How To Answer Any Q Flashcards
First look at the question with further physics and ask for assumptions
So assume no evaporation occurs, assume same mass distribution and g is constsnt etc
Then draw diagrams to make it easier in different cases
For example draw a diagram of befor and after WITHOUT ANY PHYSICS FIRST
Then make a sense of what’s happening
So talk through thoughts in head and what you expect to happen before it acc happens, and what your answe will depend on
Write a statement and move in
Now do the physics and a,the, state principles , lay down variables and make equations
Now if it reaches state,ent, then come to a conclusion
Remember world done = force x distance of object moved in direction if force
Thus f sin theta
Always draw force diagram first
Young modulus
F=kx is booked law, a spring extension is proprtinsl to firce applied until a limit of proptinal it’s
Then the gradient is spring constsnt or stiffness
In parallel springs , as force is halved or sitirbuted, the new spring constsnt is addition , as half force given ti both
But series same force given to both so they both extend, so series spring constsnt gets smaller using parallel rule
Extends until proprtinsl limit, then elastic limit, then after that I elastic and any work done area is used to deform the thing = plastic ferirmatjin
Energy = 1/2 f x , which is 1/2 kx2 if sun f for kx
It’s the same for young modulus, 1/f fx m need to rearrange those
May have to it egrets
Deforming and loading
Loading is what happens when firce applie,d unloading when removed
For a spring this should be same, until you go last elastic limit, it will return to deformed area because of atom binding and area is used to deform
Rubber is hysterics graph, it doesn’t deform, but energy is given out as heat when deforms ti orig a, kefntg
More work is done when stretching, then is r,erased when letting go, thermal energy released, so rubber thing, need to cool so pressure equalsie
Young modulis
Stress / strain, where stress is force / area and strain is x/ L
For a metal
This is a prototype thst regsrdless of dimensions will be a thing for each object, a special protects
- originally it extends according according ti hooked law, then it no longer portions until it reaches a point. Elastic limit
- here there are yield points, where the alloy big atoms unlock from small, leading ti accelrwtion and thus easier to extend so less force so goes down, then it locks again
- now it just oaksticlsly deforms until UTS point, this is max stress it can take before breaking now
- past this, it thins necking, similar to where plastering pulled, and it stenches very easily so goes down u til ti just breaks
Other stress stain groans
Iron and glass
They just break , and don’t reach any yew points
For metal
Okay and area is energy density, energy stored per unit volume = similar to stress so if they give modules can work spout strjan
Is it always good to have high young modules for enginerring?
No = aerolpkane wants made form aluminium alloy that is string and stiff so high young modules
And for example car numbers need to deform quick ti reduce time for change if moentum
Normal Newtonian pair ?
You exert reaction force down and it enters one up
Reason why no accelrwtion this plane is because nrowml from ground is equal to weight
Until there is accelrwtion snd this changes
When you jump to earth whey doesn’t earth move to you
It does
But because earth inertia so big, it resists this accelrwtion more and doesn’t seem like anything baoloend
Momentum
Is a vector quantity
Is always conserved
Total momentum before = momentum after
Principle is for a system of interacting objects, the total momentum in a specified direction and
plane is constsnt, as long as no external forces act or the system !
For a gun providing no external force, the bullet and gun is the system with koentum zero
When you shoot buller in velocity, go;will she to recoil so monetum is conserved.
Elastic collision is where kinetic energy is conserved , and I least if is where ke isnt, transferred to other forms
In an elastic collision, one will stop and otherkeep going, or two at same speed will collide and bounce off
Easktic only if a same mass I believe
Both cases total energy still conserved yh , and momentum tok ofc
Second law is rate of change if meontu
When crash your momentum gins go from something to 0 confirmed, you want the time taken to be huge so that force is less, so air bags seatbelt bumper deform to save you
He is koentum always conserved in collision or two interacting
Basically two interacting, the net force of the overall system is 0
So the change if momentum rate must be 0
Thus change in momentum gotta be 0, so koentum is conserved
Impulse ideas
Impulse is fxt which is change if koentum
This is always conserved
So momentum conserved and you can use impulse to find change of koentum
And momentum conserved in all planes so csn make equations, if momentum in y plane wa s9 etc
Cruet snd mean drift velcoutn
Current is the rate of flow of charge, can be ions etc, in which charge is x2
Ammeters no resistance so max current can go through ideally
cruet = elementary charge x mean drift velocity x number density (which is number of FREE ELECTRONS PER CUBIC METER)
So things with higher number density will tend to have higher current, this is the conductors, semi in middle and insulators bottom
And so when number density increases, when temperature breaks covalent bonds in the semiconductors, current increases and resistance thus decreases
Kirchhoff first law is that current going into a jus ti on = ti current out, so if there is no junction current same ebeyrbwer (series circuit )
This related to conservation of charge, where the charge can’t be created or destroyed, charge is fielded as charge CANT be created, the charge going in = charge going out
Volt
Remember batteries is vector addition, they cancel out if 3 and two one way and one other
Don’t conritbute ti viktsge but do for total internal resistance!
Voltmeters have to have infinite resistance so max voltage goes through them and no current goes through the voltmeter jr self
Emf is work done to charge carriers and pd is work done by charge dwrrueee
Emf = pd in a closed loop = Kirchhoff second, and relates to conservation if energy
Thermionic emissions do the electron gun, how does it work?
Small filament heated by an electric current, this gives the electrons enough kinetic energy to escape the surface of the metal = thermionic emission
If the filament is connected to an anode, filament becomes the cwbtide and if in a vacuum, the electrons will then go towards anode
If hole in cathode then you have a beam of electrons
Where the electrical work done given to vQ = keintic energy
Resistance and ohms law
Ohms law is that fir a wire metal at constsnt TEMPERTAURE (because then resitivity changes), the current of the wire is directly proptinal to voltsge
Okay if you have something insulate, more current hat goes past the current will heat snd insulator keep heat, this increase this area where the metal positive ions gain kinetic energy and can start to vibrate, acting as an impeding to current flow, this means less current fir same voltage so resistance increases
IV characteristics
Fixed resistor = straight, obeys ohms law , and gradient 1/ is resurgence
Filament lamp, as voltage uncrease and current increase heat increases in metal filament which causes vibration and impediment so less current and more resistance. More resistance = less gradient so decreases
Diodes
On,y allow curetnone way, so on or off, so controls all electrics
LEDs emits light when conduct through the,
Vi graph is that reverse it blocks do infinite resistance, and takes a bit of current before resistance evituskky tends to 0, this often threshold pd
Differn tmwteirals have different resitivities which allows different resistances , but this changes at differnt temps
Yh for semi it decreases allowing more current
Thermistor and ldr
Negative temp cieffeicnr means as the temp goes up reistsnce decreases, this because number density increases more current and so less reistsnce
Same for light
Graph of restsice. Against temp is 1/x , inversely proptinal
If charvwtristic if wnthermsitor
As current increase,so heat increases, which increases temp and thus decreases resistance so looks like it goes to infinty tan graph
Internal resistance
When batteries in series, the emf adds but so does the reistsnce, meaning you can get more pds
When in parallel, emf same, but rest is net decreases, so higher currents
For different things differnt used
Power is the thing thst kills
Potentiometer
Use potentiometer to measure v and I as you can range the whole voltage
Closer you bring it so length of resistor in wire increase,s more closer to VIn, assuming that’s only comoiej t
Better because
- compact, change in restsnce linear log
- can vary thr whole thing
Otherwise thou want something somewhere in between for maximum range
Remember in refraction what happens to wave,wntgn
Remember that refraction is that another mdieum is more dense optically and thus speed is less in there, but for same frequency this reduced wavkentgh and so waves become closer together and bend towards normal!
Deep water faster
Diffraction is oheneomea thst when a wave goes past a gap similar to their wavelength, it spreads out around
The more smaller the wavelength is compared to the gap the more diffraction is observed
This is why in young modules you want INITAL slit to be quite thin to ilium it’d other two slitd
Polarisisitom is confining electromagnetic or transverse waves into one plane, can’t be log tofunal
You’ll have a filter thst will do this and all components of it into that plane too, dlr elstive intensity falls
This is proptinal to cos theta maybe?
Anyways intend to I= power /area, where r is radius as it radiates radially, and it’s area of a sphere 4 pi r2
Intensity is then proptinal to amplitude sqaured too, and amplitude is the measure of energy
So when amplitude falls, the intensity falls too and thus becomes whiter or louder
Aroeals are often polarised to get different planes things so they don’t interfere as much
