4B1 Magnetic Fields, Forces, and Materials

Question 1

Define:

Magnetism

Answer 1

A phenomenon caused by the motion of charged particles, resulting in magnetic fields and forces.
## Footnote

This results in attractive and repulsive forces between objects.

Question 2

Fill in the blank:

A _______ is a material that generates a magnetic field and can attract or repel certain objects.

Answer 2

magnet

It can be natural (like lodestone) or artificial (like electromagnets), depending on their origin.

Question 3

What is a solenoid?

Answer 3

A cylindrical coil of wire that acts as a magnet when conducting an electric current.

Solenoids are commonly used in various electromagnetic applications.

Question 4

Define:

Magnetic moment

Answer 4

Vector quantity that measures the strength and direction of a magnet’s ability to produce a magnetic field.

Question 5

What are magnetic poles?

Answer 5

The oppositely charged sides of a magnet, labeled as north (positive) and south (negative).

Opposite poles attract while like poles repel.

Question 6

True or false:

Magnetic monopoles exist in nature.

Answer 6

False

Magnetic poles always come in north-south pairs. When a dipole is cut, two new dipoles are created, each with a north and a south pole.

Question 7

True or false:

Magnetic field lines always point from the north pole to the south pole outside the magnet.

Answer 7

True

Inside the magnet, they return from the south pole to the north pole.

Question 8

How does charge alignment contribute to magnetism?

Answer 8

Aligned electron spins in atoms create a net magnetic field.

Magnetism arises when many atomic magnetic moments align in the same direction.

Question 9

Define:

Magnetic flux

Answer 9

Measure of the total magnetic field passing through a given surface.

It is measured in Weber (Wb).

Question 10

What is the formula for magnetic flux?

Answer 10

Φ=B∗A∗cos(θ)

Where Φ is magnetic flux, B is magnetic field strength, A is surface area, and θ is the angle between B and A.

Question 11

True or false:

Magnetic flux is a scalar quantity.

Answer 11

True

Magnetic flux has only magnitude and no direction, making it a scalar quantity.

Question 12

True or false:

Magnetic flux can have a negative value.

Answer 12

True

Magnetic flux is negative when field lines pass opposite to the surface’s normal vector.

Question 13

What happens to magnetic flux if the magnetic field increases but the loop area remains constant?

Answer 13

It increases.

Magnetic flux is directly proportional to the strength of the magnetic field.

Question 14

Fill in the blank:

The area vector in magnetic flux calculation is directed _______ to the surface.

Answer 14

perpendicular

The area vector is normal to the surface by definition in flux calculations.

Question 15

Why is magnetic flux important in electromagnetic induction?

Answer 15

Changes in magnetic flux induce an electromotive force (EMF) according to Faraday’s Law.

This principle is fundamental in the operation of generators and transformers.

Question 16

What is Lenz’s Law?

Answer 16

Induction opposes any change in magnetic flux.

The direction of induced current is such that its induced magnetic field opposes the external magnetic field.

Question 17

Define:

Magnetic force

Answer 17

Force exerted on a moving charge or current-carrying conductor in a magnetic field.

This force depends on the charge’s velocity, the strength of the magnetic field, and the angle between them.

Question 18

Fill in the blank:

The magnetic force on a moving charge is maximum when the charge moves _______ to the magnetic field.

Answer 18

perpendicular

The force is given by F=qvBsinθ, where θ=90° maximizes the force.

Question 19

True or false:

Magnetic force can do work on a moving charge.

Answer 19

False

Magnetic force changes the direction of a charge but does not change its speed or do work.

Question 20

How is the direction of magnetic force on a moving charge determined?

Answer 20

Using the right-hand rule.

Point your fingers in the direction of velocity, your palm toward the magnetic field, and your thumb points to the force for a positive charge.

Question 21

What factors influence the magnetic force on a current-carrying wire?

Answer 21

• Current
• Length of the wire
• Magnetic field strength
• Angle between the wire and the magnetic field

The force is calculated as F=ILBsinθ.

Question 22

What happens to the path of a charged particle moving perpendicularly to a uniform magnetic field?

Answer 22

It follows a circular path.

This is because the magnetic force acts as a centripetal force.

Question 23

What is the Lorentz force?

Answer 23

The force experienced by a moving charge in the presence of both electric and magnetic fields.

It is the sum of electric (qE) and magnetic (qvB) forces.

Question 24

What is the formula for the Lorentz force?

Answer 24

F=q(E+v×B)

q is the charge, E is the electric field, v is the velocity, and B is the magnetic field.

Question 25

How does the **velocity of a charge** affect the magnitude of the **magnetic force** on it?

Answer 25

It is **directly proportional** to the magnitude of the velocity. ## Footnote If the velocity is zero, the magnetic force is also zero.

Question 26

# True or false: A **stationary charge** in a **magnetic field** will experience a magnetic force.

Answer 26

False ## Footnote The charge must be moving to experience a magnetic force.

Question 27

# Fill in the blank: If the **velocity** of a charged particle is \_\_\_\_\_\_\_ to the **magnetic field**, the **force is maximum**.

Answer 27

perpendicular ## Footnote At 90°, sinθ=1, resulting in the maximum force.

Question 28

What is the equation for the **magnetic component** of the **Lorentz force**?

Answer 28

F=qvBsinθ ## Footnote q is charge, v is velocity, B is the magnetic field strength, and θ is the angle between v and B.

Question 29

Why does the **magnetic force** not change the speed of a charged particle?

Answer 29

The force is **always perpendicular to the velocity**, only changing its direction. ## Footnote Magnetic forces do no work on moving charges.

Question 30

How are **Lorentz force** and **magnetic force** related?

Answer 30

The **Lorentz force** includes both the electric and magnetic components, while the **magnetic force** is only due to the interaction between a moving charge and a magnetic field. ## Footnote Magnetic force is a part of the Lorentz force when no electric field is present.

Question 31

What is the **Earth's magnetic** field modeled like?

Answer 31

A bar magnet ## Footnote The Earth has a distinct north and south pole similar to a bar magnet.

Question 32

# Fill in the blank: A \_\_\_\_\_\_\_ is a **temporary magnet** created by passing an **electric current** through a coil of wire.

Answer 32

electromagnet ## Footnote The magnetism disappears when the current is turned off. Its strength depends on the current and number of coils.

Question 33

How are **electromagnets** used in **recycling plants**?

Answer 33

Humans can extract magnetic materials, like iron, from mixed waste. ## Footnote The magnetic field can be turned on and off to release the materials.

Question 34

What is **ferromagnetism**?

Answer 34

The **ability of certain materials to stay magnetized** after the external field is gone. ## Footnote These materials have aligned magnetic domains that enhance their magnetic strength.

Question 35

# Fill in the blank: A \_\_\_\_\_\_\_ magnet is made by placing a **ferromagnetic material**, like iron, in a strong magnetic field to temporarily magnetize it.

Answer 35

temporary ## Footnote They lose magnetism once the external field is removed.

Question 36

# Define: Permanent magnet

Answer 36

Material that **retains its magnetic properties** over time without the need for an external power source. ## Footnote Common examples include refrigerator magnets and compasses.

Question 37

What causes a magnet to be classified as a **permanent magnet**?

Answer 37

**Internal alignment** of magnetic domains. ## Footnote An internal separation of charge within the material causes it to become magnetized.

Question 38

How does ferrimagnetism differ from **antiferromagnetism**?

Answer 38

* Ferrimagnetism has **unequal opposing moments**, resulting in a net magnetic field. * Antiferromagnetism has **equal opposing moments** that cancel each other out. ## Footnote Many metal alloys exhibit antiferromagnetism.

Question 39

What is the **difference** between **diamagnetic** and **paramagnetic** materials?

Answer 39

* Diamagnetic: **charged particles align perpendicular** to an external magnetic field * Paramagnetic: **charged particles align parallel** to an external magnetic field ## Footnote Diamagnetism and paramagnetism are types of temporary magnetism.

Question 40

What does the **Biot-Savart law** describe?

Answer 40

The magnetic field generated by a **steady current**. ## Footnote It applies to infinitesimal current elements in space.

Question 41

# Fill in the blank: The **Biot-Savart law** relates the magnetic field to current, the distance, and the \_\_\_\_\_\_\_ of the current element.

Answer 41

orientation ## Footnote The direction of the current element influences the resulting magnetic field.

Question 42

Which physical law is **analogous** to the **Biot-Savart law** in electrostatics?

Answer 42

Coulomb’s law ## Footnote Both describe **field generation** due to source elements, but one for electric fields and the other for magnetic fields.

Question 43

# Fill in the blank: The Biot-Savart law uses the **cross product** of the current element and the \_\_\_\_\_\_\_ vector.

Answer 43

position ## Footnote This determines the direction of the magnetic field contribution.

Question 44

According to the **Biot–Savart Law**, how is the magnetic field affected when the current is doubled?

Answer 44

The magnetic field also doubles. ## Footnote Magnetic field strength is directly proportional to current.

Question 45

The Biot-Savart law only applies to **straight current-carrying wires**.

Answer 45

False ## Footnote It applies to any current-carrying path, including loops and curves.

Question 46

What happens to the **magnetic field** strength as the **length of the current-carrying wire increases**?

Answer 46

The magnetic field **increases**. ## Footnote Longer current paths contribute more to the total magnetic field.

Question 47

# Fill in the blank: The force per unit length between two **parallel current-carrying wires** is described by \_\_\_\_\_\_\_ law.

Answer 47

Ampere's ## Footnote Ampere’s force law quantifies the magnetic force between parallel currents.

Question 48

# True or false: The **force between two wires** can create mechanical motion in some devices.

Answer 48

True ## Footnote This principle is fundamental in motors.

Question 49

Why do parallel current-carrying wires **attract** each other when their currents flow in the **same direction**?

Answer 49

Their magnetic fields combine constructively, creating a region of lower energy between them. ## Footnote This reduces the magnetic energy, causing an attractive force.

Question 50

# Fill in the blank: The magnetic force between **two wires** depends on the product of the \_\_\_\_\_\_\_ flowing through them.

Answer 50

currents ## Footnote Higher currents result in stronger forces.

Question 51

What happens if **two wires are perpendicular** to each other?

Answer 51

There is no net force between the wires. ## Footnote Magnetic fields do not interact to create a force in this case.

Question 52

What role does the **permeability of free space (μ0 )** play in the force between wires?

Answer 52

It is a proportionality constant in the force equation. ## Footnote μ0 defines the magnetic properties of a vacuum.

Question 53

# True or false: The **force** between two wires depends on the **medium surrounding** them.

Answer 53

True ## Footnote Magnetic permeability of the medium influences the force.

Question 54

What is **Faraday's law** of induction?

Answer 54

An electromotive force (emf) is induced in a circuit when the **magnetic flux through it changes over time**. ## Footnote Faraday’s law explains how varying magnetic fields produce current, which underlies the operation of transformers, generators, and motors.

Question 55

What is the primary function of a **transformer**?

Answer 55

To increase or decrease voltage in an electrical circuit. ## Footnote Transformers work on the principle of electromagnetic induction.

Question 56

# True or false: **Transformers** work with **direct current (DC)**.

Answer 56

False ## Footnote Transformers require alternating current (AC) to operate.

Question 57

What is the **basic principle** that allows an **electric motor** to function?

Answer 57

The interaction between a magnetic field and a current-carrying conductor creates force. ## Footnote This is described by the **Lorentz force**.

Question 58

How does a **generator** differ from an **electric motor**?

Answer 58

A generator **converts mechanical energy into electrical energy**, while a motor does the **opposite**. ## Footnote Generators rely on electromagnetic induction.

Question 59

What are the main **components** of an **AC generator**?

Answer 59

* Coils of wire * Axle * Rotating slip rings * Stationary brushes ## Footnote AC generators produce alternating current with specific frequencies.