4A1 Electrostatics

Question 1

Define:

Electric charge

Answer 1

Fundamental property of matter that determines how particles interact through electromagnetic forces.

Charged objects experience attraction or repulsion depending on their charge type and the presence of an electric field. The SI unit of electric charge is the coulomb (C).

Question 2

What are the two types of electric charge?

Answer 2

1. Positive
2. Negative

Protons are positively charged, while electrons are negatively charged.

Question 3

What is the charge of a single proton?

Answer 3

+1.6×10^(−19) C

This is known as the elementary charge (𝑒), the smallest unit of electric charge. An electron carries the same magnitude of charge but with a negative sign.

Question 4

Define:

Coulomb

Answer 4

Unit of electric charge in the SI system.

It is equal to the charge of approximately 6.24 × 10¹⁸ electrons. One Coulomb is defined as the charge transferred by a current of one ampere in one second.

Question 5

True or False:

Like charges repel, and opposite charges attract.

Answer 5

True

This principle is a fundamental law of electrostatics, as described by Coulomb’s Law.

Question 6

What does it mean that charge is quantized?

Answer 6

Electric charge always exists as an integer multiple of the elementary charge, ±1.602 × 10⁻¹⁹ C.

This fundamental property was demonstrated by Millikan’s oil-drop experiment.

Question 7

Fill in the blank:

A neutral object has ______ amounts of positive and negative charges.

Answer 7

equal

Neutrality is achieved when charges cancel out.

Question 8

Define:

Ion

Answer 8

Atom or molecule that has gained or lost one or more electrons, acquiring a net positive or negative charge.

Positive ions are called cations, while negative ions are called anions.

Question 9

How do electric charges behave during a thunderstorm?

Answer 9

Charges build up inside clouds, leading to lightning bolts when the forces become large enough.

Lightning carries a lot of charge and energy.

Question 10

What happens during electrical induction?

Answer 10

Charged object causes a redistribution of charges in a nearby neutral object without direct contact.

This is the principle behind charging by induction.

Question 11

Fill in the blank:

The total electric charge in an isolated system is _______.

Answer 11

conserved

This is the principle of charge conservation. Charges are merely transferred, not created.

Question 12

True or false:

Only conductors can experience induced charges.

Answer 12

False

Both conductors and insulators can experience induced charges, but the process differs.

Question 13

Why does a charged balloon stick to an insulating wall?

Answer 13

The wall’s molecules become polarized, and the positive charges within the molecules shift slightly toward the negative charged balloon.
## Footnote

This creates a localized attractive force, allowing the balloon to stick even though the wall is neutral. The charges in the insulating wall do not move freely but shift slightly within each molecule, causing the balloon to stick.

Question 14

Fill in the blank:

When a negatively charged rod is brought near a metal sphere, the _____ ______ in the sphere are repelled, leaving one side positively charged.

Answer 14

free electrons

Conductors allow free electrons to move, causing a redistribution of charge when near a charged object.

Question 15

How do conductors and insulators differ in electron movement and charge behavior?

Answer 15

• Conductors allow free movement of electrons, enabling charge flow.
• Insulators restrict electron movement, keeping charges localized.

• Conductors have free-moving electrons, allowing electric charge to flow easily, making them ideal for wires and electrical circuits.
• Insulators restrict electron movement, keeping charges localized and preventing charge flow, which makes them useful as electrical insulators.

Question 16

True or false:

The net charge of a system changes during induction.

Answer 16

False

Induction redistributes existing charges without altering the net charge.

Question 17

What happens to the total charge in a system when two objects are rubbed together?

Answer 17

It remains constant, but electrons are transferred from one object to the other, making one positively charged and the other negatively charged.

This process is called triboelectric charging.

Question 18

Fill in the blank:

When a negatively charged rod touches a neutral sphere, the sphere becomes _______ ______.

Answer 18

negatively charged

Electrons are transferred from the rod to the sphere.

Question 19

Explain the principle behind grounding an object.

Answer 19

Grounding provides a path for excess charge to flow to the Earth, neutralizing the object without violating charge conservation.

The Earth acts as an infinite reservoir of charge.

Question 20

Define:

Charge transfer

Answer 20

The process by which electric charge moves from one object to another.

Charge transfer can occur through various mechanisms.

Question 21

Explain the principle of charging by conduction.

Answer 21

It occurs when a charged object physically touches another object, transferring charge between them.

The charge transferred depends on the materials’ properties.

Question 22

What happens when you touch a metal doorknob after walking on a carpet?

Answer 22

You may feel a small electric shock because excess electrons that built up on your body while walking on the carpet suddenly transfer to the metal doorknob upon contact.

This is an example of charging by conduction, where direct contact enables the transfer of charge.

Question 23

Fill in the blanks:

Induction occurs when a charged object is brought near a neutral object, causing a redistribution of charges without _______ _______.

Answer 23

direct contact

Induction is a method of charging that relies on the influence of the electric field of a charged object.

Question 24

Define:

Electrostatic discharge

Answer 24

Rapid transfer of the electrical charge between two objects, equalizing the charge.

It often produces light, sound, or heat. It often occurs when charge build-up reaches a critical point.

Question 25

What causes a **static electricity discharge**?

Answer 25

When a charged object comes into contact with a conductor or another object at a different electric potential, **allowing excess charge to transfer suddenly**. ## Footnote This often results in a small shock, especially in dry environments.

Question 26

What does **Coulomb's law** state?

Answer 26

The **force is directly proportional to the product of the charges** and inversely proportional to the square of the distance between them. ## Footnote The equation is F = k * (q1 * q2) / r^2

Question 27

# True or false: **Coulomb’s law** applies only to charged particles in a **vacuum**.

Answer 27

False ## Footnote It can also be applied in media, but the permittivity of the medium must be considered.

Question 28

What **conditions** must be met to apply **Coulomb's law**?

Answer 28

Charges must be: * Stationary * Point particles * Non-overlapping ## Footnote These conditions ensure that Coulomb's law can be accurately applied without the complexities introduced by motion or extended objects.

Question 29

Explain the role of **distance** in **Coulomb’s law.**

Answer 29

The **force** between two charges decreases with the **square of the distance** between them. ## Footnote This is an example of an inverse square law, where doubling the distance reduces the force to one-fourth its original value.

Question 30

What is the significance of the **superposition** principle in Coulomb's law?

Answer 30

It allows the **calculation of the net force on a charge** due to multiple other charges by summing the individual forces. ## Footnote This principle is essential for analyzing systems with more than two charges.

Question 31

How does **Coulomb's law** compare to **Newton's law of gravity**?

Answer 31

Both are **inverse-square laws** and involve a product of a specific property (charge for Coulomb's law and mass for gravity). ## Footnote Unlike gravitational force, electrostatic force can be either attractive or repulsive.

Question 32

Why does the **gravitational force** dominate over the **electric force** between the Earth and the Moon?

Answer 32

Because the Earth and Moon are **electrically neutral on a large scale**, gravitational forces predominate due to their massive sizes. ## Footnote Gravitational forces are always attractive, while electric forces can cancel out when charges are balanced.

Question 33

What is the **range** of Coulomb force?

Answer 33

Coulomb's law is **valid for any distance**, and the force decreases with the square of the distance between the charges. ## Footnote The force tends to zero as the distance approaches infinity.

Question 34

# True or false: The **force** between charges is always **attractive**.

Answer 34

False ## Footnote Like charges repel, and opposite charges attract, as described by Coulomb’s law.

Question 35

# Fill in the blank: If the **distance** between two charges is halved, the **force** between them \_\_\_\_\_\_\_\_.

Answer 35

quadruples ## Footnote This follows the inverse square relationship in Coulomb’s law.

Question 36

# Define: Electric field

Answer 36

**Region around a charged object** where other charges experience a force. ## Footnote Its unit is newtons per coulomb (N/C).

Question 37

# True or false: The **direction of the electric field** is always from a positive charge to a negative charge.

Answer 37

True ## Footnote This is a convention used to represent the direction of force on a positive test charge.

Question 38

# True or false: **Electric field lines** can cross each other.

Answer 38

False ## Footnote Electric field lines cannot cross because that would imply two different directions for the field at the same point, which is impossible.

Question 39

How do **electric field lines** indicate the strength of an electric field?

Answer 39

The **density of field lines** represents field strength: closer lines indicate a stronger field, while sparse lines indicate a weaker field. ## Footnote Field line density is proportional to field magnitude.

Question 40

# Fill in the blank: The **force on a charge** in an **electric field** is given by the equation \_\_\_\_\_\_\_.

Answer 40

F=qE ## Footnote F is the force, q is the charge, and E is the electric field. This shows the interaction of a charge with an electric field.

Question 41

# Fill in the blank: The **electric field** inside a **conductor** in electrostatic equilibrium is \_\_\_\_\_\_.

Answer 41

zero ## Footnote Charges within a conductor rearrange themselves to cancel any internal field.

Question 42

What happens when a **positive test charge** is placed in an **electric field**?

Answer 42

It follows the electric field lines. ## Footnote The field lines indicate the direction of the force the charge is experiencing.

Question 43

# Define: Electric potential

Answer 43

**Amount of work needed to move a unit positive charge** from a reference point to a specific point in an electric field without acceleration. ## Footnote Its unit is the volt (V).

Question 44

What does **one volt** represent in terms of energy and charge?

Answer 44

1 Joule per Coulomb. ## Footnote This means that for every Coulomb of charge, 1 Joule of energy is released.

Question 45

# True or false: Electric potential is a **vector quantity**.

Answer 45

False ## Footnote Electric potential is a scalar quantity, unlike electric field or electric force.

Question 46

# Fill in the blank: The **electric potential** near a positive charge is \_\_\_\_\_\_ compared to farther away.

Answer 46

higher ## Footnote Electric potential decreases as the distance from a positive charge increases.

Question 47

# True or false: A **positive charge** moves naturally to regions of higher electric potential.

Answer 47

False ## Footnote Positive charges move to regions of lower potential, while negative charges move to regions of higher potential.

Question 48

# Fill in the blank: **Equipotential lines** are always \_\_\_\_\_\_\_ to electric field lines.

Answer 48

perpendicular ## Footnote This ensures that no work is done moving a charge along an equipotential line.

Question 49

What is the **difference** between electric potential and potential difference?

Answer 49

* Electric potential is the **electric potential energy per unit charge** at a specific point in an electric field. * Potential difference, also called voltage, is the **change in electric potential** between two points, which drives charge flow in circuits. ## Footnote Potential difference drives the flow of charges in electric circuits.

Question 50

Why is **potential difference** important in everyday life?

Answer 50

Potential difference powers household devices by **allowing electric current to flow through circuits**. ## Footnote This concept powers lights, appliances, and charging electronic devices. Electric utilities deliver energy by maintaining a voltage difference between supply and ground points.

Question 51

# Fill in the blank: Another term for **potential difference** is \_\_\_\_\_\_.

Answer 51

Voltage ## Footnote Voltage is commonly used to describe the potential difference in electrical circuits.

Question 52

How are **electric potential** and the **electric field** related?

Answer 52

The electric field is the rate of **change of electric potential per unit distance**; it points in the direction where potential decreases most rapidly. ## Footnote A strong electric field indicates a steep change in electric potential over a short distance.

Question 53

Describe the relationship between **electric potential** and **electric potential energy**.

Answer 53

* Electric potential energy is the **total energy** a charge has due to its position in an electric field. * Electric potential is this **energy per unit charge**. ## Footnote U=qV, where U is potential energy, q is charge, and V is electric potential.

Question 54

What is the formula for **electric potential energy**?

Answer 54

UE = (k * q1 * q2) / r ## Footnote Where UE is electric potential energy, k is Coulomb's constant, q1 and q2 are the charges of the particles, and r is the distance between them.

Question 55

What is the **SI unit of electric potential energy**?

Answer 55

Joules (J) ## Footnote Electric potential energy is expressed in kg m²/s².

Question 56

What is meant by **zero potential** in the context of electric potential energy?

Answer 56

It is an arbitrary baseline where electric potential energy is considered to be zero. ## Footnote Typically, this is measured at a radius of infinity.

Question 57

What happens to **electric potential energy** when two opposite charges are separated?

Answer 57

**Positive work must be done** to separate them. ## Footnote This is due to the attraction between a positive and a negative charge.

Question 58

What is the relationship between the charges of two particles and their **electric potential energy**?

Answer 58

Charges are **proportionally related** to electric potential energy. ## Footnote The greater the charges, the greater the potential energy.