Physics IGCSE - Electromagnetic Effect

Positive and negative charges in electrical charge

Charge

Property of matter → charged object experiences a force from another charged object or in an electric field

UOM: columbs

positive charges repel other positive charges

negative charges repel other negative charges

positive charges attract negative charges → electrostatic attraction

Electric field

a region in which an electric charge experiences a force

direction of an electric field at a point is → direction of force on a positive charge at that point

electrostatic charging by friction

friction between two insulators being rubbed against each other.

How to check for static electricity

• attracts small paper pieces

• causes an electroscope's metal leaf to rise

charging of solids by friction involves only a transfer of (electrons)

Conductor

an object that has free electrons → conduct an electric current

Conductor examples

• copper

• aluminium

Insulator

does not have free electrons and therefore does not conduct electric current

Insulator examples

• plastic

• rubber

Forces between magnetic poles

• Like poles repel:

  • North (N) repels North

  • South (S) repels South

• Unlike poles attract:

  • North (N) attracts South (S)

Forces between magnets and magnetic materials

• Magnets can both attract + repel each other (depending on poles).

• Magnetic materials → e.g. iron, nickel, cobalt are:

  • Attracted to both North + South poles

  • Never repelled

• Magnetised material → acts like a magnet (has N and S poles)

• Unmagnetised material → can be attracted to a magnet but has no poles

Magnetic field

a region in which a magnetic pole experiences a force

Direction of a magnetic field at a point → direction of force on N pole of a magnet at that point

Differences between properties of temporary magnets (made of soft iron) and the properties of permanent magnets (made of steel)

Temporary magnets	Permanent magnets
Made of soft iron Easily magnetised Easily demagnetised Used when magnetism is needed for a short time (e.g. electromagnets)	Made of steel Hard to magnetise Hard to demagnetise Retain magnetism for a long time

induced magnetism

When a magnetic material becomes temporarily magnetised when placed near a magnet

Difference between magnetic and non-magnetic materials

Magnetic	Non - magnetic
attracted to magnets → e.g. iron, steel, nickel, cobalt	not attracted to magnets → e.g. plastic, wood, copper, aluminium

how permanent magnet differs from an electromagnet

Permanent magnet	Electromagnet
always magnetic + made from materials like steel fixed strength	magnetic only when electric current flows through it can be switched on/off strength can be adjusted by changing current

Conductor moving across a magnetic field or a changing magnetic field linking with a conductor → induce an e.m.f. across conductor

Factors affecting magnitude of an induced e.m.f.

• Speed of movement → faster motion of magnet or coil → increases e.m.f.

• Strength of the magnetic field → stronger magnetic field → larger e.m.f.

• Number of turns in the coil → more turns → greater induced e.m.f.

• Angle of movement → maximum e.m.f. is induced when the motion is at right angles to the magnetic field lines.

Describe a a.c. generator (rotating coil)

• A coil of wire is rotated in a magnetic field

• As the coil turns → cuts through magnetic field lines → alternating current (a.c.) in the coil

• Direction of current changes every half turn → a.c.

Describe use of slip rings and brushes where needed

• Slip rings are connected to the rotating coil + rotate with it

• Brushes are fixed + press against slip rings

• Allows current to be transferred to an external circuit without twisting the wires

• Use of slip rings ensures that the output is smooth a.c. → instead of flipping direction suddenly

graphs of e.m.f. against time for simple a.c. generators

Solenoid

a coil of wire

Pattern + direction of the magnetic field due to currents in straight wires and in solenoids

• For a straight wire:

  • Thumb points in the direction of the current

  • Fingers curl in the direction of the magnetic field lines (concentric circles)

• For a solenoid (coil):

  • Fingers curl in the direction of the current around the coils

  • Thumb points in the direction of the magnetic field (towards the north pole of the solenoid)

Effect on magnetic field around straight wires and solenoids of changing the magnitude and direction of the current

Around a Straight Wire

• Increase current → magnetic field becomes stronger (field lines are closer together)

• Reverse current → magnetic field direction reverses (use right-hand thumb rule)

In a Solenoid

• Increase current → stronger magnetic field inside and around the solenoid

• Reverse current → magnetic field direction reverses (north and south poles switch)

Force acts on a current-carrying conductor in a magnetic field\

effect of reversing:

• current

• direction of the field

Left hand rule

Right hand rule

Current-carrying coil in a magnetic field may experience a turning effect

Effect is increased by increasing:

• number of turns on the coil

• current

• strength of the magnetic field

d.c. motor

electrical device formed of a coil in a magnetic field. When a d.c. current is passed through it, the coil rotates

Operation of an electric motor

• coil of wire placed in magnetic field → connected to a direct current (DC) supply

• When current flows through coil → force acts on both sides of coil (using Fleming’s Left-Hand Rule)

• forces are in opposite directions → causes coil to rotate

Action of a split-ring commutator and brushes

Role of Split-Ring Commutator

• Reverses direction of current every half turn

• Keeps coil rotating in same direction

• Ensures continuous spinning motion

Role of Brushes

• Maintain electrical contact between rotating split ring + external circuit

• Allow current to flow into spinning coil without twisting the wires

Transformer

• A soft iron core → easily magnetised/demagnetised

• Two coils: primary + secondary

• Works via electromagnetic induction

Step - up transformer

more turns on the secondary coil → increases voltage

Step - down transformer

Fewer turns on the secondary coil → decreases voltage

Transformer equation

Transformer efficiency equation

This assumes 100% efficiency → no energy lost

Use of transformers in electricity transmission

• Transformers reduce loss in cables

• Step - up transformers → increase voltage → decrease current

• Step down transformers → reduce voltage

Power loss equation

• Power loss increases with current

• Increasing voltage reduces current → smaller losses

• High voltage → used for long distance transmissions