Unit 7 - Magnetism

What is a magnet?

• An object that produces a magnetic field

• Has a North and South pole

Temporary vs Permanent magnet

Temp:

• Only has magnetic properties in the presence of an external magnetic field

• Made of ferromagnetic elements like Fe, Ni, Co

Perm:

• Retain their magnetic properties regardless of presence of field

What causes magnetic poles?

• Atoms all have small magnetic fields, when “neighborhood” of theme atoms all point poles in same direction, creates a DOMAIN

• Because all domains point same direction, field lines of atom 1 flows through N pole to S pole of atom 2, etc

  • Effect COMPOUNDS to whole magnet

Domain theory

• Ferromagnetic elements like iron, nickel, and cobalt are made of atoms with tiny magnetic fields

• In the presence of a magnetic field, poles spontaneously point the same direction

• NOTE: if a magnet is split in half, the new magnets retain poles but are less strong (less domains per)

What 2 factors can cause a permanent magnet to lose its magnetism?

1. Dropping it - causes domains to become misaligned, loses poles

2. Heating past Curie temperature - increases kinetic energy, domains become misaligned too

What does the strength of a magnetic field (measured in Tesla, T) rely on?

Strength and distance of source magnet

What direction do field lines always travel?

From North to South

Why can’t magnetic field lines every intersect?

Because that would mean that at the point of intersection, the magnetic field would be going in two directions, implying that point experiences force in two directions as well- this is impossible.

• Imagine a charged particle moving thru the magnetic fields, and at the point of intersection, it must be in two different directions (due to two forces). IMPOSSIBLE

• Or a compass pointing towards the point of intersection- it must be in two different places, IMPOSSIBLE

Magnetic N/S vs Geographical N/S

• Magnetic poles are our “real” poles, earth is a giant magnet

• North pole is actually magnetic south - our compasses’ north pole is attracted to earth’s south pole, and vice versa for south pole

RHR for straight conductor

• Thumb in direction of current

• Fingers wrap around field lines

RHR for coiled conductor

• Thumb in direction of North pole

• Fingers wrap around current

RHR for electromagnetic force

• Thumb in direction of current

• Fingers for magnetic field lines (N → S), fingertips south

• Palm in directionf of force

Motor principle

When a current carrying conductor is placed in an external magnetic field, the conductor experiences a force perpendicular to both itself and the external magnetic field.

• EXPLAINED BY THE RHR FOR FORCE

Formula for relating strength of magnetic force created by helix to # of loops per unit length

$$\frac{F1}{F2}=\frac{N1}{N2}=\frac{I1}{I2}$$

Formula for force that conductor experiences as a result of motor principle

$$\frac{F1}{F2}=\frac{I1}{I2}=\frac{B1}{B2}$$

Synonyms for helix

Solenoid, coil, electromagnet

Lenz’s law

Magnetic field created by induced current always opposes the motion of the magnetic field that produced it. (Wants field lines to be in equilibrium, resists change)

• If magnet moving towards with N pole facing = develops N pole

• If magnet moving away with N pole facing = develops S pole

How is Lenz’s law consistent with the law of conservation of energy?

If the magnetic field created by the induced current is attracted to the magnetic field which created it, the two objects would move together without any inputted energy and produce a current.

• Impossible, as the KE and electrical E produced came from nowhere

• Energy cannot be created- therefore the above scenario is impossible

What is difference between Alternating Current (AC) and Direct Current (DC)?

Alternating:

• Current keeps switching directions

Direct:

• Current flows in one direction

Why can’t DC be used in transformers?

Because they rely on changing current to produce a changing magnetic field, which can then induce another current.

• As current moves back and forth (alternating) causes oscillating magnetic field bc when e^- briefly stop to switch direction, magnetic field collapses to zero

• Changing magnetic field is necessary to induce another current

Transformer formulas, relating potential difference, # of coils, and current between primary and secondary coil

$$\frac{Vp}{Vs}=\frac{Np}{Ns}=\frac{Is}{Ip}$$

Step-up vs Step-down Transformer

Step-up:

• Secondary has higher V

Step-down:

• Secondary has lower V

DC motor pieces

• Armature

• Conductor

• Carbon brushes

• Split-ring commutator

• Permanent magnets

How do DC motors work?

• Relies on alternating poles constantly repelling then attracting, causing the armature to rotate

• Current switches directions via carbon brushes alternating contact with commutator plates

  • Changing direction causes flipped poles, which continues to rotations

Mentally draw a DC motor