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Chapter One

Electrostatic

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Objective

Definition

Overview

Electrification(Charging)

Conservation of Charges

Coulomb’s Law

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Definition

Electrostatics: is the study of electric charges at rest.

atom

–

nucleus

Electrostatic phenomena arise from the forces that electric charges exert on each other. Such forces are described by Coulomb's law.

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Overview

Everything has electric charges, both positive (protons), and negative (electrons).
Usually, objects have an equal number of positive and negative charges, therefore they are neutral.
If an object loses electrons, it will become positively charged object.
If an object gain electrons, it will become negatively charged object.
Only electrons (-) move, but protons (+)do not move.

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Overview

Law of Electric Charges:

Unlike charges attract each other.
Like charges repel each other.

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Overview

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Types of Electrification

Electrification: is the process of charging objects by adding or removing electrons.

Electrification (Charging) by friction.
Electrification (Charging) by contact (conduction).
Electrification (Charging) by induction.

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Types of Electrification :

Electrification by friction:
If we rub an object (A) with another object (B), then electrons transfer from one of them to the other; causing the two bodies to become charged.
If electrons move from A to B, then: body (A) loses electrons (A has deficit of electrons) to become positively charged, and body (B) gains electrons (B has excess in electrons) to become negatively charged.

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Types of Electrification :

At the end of friction A attract B (unlike charges).

During rubbing piece of glass with piece of silk, electrons transfer from glass to silk.

During rubbing piece of plastic with piece fur, electrons transfer from fur to plastic.

Notes:

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Types of Electrification :

Example 1: charging by friction:

we are Rubbing an ebonite rod(Kind of rubber) with fur piece.

Explain what happens ?

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Types of Electrification :

Solution of Example 1:

After rubbing, the ebonite rod with fur piece, some electrons moves from the fur piece to the ebonite rod, then:

The ebonite become containing an excess of electrons (negatively charged).
And the fur become containing a deficit in electrons (positively charged).

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Types of Electrification:

Electrification by contact (Conduction):

When a negatively charged body (A) comes in contact with neutral body(B), electrons can move from body(A) to the neutral body(B); Then the two bodies (A) and (B) become negatively charged.

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Types of Electrification:

Example 2: Electrification by contact:

Suspend a neutral metalic ball by one end of an insulating string, the other end is fixed (this setup called pendulum). Put a negatively charged ebonite rod in contact with the neutral ball. Explain what happens?

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Types of Electrification:

Solution of Example 2:

electrons move from the ebonite rod to the ball. The ball becomes negatively charged. After contact both ball and the ebonite become negatively charged. they repel each other.

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Types of Electrification:

Example 3: electrification by contact:

we put a positively charged glass rod in contact with the neutral ball Suspend by an insulating string. Explain what happens?

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Types of Electrification:

Solution of Example 3:

The electrons move from the neutral ball to the glass rod.
After contact both ball and the glass rod becomes positively charged, so they repel each other.

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Types of Electrification:

Electrification by Induction:

During charging by induction, no contact occurs between the two objects. The charged object influences a neutral object. A Redistribution of charges occur in the electroscope.

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Types of Electrification:

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Types of Electrification:

Example 4: we put a negatively charged object (ebonite rod) close to a neutral object (ball of the electroscope). Explain what happens?

Solution of example 4:

the electrons on the negatively charged object repel the electrons on the neutral ball

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Types of Electrification:

Solution of Example 4:

Redistribution of charges occurs on the electroscope.
a temporary positive charge on the neutral object.
Then the leaves of the electroscope diverge.
When the ebonite rod is taken away; the two leaves keep the same divergence.

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Types of Electrification:

Example 5:

a positively charged object (glass rod) is brought close to a neutral ball of the electroscope. Explain what happens?

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Types of Electrification:

Solution of Example 5:

The positive charges of glass rod attract the electrons in the neutral object (electroscope).
The negative charge arranged on the surface of the ball (Redistribution of charges).
Then the leaves of the electroscope become positively charged then diverge.

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What is Grounding?

A charged object is grounded when it is connected to the earth through a connecting wire.

If a charged object is grounded, it will become neutral.

Note:

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How does Grounding occur?

When we touch a metal ball of positive charge:

–

What is Grounding?

The Metal ball becomes neutral.

electrons flow from the earth to the metal ball to neutralize it.

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How does Grounding occur?

Similarly, if the metal ball is of negative charge.

–

What is Grounding ?

electrons flow from the metal ball to the earth.

the metal ball becomes neutral.

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Conservation of electric charges:

Charge is never created or destroyed it is only transferred from one object to another.

The total number of charges of a body is conserved.

Law of conservation of charges:

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Conservation of electric charges:

Example 6:

Two identical charges A (q₁=-8×10^-6C) and B (q₂=5×10^-6C) are 10cm apart.

q₁and q₂ are placed in contact with each other.

Calculate the new charges of A and B using Law of conservation of charges.

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Conservation of electric charges:

Solution of Example 2:

Law of conservation of charges:

But because the two bodies are identical then:

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Coulomb’s law (law of interaction of bodies):

Statement of Coulomb’s law:

The force of interaction between the two charges is proportional to the product of absolute values of the charges and inversely proportional to the square of the distance separating them.

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Coulombs law (law of interaction of bodies):

Example 7: Force of Interaction

Given a point charge (A) of charge Q1 = +1.23x10^–8 C and a point charge (B) of charge Q2 = –2.0x10^–6C are separated by a distance r =30 cm.

calculate the attractive force between the two charges.

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Coulombs law (law of interaction of bodies):

Solution of Example 7:

Applying Coulomb’s Law:

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Grade 10 Solving problems 2019 – 2020

Exercise 1: Electrification

A plastic rod is rubbed against a piece of fur.

Indicate the type of electrification.
Describe what will happen during rubbing.

Specify the type of charge on the plastic rod and on the piece of fur after rubbing.

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Grade 10 Solving problems 2019 – 2020

Solution of Exercise 1:

Electrification by friction.
During rubbing a transfer of electron will take place from fur to plastic.
Fur: become have deficit of electrons, so it is positively charge.

And Plastic: become have excess of electrons, so it is negatively charged.

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Grade 10 Solving problems 2019 – 2020

Indicate the type of electrification of the electroscope.
Explain what will happen in the electroscope.

Exercise 2: Electrification

A negatively charged plastic rod approached near the knob of a neutral electroscope, but not touching it as shown in the adjacent figure.

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Grade 10 Solving problems 2019 – 2020

Solution of Exercise 2:

Electrification by induction.
A Redistribution of charges take place on the electroscope.

The electrons of the plastic rod will repel the electrons of the electroscope. The electrons will move from the knob to the leaves, so the leaves become negatively charged and diverge.

The knob have a deficit of electrons, so it become positively charged.

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Grade 10 Solving problems 2019 – 2020

Exercise 3: Electrostatic Force

Two identical metallic spheres (A) has an excess of 10¹⁵electrons, and (B) has a deficit of 2× 10¹⁴electrons, are 20 cm apart.

Given: K=9×10⁹ SI, and the elementary charge is e= 1.6×10^-19 C.

1. Calculate q_A and q_B the charges of (A) and (B) respectively.

2. Calculate the magnitude of the electrostatic force of interaction between (A) and (B).

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Grade 10 Solving problems 2019 – 2020

Solution of Exercise 3: Electrostatic Force

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Grade 10 Solving problems 2019 – 2020

Exercise 3: Electrostatic Force

The previous identical spheres (A) and (B) are now brought in contact and then separated.

Calculate the final charges q’_A and q’_B after contact.
Deduce the number of electrons (deficit or excess) corresponds for each sphere after contact.

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Grade 10 Solving problems 2019 – 2020

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Grade 10 Solving problems 2019 – 2020

Exercise 4: interaction of Charges

Given two identical balls (A) and (B) carrying charges q_A = 5×10^-6C and q_B = -3×10^-6C respectively. (A) and (B) are placed 50cm apart.

Draw, label and write the characteristics of the Forces between (A) and (B). Find their magnitudes.
What becomes the forces between (A) and (B) if:

The distance is r1= 25cm apart.
The distance is r2 = 100cm apart.

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Grade 10 Solving problems 2019 – 2020

Solution of Exercise 4:

Characteristics of F_A
Point of application: center of the point charge A
Line of action: horizontal.
Direction: to Right
Magnitude: FA = 540 × 10^-3 N.

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Grade 10 Solving problems 2019 – 2020

Solution of Exercise 4:

Characteristics of F_A
Magnitude: FA = 540 × 10^-3 N.

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Grade 10 Solving problems 2019 – 2020

Solution of Exercise 4:

The force FA become:
if d1 = 25cm = d/2 then:

If d2 =100cm =2d then:

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Time: 30min Quiz in physics 2019 – 2020

Exercise 1 (9 points): methods of Electrification

Two groups of students performed two experiments using the following materials: Piece of fur, glass rod, plastic rod and electroscope.

First Experiment: (4pts)

The first group of students rubs a neutral plastic rod with neutral piece of fur, 10⁹ electrons transfer from the fur to the plastic rod. (e =1.6x10^-19C)

What do we mean by “A neutral plastic rod”? ( 0.5point).
Name the method of electrification that is followed. (0.5 point)

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Time: 30min Quiz in physics 2019 – 2020

Solution of Exercise 1:

A neutral plastic rod: number of negative charges equal number of positive charges.(0.5 point).
Electrification by friction. (0.5 point).

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Time: 30min Quiz in physics 2019 – 2020

Exercise 1 (9 points): methods of Electrification

First Experiment: (4pts)

After rubbing, which body become positively charges and which one become negatively charged? Justify. (1 point)
Determine the charge (Q₁) carried by the plastic rod after rubbing. (0.5 point)
By applying law of conservation of the electric charges, determine the charge carried by the piece of fur after rubbing.(1.5 points).

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Time: 30min Quiz in physics 2019 – 2020

Solution of Exercise 1:

After rubbing; electrons transfer from fur piece to plastic rod then: the fur piece have deficit of electrons so it become positively charged, and plastic rod have excess of electrons so it become negatively charged. (1point).

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Time: 30min Quiz in physics 2019 – 2020

Solution Exercise 1:

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Time: 30min Quiz in physics 2019 – 2020

Exercise 1 (7.5 points): methods of Electrification

Second Experiment : (3.5pts)

The students put the positively charged plastic rod in touch with the knob of the neutral electroscope. The leaves become divergent.

Explain why the leaves become diverge. (1 point)
Name the method of electrification followed in this experiment. (0.5 point)
If the rod is moved away, what will happen to the leaves? (0.5 point)

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Time: 30min Quiz in physics 2019 – 2020

Solution of Exercise 1:

Second Experiment :

When the positively charged touches the neutral knob; the positive charges of the rod attract the negative charges of the electroscope, so negative charges move from the leaves to the surface of the knob leaving the leaves with positive charges, then the leaves diverge. (1 point)
Electrification by contact. ( 0.5point)
After moving the rod, the leaves still diverge, because they are positively charged. (0.5 point)

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Time: 30min Quiz in physics 2019 – 2020

Exercise 1 (7.5 points): methods of Electrification

Second Experiment : (3.5pts)

One student touches the knob of the above charged electroscope.
Name this method. (0.5 point)
What will happen to the leaves? Justify. (1 point)

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Time: 30min Quiz in physics 2019 – 2020

Solution of Exercise 1:

Second Experiment :

.
Grounding.
Electrons moves from the ground to the leaves so the leaves become neutral; then they collapse.

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Time: 30min Quiz in physics 2019 – 2020

Exercise 2 (6 points): methods of Electrification

Two identical point charges (A) and (B), placed on an isolated support. Given: Q_A = +5 x 10 ^{– 5}C and Q_B = – 7 x 10^–5 C; K = 9 x 10⁹ Nm²/C²; │e│ = 1.6 x 10^–19C.

The force of attraction between (A) and (B) is FA = FB =31.5N when the distance between them is d.

Determine which point charge has deficit of electrons, then Calculate its value.
Calculate the distance (d) separating (A) and (B).

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Time: 30min Quiz in physics 2019 – 2020

Solution of Exercise 2:

The point charge A has deficit of electrons because its charge is +5×10^-5C.

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Time: 30min Quiz in physics 2019 – 2020

Exercise 2 (6 points): methods of Electrification

Calculate the new force exerted by (A) on (B) if the distance separating them is doubled.
The charges (A) and (B) are left to touch each other. After a short period of time they are found to repel each other.
Explain the observed phenomena.
Calculate the new charge of (A) and (B).
Deduce the number of electrons transferred from (B) to point charge (A).

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Time: 30min Quiz in physics 2019 – 2020

Solution of Exercise 2:

When (A) touch (B); electrons transfer from (B) to (A).

the point charge (A) become negatively charged.

(A) And (B( repel each other.

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Time: 30min Quiz in physics 2019 – 2020

Solution of Exercise 2:

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Grade 10 physics 2019 – 2020

End of Chapter 1

Electrostatic interaction