Physics/AP Flashcards
what is a charged object and when is an object positively or negatively charged
- a charged object is one which has gained or lost electrons
- a negative object has gained electrons
- a positive object has lost electrons
how can we charge insulators and why does this happen
- insulators can be charged by friction, when two insulators are rubbed together their electron clouds interact and there is a transferal of electrons
what occurs due to this transferal of electrons in insulators
- you get two equal and oppositely charged objects
- they don’t immediately discharge because electron movement in insulators is low
what are some dangers/problems with static electricity/charge and how do we prevent them
- dirt can cling to statically charged monitors
- refueling aircraft can be dangerous if a static charge builds up in the pipe used to transfer the fuel
- we use earthing cables to carry the charge away to prevent this
what is the difference between ac and dc
- DC is the constant flow of current in one direction, it is a constant voltage circuit
- AC is the flow of electrical charge which periodically (sinusoidal) changes direction
what is the IV graph for a resistor and filament lamp
a resistor has a straight line where grad = 1/R
a filament lamp has -ve N^2 shape for a half curve where x>0 and +ve N^2 shape where x<0 for half a curve
how does this change with VI graphs
resistor = the same but gradient is R
filament lamp has increasing/decreasing gradient – opposite to IV
define a magnetic field and how we can detect one
“a magnetic field is a field surrounding a permanent magnet or current carrying conductor in which magnetic objects will experience a force”
- we can detect them using a plotting compass
what to remember when using magnetic field lines to plot magnetic fields
- lines go North Pole to South Pole
- equally spaced = uniform field
- closer lines = stronger field
- strongest field usually at both poles
- N-N and S-S repel
- N-S and S-N attract
what happens when a conductor carries a current
a magnetic field is created around the wire
why does this field around a current carrying conductor occur and what is the difference to permanent magnets
- electrons in the wire have a charge
- when any charged particle moves it creates a magnetic field around it
- in permanent magnets, it’s due to aligned fields around the nuclei of the atoms due to the electrons orbiting them
what does a magnetic field around a current carrying conductor look like and how can we remember it
- for a current carrying conductor, the magnetic field lines are concentric circles at 90degrees to the wire
- right hand rule = hold out your right hand as a thumbs up, the thumb is the direction of the conventional current and the fingers show the direction of the magnetic field lines around the wire
what does the magnetic field look like if its a long coil of wire (solenoid)
- it occurs as straight evenly spaced lines within the coil (uniform field)
- but then some lines loop back round to the other end
- outside of the coil between poles it is also uniform
what is a magnetically hard material
one which can be permanently magnetised
what is a magnetically soft material
one which can only be temporarily magnetised
what is an induced magnet
- an induced magnet only becomes magnetised when in a magnetic field
- they can’t be repelled, only attracted by other magnets
- they lose their magnetism once removed from the field
what are the factors on the strength of electromagnetism
- number of loops in the electromagnet
- adding/changing a metal core
- current, a greater current = more electrons = stronger field
- increasing wire size can decrease resistance so increase current
what occurs to a current carrying conductor when placed in an external magnetic field
- it experiences a force as the magnetic field produced by the wire interacts with the external magnetic field
how can we remember the important bits about a CCC in an external field
- Fleming’s left hand rule
- use thumb, index finger and middle finger all at right angles
- Mr Fleming’s Cat going down
- i.e. Motion on thumb, external Field on index, Current on middle
what are the magnitudes of the forces experienced by the wire in comparison to the other magnets
- the wire experiences a force of F
- the two magnets experience forces of F/2 each
what are the factors on the force experienced by a CCC in an external magnetic field and formula
- B, the magnetic flux density of the field
- I, the current in the wire
- L, the length of the wire in the magnetic field
- Sin(theta), the angle between the field and current direction
F = BILsin(theta)
what is this formula (know) when the angle between the current direction and field direction is 90 degrees
F = BIL
When is an EMF induced in a wire
- when there is a change in magnetic flux (same as work function symbol)
M.F. = BAcos(theta)
flux linkage = M.F. x number of coils
EMF is directly proportional to rate of change of flux linkage
how can an AC be induced in a wire
- repeatedly move a magnet towards and away from a wire wrapped around a core
what are the factors on the AC produced
- number of coils in wire
- speed of movement of magnet
- rate of change of cutting magnetic field lines/ rate of change of flux linkage
- length of conductor
how can a DC motor be produced
- split ring with brushes delivering current
- ring connected to split ring inside an external magnetic field
- every half turn the current changes direction, this spins the ring
how to produce an AC motor
- non-split ring with brushes delivering current connected to a ring placed in an external field
- this means the ring only rotates a half turn before rotating back
How can we increase the force produced by the DC motor
- increasing current
- more coils
- stronger magnet
- less friction
name some uses of electromagnets
- loudspeakers
- transformers
- MRI machines
- motors etc.
what do transformers do
- they use electromagnetic induction to change voltage
what is the formula of coils and voltage for an ideal transformer
Ns/Np = Vs/Vp
what is the formula for I and V for an ideal transformer
VpIp = VsIs
- Because in theory all power is transferred (100% efficiency)
what is thermal radiation
thermal radiation is EM waves in the IR region of the spectrum (usually)
- temp can change freq.
what are the 4 factors on emission and absorption of thermal radiation
- darker colours emit and absorb more radiation than light colours
- a greater surface area increases the rate of absorption and emission
- matte surfaces give a higher absorption and emission
- temperature, the higher the temperature the greater the rate of IR emission
what is the Doppler effect
- As an object emitting a wave travels towards you, the waves become squashed, this decreases the wavelength
- as it travels away from you the waves are stretched and the wavelength increases
what is the equation for the Doppler effect
FO = F1(V/V+-VS) FO = freq for observer F1 = freq for source V = velocity of wave VS = velocity of source
what is a nuclide and give an example of different nuclides
- A nuclide is an atom or species with a specific number of protons and neutrons in its nucleus
- e.g. C12 is different to C13
what determines how stable a nucleus is and what happens if its unstable
- the balance of protons and neutrons
- if a nucleus is unstable it undergoes radioactive decay
what are the two main features of radioactive decay
- it’s spontaneous, cannot be sped up/slowed down and we can’t change it
- it’s random, we don’t know when a particular nucleus will decay and each nucleus decays independently
what is alpha radiation
- 2 Protons and 2 Neutrons
- He nucleus
- it has a mass number of 4 and an atomic number of 2
- has a 2+ charge
what is Beta - radiation
- a stream of fast moving electrons
- formed when a neutron decays into a proton and an electron, has a mass number of 0 and an atomic number of -1 (in equations)
- has a charge of -e
what is beta + radiation
- a stream of fast moving positrons
- formed when a proton decays into a positron and a neutron
- has a mass number of 0 and an atomic number of 1 (in equations)
- has a charge of +e
what is gamma radiation
- high frequency EM radiation
what is the relative penetrating power of the types of radiation
- alpha = low = stopped by paper
- Beta + or - = medium = stopped by aluminium
- Gamma = High = stopped only partially by a lot of lead
what are the relative ionising abilities of the types of radiation
- Alpha = large mass/charge = high
- Beta = low mass/charge = low
- Gamma = negligible ionisation
how do the different types of radiation deflect in a magnetic/electric field
- gamma doesn’t
- Alpha deflects slightly to negative
- Beta + deflects lots to negative
- Beta - deflects lots to positive
what to remember about nuclear decay equations
- EVERYTHING must be conserved
- if a neutron is fired at the start, don’t forget it from your mass equations
what is background radiation and what causes it
- it is radiation that is constantly around us
- it’s mostly caused by rocks, domestic gases, CMB
what are the risks associated with ionising radiation
- it can cause cell/DNA mutations
what is a half life
“the half life of an isotope is the average length of time required for half the active nuclei in the sample to decay”
what is the decay constant and the equation with it
it is the probability of decay of an individual nucleus per unit time (lambda)
A = Lambda x N activity = decay constant x number of active nuclei
what is the general equation for dealing with half lives
N = N0 x e^(-lamdba x t)
how can the generalised equation be rearranged to include the half life
substitute in N0/2 for N
and T1/2 for t
this gives
lambda x t(1/2) = ln(2)
what are decay products
- when a nucleus starts decaying, some of its decay products will also decay
- due to the nature of half lives, this can all occur simultaneously
what to do if it has a non-100% efficient motor and energy loss elsewhere
- if it asks for total energy loss then do
energy loss to surroundings + energy loss through efficiency = total energy loss
what to do if the difference of two lengths is calculated questions
- if asking displacement from centre point then half the difference between the two distances
units for energy
kgm^2s^-2
E= MC^2
units for force
kgms^-2
F= MA
units for momentum
kgms^-1
equation for voltage
V = IR
3 equations for power
P = IV P = I^2 R P = V^2/R
equation for elastic potential energy
Ep = 1/2 kx^2
equation for force vs extension
F = kx
equation and proportionality for voltage against speed of an electron and what this actually means
eV = 1/2 mc^2 thus v is prop to c^2 so sqrt(voltage) = k c e.g. if voltage x 1/2 c x 1/sqrt(2)
equation for work done
w = fxcos(theta)
which direction do resistive forces act on an inclined plane
parallel to the plane
how does energy get transferred in liquids
through collisions of particles with different amounts of kinetic energy, not through vibrations, that is a solid
which way does a wave bend when it enters a more optically dense medium
towards the normal
what is the relationship between time and number of waves to give frequency and period
number/time = frequency
time/number = period
how to solve suvat questions where they need to be at the same displacement i.e. where they meet at
- form equations for both objects including any initial displacements (pick a start point)
- remember negatives
- set them equal and solve for whatever you need to
how do circuits where you have 2 loops where the centre is a part of both loops and the batteries face in opposite directions work
- use kirchoff’s laws of current to show that the centre has a current equal to that of both the outer loops
- consider the outer loops separately
- simultaneous equations may need to be used
what are the relevant equations for calculating the pressure exerted by falling water onto a surface
- hydrostatic pressure
hydrostatic pressure = density x gh - other pressure
f/a = deltaP/(deltaT x A)
often in these cases, delta t we ignore
what to remember when working with multiple springs connected in series with each other
if there is a mass attached to the top spring and the bottom spring
the top spring experiences both masses
the bottom spring only experiences one of them
what is the angle of incidence and refraction in a longitudinal wave
take the angle between the lines representing the peaks and the region boundary, because of right angles and right angled triangles, this works
can thermal radiation travel in a vacuum
yes
is a shiny surface a good emitter of radiation
No
if the mass of an object decreases at a constant rate and the force remains constant, what happens to the acceleration
it increases at an increasing rate, think of reciprocal graphs
what 3 factors do we want for minimum rate of thermal conductance through a bar
longest distance/length
minimum diameter/radius
lowest temperature difference
what are the useful equations for transformers
VpIp = VsIs (where 100% efficiency) Np/Ns = Vp/Vs Is/Ip = Ns/Np
