Medium is the Message Flashcards
Sensors that respond to the environment
doorbell responds to pressure of a finger, street lamps have sensors to measure the brightness of the sky, weather conditions
Position sensor
measures how far something has moved from a reference position - simple position sensor depends on rotary potentiometer
Where are fibre optic cables used most?
in long distance communication systems such as telephone systems and computer networks
How is most communication transferred?
by signals sent along a cable - electrical signals can be sent along cables made of copper or other good electrical conductors, alternatively visible or infrared radiation can be sent along fibre optic cables
How do fibre optic cables work?
the signal travels inside a thin transparent fibre meeting the surface at an angle of incidence greater than the critical angle therefore it undergoes total internal reflection and is confined to the interior of the fibre
Dispersion in a fibre optic
spreading means that a signal that starts as a sharp pulse is smeared out after travelling along the fibre
Multipath dispersion
the refractive index is the same throughout the fibre so the speed of propagation of the light through it is the same regardless of the route. Therefore when different bits of radiation take different routes the result is that a sharp pulse gets spread out and the longer the fibre the worse things get
How can multipath dispersion be reduced?
by limiting the range of angles of incidence that result in total internal reflection - this can be achieved by surrounding the core fibre with a cladding material whose refractive index is slightly lower tan that of the core to produce a stepped index fibre
Attenuation
loss of intensity - in metal cables occurs where there is a transfer of energy from the signal to the cable due in part to its resistance and leakage through the insulation
What are charge coupled devices (CCD) used for?
used to obtain 2D images
How do CCDs work?
They’re made up of pixels. When a photon is absorbed by a pixel, a photoelectron is released from the semi-conductor but remains trapped in the pixel. As more photons are absorbed, more electrons are released. If an image is projected onto a CCD then charge builds up in each pixel according to the number of photons that have reached it, in this way recording the image.
How is a CCD image read?
the electrons are electrically shunted along the CCD step by step producing a small pulse of current as each ‘package’ reaches the edge. The size of each pulse can then be processed and transmitted
CCD images
If the image is bright then only a short exposure time is needed as the CCD images are produced so rapidly there is no noticeable time delay
Capacitor
device that stores charge (energy - by separating electric charge)
When a capacitor becomes charged…
…electrons simply redistribute themselves between the plates making one positive and one negative - overall no net charge
Capacitance =
Charge / Voltage (C=Q/V)
Energy in a capacitor can be worked out by…
…the area under a charge voltage graph
CCD read-out
Each packet of charge has to be moved to an output system and measured. The CCD is designed so that each tiny light sensitive capacitor is separated from its neighbour by another capacitor that remains unaffected by light. First a temporary voltage is applied to the ‘empty’ capacitor so that the charge spreads into it. The first capacitor’s voltage is then reduced to zero, leaving all the charge on the adjacent one. This process is repeated until each capacitor’s initial charge gets to an output sensor where the voltage across the capacitor is finally measured.
Are LEDs conductors?
they’re semi-conductors
Semi-conductors
are materials that conduct less well than metals but they are not insulators
Doping
Adding atoms of another element - Semi-conductors conduct much better if a small number of atoms of another element are introduced to the crystal to make the average number of electrons per atom slightly more than four or less than four (this lowers the resistivity)
P type semi-conductor (p stands for positive but the atom is actually neutral)
the average number of outer electrons per atom is less than four. Some of the atoms have only 7 electrons around them with space for another one - a hole. An electron from a neighbouring atom can easily move into the ‘hole’, allowing good electrical conduction and leaving a hole next to another atom. If an electron moves from the left into a hole, then the hole appears to move to the right.
N type semi-conductor
If the average number of electrons per atom is more than 4, then the extra electrons cannot fit into the shell containing 8 electrons so they have to occupy a higher energy level shell. These higher energy electrons can move more easily, reducing the resistivity
A place where the 2 types of semi-conductor meet is called…
a junction (this is used in an LED)
Liquid crystal
is a liquid in which the molecules arrange themselves in some sort of ordered pattern
polar molecules
are charged - they’re neutral overall but the charge is not evenly distributed
Electric field strength
the force experienced by a charge of 1 coulomb, the force per unit charge
Electric field
a region in which a charged object experiences a force
E =
F/Q (force / charge)
E =
V/d (voltage / distance)
The field strength can also be equal to…
the potential gradient - the rate at which the potential changes with distance across the gap
Electric field lines show…
the path and direction a free positive charge would take in the field
The spacing between electric field lines show…
the strength of the field (closer together, the stronger the field)
The electric field between two parallel plates is called…
a uniform electric field (because the electric field strength is the same at every point between the plates and so all of the field lines are the same distance apart between the plates)
radial electric field
field around a point charge
Field lines of a negative charge
point inwards
Field lines of a positive charge
point outwards
The field of a point charge is…
not uniform - it’s stronger near the point charge where the field lines are closer together
Lines of equipotential
points around a point charge that are all at the same potential
Direct current
the voltage and current are always in the same direction
Alternating current
alternate directions
Average voltage or current is called
root mean sqaure
Inside a cathode ray tube there is…
an electron gun, vacuum, fluorescent screen, anode and cathode
How does a CRT work?
Electrons are released from an electron gun; they’re released from a negative electrode and accelerated in an electric field between the cathode and anode. The resulting stream of high energy electrons hits a phosphor screen causing it to glow. Around the outside of the tube are deflection coils; current in these coils produces a magnetic field that deflects the electron beam
Electron gun
An electric current heats the filament. During a process known as thermionic emission, electrons that have enough energy escape from the filament’s surface. These electrons and the radiation from the filament heat up a nickel cathode resulting in the thermionic emission of a large number of electrons from the cathode. A p.d. between the cathode and anode produces an electric field that accelerates the electrons.
How is an image produced using a CRT?
The electrons hit the phosphor screen and are brought to rest. Their kinetic energy is transferred to the atoms and molecules in the phosphor screen coating which becomes excited and then lose energy by emitting photons
Can use Fleming’s left hand rule in CRT…
to show force that arises due to the motion of the charged particles in the magnetic field - shows why electron beam is deflected in a magnetic field
Dispersion
causes a reduced signal/ changes to signal
In a fibre optic cable even is waves are emitted from the same source, they could travel different paths so…
…may take longer to reach the same point - due to path difference superposition may occur
When does a capacitor stop charging?
when it reaches the voltage (EMF) of the cell
What is an electrolitic capacitor?
the insulator, the diolitic only flows one way
What happens when a capacitor is connected to a power supply?
the electrons are pushed around the circuit to one side of the capacitor. They will repel the electrons on the other plate making them move away and round to the negative side making the other plate positive. When connected to the positive side the electrons flow back round the circuit redistributing the charge evenly
What is capacitance measured in?
Farads - 1 Farad = 1 Coulomb / 1 Volt
Energy
E = 1/2QV or E = 1/2CV^2
If capacitors are in parallel…
add the capacitance (opposite to resistance)
N type semi-conductor example
Silicon conducts very poorly, it has 4 electrons in its outer shell. Adding a small number of atoms of another element can increase the conductivity. If an element with 5 outer shell electrons is added, the extra electron cannot fit in the shell so occupies a higher energy level - free electron
P type semi-conductor example
Adding boron to silicon means an electron hole is formed. An electron from a neighbouring atom can easily move into the hole and the hole moves to the atom the electron left - drop in energy emits a photon
Equipotential
a line of constant potential - if a charge moves across an equipotential its energy doesn’t change
Electric fields can be…
…attractive and repulsive
Bubble chamber
Is a tool for research - a magnetic field will curve the trajectory of charged particles, oppositely charged particles will curve different directions and neutral don’t leave a trail so aren’t seen in the photograph
Magnetic force is always…
…perpendicular to the velocity; centripetal force
AC
alternating current, alternates between positive and negative
