PHYSICS 30

what are systems?

• opem
  can exchange both matter and energy with surroundings
  external forces act
  mechanical energy is not conserved but overall energy is

• closed
  can exchange energy but not matter
  not subjected to external forces

• isolated
  neither matter nor energy can be exchanged
  not subjected to external forces
  no energy gained from applied forces
  
  

What is work?

• ability to move object through a distance

• scalar

• W = Fdcoso

What is mechanical energy?

• sum of potential/kinetic energy (Principle 5)

• remains constant in an ideal/isolated system

• law of conservation of energy Ei = Ef

• mgh + 0.5mk² + 0.5mv² = mgh + 0.5mk² + 0.5mv²

• mechanical energy not conserved if system isn’t isolated

• work-energy principle: W=E

What is linear momentum?

• product of mass and velocity

• vector → direction is the same as that of velocity

• p = mv

• p = momentum (kg m/s)

• m = mass (kg)

• v = velocity (m/s)

Momentum will change if:

• mass changes
  force applied to change either magnitude or direction of velocity

• change in p = m(change in velocity) = m(vf-vi)

what is newton’s second law, regarding momentum and impulse?

• rate of change of momentum of a body is equal to net force applied to if

• F = ma, but a = (vf-vi)t

• so F = m(vf-vi)/t → F = change in p/t → Ft = change in p

what is impulse (Ns)

• = change in momentum (kg m/s)

• increasing time of impact and resulting acceleration

• if m(vf-vi) constant, then force will go up as time goes down and vice versa

• ex) airbags and bouncing

• mathematically = change in momentum

• considered of discussing force and time

• direction of impulse = direction of force on object in question

How to interpret graph areas?

• force-displacement graph
  area between graph/x-axis = work
  

• force-time graph
  area between graph and x-axis = impulse

• divide graph into shapes of unknown area to calculate

What are the three conservation of momentum scenarios?

• 1. Hit and bounce
  pi1 + pi2 = pf1 + pf2

• 2. Hit and stick
  pi1 + pi2 = pf(1+2)

• 3. Explosion
  pi(1+2) = (usually 0) = pf1 + pf2

What is momentum conservation?

• law of conservation of momentum
  total momentum of system remains constant if no external forces act

• pi = pf

• ex) rocket propulsion → backward momentum of expelled gases balanced by forward momentum of expelled gases balanced by forward momentum gained by rocket

what is 1d conservation?

• pi = pf

what is 2d conservation?

• momentum conserved in both x and y directions

• pxi = pxf and pyi = pyf

• use trig rations to determine components of initial momentum (soh cah toa)

  • use pythogoras to determine magnitude and direction of unknown momentum

what is conservation of energy in collisions?

• total energy conserved by collisions classified based on Ek conservation

what are elastic collisions?

• total kinetic energy conserved

• occurs when objects collied being permanently generating

inelastic collisions occur when:

• objects permanently deformed

• total kinetic energy not conserved
  total energy still conserved therefore momentum still conserved → energy converted to other forms such as thermal or potential

• completely inelastic collision → occurs when two objects stick together

• LOOK AT NOTEBOOK FOR SYSTEM OF COLLISIONS TABLE

p of object changes when?

• if there is net force exerted on object by other objects (consider - is Q about system or objects?)

• internal forces are forces exerted BETWEEN OBJECTS in the system

• internal forces will be action-reaction due to newtons third law

What are electrostatics?

• electricity at rest

• electric charge in an atom
  atoms electrically neutral
  # protons (+) = # electrons (-)

What is a charged atom?

• ion
  positive ion → lost one or more electrons
  negative ion → gained one or more electrons

What is rule of charge?

• opposite charges attract

  • like charges repel

What are conductors?

• materials in which outermost electrons free to move and flow

• most metals

How will charge distribution change on conductors?

• solid sphere: evenly on surface

• solid flat conducting plate: evenly on surface

• hollow conducting object: distribute evenly on outer surface - no excess charge on inner surface (regardless of shape)

• irregularly shaped solid: greatest charge density at points/curves of surface

• parallel plates: evenly on inner surface

What are semi-conductors?

• materials that allow few electrons to pass through

• silicon, germanium → used in transistors and computer chips

Insulators

• materials that do not allow electrons to easily pass through (electrons tightly band to nucleus)

• charge distribution - not even → of charge transferred to particular location, e- remain at that location

• ex) compare rubber insulator to conductore: wood, rubber, glass, (can conduct electricity very slightly)

What is the law of conservation of chare?

• net amount of electric charge produced in any process is zero

• electrons cannot be created nor destroyed, only transferred from one material to another

• q = chare

• q individual = (q1 + q2 + q3 +…)/# of objects

• * objects that are similar in size will share charges evenly, whereas much larger objects in contact with smaller objects will not

What are the types of charging?

• friction

• induction → objects do not touch

• conduction → objects do touch

What is charging by friction?

• objects charge by rubbing together
  glass (+), silk (-) → unique instance
  amber (-), wool (+)
  plastic (-), cotton (+)
  rubber (-), fur/wool (+)

• usually rougher objects become positively charged

What is charging by induction?

• temporary charging
  - hold charged rod near neutral object → causes charge polarization (separation) within object (overall object still neutral, where charges distribute by separating within object)

• if rod removed, object remains neutral

• if object broken in half before rod removed, now would have 2 permanently charged objects

• greater the charge on rod, the greater the separation

What is an electroscope?

• device used to detect charge

what is permanent charging?

• hold charged rod near neutral object causing charge separation

• ground object to induce permanent charge
  ground → provide path for electrons to enter or leave object

• 1. remove ground

• 2. remove charged rod

• if charged rod removed first → object will remain neutral as electrons leave/return through the ground.

How to determine the unknown charge of a rod?

• induce known charge on es (not neutral)

• bring unknown rod towards es

• if leaves converge (come together)→ charge is opposite as that of es

• if leaves diverge → charge is same as that of es.

What is charging by conductino?

• electrons transfer from one material to another when objects touching

• ex) if pith ball suspended by string and negatively charged rod brought near → charge separation on pith, therefore rod and pith will attract

• when objects touch → e- transfer to pith causing instant repulsion → pith and rod now both negative

What is electric force?

• F prop to q1q2, F prop to 1/r²

• Electric F = (kq1q2)/r²

• Grav F = (Gm1m2)/r²

Misc. for electric force?

• 1C = charge of 6.24 × 10^18 electrons

• elementary charge for prot/elec = 1.6 × 10^-19
  protons → (+)
  electrons → (-)

• when calculating → use absolute value of charge

• use sign for determining attractive vs repulsive force.

what is a field?

• invisible region of influence that causes objects within that region to experience a force

• explains how force acts ata distance

• concept developed by michael faraday → extensive work in electromagnetism

what is electric field’s test object?

• positive charge small enough so as not to affect field of producing charge

• this means that positive fields push that test object (radial outwards)

• and negative fields pull test object (radial inwards)

• negative charges used to determine direction, but not included as part of calculation