Magnetism + Electromagnetism (1, 2)

What are the ends of magnets called, and which direction do the lines of force go?

Lines of force/flux go from the north pole to the south pole

What are the lines of a magnetic field called, and can they be broken?

Magnetic lines of flux/force, no because each magnetic line forms independent closed loop i.e. they do not merge or cross each other

Why is magnetic field strongest at poles?

Flux lines most concentrated at poles, get exponentially weaker further away due to increasing line spacing and reluctance of air

What path do magnetic lines of force always take?

Path of least reluctance (magnetic resistance) i.e. permeability of path material, long ferrous material has lower reluctance than short air gap

How can magnets be attracted/repelled?

Opposites attract, sames repel

e.g. What happens when a steel bar spans across a horseshoe magnet's poles?

Steel bar will have induced magnetic poles opposite to magnet, allows magnetic attraction to ferrous non-magnetised object

What are considered the most important magnetic materials, and which has the strongest effect?

Iron + alloys, cobalt, nickel, are ferromagnetic, rare earth materials are magnetic too. Rare earth materials have strongest effect

Difference between soft/hard ferromagnetic materials?

Soft are easily magnetised but need external force to stay magnetised, Hard permanently (without specific process) retains magnetisation

What is magnetism remaining in material after magnetising force has been removed called?

Residual magnetism

What does permeability mean?

Ease with which a material allows passage of flux

What does coercivity mean?

Strength of external magnetic field required to magnetise/demagnetise material

What does retentivity mean?

Ability of material to resist magnetism, or retain magnetism after magnetising force is removed

How many rare earth elements are there, and which can produce very strong but brittle magnets?

17, Neodynium

What is the difference between ferromagnetic, paramagnetic and diamagnetic?

Ferromagnetic can create own flux with/without magnet, paramagnetic is only slightly magnetic when affected by flux, diamagnetic creates opposing magnetic field (considered non-magnetic)

How is sensitive equipment protected from magnetic fields?

Magnetic shielding: highly permeable material provides alternate path for magnetic field around an object

One example of when magnetic shielding is used?

Shielding cables on variable frequency motor drives to prevent interference to motor, and to minimise interference between electrical and comms cable

How does a reed switch work and what is it used in?

Two contacts slightly apart in sealed tube, when magnet comes close the field causes contacts to close. Used mainly in alarm systems

What two effects are always present in current carrying conductor?

Heat waste due to resistance, magnetic effect

What is the right-hand grip rule?

Right hand indicating flux direction wrapped around energised conductor with thumb sticking up along current direction

What symbols are used for direction of current flow?

Dot for current out of page, cross for current into page

What is a conductor wound into many loops called and what effect do the loops have on the magnetic field?

a COIL or SOLENOID has a stronger magnetic field when wound

What is the Right-Hand Solenoid Rule?

Right hand indicating which direction current travels through the winds with thumb sticking up along north pole direction

How does the flux in two parallel energised conductors affect each other when current is travelling in the same direction?

Flux rotates in the same direction, so they attract each other and can combine field lines running close enough to be attracted to each other

How does the flux in two parallel energised conductors affect each other when current is travelling in opposite directions?

Flux rotates in opposite directions, so they repel each other

How to calculate force between conductors carrying current? (Newtons, amperes and metres are used)

Force between conductors = (permeability of free space (2x10-7) * current in first conductor * current in second conductor) / distance separating

How to calculate magnetomotive force? (Ampere-turns and amperes are used)

Magnetomotive force = current flowing * number of turns on coil

How do relays and contactors use electromagnetism to operate?

Current passes through coil, generates magnetic field around it. This field then operates a mechanical switch which controls a larger electrical circuit

How do solenoids work?

When energised, generated magnetic field moves iron plunger which performs mechanical function

How does a maglev train work?

Trains suspended by magnetic fields, magnets provide levitation as well as propulsion

How does induction heating work?

Magnetic fields used to induce current in object to be heated. Current flow results in heating (Note: heated item must be metallic)

In a circuit, where should/can a voltmeter and ammeter go?

Volts are same in parallel, so total volts can be measured from both battery terminals. Amps can only be measured in series so Ammeter has to be part of the main circuit

How can you change a solenoid's polarity?

Changing direction of current through coil

How can you change a solenoid's magnetomotive force?

Higher current means stronger mmf, lower current means weaker mmf

What is the difference in magnetic field strength between a single conductor and a coil?

Coil is much stronger than single conductor due to the field being increased by the adjacent windings

If current in one conductor alongside a parallel conductor was increased, what would happen to the flux between them?

Would also increase

Large power cables are clipped to supports to prevent moving due to?

Forces created by fault currents (currents much higher than normal load)

What scientific notation is micro (µ)?

10-6