Topic 15 - Electromagnetism Flashcards
What is an alternator?
An alternating current generator
What is a dynamo?
A direct current generator
What is an electromagnet?
An insulated wire wrapped around an iron bar that becomes magnetic when there is a current in the wire.
What is electromagnetic induction?
The process of inducing a potential difference in a wire by moving the wire, so it cuts across the lines of the magnetic field.
What is Fleming’s Left Hand rule?
Thumb = Movement, Index finger = Field, Middle finger = Current. Force, field current.
What is the generator effect?
The production of a potential difference using a magnetic field.
What is induced magnetism?
A non-magnet is induced to become a temporary magnet when in the presence of a magnetic field.
What is a magnetic field?
The space around a magnet or a current-carrying wire.
What is a magnetic field line?
A line in a magnetic field along which a magnetic compass points.
What is magnetic flux density?
A measure of the strength of the magnetic field.
What is the motor effect?
When a current is passed along a wire in a magnetic field in a magnetic field and the wire is not parallel to the lines of the magnetic field a force is exerted on the wire by the magnetic field.
What is a solenoid?
A long coil of wire that produces a magnetic field in and around the coil when there is a current in the coil.
What is a split-ring commutator?
Metal contacts on the coil of a direct current motor that connects the rotating coil continuously to its electric power supply.
What is a step-down transformer?
An electrical device used to step-down the size of an alternating potential difference.
What is a step-up transformer?
An electrical device used to step-up the size of an alternating potential difference.
What is a transformer?
An electrical device used to change an alternating voltage.
What are the two poles of a magnet?
North and south
What direction do magnetic field lines go?
North to south
Where are magnetic fields strongest in a magnet?
Around the poles
What do two of the same poles (north and north or south and south) do when in contact?
They repel
What is the general rule for magnetic poles?
Like poles repel.
Unlike poles attract.
What are lines formed at the end of magnets called?
Magnetic field lines.
What does a compass point towards?
It points south and away from the north direction.
What does induced magnetism cause?
A force of attraction between any unmagnetised magnetic material placed near one end of a bar magnet.
What is a solenoid?
A long coil of insulated wire.
Where are solenoids used?
In devices where a strong magnetic field needs to be produced.
When is a magnetic field produced in a solenoid?
When a current is passed through the wire.
How do you increase the magnetic field in a solenoid?
Increase the current.
Reverse the direction of the current.
What are the field lines like in a solenoid? What effect does this have on the magnetic field strength?
The lines are parallel to the axis of the solenoid and are all in the same direction.
As a result the magnetic field is much stronger.
What are the magnetic field lines like outside the solenoid?
The lines bend around from one end of the solenoid to the other end of the solenoid. The magnetic field is like the field of a bar magnet.
What is an electromagnet?
A solenoid in which the insulated wire is wrapped around an iron core. When a current is passed along a wire, a magnetic field is created around it.
How are electromagnets used scrapyard cranes?
Scrap vehicles are lifted using powerful electromagnets attached to cranes. The steel frame sticks to the electromagnet when current passes through the coil of the electromagnet.
How do circuit breakers use electromagnets?
It is a switch in series with an electromagnet. The switch is held closed by a spring. When current is too large, the switch is pulled open by the electromagnet and stays open until manually reset.
How do electric bells use electromagnets?
When an electric bell is connected to a battery, the iron armature is pulled onto the electromagnet. This opens the make-and-break switch and the electromagnet switches off. The armature then springs back and the make-and-break switch closes again making a repeated ringing sound.
How does a relay use electromagnets?
The relay is used to switch an electrical machine on or off. A small current through the coil of the electromagnet magnetises the iron core, pulling the armature onto the electromagnet.
What is the motor effect?
When a force can act on a wire in a magnetic field when a current is passed through the wire.
How can you increase the size of force in a solenoid?
Increasing the current
Using a stronger magnet.
What things about the wire affect the motor effect?
When the wire is perpendicular to the magnetic field, the force is greatest.
When the wire is parallel to the magnetic field lines, the force is zero.
What unit is magnetic flux density measured in?
Tesla
What does size of force in a conductor depend on?
Current in the conductor.
Length of the conductor.
Magnetic flux density of the magnetic field.
What is the equation for magnetic flux density?
Force = magnetic flux density x current x length
What is electromagnetic induction?
When a potential differnce is created across the ends of a wire when it crosses through magnetic field lines.
Why does an ammeter deflect when one end of a bar is pushed into the coil?
The movement of the bar magnet causes an induced potential difference in the coil.
The induced potential difference causes a current, because the coil is part of a complete circuit.
When is induced potential difference greatest in an alternator?
When the plane of the coil is parallel to the direction of the magnetic field.
When is induced potential difference zero in an alternator?
When the plane of the coil is perpendicular to the magnetic field lines.
What happens when the coil rotates faster?
The bigger the alternating current of the coil.
The bigger the peak value of the alternating value.
What are the differences between dynamos and alternators?
Dynamos are direct current, alternators are alternating current.
Dynamos have split-ring commutators whereas alternators have slip rings.
What kind of potential difference does a microphone generate?
Alternating potental difference.
What type of potential difference does a loudspeaker generate?
An alternating potential difference.
What is involved in a transformer?
Two coils of insulated wire, both wound around an iron core.
The primary coil is connected to an a.c. supply. When an a.c. is induced in the primary coil, an a.c. is induced in the secondary coil.
Why does an alternator work?
A.C. 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 a.c. between the terminals of the secondary coil.
What type of current is utilised in a transformer?
Alternating current.
What is the equation for potential difference across a primary and secondary coil?
P.D across primary coil / P.D across secondary coil = number of turns on primary coil / number of turns on secondary coil
What does electrical power supplied to an appliance depend on?
The current and potential difference.
What is the heating effect in a resistor proportional to?
The square of the current.
What happens if potential difference is made 4 times bigger using a step-up transformer?
The current needed to transfer the same amount of power is reduced by a factor of 4.
The heating effect in the grid is 16 smaller.
So power wasted due to the heating effect of the current is 16 times less.
This leads to an increase in efficiency.
