Magnetism

Question 1

Measurement

Answer 1

Measurement is one of the most important aspects of science.

1) size (length of car, vs length of ant)
2) units (m, N, dB etc) and
3) reference (20 km north of Montreal references Montreal as a starting point/origin).

Question 2

Magnetism – 2 basic types

Answer 2

Ferromagnetism (passive) and Electromagnetism (active)

Question 3

Ferromagnetism

Answer 3

involves certain metallic elements, and describes magnets that do not require an active electric current

Question 4

Electromagnetism

Answer 4

describes the phenomena resulting from the interaction between electricity and magnetism

Question 5

What is a property of ferromagnetics?

Answer 5

Only certain metals can experience ferromagnetic effects. Fridge magnets are attracted to the metal casing of your refrigerator, but those same magnets feel no effect when placed on wooden cupboards.

Question 6

What are ferromagnetic metals?

Answer 6

Generally, the best magnetic materials are: Iron, Cobalt and Nickel, as well as some of the alloys containing those metals. Magnetically susceptible materials are called Ferromagnetic.

Question 7

What are the strongest types of magnets?

Answer 7

The strongest types of magnets are called rare earth magnets (typically neodymium magnets: Nd2Fe14B). Rare earth magnets are many times stronger than magnets made of iron, cobalt or nickel, but they do not have the same range of operating conditions.

Question 8

Metals

Answer 8

As we know, metals are conductors and allow electrons to flow within them.

Question 9

What is current electricity?

Answer 9

Current electricity is the movement of electrons within a conductor (controlled using voltage sources and resistors for example).

Question 10

What causes the phenomenon of magnetism?

Answer 10

The phenomenon of magnetism is also caused by the movement of electrons, hence the broad category of “Electromagnetism”.

Question 11

Many phenomena are electromagnetic in nature:

Answer 11

Many phenomena are electromagnetic in nature: static & current electricity, magnetism, light, chemistry*.

Question 12

What are the 4 fundamental forces of nature?

Answer 12

Electromagnetism is one of the 4 Fundamental Forces of Nature, along with gravity and the strong and weak nuclear forces.

Question 13

What are domains in ferromagnetism?

Answer 13

Domains are small regions where the movement of electrons is organized, and the magnetic field generated by their movement is consistently oriented. Domains are tiny magnetically oriented regions

Question 14

Unmagnetized domains vs Magnetized domains

Answer 14

Unmagnetized materials have domains that are randomly oriented.
Magnetized metals have domains that are mostly oriented the same way.

Question 15

Instead of Positive (+) and Negative (-) charges, magnets have ___________

Answer 15

poles (North and South). (These labels are historic and arbitrary)

Question 16

How are magnetic forces similar to electric forces?

Answer 16

Magnetic forces behave similarly to electric forces of attraction and repulsion: opposites poles attract, similar poles repel.

Question 17

Two Laws of magnets

Answer 17

1) Magnets always come with both poles (breaking a magnet produces two smaller complete magnets).

2) “Conventional” matter cannot produce a magnetic monopole (N or S by itself).

Question 18

Field lines

Answer 18

Similarly to electric fields, we use directional lines to represent magnetic fields.
Magnetic field lines are drawn with arrows pointing away from N, and towards S.
Magnetic field direction arrows show the effect on a N pole (moves away from N, towards S).

Question 19

Line crowding in field lines explained

Answer 19

When drawing magnetic field lines, the line crowding is a visual representation of magnetic field strength.

Crowded field lines represent a strong magnetic field

Sparse field lines represent a weaker magnetic field

Question 20

Compasses

Answer 20

A compass is a magnetic indicator, the North pole of the compass needle is usually emphasized with color or shape.
In the absence of applied magnetic fields, compasses align themselves with the Earth’s magnetic field which is relatively weak but always present.
The red arrow of a compass is magnetized to N, and will align itself towards S.
This means that the geographic North pole of Earth is actually a magnetic South pole! (most people find this surprising)

Question 21

What do compasses do and what are they used for?

Answer 21

Compasses, like all magnets, try to align themselves with surrounding magnetic fields. They are used in experiments to detect the orientation of applied magnetic fields.

Question 22

Ferromagnetic _____ can acquire _____ or __________ magnetism.

Answer 22

Ferromagnetic materials can acquire temporary or permanent magnetism.

Question 23

What happens when a ferromagnetic material is in the presence of a magnetic field?

Answer 23

When a ferromagnetic material is in the presence of a magnetic field, the domains within the material may align themselves with the field.

Question 24

Magnetic Remanence

Answer 24

The ability of a material to align itself with a magnetic field is referred to as Magnetic Remanence.

Question 25

Materials that gain and lose alignment easily

Answer 25

Materials that gain and lose alignment easily have low remanence.

Question 26

Low magnetic remanence example

Answer 26

The low remanence metal in your fridge temporarily aligns itself with the field of the fridge magnet when it is near. This allows the magnet to “stick” to the metal of the fridge. This alignment is not permanent, and once the magnet is removed the fridge metal returns to a disordered non-magnetic state.

Question 27

High remanence

Answer 27

It is very difficult to change the alignment of materials with high magnetic remanence. This means that once the domains are successfully aligned, they will maintain this alignment, creating a “permanent” magnet.

Question 28

How to make a magnet

Answer 28

Typically, to make a permanent magnet, we first heat the metal to very high temperatures which makes the electron domains more mobile and easier to re-align. The material is then subjected to very intense magnetic fields until the domains align with it. The material is finally quenched (cooled rapidly) and the domain alignment is frozen in place, creating a permanent magnet. (the high magnetic remanence prevents the domains from altering their aignment).

Question 29

Natural metals

Answer 29

Natural magnets found in the earth have been magnetized over long periods of time by the Earth’s weak magnetic field.

Question 30

Electromagnetism

Answer 30

Electricity and Magnetism are fundamentally connected (via light)
Each phenomena can be studied in isolation (to a degree)
For example:
-We can largely ignore magnetic effects when studying circuits
-We can experiment and make conclusions with ferromagnetic materials without going into details of the electron motion within domains.
Notice that the magnetic field and electric field are always at right angles to one another.
A changing electric field causes a magnetic field, and vice versa

Question 31

2. Electromagnetism – link between current and magnetic field

Answer 31

First discovered by accident. It was observed that current flowing through a wire deflected a nearby compass needle. When the current was switched off, the compass needle re-aligned itself with Earth’s magnetic field.
Further experimentation was able to map out the magnetic influence on the space surrounding a wire subject to current.
The magnetic field surrounding a charge carrying wire is circular, and at right angles to the direction of current.

Question 32

Conventional vs true current

Answer 32

As mentioned in the dynamic electricity unit (current electricity), we have two options for representing current in a circuit or wire: ”conventional” current or “electron”/”true” current.
“Electron” current is a description of the actual motion of electrons. They move from the negative terminal of the battery towards the positive end.
(- → +)
“Conventional” current was formulated before we understood the nature of moving charges within matter.
Conventional current flows in a direction from the Positive end of the power source towards the Negative end of the power source. (+ → -)
We will use Conventional current in this class. (right hand rule)

Question 33

Right hand rule

Answer 33

“Hand” rules in science are common and help us when several quantities are perpendicular to each other.
Extending your thumb, index and middle finger, you can create several right angles.
For our studies of magnetic fields, we will curve our fingers together instead of extending:

Thumb = current in straight wire
Curved fingers = circular magnetic field surrounding wire
The right hand rule only works with conventional current.
Right hand rule allows us to know the direction of magnetic field when the current direction is known, and vice versa.

Question 34

The right hand rule tells you:

Answer 34

Right hand rule can be used in two ways on a straight wire:

Current is known – place thumb in direction of current (+ to -), fingers curve in direction of magnetic field (fingers point towards N)

Field is known – place curved finger in direction of magnetic field (pointing towards N), thumb points in direction of current (+ to -)

Question 35

Electromagnetism Applications

Answer 35

Using electric current, we can create electromagnets.
Since current is easily controlled via circuit elements (switches, resistors, voltages sources, etc), we can control the magnetic fields produced.
Examples:
Electronic door lock
Scrapyard electromagnet
Scientific apparatus (MRI)

Question 36

Solenoid

Answer 36

The most common way to create an electromagnet is to make a coil of wire called a solenoid.
Electrons moving in a circular path create a linear magnetic field (just like electrons moving in a linear path create a circular magnetic field).
We can control the strength of the magnetic field created by the solenoid by modifying the physical parameters of the solenoid.
Solenoids are often fitted with a core that the wires are wrapped around. The magnetic remanence of the core will affect the potential strength of the electromagnet.

Question 37

How to increase magnetic field

Answer 37

Increasing the current in the coil will increase the magnetic field created.
Increasing the number of loops/turns in the solenoid will create a stronger magnetic field.

Question 38

How to strengthen the electromagnet

Answer 38

Using a core with low magnetic remanence will create a stronger electromagnet (easier to align domains).

Question 39

2. Electromagnetism – applications (solenoid)

Answer 39

Right hand rule can be used in two ways on a solenoid:
Current is known – curve fingers in direction of current (+ to -), thumb will point direction of magnetic field (thumb points towards N)
Field is known – place curved thumb in direction of magnetic field (pointing towards N), curved fingers point in direction of current (+ to -)