physics topic 7 Flashcards
what are the poles of a magnet
Where magnetic forces are strongest.
what are permanent magnets
they produce their own magnetic fields. Can attract and repel.
what are induced magnets
Material that becomes magnetic when placed in a magnetic field.
Induced magnets can only attract.
give examples of metals that are magnetic
Iron, steel, nickel and cobalt
what is a magnetic field
The region around a magnet where a force acts on another magnet (or magnetic material).
describe the direction of a magnetic field
The direction of a magnetic field line is always from north pole to south pole.
how can the magnetic field direction of a magnet be seen
Placing a magnetic compass in the magnetic field of a bar magnet causes the compass to point from north to south
how can the magnetic field shape of a magnet be seen
Place the bar magnet under a piece of paper and sprinkle iron filings over
the paper. Tapping the paper will produce the magnetic field pattern
how does a magnetic compass work
A magnetic compass contains a small
bar magnet which points in the direction of the Earth’s magnetic field.
what does the magnetic field pattern produced by compasses lead us to believe
Earth’s core is magnetic.
what is thought to be the origin of the earths magnetic core
movement of molten iron in the core.
what happens when a current flows through a conducting wire
a magnetic field is produced around the wire.
what does increasing the current flowing through an electromagnet do
increases the strength of the magnetic field.
what does Increasing the distance from the wire in an electromagnet do
decreases the strength of the magnetic field.
what is the right hand rule
Thumb points to direction of current
(+ to -)
Fingers curl to show direction of magnetic field
what is electromagnetism
When a current flows through a conducting wire a magnetic
field is produced around the wire.
what is a solenoid
a coil of wire
how can you increase the strength of an electromagnetic field
Have more turns on the COIL
Increasing the CURRENT
Have an IRON CORE
what is an electromagnet
A solenoid with an iron core is an electromagnet.
what does a relay switch do
It uses a solenoid to attract an iron
armature
why are relay switches used
Relays are used so that a
small current can turn on a
larger current in an isolated
circuit.
This reduces the amount of
thicker, more expensive,
wires needed.
Explain how an ignition circuit
works.
- closing the switch causes a current to pass through the electromagnet
- the iron core (of the electromagnet) becomes magnetised
- the electromagnet / iron core attracts the iron arm
- the iron arm pushes the (starter motor) contacts together
- the starter motor circuit is complete
- a current flows through the starter motor (which then turns)
describe how an electric bell works
Close Switch
Current flows through circuit
Making an electromagnet
Attracts iron strip (armature)
Causing hammer to hit gong
Circuit breaks
Electromagnet deactivates
Iron strip (armature) returns to original position
describe what happens when you put a wire carrying a current in a magnetic field
(force on a conductor)
A wire carrying a current
Has a magnetic field around it (right hand thumb rule)
If you put this wire into ANOTHER magnetic field
The magnetic fields will interact with each other (attract and repel)
Causing the wire to MOVE
what factors effect a force on a conductor
Magnetic Flux Density (B) in tesla
Current (I) in amperes
Length of Conductor (L) in
metres
how are the factors that effects a force on a conductor linked in an equation
Force (N) = Magnetic flux density (T) x Current (A) x Length(m)
when does the maximum force on a wire occur
Occurs when wire is perpendicular to the magnetic field Because it cuts through more of the magnetic field
describe the motor effect
When a conductor carrying a current is placed in a magnet
field the magnet producing the field and the conductor exert a force on each other.
describe Flemings left hand rule
THUMB = Direction of Force (motion)
INDEX FINGER = Magnetic field (N to S)
SECOND/MIDDLE FINGER = Current (+ to -)
how do headphones and loudspeakers use the motor effect
Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves.
how do speakers work
Alternating current creates a magnetic field around the coil
which interacts with the permanent magnet field
producing a force causing the coil/cone to move
Leading to vibrations which produces sound
when the direction of the current reverses, the direction of the force reverses
what is a generator
If you move the coil of wire in a magnetic field it generates an induced current
(electricity)
How to increase the size of the induced potential difference in a generator
Rotate coil faster
Stronger magnetic field
Increase number of turns of wire on coil
what is the Direction of Induced
Potential difference in a generator dependant on
dependent on the direction of rotation of the coil and the orientation of the magnetic
field.
what does Reversing the direction of rotation of the coil in a generator do
will reverse the direction
of the induced potential difference/induced current.
what does Swapping the polarity of both magnets do in a generator
will reverse the direction
of the induced potential difference/induced current.
describe how Generators/Alternators work
Alternators rotate a coil through a magnetic field
This induces a potential difference
As there is a complete circuit, a current is induced
Generators have a slip-ring commutator – allows coil to always remain in contact to the circuit
This induces an alternating current in the coil
Every half turn, the current and potential difference reverses direction (A.C)
describe how Dynamos work
Dynamos rotate a coil through a magnetic field
This induces a potential difference
As there is a complete circuit, a current is induced
Dynamos have a split-ring commutator –the two ends of the coil swap from one
brush to the other
so the direction of the induced current does not reverse every half rotation (D.C)
what angle should wire be to get max voltage in alternator and dynamo outputs
90 degrees
how do microphones work
Pressure from sound waves
Causes the diaphragm to vibrate
Which causes the coil to vibrate
Coil moves through the magnetic field
Which induces a potential difference
and current
why is the core of an electromagnet iron
because iron is quickly magnetised
describe step up transformers
Increase the potential difference so Vs > Vp
More turns on the secondary coil
describe step down transformers
Decrease the potential difference so Vs < Vp
More turns on the primary coil
how do transformers work
insulation prevents electricity/current flowing through the iron core
alternating current (a.c) in the primary coil
produces a changing magnetic field in the iron core
Which produces magnetic field in the secondary coil
induces an alternating potential difference across the secondary coil
and therefore an alternating current (a.c) in the secondary (coil)
how are transformer used in the national grid
to make the transmission
of electricity more efficient.
what does increasing the potential difference do to overhead power lines
decrease the current in the
overhead powerlines.
Potential difference and current are what
inversely proportional
