Using Magnetic Fields To Keep Things Moving Flashcards
What is the magnetic field
The region around the magnet in which a piece of iron or steel will be attracted to it
What are the lines formed when iron fillings placed near a magnet form a pattern called
Lines of force or magnetic field lines
A plotting compass placed in the magnetic field will always point ……
Along a field line
What happens when like poles are brought together
The repeal each other
What happens if unlike poles are brought together
The attract each other
What is produced when a current flows through a wire
A magnetic field is produced around the wire
How is an electromagnet made
By wrapping insulated wire around a piece of iron called the core
What happens to the iron when a current flows through the wire
The iron becomes slowly magnetised
When the current is turned off what happens to the iron
It loses its magnetism
Where are electromagnets used
Scrapyard cranes, circuit breakers, electric bells and relays
What is the motor effect
When a current is passed along a wire 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 a magnetic field
When is the force from the motor effects a maximum
If the wire is at 90 to the magnetic field and zero if the wire is parallel to the magnetic field
What is Flemings left hand rule used for
To determine the direction of the force
Explain the fingers in Flemings left hand rule
The first finger represents the magnetic field (north to south)
The second finger represents the current (pointing poz to neg)
The thumb is the direction of force
What is the size of the force increased by
Increasing the strength of the magnetic field
Increasing the size of the current
When is the direction of the force reversed
If either the direction of the current or the direction of the magnetic field is reversed
How is the speed of a motor increased
Increasing the current
How can the direction of the motor be reversed
By reversing the direction of the current
Why does a coil spin when a current passes through the coil
A force acts on each side of the coil due to the motor effect
The force on one side of the coil is in the opposite direction to the force on the other side
What does the split ring commutator do
Reversed the direction of the current around the coil every half turn.
Why is the coil always pushed in the same direction in a split ring commutator
Because the sides swap over each half turn
What happens if an electrical conductor cuts through magnetic field lines,
a potential difference is induced across the ends of the conductor
What is electromagnetic induction
if a magnet is mov into a coil of wire A PD is induced across the ends of the coil. If the wire or coil is part of a complete circuit a current passes through it
What happens if the direction of movement of the wire or polarity of the magnet is reversed
The direction of the induced PD is also reversed. A PD is only induced while there is movement
How is the size of the induced PD increased
The speed of the movement
The strength of the magnetic field
The number of turns on the coil
What does a transformer consist of
2 coils of insulated wire, called the primary coil and the secondary coil, which are wound into an iron core
What happens when an ac passes through the primary coil in a transformer
It produces an alternating magnetic field in the core, this field continues to collapse and expand
What happens when the alternating magnetic field lines pass through the secondary coil
It induces an alternating PD across its ends
What happens if the secondary coil is part of a complete circuit and the alternating magnetic field lines pass through it
An alternating current is produced
Why are the coils of wire insulated
So the current does not short across either the iron core adjacent turns of wire but flows around the whole coil
What do step up transformers do
Make the PD across the secondary coil greater than the PD across the primary coil. It’s secondary coil had more turns than its primary coil
What do step down transformers do
Make the PD across the secondary coil less than the PD across the primary coil. It’s secondary coil has fewer turns that it’s primary coils
What does a switch mode transformed have
A ferrite core
What does a switch mode transformer do differently to traditional transformers
Operates at a much higher frequency
Is lighter and smaller
Uses very little power when there is no device connected across its output terminals
What would happen if if transformers work with dc not ac
It do passes through the primary core a magnetic field is produced in the core but it would not be continually expanding and collapsing so no PD would be induced in the secondary coil
Why does Ypres national grid use step up transformer
The higher the PD at which electrical energy is tenasmitted across the grid, the smaller the energy wasted in the cables
Why are step down transformers used
To make the PD safe for household use
A step down transformer has less …..
Turns on the secondary coil that the primary
A step up transformer has …….
More turns on the secondary coil than on the primary coil
What efficiency do transformers have
Almost 100%
What does an electrocardiograph measure
The PD generated by the heart
What do digital thermometers measure
Temperature
What do mr scanmers do
Use mpradio waves to produce detailed pictures of the body
What is endoscopy for
Seeing inside the body without incisions
What are X-rays for
To take pictures of suspected Brocken bones
What are cut scanners used for
To build up digital pictures of a cross section through the body
What are the ends of magnets called
Magnetic poles
