Magnetic Fields: Strength, Shielding and Alignment

Force Between Magnetic Poles

• Magnets have an S-pole and N-pole, regardless of size.
• Like poles repel each other, unlike poles attract.
• Materials with magnetism attract opposite poles of a magnet.
• Magnetic materials attract opposite poles of other magnetic materials.

Magnetic Field

• Monopoles do not exist in magnets; magnets are bipolar, having N-pole and S-pole.
• Every magnet has a space around it where it exerts influence, known as its magnetic field.
• The region or space around a magnet where it exerts a force on other magnetic poles constitutes a magnetic field.
• Intensity of magnetic field (B) at any point is measured in Tesla (T).
• Magnetic field is represented by field lines.
• Direction of magnetic field is indicated by the direction of force on an N-pole.
• Magnetic field lines are curved; direction at a point is found by drawing a tangent.
• The magnetic field of a current-carrying wire can be found by placing iron fillings around the wire.
• Concentric circles indicate magnetic field around a current-carrying wire.
• Right-hand rule determines the direction of the magnetic field; thumb points in the direction, the current and curled fingers show the direction of magnetic field.
• A bar magnet is a material with permanent magnetic properties
• Magnetic field pattern illustrates the magnetic field.
• Magnetic field lines originate from the N-pole and terminate at the S-pole, traveling within the magnet from S-pole to N-pole.
• The magnetic field of a bar magnet can be observed using iron filings or compass needles.

Relative Strength of Magnetic Field

• Field lines indicate the strength of the magnetic field at any point.
• Stronger fields have closer lines; weaker fields have lines that are farther apart. -Magnetic field lines provide the direction and the strength of the magnetic field.
• Bringing two N-poles close can weaken the field; placing an N-pole near an S-pole strengthens with the field.

Magnetic Shielding

• By adjusting magnets, a field-free region called a "neutral zone" can be created.
• Neutral zones occur where magnetic field lines repel each other, thus shielding the magnetic field.
• Magnetic shielding is used to protect devices from external alteration of function by shielding devices from external magnetic fields
• Materials for magnetic shielding are called shields.
• Shields protect sensitive circuits from parasitic fields.
• Materials commonly used in magnetic sensors, EMI: Iron, Cobalt and Nickel.
• Shields typically have rounded corners.
• Rounded corners help magnetic field lines turn 90 degrees.

Induced Magnetism

• Induced magnetism is the phenomena where a non-magnetic material turns magnetic using certain techniques.
• An induced magnet is "A material that becomes a magnet when placed in magnetic field".
• Magnetism is induced by stroking a material with a magnet, hammering it in a magnetic field, heating it, or putting it in a coil with direct current.
• Stroking Method aligns poles in a material through stroking for induced magnetism.
• In stroking, bring a permanent magnet bar to the metal, rubbing one pole to the other then lifting the magnet.
• As a result of stroking, one end becomes south pole.
• The Hammering Method involves placing a metal bar in a strong magnetic field and hammering gently.
• Hammering causes domains to align with the applied field, magnetizing the metal.
• The Hammering Method is primarily used for steel magnetization.
• Heating the metal slightly before hammering can increase magnetization.
• Demagnetization is achieved with the Heating Method; heat speeds up the movements of existing domains resulting in misalignment and loss of magnetization.
• The phenomenon in which a magnetic field results from heating is called the "magnetic Seebeck effect" or "thermo-magnetism".

Temporary and Permanent Magnetics

• Temporary Magnets are magnetic materials which do not retain magnetization without external magnetic field or applied current.
• Solenoid behaves as a temporary magnet because upon current removal it loses its field and is often referred to as an electromagnet.
• An electromagnet is a magnet that produces a magnetic field through electric current.
• Iron nails, screws, metal bolts, and kitchen utensils are examples of temporary magnets.
• Permanent magnets and temporary magnets have the following differences:
  • Permanent magnets: Are permanently magnetized.
  • Electromagnets: Are temporarily magnetized.
  • Permanent magnets are made of hard magnetic materials.
  • Electromagnets are made of soft magnetic materials.
  • The magnetism of permanent magnets does not vary in strength
  • The strength in electromagnets can be adjusted based on need.
  • The poles of permanent magnets cannot be altered.
  • The poles of electromagnets can be altered.

Solenoid

• A solenoid is a coil of wire wrapped around a cylinder and used for metal magnetization.
• The magnetic field of a solenoid resembles that of a bar magnet.
• A conductive wire wrapped around a metal with insulation aligns domains and generates a magnetic field.
• As long as current flows, the coil of wire acts as a magnet.
• Direct current (DC) maintains the coil polarity, hence magnetizing solely in one direction.
• When using alternating current (AC), solenoid polarity switches after each half cycle: Material is magnetized/ demagnetized in one direction in first half cycle, then in opposite direction in second half.
• Soft magnetic materials like iron easily magnetize and demagnetize.
• Hard magnetic materials, such as steel, don't magnetize or demagnetize.
• Electric motors in hair dryers, electric razors, and trimmers rely on magnetic force.
• Electric motors generate a magnetic field via electric current through a coil, causing movement or spinning that runs the motor.
• The magnetic field of a solenoid is mathematically defined as B = µ₀nI.
  • B represents magnetic field strength with Tesla (T) as its unit (equivalent to Newton per Ampere per meter).
  • µ₀ represents material permeability, which shows how readily a material allows a magnetic field to pass.
  • I represents current flowing through solenoid.
  • n represents number of solenoid turns per unit length.

Formulas for Magnetism

• Magnetic field Formula is: B = µ₀I / 2πr
  • Where 'r' is the distance from the wire center and I is magnitude of current flowing.
• The value of the permeability for vacuum is: µ₀ = 4π x 10⁻⁷NA⁻²

Magnetic Field of Solenoid

• A student makes solenoid with 15 turns by wrapping copper wire around a 50cm iron rod, connecting copper wire end to a battery that provides 1.2 A.
  • Length 'L' = 50 cm = 0.5 m
  • Number of turns 'N' = 50
  • Current 'I' = 1.2 A
  • Permeability 'µ₀' = 4π x 10⁻⁷ N/A²
• The number of turns per unit length is calculated as: n = N / L = 50 / 0.5 = 100 m⁻¹
• Putting these values into B = µ₀ n I, one can find: B = (4π x 10⁻⁷ N A⁻²)(100 m⁻¹)(1.2A)
• Therefore: B = 1.5 x 10⁻⁴ T

Uses of Permanent Magnets and Electromagnets

• The need dictates the use of permanent magnets or electromagnets.
• Permanent magnets retains magnetization and it has its own field.
• Because permanent magnets do not need continuous electricity, they are used in applications where continuous supply of electricity is unavailable or costly to maintain.
• Permanent magnets have lower magnetic field strength but retain field without electricity.
• Permanent magnets are used in induction cookers, MRI machines, particle accelerators, transformers, automotive, aerospace, medical, semiconductor, and energy industries.
• Electromagnets produce a magnetic field via electric current.
• Electromagnets act as temporary magnets, functioning only when current flows and losing their magnetic field when the current stops.
• Electromagnets have wide daily applications in electromechanical and electronic devices.
• Home appliances like fans, electric motors, and doorbells use electromagnetism.
• Medical fields also use electromagnets in MRI scans, communication devices, and power circuits.

Magnetic Recording

• Magnetic recording saves sounds, pictures, and data as electrical signals using selective magnetization.
• To write data, a magnetic tape head moves onto the tape, aligning magnetic domains with applied current.

Magnetic Recording Materials

• In magnetic recording, materials commonly used are iron-oxide, cobalt, and chromium oxide.
• Magnetic tape and disk recorders store and reproduce audio, video, and computer data.
• Computer storage devices such as magnetic drums, core and bubble units are examples of magnetic recording materials.

Speakers

• Speakers use magnets to produce vibrations resulting in sound.
• Two magnets, one permanent magnet, one electromagnet, are needed to produce sound from speaker.
• The conical structure made of flexible material to produce vibrations is called diaphragm.
• The permanent magnet is fixed at the center of the cone (diaphragm).
• An electromagnet is attached at the center of the permanent magnet capable of moving to and fro.
• The electromagnet generates alternating the magnetic field producing interaction with field of permanent magnet.
• As the cone aligns with the electromagnet by vibrating back and forth, it vibrates and produces sound.
• Larger permanent magnets allow louder sound production.
• Neodymium is a commonly used permanent magnet material in speakers.
• Microphones also use magnets but in reverse order.
• In microphones, the diaphragm vibrates due to sound, which produces movement in the electromagnet within the magnetic field of the permanent magnet to produce an electrical signal for the speaker.

Anatomy of Speaker

• The speaker anatomy includes the frame, cone (diaphragm) which is 20-30 cm wide, spider, leads, voice coil, and permanent magnet

Door Locks

• Door locks use magnets; they have an electromagnet fixed to the door frame and a metal plate fixed to door.
• When the door is closed, the plate connects with the electromagnet.

Domain Theory of Magnetism

• Magnetism has been around since 600 BC when ancient loadstone which attracts iron, was discovered in Magnesia from which name, 'magnet' derived.
• Scientists started to understand magnetism in twentieth century and developed technologies based on this understanding.
• Magnetism originates from the motion of charge particles, like electrons.
• Electrons move around nucleus in atoms and charges move in wires in the electric current.
• Atoms have a positively charged nucleus and light particles called electrons that orbit nucleus.
• In the twentieth century it was found that motion of charge particles produce magnetism
• Each electron produces a small level of magnetism.
• Each electron loop or single atom produces a tiny magnet with two poles called north-pole (N pole) and south-pole (S pole).
• Spinning nucleus produces little magnetism.
• The motion of electrons also produces tiny magnetism.
• For atoms, magnetism results from the orbital motion of electrons.
• In some atoms, electrons are oriented to add their magnetic field, making the whole material magnetic.
• Materials containing atoms with parallel magnetic fields create 'domains' (about 10^12atoms and a few millimeters).
• In unmagnetized material domains are randomly organized; in magnetized material, they're aligned.
• "A domain is the group of atoms in a material which have n-poles pointing in the same direction."

Explore magnetic field strength indicated by field line spacing. Understand neutral zones, magnetic shielding importance and materials. Learn about compass needle alignment near magnets and the origin of magnetism at the atomic level.