LAST MIN Flashcards
How are transformers used
The high current heats up wires so energy can be wasted so to reduce this transformers are used and high voltage cables.
- Step up make Voltage really high at 400,000 V and keep current low (usually in less comercial areas like across country
- step down decrease voltage to safe usable levels for consumers like in towns and homes
Describe how a power station generates electricity for the national grid
- burning fuel heats water which creates steam
- steam powers turbine
- turbine connected to powerful magnet inside generator
- the magnet spins, inducing a large voltage and a.c current and single output generated
- generator joined to national grid
How do loudspeaker work
coil wrapped around pole of permanent magnet
so a.c signal causes a force on the coil
-when current reversed the force acts i different direction
-this moves the paper cone as it vibrates it as pressure causes sound waves
How do microphones work
- using electromagnetic induction
- sound waves hit diaphragm which is attached to coil of wire which makes both move creating a current in the coil
How do Alternators work
- similar way to dynamos, force applied to rotate coil in magnetic field with an induced current in coil
- but instead of split ring has slip rings and brushes so the contacts dont swap every half turn which produces and alternating current
How do dynamos work
- generate a direct current by applying a force to rotate coil in magentic field (like in a motor)
- current is induced in coil which changes direction every half turn due to split ring communicator to keep current flowing in same direction
How does a transformer work
-use electromagnetic induction to change size of Potential difference of an alternating current by: increasing number of coils on secondary coil than primary coil in step down
what two situations can you get electromagnetic induction
when electrical conductor (coil of wire) and magnetic field are relative to each other
-this can be done by rotating/ moving/ changing polarity of magnet
when magnetic field through conductor changes (like in a transformer)
-this can be done by increasing STRENGTH of magnetic field, increasing SPEED, increasing TURNS ON COIL
what is electromagnetic induction in terms of Potential difference
induction of potential difference in a wire which is experiencing a change in magnetic field
What is a solenoid
describe the field lines:
a long coil of current carrying wire
- the field lines around each separate coil of wire line up in same direction so the field is strong and uniform
how can the direction of a motor be reversed
by swapping the polarity of the D.C supply or swapping the magnetic poles over
Describe how a motor works
- forces (which act on current in magnetic field) act on two arms of a current carrying coil
- they act in opposite directions so coil rotates
- the split ring commutator swaps the contacts every half turn to keep motor rotating in same direction
How can you find the size of a force
Force = magnetic flux density x current x length (of conductor)
in order to experience full force in a motor the wire needs to be at……
90 degrees to the magnetic field
if it runs along field it wont experience any force
When is air not an insulator
when its ionised enough that it becomes more conductive and sparks are made
What is ionisation in a electric field
Electrons in the air particles to be removed
How can sparking be explained with electric fields (and therefore static)
- When an object is statically charged it creates its own electric field
- interactions in this field cause sparking
What do electric charges do
Create an electric field around any charged object
stronger as you get closer
How does attraction by induction occur
If a negatively charged object is used to charge a neutral object by induction, then the neutral object will create a positive charge.
(The charged object that is brought near will always repel like charges and attract opposite charges.)
how does rubbing an insulator transfer electrons and create a static charge
negatively charged electrons are scraped off one material onto another
- as its an insulator the electrons aren’t free to move
- this builds up static electricity
- the material becomes electrically charged with a positive static.
How can energy of a system be changed
Mechanically (forces doing work), Electrically (by moving charge doing work), by heating or by radiation
What does power mean
How much work is done per second
-It is the rate of energy transfer and is measured in Watts
Whats is the effect of a gravitational field on objects
non contact force- the gravittional attraction between objects is caused by their gravitational fields interacting
What is the effect of magnetic fields on objects
and electrostatic force
interacting magnetic fields cause attraction and repulsion between magnetic objects
-electrostatic force causes attraction and repulsion between electrical charges and their electric field
What are the forces that occur when objects are in contact with each other
both objects feel an equal and opposite force called an interaction pair (3rd law)
-these pairs can be shows with vectors (arrows diagram)
Objects in equilibrium have a resultant force of…..
zero (this can be stationary or moving at constant speed)
Describe a situation where energy is transferred electrially
electric ions carry charge electrically from mains power supply to thermal energy store of metal plates
Describe a situation where energy is transferred by radiation
Toaster- coil of wire with high resistance heats and glows giving off infrared radiation and cooks the bread
What is a moment
The turning effect of a force
How do you calculate a moment
force x distance
How would someone get the maximum moment/turning effect
you could push at right angles as any other angle means smaller distance so smaller moment
What is the principle of moments for a an object in equilibrium
Sum of clockwise moments is = sum of the anticlockwise moments
What do leavers do
They make it easier to do work by transferring the turning effect by increasing the distance from the pivot where a force is applied
Explain how gears transmit rotational effects of forces
They fit together to tramit turning effects by interlocking,
a larger gear is slower but creates a large moment
a small gear is faster but has smaller moment
How can we work out speeds and moments of gears
gear ratios
