Physics SAT3 - Magnetism

Flow of Magnetic Field

Out of north pole and into south pole

Producer of Magnetic Fields

Moving charged particles, such as electrons in a magnet

Right Hand Grip Rule

Thumb is conventional current, curl of fingers is direction of circular magnetic field

Solenoid

Many loops or coils stacked next to one another

Solenoid Right Hand Grip Rule

Curl of fingers is direction of current, thumb is direction of magnetic field

90 Degrees to B Field

When force is maximised on a current carrying wire

0 Degrees to B Field

When force is minimised on a current carrying wire

Right Hand Palm Rule

Thumb is direction of current, fingers are direction of B field, palm is direction of force

Right Hand Palm Rule for Charges

Thumb points in direction of positive charge movement

Motion of Charged Particle in B Field

Force is always perpendicular to velocity, so F constantly changes as v changes, resulting in circular motion

Cyclotron

Particle accelerators composed of dees inside of two electromagnetic plates

Dees

Connected to an alternating potential difference which generates a uniform electric field between the dees

Direction of E-Field in Cyclotron

Alternates direction while ion inside dees due to alternating potential difference

Mechanics of Cyclotrons

Ion accelerates into dees due to E-field. While in dees, ion experiences circular motion due to B-field. Ion travels in semi-circle and re-enters. E-field is flipped, accelerates and gains KE. Repeat until radius of curvature big enough for ion to hit target.

Magnetic Flux

A measure of the number of magnetic field lines passing through an area

Most Flux

Area perpendicular to B field

Least Flux

Area parallel to B field

Emf

As uncharged conductor moves through B field, free electrons experience force and travel to one end of the conductor to induce potential difference

Faraday’s Law

The induced emf is equal to the rate of change of the magnetic flux linkeage

Causes of Flux Change

B field strength changing, area changing or angle to B field changing

Lenz’s Law

The induced emf will create a current that opposes the change in flux

Right Hand Grip Flip

Point thumb in B field direction. If flux increasing, keep thumb. If flux decreasing, flip thumb. Flip thumb direction - curl of fingers is induced current

LOCOE

Induced current is electrical energy, so KE is converted to electrical energy and slows down moving object, making opposing force

Eddy Currents

Circular induced current produced through induction on unbroken surfaces

Generators

Devices that convert rotational kinetic energy into electrical energy

AC Generator

Alternating current, has slip-ring commutators

DC Generator

Current goes in one direction, has split-ring commutators. Induced current is reversed every half rotation

Iron Core

Inside wires of generator, maximises B field

Rotor Coils/Armature

Loops of wire around the iron core, current is induced as they rotate

Brushes

Graphite blocks on either side of the commutator. Provide point of contact for the slip/split rings

Axle

Rod that passes through the iron core and rotor coils, provides axis of rotation

Transformers

Devices that take an AC input voltage and deliver a different output voltage

Step-Up Transformer

Has a greater number of loops and produces a higher voltage

Step-Down Transformer

Has fewer loops and produces a smaller voltage

Principles of Transformer

Circuit 1 has a solenoid that produces B field as current flows. B field constantly changing (AC source) therefore emf is induced, which induces current in circuit 2. emf proportional to number of loops, induced current changes due to circuit 2 loops.

Increased Current, Decreased Voltage

Step-down transformers

Decreased Current, Increased Voltage

Step-up transformers