0 3) 1/3 4) 6.0 5) 1/9

A field is an area or volume that has a number, representing some quantity, assigned to every location.

That number can be a scalar or a vector.

A football field has numbers assigned in one dimension

Define the concept of a field

A weather map is an example of a scalar field.

Every point on the map has a scalar quantity associated with it. In this case, it’s temperature.

For example, the temperature in NP

is about 75o

We can also define a field in terms of vectors.

Vector Field Examples - Has an amount and a direction associated with each position.

Example: Earth has a Gravitational Field

• Any charge will set up an electric field around it.

• It exerts an electric force on any other charged object within the field.

• It is defined as the force per positive charge.

Don’t confuse the charge that creates the field with the charge that reacts to the field

The Electric Field (E) is a vector field

Look at phet “Charges and Fields”

All electric charges set up an electric field around themselves. To determine the direction of an electric field at any given point, a positive point charge or test charge is used.

A positive point charge is like a point or an infinitely small spot that has a single positive charge.

Convention states that when testing an electric field, always use a positive point charge, never a negative one.

Draw the electric field around a positive charge.

+

Electric field for a positive charge

Draw the electric field for a negative charge.

Rules for Drawing Electric Fields

1. The lines must begin on positive charges and end on negative charges, or at infinity.

2. The number of lines drawn leaving a positive charge or approaching a negative charge is proportional to the amount of charge.

3. Field lines may not cross or touch each other.

4. Field lines must meet conductors or charges perpendicular to the surface of the conductor or charge.

This implies that the vector sum has two

values---which it can’t

Less ChargeExample: 2 charges

Q

Example: Charge & Plate

This is an equipotential line. Do not worry about it for now.

Back to Earth’s Gravitational Field:

Near the surface of the Earth, Earth’s gravitational field is 9.8 N/kg downwards, toward Earth’s surface.

Do larger masses experience a larger gravitational field from Earth?

What two variables does gravitational field depend on?

FgGM

1M

2

R 2M

1

GM2

R 2

Calculation of a Gravitational Field (on

Earth 9.8 m/s2)

Mass CREATINGthe field

Mass EXPERIENCING

the field

+

Consider a positive charge in space.

The Electric Field is used to describe the effect of this charge at

some point in space.

+


+

If a positive point charge (+1) is placed at that point, a force will be exerted on it by the original charge.

Let’s say, the force is 10 N.

10 N

+


Then we can say, whenever a charge is placed at that point, for every coulomb of charge, it will have a force of 10 N act on it.

We say the electric field at that point is 10 N per coulomb.

E = 10 N/C

+


In general, we say F = Q E

If a 3 C charge is placed there, the force on it would be 3 C x 10 N/C or 30 N.

QEF

221

d

QkQF

Qd

kQF

2

Therefore, this part of Coulomb’s Law must calculate the Electric Field

Q

Another equation for Electric Field:

According to Coulomb’s Law:

Another way to calculate electric force is this:

This charge CREATES the

field

This charge EXPERIENCES the field

The electric field produced by a point or spherical charge is given by….

2d

QkE

K = Coulomb’s Constant (9.0x109 N m2/C2)

Q = The charge producing the field. Given in Coulombs

d = The distance to the point in question

The direction of the electric field is based on the direction of force for a positive charge.

Q

.0025 C

+ 20.00 m

E 9.0109 .0025C

20.002m 2

2d

QkE

= 5.6x104 N/C

Q

What is the electric field 20.00 m to the right of a (+) 0.0025 C point charge?

E=?

1) 20.m 2) 5.0m 3) 25m 4) 40.m 5) 32m

The electric field produced by a point charge is 16 N/C at a distance of 10. m. At what distance will the field be 4.0 N/C?

The electric field produced by a point charge is 16 N/C at a distance of 10.m. At what distance will the field be 4.0 N/C?

1) 20.m 2) 5.0m 3) 25m 4) 40.m 5) 32m

E kQd 2

16N

C(9.010

9 Nm 2 /C 2 )Q

(10.m)2

Q 1.77810 7C

4.0NC

(9.0109 Nm 2 /C 2 )(1.77810 7C)

d 2

d 20.m

1

2

3

4

+ -

Two charges, +Q and –Q, are located two meters apart as shown. Which vector best represents the direction of the electric field at the point above them?


1

2

3

4

+ -


+ -

0.0 m

Two charges are along the x-axis. Q1 is 3.0 m from the origin and has a charge of -12.0µC. Q2 is 4.5 m from the origin and has a charge of +4.0µC. (all charges are along the positive x-axis)a) Calculate the electric field 8.0 m from the origin.

+Q2 = + 4.0x10-

6C

4.5 m

-Q1 = - 12.0x10-6C

3.0 m

E = ?

8.0 m

𝐸1=|𝑘𝑄1

𝑑2 |𝐸1=|9×109(−12.0×10−6)

5.02 |𝐸1=4320𝑁 /𝐶

𝐸2=|𝑘𝑄2

𝑑2 |𝐸2=|9×109(+4.0×10−6)

3.52 |𝐸2=2939𝑁 /𝐶

𝐸1=4320𝑁 /𝐶

𝐸2=2939𝑁 /𝐶

0.0 m

+Q2 = + 4.0x10-

6C

4.5 m

-Q1 = - 12.0x10-6C

3.0 m

E = ?

8.0 m

=

𝐸𝑡𝑜𝑡𝑎𝑙=1400𝑁 /𝐶

b) What force will a - 9.0 mC charge experience if it is placed 8.0 m from the origin?

0.0 m

+Q2 = + 4.0x10-

6C

4.5 m

-Q1 = - 12.0x10-6C

3.0 m

-Q3 = - 9.0x10-6C

8.0 m

E = 1381 N/C F = QE

F = (-9.0x10-6C)(1380N/C)F = 0.012 N

Two charges, - 4.0 μC and - 5.0 μC, are separated by a distance of 20.0 cm. What is the electric field intensity halfway between the charges?

Two point charges, separated by 1.5cm, have charges of +2 and -4C. Suppose we determine that 10 field lines radiate out from the +2C charge. If so, what might be inferred about the -4C charge with respect to field lines?

1) 20 radiate out 2) 5 radiate out 3) 20 radiate in 4) 10 radiate in 5) 5 radiate in

How could you determine the mass of

one M&M without opening the

M&M bag?

A few assumptions:

1. All M&Ms have the same mass.

2. There are no broken M&M’s in the bag

3. The mass of the package is minimal.

4. The number of M&M’s per pack varies considerably.

A quick thought experiment:Given: Several hundred packs of M&M’s

A Scale

Experiment with M & M's

0102030405060708090

100

1 2 3 4 5 6 7 8 9 10 11 12

Pack Number

Mass (

g)

Smallest difference equals 1.0 grams

This is exactly what was done to determine the charge of an electron. • In 1909 Robert Millikan

measured the charge of an electron using an oil drop experiment

• In 1923 he received the Nobel Prize for his work.

Darn oil drops make a mess!

Millikan’s set up:

Negatively charged oil drops are attracted to the positively charged plate. The more charge the oil drop has, the more attracted to the plate it is.

F = QE

F = mg

Charged oil drop in an electric field

Millikan varied the electric field in between the plates, until the electric force on the “target” oil drop balanced the force of gravity and the oil drop stayed suspended between the plates.

Example: A 3.2x10-8 kg oil drop is suspended in an electric field of strength 1.31x1011 N/C. (a) What is the charge on the oil drop? (b) how many extra electrons does the oil drop have?

FgF

e

mg QE

(3.210 8kg)(9.8ms 2)(1.311011 N

C)Q

Q 2.410 18C

1electron1.610 19C

15electrons

Shocking Fingers and Lightning Rods

On a regularly shaped object, charges are evenly spread.

On an irregularly shaped object, charge tends to accumulate at areas of largest curvature/smallest radii.

Big radius, small curvature

Small radius, large curvature

In other words, charge accumulates at

sharp points

Ben Franklin invented the lightning rod.

Q

At what point is the charge per unit area greatest on the surface of an irregularly shaped conducting object?

1) Where the surface curves inward

2) Where the surface is flat

3) Where the curvature is greatest (smallest radius)

4) Where the curvature is least (largest radius)

At what point is the charge per unit area greatest on the surface of an irregularly shaped conducting object?

1) Where the surface curves inward

2) Where the surface is flat

3) Where the curvature is greatest

4) Where the curvature is least

2. Faraday Cagehttp://www.youtube.com/watch?v=Zi4kXgDBFhw

http://www.youtube.com/watch?v=Zi4kXgDBFhw

Conducting Cup

Insulating Stand

The Electric Field inside a conducting surface is zero.


+Charged Ball


++++++ + + +

+++++-

---- - - --

----

Polarized Charged


+++++ + + +

+++++

Charge remains on the outside

only

The negative charges from the polarized inside get neutralized as the positive ball comes in contact with them. The charge from the positive ball is now left on the outside of the cup.

A negative charge comes close to the conductor and the conductor polarizes.

-

-

-

-

-

-

+

+

+

+

+

+

++

+

+

+

+

+

+

+

+

+

+

+

-

-

-

--

-

-

--

-

--

-

-

But not all the negative charge can accumulate on the far side because then the far side would be “too” negative so some stays in the center leaving the center neutral.

-

-

-

-

-

-

+

+

+

+

+

+

++

+

+

+

+

+

+

+

+

+

+

+

-

-

-

--

-

-

--

-

--

-

-

The rod touches, the electrons transfer, and the outside is left negative while the inside is still neutral.

+

+

+

+

+

+

++

+

+

+

+

+

+

+

+

+

+

+

-

-

-

--

-

-

--

-

--

-

-

------

-

--

- -

A faraday Cage is a metal enclosure in which charge will always flow to the outside, thus leaving the inside neutral.

Not only do Faraday cages block charge, they more importantly block electromagnetic radiation.

Applications of a Faraday Cage

1. Microwave Oven

2. Electronic shielding

3. Lightning protection

A faraday Cage is a metal enclosure in which charge will always flow to the outside, thus leaving the inside neutral.

During a lightning storm you are relatively safe in…

A metal framed building.

A car. (not a convertible however)

Watch Car being Struck by Lighting

The Strong Force The Weak Force. The Electromagnetic Force Gravity The strong and weak forces are...

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