Section 6 - Electric and Magnetic Fields Flashcards
What causes a build-up of static?
Friction
What happens when certain insulating materials rub?
They pass negativelt charged electrons from one to the other
Why are the electrons not free to move in insulating materials?
The materials are insulators
What do electrically charged objects do?
Exert a force on one another
What does too much static cause?
Sparks
Why does static electricity cause sparks?
There is a potential difference between the two which will eventually cause the electrons to flow
Jake removes his jumper in a dark room. As he does so, he hears a crackling noise and sees tiny sparks of light between his jumper and his shirt
Explain the cause of this
As the jumper rubs the shirt, a static charge is built up where electrons are removed from one and deposited on the other. The charge will eventually build uo where the charge is large enough to jump across the air gap causing sparks
What uses static electricity?
Photocopiers
Electrostatic sprayers
What type of charges are in insulators?
Fixed Charges
What type of charges are in conductors?
Mobile Charges
What is the difference between conduction and induction?
Conduction is direct contact
Induction is no contact
How does a negative balloon stick to a neutral wall?
The electrons move from the protons in the wall, causing the balloons electrons to be attracted to the protons
How do insecticide sprays work?
The gun is charged
Each drop is charged, so each drop repels eachother meaning the spray is fine and spread
Whats special about painting car and bikes?
The object sprayed is often the opposite charge to the spray so the spray attracts to the object
When can static electricity be dangerous?
Refueling cars
Static on airplanes
Lightning
How can refueling cars be dangerous?
As fuel enters the car the static can build up this can lead to a spark
How can static on airplanes be dangerous?
It can interfere with communication equipment
How do airplanes build up static?
As they fly through the air friction between the air and the plane causes the plane to become charged
How does lightning form?
Raindrops and ice bump together inside clouds, leaving the top positive and the bottom negative leading to a high voltage and a big spark
How can electrostatic charge be stopped?
By earthing an object
What is earthing?
Connecting a charged object to the ground using a conductor
What does earthing do?
Provide an easy route for the static charges to travel into the ground
How do electrons flow when earthing if the charge is negative?
Away (down)
How do electrons flow when earthing if the charge is positive?
Towards (up)
Give two uses of static electricity
Any two from: Photocopiers Reducing pollution Painting cars Electrostatic sprayers Insecticide
What do electric charges create?
An electric field
Where is an electric field created?
Around any electrically charged object
When drawing field lines, what direction are the lines drawn?
Positive to negative
When drawing field lines, what angle are the lines drawn at
90*
When drawing field lines, what do the distance between the lines resemble?
The strength of the field
How many lines must you draw when drawing an electric field?
8
Where is the strength and direction of the field different?
The end of the plates
What is an electric field?
The region where an electric charge
experiences a force
What is a magnetic field?
A region where other magnets or magnetic materials experience a force+
What direction do magnets always go?
North to south
What do magnetic fields cause?
Forces between magnets
How many lines should be drawn for a uniform magnetic field?
3
What does a compass needle always line up with?
The magnetic field it’s in
How do you build a picture of a magnetic field?
Put a magnet on a piece of paper and draw round it
Place the compass on the paper near the magnet, the direction of the magnet represents the field line at this position
Mark the position of the magnet and then move the compass so in the direction it points
Repeat this until you get several field lines
When do magnets not point to the north?
When they’re not near a magnet
Why do compasses work on the Earth?
The core is magnetic so it generates its own magnetic field
What are the three main magnetic elements
Iron, nickel and cobalt
Explain the difference between permanent and induced
magnets
Permanent magnets are always magnets whilst induced magnets are only magnets in the presence of an magnetic field
What can induced magnets not do?
Repel
What always happens between a magnet and a magnetic material?
Attraction
What are permanent magnets?
Magnets that produce their own magnetic field all the time
Explain how the behaviour of a magnetic compass is related to
evidence that the core of the Earth must be magnetic
Without another nearby existing magnetic field, a magnetic compass always points to the north of the Earth, suggesting the core of the Earth is magnetic
What are induced magnets?
Magnets that only produce a magnetic field while they’re in another magnetic field
What happens if you put magnetic material into a magnetic field?
It becomes an induced magnet
Why is magnetic induction always attractive?
Because the south pole of the magnet will induce a north pole in the material and vice versia
What happens when you remove a induced magnet from a field?
It returns to normal and stops producing a magnetic field
What does how quickly a material loses its magnetism depend on?
How hard or soft they area
How fast do magnetically soft materials lose their magnetism?
Very quickly
How fast do magnetically hard materials lose their magnetism?
More slowly
What are permanent magnets made from?
Magnetically hard materials
What are the common uses of magnetic materials?
Fridge doors Cranes Doorbells Magnetic separators Maglev trains MRI machines Speakers and microphones
Give two differences between permanent and induced magnets
Perms produce their own fields but induced become magnets when they’re in a field
Force between a perm and induced is ALWAYS ATTRACTIVE
Force between a perm and perm can be attractive OR repulsive
What does a moving charge create?
A magnetic field
What must a current flow through to create a magnetic field?
A long, straight conducter
Describe the fields that are made when a wire has a current?
Concentric circles perpendicular to the wire
How do you work out the direction of the field after changing the direction of the current?
Right hand thumb rule
What is the right hand thumb rule?
Point your thumb in the direction of current and curl your fingers
The direction of your fingers is the direction of the field
How is a full force experienced in a motor?
The wire has to be at 90* to the magnetic field
What is flemings left hand rule?
thuMb is for Motion
First finger is for Field
seCond finger is for Current
What is magnetic flux density?
How many fields (flux) lines there are in a region
What does magnetic flux density show?
The strength of the magnetic field
How do you calculate the force from a motor?
F = B x I x L Force = MFD x Current x Length N = T x A x m
What is the length in the F = BIL equation?
The length of the condutor that the field is in
When does the F = BIL equation only work?
When the current is at 90* to the magnetic field it is in
A 35cm long piece of wire is at 90* to an external magnetic field. The wire experiences a force of 0.98N when a current of 5.0A is flowing through it
Calculate the MFD of the field
35cm = 0.35m
F = B x I x L B = F / I x L B = 0.98 / 0.35 x 5 B = 0.57 T
What are teslas also equal to?
N / Am
What is the equation for MFD?
MFD = Force / Current x Length Teslas = N / Am
Why do motors rotate?
Because the forces act in opposite directions
What is the function of the split-ring commutator?
It swaps the contacts every half turn
How can the direction a motor be reversed?
By swapping the polarity of the d.c. supply or swapping the magnetic poles over
How can you increase the strength of the magnetic field produced by a length of wire?
By wrapping it into a long coil with lots of loops
What is a solenoid an example of?
An electromagnet
Why does an iron core increase the field strength of a solenoid?
The iron core becomes an induced magnet whenever current is flowing
What is electromagnetic induction?
The induction of a potential difference in a wire which is experiencing a change in magnetic field
Explain how to induce a current via electromagnetic induction?
By moving/rotating a magnet in a coil of wire or a conductor in a magnetic field
How do you produce alternating current through electromagnetic induction?
By moving backwards and forwards or by rotating in the same direction
When can you get induction?
When a electrical conductor and a field move relative to each other
When a field through an electrical conductor changes
How do you increase the size of the induced potential difference?
By:
Increasing the strength of the field
Increasing the speed of movement/change of field
Having more turns per unit length
Describe how the magnetic field produced attempts to oppose the original change?
The current produces its own field which acts against the change that made it
What do transformers do?
Change the voltage
What can transformers only do?
Change the voltage for alternating current
What do transformers use?
Induction
What do transformers use induction to do?
Change the size of the PD of an alternating current
What do all transformers have?
Primary and secondary coils joined with an iron core
What is the role of the iron core in induction?
It is a magnetic material that produces an alternating magnetic field due to the coil which is also producing an alternating magnetic field
This changing MF induces a PD in the second coil
What do step up transformers do?
Step the voltage up
What do setp down transformers do?
Step the voltage down
Describe the coils in step-up transformers?
More secondary than primary
Describe the coils in step-down transformers?
More primary than secondary
How efficient are transformers?
100%
What is the equation for power?
Power = Current x Voltage Watts = Amperes x Volts
What is the transformer equation?
V1 x I1 = V2 x I2
or
VP x IP = VS x IS
A transformer has an input voltage of 1.6V, The output power is 320W
Calculate the input current?
P = I x V I = P / V I = 320 / 1.6 I = 200
State the input power equation for transformers?
Input Power = Output Power
What do dynamos do?
Generate direct current
How do dynamos work?
They apply a force to rotate a coil in a magnetic field and as the coil spins, a current is induced in the coil
What do alternaters do?
Generate alternating current
How do alternaters work?
They apply a force to rotate a coil in a magnetic field and as the coil spins, a current is induced in the coil
How are dynamos and alternators different?
Alternators have slip rings and brushes so the contacts don’t swap every half turn whilst dynamos have split-ring commutator
What do dynamos have that alternators don’t?
Dynamos have split-ring commutators whilst Alternators have slip rings and brushes
What do microphones use?
Electromagnetic induction
Explain how microphones work?
The sound waves hit a flexible diaphragm that is attached to a coil of wire which is surrounded by north and south of a magnet
As the coil moves, the a current is generated
What do microphones convert?
The pressure variations of a sound wave into current in a electrical circuit
In a loudspeaker, what is the diaphragm replaced with?
A paper cone
Explain how a speaker works?
The coil is wrapped around one pole of a magnet, so the AC signal causes a force on the coil, and when the current is reversed the force acts in the opposite direction which makes the cone vibrate, which makes the air around the cone vibrate
What do loud speakers convert?
Variations in current into variations in pressure
How do power stations work?
A fuel is burnt, heating up water which is converted into steam which then turns a turbine
The turbine is then connected to a powerful magnet inside a generator
As the turbine spins, the magnet spins with it inducing a large V and AC
What is the only type of power generator that doesn’t use a turbine?
Solar
What is the turbine connected to?
A powerful magnet
Why is fuel used in power stations?
To heat water
What is the national grid?
A network of wires and transformers that connects UK power stations to consumers
In the national grid, why are high voltages/currents needed?
To transfer loads of energy each second, a high power is needed which requires a high voltage or current
Why isn’t the national grid at such a high current?
High current makes wires heat up so loads of energy is wasted to thermal stores
How do you calculate the power lost due to resistive heating?
Power = Current² x Resistance
What is the equation linking input voltage and number of turns?
V1 / V2 = N1 / N2
or
VP / VS = NP / NS
Explain why step-up and step-down transformers
are used in the transmission of electricity in the national gri
To ensure the national grid has extremely high voltage, to reduce current and allow consumers to have a much smaller voltage if 230V
How does the rubbing together of materials cause static electricity to build up?
Friction will cause the transfer of electrons
Expalin the behaviour of a plotting compass that is far away from a magnet?
Will point towards the magnet
Describe the magnetic field around a current-carrying wire?
Concentric circles that are perpendicular to the wire
Explain why a current-carrying conductor in a magnetic field experiences a force?
Due to the interaction between the field and the magnetic field produced by the moving charges in the wire
