6) Magnetism & Electromagnetism Flashcards
Poles of a magnet
-north
-south
Law of magnetism
-Two like poles (S and S or N and N) repel each other
-Two unlike poles (S and N) attract each other
-The attraction or repulsion between two magnetic poles is an example of a non-contact force
Permanent magnet
-magnet that can produce its own magnetic field and will repel and attract other magnets when it is in contact with them
-made from magnetic material
-cannot be demagnetised
Soft magnetic materials
-get magnetised when an external magnetic field is applied
-demagnetised when the external field is removed
-electromagnets are made of soft magnetic materials
Hard magnetic materials
-magnetised when an external magnetic field is applied
-stays magnetised permanently when the external field is removed
Magnetic material examples
-iron
-cobalt
-nikel
-steel (iron + carbon)
Magnetic field
The region around a magnet where a force acts on another magnet or on a magnetic material
-strongest at the poles where field lines are closest
-weaker as distance from the magnet increases, field lines further apart
Magnetic field lines
-represents strength and direction of magnetic field
-lines close together: strong
-lines far apart: weak
-always go from N to S
-must never touch/ cross other field lines
Uniform magnetic field
Produced in the gaps between opposite poles
-same strength, direction at all points
-equal spacing between field lines
-arrow on each field line going from N to S
Induced magnets
When a magnetic material is placed in a magnetic field, the material can temporarily be turned into a magnet.
Investigate magnetic field pattern - bar magnet
- Place a compass on a piece of paper at the end of the North pole of a magnet
- Draw a point where compass points
- Place the compass next to the point so the needle points away from the dot
- Draw a new dot where the compass points
- repeat until there is a chain of points from one end of the magnet to the other
- Link the points using a smooth curve
- Repeat to create several other magnetic field lines
Electromagnetism
-When a current flows through a conducting wire a magnetic field is produced around the wire
-circular field pattern - no poles
-distance from wire increases, circles get further apart
Right hand thumbs (up) rule
Thumb - direction of current
Other fingers - direction of field
-reverse direction of current, reverse direction of magnetic field
Factors affecting electromagnetic field strength of current carrying wire
-size of current
-distance from long straight conductor (wire)
Motor effect
-A current carrying wire is produces a magnetic field
-this field interacts with other magnetic field (that it is placed in) so a force is applied on the wire
-force pushes the wire so it rotates
Fleming’s left hand rule
-thumb - direction of force
-pointer- direction of magnetic field
-middle - direction of current
Split ring commutator
-swaps the contacts of the coil
-reverses the direction current is flowing
-reverse the direction of force
-coil will continue to rotate
Motor effect - Speed which coil rotates can be increased by
-increase current
-increase strength of magnets
Motor effect - force supplied by the motor can be increased by
-increase current in the coil
-increase strength of magnet
-adding more turns to the coil
Magnetic field patterns - solenoid
-inside a solenoid, the fields from individual coils add to form a very strong almost uniform field along the centre of the solenoid
-one end acts like N pole - current travelling anticlockwise
other end acts like S pole - current travelling clockwise
-current enters: S/ current leaves: N
Magnetic force on a charge
When a charged particle in a parallel wire passes through a magnetic field, the field can exert a force on the particle, causing it to deflect
-parallel to field lines - no force
-at angle - small force
Motor effect - Loudspeakers
- a current in the coil creates a magnetic field which interacts with the permanent magnet generating a force, pushes the cone outwards
- the current is made to flow in the opposite direction
- the direction of the magnetic field reverses
- the force on the cone now pulls it back in
- repeatedly alternating the current direction makes the cone vibrate in and out
- the cone vibrations cause pressure variations in the air - which are sound waves
Increase strength of electromagnet’s magnetic field
-increase current in the coil
-add more turns to the coil
Increase strength of magnetic field around a solenoid
-increase size of current
-increase number of coils
-add an iron coil through the centre of the coils - iron will become an induced magnet
Electromagnetic induction
A voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it
Moving the wire into a fixed magnetic field
-wire cuts through the field lines
-induces a potential difference in the wire
Moving a magnet into a fixed conductor (coil)
-As the magnet moved through the coil, the field lines cut through the turns on the coil
-This induces a potential difference in the coil
Electromagnetic induction - Factors affecting induced pd
- The speed at which the wire, coil or magnet is moved
- The number of turns on the coils of wire
- The size of the coils
- The strength of the magnetic field
Alternator
-type of generator that converts mechanical energy to electrical energy in the form of alternating current
-slip rings and brushes provide a continuous connection between the coil and the meter
-induced pd in the circuit repeatedly changes direction as coil spins
-current deflects in both negative and positive
Dynamos
-direct-current generator
-same as alternator except has a split ring commutator
-changes the coil connections every half turn as induced pd is about to change direction
-current in external circuit always flows in the same direction
-varies from 0 to max value twice each cycle of rotation, never changes polarity
Transformer
An electrical device that can be used to increase or decrease the potential difference of an alternating current
Consists of:
-primary coil
-secondary coil
-iron core - easily magnetised
How transformers work
- An alternating pd is applied to the primary coil
- generating a changing mf around a coil
- mf induces an alternating mf in the iron core
- Iron core’s mf induces a pd across the secondary coil
- causes a current to flow along secondary coil
Step up transformer
-increases pd of a power source
-more turns on secondary coil than primary coil
-for electricity transmitted over large distances, current in wire heats them, energy loss
-transferred at high voltages from power stations
Step down transformer
-decreases pd of a power source
-fewer turns on secondary coil than primary coil
-for domestic uses
Relationship between output, input pd, number of turns
Pd across primary coil/ pd across secondary = no. of turns on primary coil/ number of turns on secondary
