P15(Electromagnetism) Flashcards
What is a magnetic field
The region around the magnet
Like poles …
Repel
Unlike poles …
Attract
Where is the magnet the strongest
At the poles because magnetic field lines are denser at the poles
Where is the magnet the weakest
In the middle because field lines are less dense
What are the 3 magnetic metals
Iron, nickel and cobalt
What is induced magnetism
When an unmagnetised magnetic material can be magnetised by placing it in a magnetic field
The Magnetic filed lines of a bar magnet curve around from the ….
North Pole of the bar magnet to the South Pole
For a freely suspended magnet its north seeking pole will point …
Towards geographical north
For a freely suspended magnet its south seeking pole will point …
Towards geographical south
How to do the right hand rule
-A current carrying conductor has a magnetic field around it
-Point a thumb towards the direction of the current
-Wrap fingers around the conductor
-this gives the direction of the magnetic field
What is a solenoid
A hollow coil of insulated wire
The magnetic field lines around a wire are …
Circles centred on the wire in a plane perpendicular to the wire
The magnetic field lines in a solenoid are …
Parallel to its axis and are all in the same direction
What is a uniform magnetic field
the magnetic field lines are parallel
How do you produce a magnetic field around a solenoid
When a current is passed through the wire
Increasing the current in a solenoid …
Increases the strength of the magnet
Reversing the direction of the current in a solenoid …
Reverses the magnetic field lines
What is an electromagnet
a solenoid in which the insulated wire is wrapped around an iron core
Where are electromagnets used
-scrapyards
-circuit breakers
-electric bells and relays
How does an electromagnet work in a circuit breaker or electric bell
Attracts an iron armature which opens a switch
An electric motor has a …
Coil that turns when a current is passed through it
What is the motor effect
When a force can act on a wire in a magnetic field when current is passed through the wire
How do you increase the size of the force in the motor effect
-increase the current
-use a stronger magnet
-increase the number of coils
What does the size of the force depends on
The angle between the wire and the magnetic field lines
The force (in motor effect) is ….
-Greatest when the wire is perpendicular to the magnetic field lines
-zero when the wire is parallel to the magnetic field lines
What does Flemings left hand rule show
The direction of the force on the wire
How to do the left hand rule
What does the size of the force on the conductor depend on
-the current in the conductor
-the length of the conductor
-the magnetic flux density of the magnetic field
What is the formula that relates force, magnetic flux density,current,length
What is magnetic flux density
A measure of the strength of a magnetic field
How does the coil spin when a current is passed through the coil
-A force acts on each side of the coil due to the motor effect
-The force on one side is in the opposite direction to the force on the other side
What are the parts of an electric motor
-concave shape magnets
-coil(armature)
-split ring commutator
-brushes
What do the concave shape magnets do
keep rotating the coil perpendicular to the direction of the magnetic field to exert maximum force
What is the coil(armature)
Rectangular coil of insulated wire which is forced to rotate
What is the split ring commutator and what does it do
-made up of metal contacts attached with a coil and rotate with it
-their job is to reverse the direction of the current every half turn so that the coil rotates in the same direction
What are brushes and what do they do
-made from graphite which is a conductor and slippery and low friction
-they make contact between the slip rings commutator and battery contacts
Forces on either side of the coil ….
-equal in magnitude
-opposite direction
-forces do not share the line of action
What is the generator effect
When there is a relative motion between the conductor and a magnet, alternating PD is produced
A generator contains …
Coils of wire that spin a magnetic field
What is electromagnetic induction
When a conductor crosses through the lines of a magnetic field a potential difference is induced
The direction of an induced current always…
Opposes the original change that caused it
How do you increase the induced PD in the generator effect
-stronger magnet
-move the magnet/conductor faster
-longer conductor
How is a simple AC generator made
Made up of a coil that spins in a uniform magnetic field
How do AC generators work
-As the coil rotates, Magnetic flux linking to the coil changes which induces alternating pd
-When the side of the coil moves parallel to the field lines, induced pd is zero
-As the coil rotates, magnetic flux linking to the coil increases and so does induced pd
-When sides of the coil cuts through the magnetic field lines at right angles, induced pd is maximum
-For a next 1/4 turn pd reduces to zero again
-For next 1/2 turn, induced pd reaches maximum and then to zero but in the negative direction
How is a simple DC generator made
Has a split ring commutator instead of two slip rings
How do DC generators work
-As the coil rotates, split ring commutator reconnects the coil in the opposite direction in the circuit every half turn
-This happens each time the coil is perpendicular to the magnetic field lines
-Induced PD does not change direction
-Induced PD varies from max to zero twice each cycle and does not change polarity
What is a microphone
A microphone converts sound signals to electrical signals
How does a microphone work
-An insulated voice coil placed in a magnetic field moves back and forth, when sound hits the diaphragm
-There is relative motion between a coil and a magnet
-This induces electrical signals in the voice coil
How does a loudspeaker work
-Voice coil is placed between the poles of a permanent magnet
-Diaphragm is connected to the voice coil
-Electrical signals are passed on to the coil in the magnetic field
-Diaphragm feels a force back and forth due to AC
-Cone vibrations produce sound waves
Transformers are used to …
Increase or decrease the size of an alternating potential difference
A transformer only works with an …
AC because a changing magnetic field is necessary to induce AC in the secondary coil
What are transformers made from
-a primary coil
-a secondary coil
-an iron core
How does a transformer work
-Alternating current passing through the primary coil generates an alternating magnetic field in the iron core
-The lines of the alternating magnetic field in the iron core pass through the secondary coil
-The magnetic field in the secondary coil induces an alternating potential difference
What do step up transformers do
Increase the size of an alternating potential difference
What do step down transformers do
Decrease the size of an alternating potential difference
What is the transformer equation
For a step up transformer the number of secondary turns is …
-Greater than the number of primary turns
-So the potential difference across secondary coil is greater than the potential difference across primary coil
For a step down transformer the number of secondary turns is …
-Less than the number of primary turns
-So potential difference across secondary coil is less than the potential difference across the primary coil
Transformers are …
100% efficient
What is the formula if 100% efficiency is assumed
How is PD induced in the secondary coil in the transformers
-Alternating PD in the primary coil induces alternating magnetic field
-This alternating magnetic field links with the secondary coil
-This induces alternating PD in the secondary coil
-If there are more coils in secondary coil, induced PD is higher
