Key definitions, laws and relationships Flashcards
Scalar
They only have a magnitude
Vector
They have both magnitude and direction.
What does “average” mean in a Motion question?
The acceleration isn’t constant. For this, you can’t use the SUVAT equations
When can you use the SUVAT equations?
When the acceleration is constant.
What is the gradient in a displacement-time graph
Velocity
What is the gradient in a distance-time graph
Speed
Define Acceleration
The rate of change of velocity.
What is the area under a velocity-time graph
The displacement
What is the gradient of a velocity-time graph equal to
The acceleration
What is the thinking distance
The distance travelled between the moment the trigger is seen to the moment you use the brakes.
What is the braking distance
The distance travelled from the time the brake is applied until the vehicle stops
What is free fall
When an object is accelerating under gravity, with no other force acting upon it.
For a steel ball and an electrical trap door, what equation is used?
s=ut +0.5 at^2
For using light gates and paper card, what equation is used?
v^2 = u^2 +2as
Why may the horizonal force not change in projectile motion?
Because there is no horizontal force acting upon that would cause it to accelerate. The acceleration in the vertical component doesn’t change the velocity in the horizontal component.
What are compressive forces
They are used to shorten objects. And they give a compressive force.
How would the graph for extending a spring that had been stretched past its elastic limit look like, when the forces are being removed?
From the point past its elastic limit, it would climb down parallel to the elastic part, and finish a bit after the starting point, because it has been altered.
What is the force constant
A measure of the stiffness of the spring.
Hooke’s Law
The extension caused is proportional to the force applied, for as long as the elastic limit hasn’t been surpassed.
How to improve the accuracy of investigating Hooke’s Law
- Use a set square
- Take at least 7 different readings, and repeat each 5 times (for the length or etc- YOU NEED TO SAY WHAT TO REPEAT THE READINGS FOR!)
- Take readings at eye level (reduce parallax error)
- Use a long piece of wire or coil (TO REDUCE THE% UNCERTAINTY IN THE MEASUREMENTS)
What is elastic potential energy
When a spring is stretched, the work done is transferred here within the spring. This energy is fully recoverable.
Extension and Energy relationship
Energy is squarely proportional to the extention.
Extension of 2, x4 the energy stored.
Loading and unloading graph for a metal wire
Loading graph is normal and then unloading graph is parallel to the elastic part and it doesn’t return to the original length.
Loading and unloading graph for polythene
Doesn’t obey Hooke’s Law.
They are easy to stretch and they suffer plastic deformation easily.
Curved graph for loading, then straight down for unloading.
Loading and unloading graph for a rubber band
Doesn't obery Hooke's Law Returns to the original length Elastic deformation Loading and unloading curves are different, but it returns to the original position. Forms a hysteresis loop.
Define tensile stress (Greek letter sigma)
The force applied per unit cross-sectional area.
Define tensile strain (Greek letter epsilon)
The fractional change in the original length of the wire
Stress-strain graph for a metal
For a ductile material.
It goes up (Limit of proportionality, then the elastic limit), curves and then accelerates even further to reach Yield points 1 and 2, and then UTS.
Define the yield points
They are points when a material extends rapidly.
Define necking
Beyond the UTS, the material become longer and thinner at its weakest point. This is called necking.
What is the Young Modulus
It is a ratio of the tensile stress to the tensile strain as they are proportional.
Define current
The amount of charge passing a given point in a circuit per second
Define charge
The electric charge flowing past a point in 1 second when there is an electric current of 1 ampere.
How to find the number of electrons in a charge
Charge/ 1.60 x 10^ -19
How can you get a large current going through a wire
Increase the number of electron moving per second.
Increase cross-sectional area
Increase the speed of the electrons.
Kirchoff’s First Law
The sum of current going into a node is equal to the sum of current leaving that node.
Define the number density
The number of free charge carriers per cubic metre of a material.
Cross sectional area and mean drift velocity relationship
They are inversely proportional.
If the radius halves, the cross sectional area decreases by a factor of 4.
So the mean drift velocity increases by a factor of 4.
How does a light turn on so quickly
They collide repeatedly with the positive metal ions. Lights turn on so quickly because all the free electrons move all at once.
SI unit for V
J C ^-1
Define P.D
The energy transferred per unit of charge.
Define EMF
It is the work done on the charge carriers. The sources of EMF include the battery etc.
What is an electron gun
It is used to produce a narrow beam of electrons.
How does an electron gun work?
A small filament bulb is heated and the electrons gain Kinetic energy. Some gain enough kinetic energy to leave the metal. A high p.d is applied between the filament and the anode, the electrons will accelerate towards the anode, gaining Kinetic energy. They pass through a small hole, creating a beam of electrons with a specific kinetic energy.
Electron transfers energy equation
eV = 0.5 mv ^2
At what V can a diode work
0.7 V
At what V can a diode give negative current?
-50V
NTC
Negative temperature coefficient.
The resistance drops are the temperature increases.
Where are thermistors used?
Thermometers
thermostats
ovens etc.
IV graph of a thermistor
Random curve. mirrored S.
Kirchoff’s Second Law
The sum of the EMFs is equal to the sum of the pds around a closed loop.
What is Internal Resistance
When the materials in the cells have a resistance and this causes the terminal pd to be lower than the EMF.
This is called ‘lost volts’.
Resistance and Length relationship
Directly proportional.
Cross sectional area and resistance relationship
They are inversely proportional
Define the moment of a force
The force multiplied by the perpendicular distance
Conditions necessary for an object that is in equilibrium
The SUM of anti-clockwise moments is equal to the SUM of clockwise moments.
And the net force is 0.
If you ever have more than 1 force acting upon an object, what do you do ?
ALWAYS RESOLVE VECTORS.
What is the work done by a force
The force multiplied by the distance in the direction of the force.
What happens to the acceleration when a trolley is heavier than one thatis lighter
There is no change in acceleration as the mass of the trolley is independent of the acceleration.
What is the centre of gravity
The point where the mass appears to act through.
Where possible, what should you always do when explaining something
Use a relationship or an equation.
If a lift has an upward acceleration , then how can you calculate the normal contact force
Find the initial force by calculating the mass multiplied by the acceleration of the lift. Then add this onto the weight.
Why does this happen?
Because the force needs to be greater than the weight to be able to pull it up.
As a lift starts to decelerate, what happens to the normal contact force
The normal contact force begins to decrease, less than the weight of the person.
When 2 springs are connected in series, explain why their force constant is k/2
Because the springs need half the force to give the same extension.
When 2 springs are connected in parallel, do they both get the same force or half the force?
They only get half the force.
For the same force, you get half the extension.
Spring constant increases.
When 2 springs are connected in series, do they both get the same force or half the force?
They get the full force that you put in.
What is potential difference
The available energy per unit of charge from electrical to other forms
Why is a parallel circuit more favourable than a series circuit?
If a specific component stops working, then the rest of the components will still keep working.
Define the work function
The minimum amount of energy neeeded to release an electron from the surface of the metal.
Define displacement of a particle in a wave
Any distance moved from the equilibrium of a point on a wave.
Define amplitude
The maximum possible displacement caused by a wave motion.
What are the conditions necessary to produce a visible pattern on the screen
Coherent sources
They must have a constant phase difference
The slits must be narrow or close together so that a diffraction pattern can overlap.
Light waves from two slits must have similar amplitudes or intensities.
If one of the slits in Young’s Double Split Experiment is covered up, what pattern is shown on the
screen?
There is no pattern being caused at all.
Because there is no interference.
So the lights spreads out over the entire screen, due to diffraction.
The light intensity is less
It becomes quite bright in the middle and a lot dimmer at the edges.
Describe a plane polarised wave
Transverse waves have particles that travel in a perpendicular direction to that direction of energy transfer. So the vibrations are only confined to a single plane.
What is Ohm’s Law
That the pd is proportional to the current, for as long as the temperature is kept constant. To prove this, the graph has to have a straight line that also goes through the origin.
what is torque
A force of a couple multiplied by the perpendicular distance between the forces.
Define the term E.M.F?
It is the energy transferred from the source to electrical per unit charge
What factors affect the magnitude of the drag force that an object experiences?
Speed of balloon
Frontal area
Texture of balloon
Temperature of the air or the density of the air.
When thinking of momentum, what do you have to take into consideration?
Is the momentum being changed from a vertical or a horizontal way ?
What do you need to be able to undergo TIR?
If the angle of incidence is greater than the critical angle.
The first medium must be denser than the second medium.
When light goes from a dense material into a less dense material, does the light bend towards or away from the normal.
From a dense to a less dense material, it bends away from the normal.
What is proof of light being a wave
Refraction and interference
What is proof of light being a particle
The photoelectric effect
What equation shows how light as a wave can behave as a particle?
E= hf
What equation shows how light as a particle can behave as a wave?
Wavelength = H/ momentum
What experiment proves that electrons have wave-like properties (that behind lambda= h/p)
They shot electrons towards a piece of graphite (which acted as a diffraction grating). On a phosphor coated screen, the electrons interfered with each other to create a concentric circles pattern.
A firework rocket is fired vertically into the air and explodes at its highest point.
What are the changes to the total kinetic energy of the rocket and the total
momentum of the rocket as a result of the explosion?
kinetic energy increases and the momentum remains unchanged.
Describe what happens during the polarisation of an EM wave
Before being polarised, they have oscillations in many directions. After being polarised, the oscillations are all confined to 1 direction, which is perpendicular to the direction of propagation.
Explain why there is 0 signal when microwaves go through a metal grille at 90 degrees.
The waves are polarised.
At 0 degrees,the plane of polarisation is parallel to the plane of polarisation of the grille, so they can pass through. But, when it’s turned, the plane of polarisation of the filter is perpendicular to that of the waves, so the waves are ABSORBED.
Upthrust equation
Upthrust = mass of liquid displaced * g
Explain how a diffraction pattern would change if a green light had been used rather than a red one.
Because a green filter has a smaller wavelength than the red one. It diffracts less than the red, and so the maxima produced would be closer together.
Explain how the absorption spectrum is created
It has dark lines at certain frequencies. The photon energy is equal to the energy level difference in the atom. The photon can be absorbed by an electron to move to a higher level. The spectrum is created because the frequencies of the absorbed photons are missing from the spectrum.
When they tell you to compare a number of methods during a PAG, what is one thing to think about?
Think about the uncertainties involved. For larger angles, the percentage uncertainty is smaller.
A narrow ray will reduce the uncertainty of the ray.
Define 1 kWh
The energy used by a 1kW device in 1 hour.
The escape velocity from the Earth is 11 km s–1. The escape velocity is the minimum vertical
velocity a particle must have in order to escape from the Earth’s gravitational field. Explain
why helium atoms still escape from the Earth’s atmosphere.
Helium atoms have a range of speeds, so it is possible that some have speeds higher than the escape velocity.
State how the observations lead to conclusions about the nature and properties of the
molecules of a gas.
- movement of smoke particles caused by (being hit by) randomly moving air molecules
- smoke particles are continuously moving because the air molecules are continuously moving
- smoke particles are visible but air molecules are not hence air molecules must be (very) small.
- small movement of smoke particles is due to the large numbers of air molecules hitting from all sides
Describe an electrical experiment to determine the specific heat capacity c of a liquid.
Image showing the ELECTRICAL HEATER connected to the circuit etc. Measure: - Mass of liquid - Initial and final temperatures - Time - Voltage - Current
Since E=mcΔT, c= E/mΔT.
Uncertainties:
- Mix the liquid to get a correct temperature reading.
- Use insulation
- Take the thermal capacity of the vessel into consideration
- Take the highest temperature if it continues to rise after the heater is turned off.
If there are 2 balls that collide into each other, when 1 is stationary, and unless they have told you what will happen after, what can you assume (esp. if there a smooth surface)?
The ball that was originally moving will stop, and the second ball will move on ahead.
If they ever ask you “why is x an outlier”
Say it’s because it is 2 times the spread from the mean.
How could an experiment about calculating the terminal speed be improved?
method assumes at v terminal for whole drop / may not
reach terminal velocity drop ?
method underestimates the v terminal systematically ?
or reaction time (at start and/or end)
links smaller time to larger v terminal or vice versa
or incorrect height measurement due to plausible reason
links smaller distance to smaller v terminal or vice versa
Systematic errors
Poorly calibrated thermometer
Parallax error
Equation to use for angles and diffraction gratings:
lambda = d sin (theta). where d= distance between diffraction gratings.
When describing the phase difference, what is the one thing you should remember
- put down pi RAD!!!
What path difference means constructive
any multiple of λ
What path difference means destructive
an odd multiple of 1/2 λ
What phase difference is constructive
Either 360 degrees of
2π RAD
What phase difference is destructive?
Either 180 degress of π RAD.
In OCR B questions, if they ever ask you to evaluate something, and there are numbers provided, then what should you do?
Usually compare it with something else. Find the % error or uncertainty and compare.
6 marker for the light gates and trolley PAG
Uncertainty:
Use a longer card to reduce the % error
Measure the length of the card with greater precision.
Ensure the card cuts the laser properly.
Repeats:
Carry out repeats
Calculate mean and uncertainty values for xyz.
Quantify energy losses.
Experiment:
Use a newtonmeter to ensure you give the same force each time.
Use a weight/ pulley with a release
Use a frictionless ramp.
Whenever you have a graph that is non-constant and you are asked to describe it…
don’t say it’s non-linear. Say, it increases first, then decreases etc. And GIVE NUMBERS!!! Refer back to the graph.
What is a wavefront
A set of points that are all in the same phase.
Relationship between Momentum and Kinetic energy
p = √ 2 m * Ek.
Experiment for finding the internal resistance, key things to note
Make sure you write down the equation of the graph is V= E - Ir. and this is why the y- intercept is E and the r is the gradient.
Often when comparing things…
If you will say x is because yz, make sure to say that and do a isn’t bc.
When talking about standing waves…
STATE THE OBVIOUS!
Say that a stationary wave has formed. A node is here, an antinode is here.
How can you calculate the uncertainty when the values aren’t given?
Uncertainty = 0.5 x range
Calculating the Potential energy
Make sure to add the entire change in length.
How would adding a switch in a V =E -Ir experiment change uncertainty etc.
You would reduce the heating effect.
You would lose less energy.
Improves reliability.
How would adding another cell in a V =E -Ir experiment change uncertainty etc.
Increase pd, so increase in current.
Change to overall resistance, meaning there would be greater values and so, less % uncertainty.
The graph would be steeper and have a higher gradient.
How would adding a reversed cell in a V =E -Ir experiment change uncertainty etc.
Reduce the p.d and reduce the current.
The % uncertainty would increase.
What is interference
When 2 coherent waves superpose.
What is a conductor
It allows a current to flow through it.
What is an insulator
It doesn’t allow a current to flow through it.
What does uniform mean?
The mass acts at the centre
Final equation for calculating moment
force * the DISTANCE THAT IS PERPENDICULAR TO THE FORCE!!!
So any force acting as a reaction force on a slope, will have a perpendicular distance that is parallel to the slope.
But the mass of an object that is standing on the slope (or the slope itself) will have a perpendicular distance from the hinge to the floor where it acts. For this, you will have to resolve the forces to calculate what the distance will be, and then multiply it by the force of the object (or the slope/ rod)
What does elastic mean
The material returns to its original length when the load is removed.
What does plastic mean
The material doesn’t return to it’s original length
What does brittle mean
It will behave elastically only up to it’s breaking point.
What does ductile mean
The elastic and plastic regions before the material breaks are able to be permanently deformed.
What does UTS mean
Maximum tensile stress a material can withstand.
Describe the behaviour of electrons which demonstrates that they have wave properties.
- Electrons may be diffracted by a graphite lattice.
- to produce rings / circular interference fringes
- diffraction of electrons occurs when the wavelength is comparable to the size gap
- changes in the electron’s speed/energy change the
size of the rings. - electrons have a (de Broglie) wavelength given by
λ=h/p
PAG for an interference pattern using speakers
Diagram of:
- 2 Loudspeakers
- a signal generator connected to the loudspeakers.
- microphone and oscilloscope/sound sensor moved
between loudspeakers
- Wear ear defenders to protect from noise.
- Change the frequency and repeat measurements for x.
- Make sure that D»_space; a
Measurements: - frequency determined from reading from signal generator - use of rule(r) to measure distances a, D and x - measures over several maxima/minima
Analysis:
- rearrangement of equation for v or into y=mx
- plot a graph of x against 1/f or equivalent
- straight line through origin confirms relationship
- gradient = vD / a
How do you confirm a relationship?
If you plot a graph of it, and it crosses the origin, the relationship is confirmed.
On a F-t graph, how can you tell how the acceleration will change?
F is proportional to the a.
So, as the y axis changes, the a will change.
If there is a straight line up, the aceleartion will also increase at a constant rate.
When something falls down, how does the acceleration change?
At the beginning, acceleration is equal to g.
But then, drag will increase, and it may reach terminal velocity?
But, overall as drag increases, the net force downwards will decrease, meaning that the acceleration will decrease.
Define impluse
The force x time for which the force acts on the object
Using Newton’s 3rd law, explain how the impulse of 2 balls colliding compares to each other?
Due to newton’s 3rd law, that states the force on one ball is equal and opposite to that on the other. Since they also have the same time, and impulse = ft, hence, they have equal but opposite (in direction) impulse.
Describe in terms of the forces acting on the driver how wearing a seat belt and having
an airbag in a car can help to protect the driver from injury in a head on collision.
Prevents the driver from hitting the steering wheel / windscreen
Deflates quickly to prevent whiplash
Increases the time/distance to stop
Decreases the (impact) force on the driver
Much wider area of the bag reduces the pressure
Explain what the normal force may do no work on an object at an angled slope
Because it acts at right angles to the motion.
How to measure h and t when dropping a ball to measure gravity.
H can be measured using a metre ruler.
T can be measured using an electromagnet to turn off, and the ball will fall, starting the timer.
Then, a trap door to stop the timer.
The timing will be stored in a data logger.
How would you precisely measure the diameter of a wire?
You should use a vernier calliper and take many readings at different lengths of the wire.
Then, use an average of them all.
If the distance between an adjacent maxima and minima is 10mm, what is the wavelength
The distance between the adjacent maxima and minima are 1/4 of the wavelength, meaning that the wavelength is 40mm.
What is hf
The energy of a photon.
How to calculate the critical angle between a boundary?
Sin C = refractive index incoming / refractive index going into.
When you decrease the slit spacing, what happens to the interference pattern?
The fringe spacing will increase.
This will give you bigger values for the fringe spacing, meaning that there will be less % uncertainty
When you decrease the distance to the screen, what happens to the interference pattern?
The fringe spacing will decrease.
This will give you smaller values for the fringe spacing. As there are smaller values, the relative uncertainty is a big proportion of the total values, leading to a greater % uncertainty.
Define internal resistance
When there is resistance inside the battery, which causes the terminal p.d to be lower than the e.m.f due to the lost volts.
What is the wavelength
The minimum distance between neighbouring identical points on a wave.
Resolution of a meter ruler
1 mm
Resolution of vernier calliper or micrometer
0.1 mm
On a IV graph, why does the resistance of a filament bulb increase as you increase the potential difference
As you increase the potential difference, the filament bulb gets hotter, meaning that the electrons collide more often per second with the ions.
This increases the resistance.
What is Newton’s 3rd law
When one body exerts a force on another the other body exerts an equal (in magnitude) and opposite (in direction) force on the first body
Define Simple Harmonic Motion
Acceleration of the body is proportional to the displacement, but in the opposite direction to the displacement.
Finding the SHC of a liquid PAG
Main things:
- Measure mass of the liquid, initial and final temperatures, current, time, and potential difference.
- Then, c= IVt/mT.
Areas of errors:
- Stir the liquid to ensure that there is no false temperature reading.
- Measure the highest temperature reading if the temperature continues to increase after the heater is turned off.
- Use insulation to reduce the heat loss.
- Take into account the thermal heat capacity of the vessel in the calculations.
Explain why the increase in temperature causes an increase in volume if the pressure is to stay constant.
- There is increased Kinetic energy.
- So the particles move around faster and have more collisions PER SECOND with THE WALLS.
- Exerting MORE FORCE PER SECOND with the walls.
- For a constant pressure, there has to be the same number of collisions with the walls.
- So, by increasing the pressure, you can decrease the number of collisions per second happening with the walls.
Define random errors
errors of measurements in which the measured quantities differ from the mean value with different magnitudes and directions.
Define systematic errors
errors of measurements in which the measured quantities are displaced from the true value by fixed magnitude and in the same direction.
Define accuracy
a measure of how close the results of an experiment agree with the true value.
Define precision
a measure of how close the results of an experiment agree with each other.
Define internal energy
a function of state and the total microscopic kinetic and potential energies of the particles composing the system.
First law of thermodynamics
the increase in internal energy is equal to the sum of the heat supplied to system and work done on system.
Define Hooke’s law
within the limit of proportionality, the extension produced in a material is directly proportional to the force applied.
Define the principle of moments
when an object is in equilibrium, the sum of anticlockwise moments about any point equals the sum of clockwise moments about the same point for a FIXED SYSTEM.
Define the moment of a force
the product of the force and the perpendicular distance between the axis of rotation and the line of action of the force.
Define a couple
a pair of forces, equal in magnitude but opposite in direction, whose lines of motion do not coincide.
Define the centre of gravity
the point on an object through which the entire weight of the object may be considered to act.
Define the Upthrust/buoyancy force
an upward force on a body produced by the surrounding fluid (i.e., a liquid or a gas) in which it is fully or partially immersed,
Define Archimedes’ Principle
the upthrust experienced by an object partially or entirely immersed in a fluid is equal to the weight of the fluid displaced by the object.
Newton’s first law of motion
a body will continue in its state of rest or uniform motion in a straight line unless an external resultant force acts on it.
Newton’s second law
the rate of change of momentum of a body is proportional to the resultant force acting on it and the change takes place in the direction of the force.
Newton’s third law
If body A exerts a force on body B, then body B exerts a force of equal magnitude but in the opposite direction on body A
principle of conservation of momentum
the total momentum of a system of objects remains constant provided no resultant external force acts on the system.
Define Angular displacement
the angle subtended at centre of a circle by an arc of equal length to the radius.
Define Angular velocity
the rate of change of angular displacement with respect to time.
Define Displacement on a wave
the distance moved by the particle from its equilibrium position.
Define the amplitude
the maximum displacement of the particle from its equilibrium position.
Define the wavelength
the distance between 2 successive points on a wave which are in phase with one another.
Define the phase difference
Between two particles or two waves tells us how much a particle (or wave) is in front or behind another particle (or wave).
Define the periodic motion
The regular, repetitive motion of a body which continually retraces its path at regular intervals.
Define the angular frequency
the rate of change of angular displacement with respect to time.
Define the amplitude of SHM
the magnitude of the maximum displacement of the oscillating particle from the equilibrium position.
Define SHM
the oscillatory motion of a particle whose acceleration a is always directed towards a fixed point and is directly proportional to its displacement x from that fixed point but in the opposite direction to the displacement.
Define damping
the process whereby energy is taken from the oscillating system.
Define natural frequency
the frequency at which it will vibrate freely (with no forces acting upon it).
Principle Of Superposition
when two waves of the same kind meet at a point in space, the resultant displacement at that point is the vector sum of the displacements that the two waves would separately produce at that point.
Newton’s Law Of Universal Gravitation
every particle in the Universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
Define the Gravitational field strength
a point is defined as the gravitational force per unit mass acting on a mass placed at that point.
Define the Gravitational potential energy
The work done by an external force in bringing that point mass from infinity to that point.
Define the Gravitational potential
a point in a gravitational field is the work done per unit mass, by an external force, in bringing the mass from infinity to that point.
Define the Escape speed
the minimum speed with which a non-propelled mass should be projected from the Earth’s surface in order to escape Earth’s gravitation field.
Define coulomb’s law
the electrostatic force between two point charges is proportional to the product of their charges and inversely inversely proportional to the square of the distance between them.
Define the Electric field strength
the electrostatic force per unit charge experienced by a small positive test charge placed at that point.
Define the Electric Potential
the work done per unit charge by an external agent in bringing a positive test charge from infinity to that particular point without acceleration.
Define the Electric potential energy
the work done by an external force in bringing the charge from infinity to that point a distance r away.
Ohm’s Law
The potential difference is directly proportional to the current flowing through a circuit , for as long as the temperature stays constant.
Kirchhoff’s First Law
the total current entering a junction is equal to the total current leaving the junction. OR The algebraic sum of currents at a junction is zero.
Kirchhoff’s second law
the net electromotive force around a closed circuit loop is equal to the sum of potential drops around the loop.
Define Magnetic flux density
the force acting per unit current per unit length on a wire placed at right angles to the magnetic field.
Define the Magnetic flux
the product of the magnetic flux density normal to the surface BN and the area A of the surface
Define the Magnetic Flux Linkage
The product of the number of turns N of the coil and the magnetic flux linking each turn.
Faraday’s law
the induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage or rate of cutting of magnetic flux linkage.
Lenz’s Law
the induced e.m.f. will be directed such that it opposes the change that is producing it.
Define the root-mean-square (r.m.s.) value of an alternating current
equivalent to the steady direct current that converts electrical energy to other forms of energy at the same average rate as the alternating current in a given resistance.
Define rectification
The process in which an alternating current is forced to only flow in one direction.
Define the Photoelectric Effect
the emission of electrons from metal by electromagnetic radiation.
Define work function
the minimum amount of the work necessary to remove a free electron from the surface of the material.
Define the threshold frequency
the minimum frequency of an incident radiation required to just remove an electron from the surface of a metal.
Define scanning tunnelling microsope
non-optical microscope which uses the concept of quantum tunnelling by electrons to study surfaces of conductors or semi-conducors at the atomic scale of about 2 Å or 0.2 nm.
Define a mass defect
the difference between the mass of the separated nucleons and the combined mass of the nucleus.
DefineNuclear fission
the disintegration of a heavy nucleus into two lighter nuclei of approximately equal masses.
Define Nuclear fusion
the combining of the two light nuclei to produce a heavier nucleus.
Define Radioactive decay
the spontaneous disintegration of the nucleus of an atom which results in the emission of particles.
Define Background radiation
to ionizing radiation emitted from a variety of natural and artificial radiation sources.
Define the activity of a source
the average number of atoms disintegrating per unit time.
Define the decay constant (symbol is lambda)
its probability of decay per unit time.
Define the half life of a source
The time taken for half the original number of radioactive nuclei to decay.
What must you need to show that a law has been validated or is correct
A graph and the graph must go through the origin to show validation of the law.
What is Boyle’s Law
For a fixed mass and temperature, the temperature remains inversely proportional to the volume of a gas.
What is the P - v graph to show a gas obeys Boyle’s law
A parabola, doesn’t cross the x- axis.
If they ask how can a decelerating force be applied to a rocket/ satellite to allow it to reach a lower orbit
They apply a force in the opposite direction to the motion so as to decrease the net resultant force in that direction ,a Dan hence the acceleration in that direction.
SHM- how can you calculate the maximum velocity
V max = Amplitude x 2 pi x frequency
State how the observation of Brownian motion of smoke particles led to the properties of air particles that we now know
movement of smoke particles randomly moving.
They are continuously moving because the air molecules seen were also continuously moving
Smoke particles are visible, but air molecules aren’t, so the air molecules must be smaller.
Small movement of the smoke particles is due to the air particles hitting them from all sides.
When to use Flemings’ LHR and RHR
Use the LHR:
For motors. Or when you want to find the force.
Use the RHR:
For generators. Or when you want to get the current.
Think of it as Motors drive on the left in Britain
Fleming’s RHR
Thumb : Direction of the velocity
First finger: Direction of magnetic flux density
Second finger: Direction of the force.
Why may a force be unable to create a change to the velocity of a particle?
Because it acts in a perpendicular direction the motion of that particle, so no work can be done on the particle.
How does the charge of capacitors in series work?
It’s all the same.
Why may a collision not conserve kinetic energy?
Because energy is transferred to heat or thermal energy.
How do particles exert pressure on the ground or any surface
The molecules collide with the surface, exerting a force per unit area. Each bounce transfers momentum to the surface. Force on the surface is the rate of transfer of momentum. Pressure is the force exerted per unit area.
In a capacitor, what does ε0 mean
The permittivity of free space.
This is the energy required for an electron to move freely through a vacuum.
If they give you εr (relative permittivity of a specific insulator)
Then, use ε = ε0 x εr.
ε0 = Permittivity of free space.
In a capacitor, what does ε mean
The energy required for an electron to move freely through an insulator.
What exactly is “damping”?
It is using friction to reduce the resonance. This can cause the system to slow down though, meaning it reduces efficiency.
What the SI base units? (btw- RTFQ if it even asks for answer in terms of SI base units?)
meter, kg, seconds, Ampere, kelvin and mole.
When is the speed of a planet orbiting The Sun the fastest?
When it’s closest to it.
Always write gravitational potential energy as:
-GPE (for gravitational fields)
Why are the velocities of Helium atoms different?
Because the atoms will have collisions, which causes exchanges of energy.
This exchange of energy happens randomly, so they have different energy, and hence velocities as well,
Another equation for finding the force constant, but using SHM information
k = mass x w^2.
Whenever comparing Kinetic energies
Say that they are different, and that x is bigger than y.
DONT JUST SAY THAT THEY ARE UNEQUAL.
What conditions are required for TIR to happen
The first medium must have a higher refractive index than the second one.
And the angle of incidence must be greater than the critical angle
How to calculate the critical angle
Sin c = n of first medium / n of second medium.
What are the 4 main differences between a stationary wave and a progressive wave?
- Energy transfer (stationary waves have no net energy transfer, whereas progressive waves do.)
- Wavelength (Stationary waves: Is the distance between adjacent nodes x 2)
- Phase difference (Stationary waves: All parts of the wave between a pair of nodes are in phase, and on different sides of a node, they are in anti phase.)
- Amplitude (Stationary waves: maximum amplitude is at an antinode.
But in a progressive wave: all parts of the waves have the same amplitude.)
A mass is hung from the bottom end of a flexible spring.
Describe and explain how the mass can be made to show resonance.
Apply a periodic force onto the mass.
When you apply a frequency that is equal to the natural frequency, it will reach maximum amplitude.
If an oscillator is damped, at what time is the most energy dissipated?
At t=0, because the oscillator has the maximum speed, and the greatest friction acting upon it.
How do you add uncertainties
Add the absolute uncertainties (the number ones)
How do you multiply or divide uncertainties
Add the percentage uncertainties.
If an uncertainty is multiplied by a constant number, what do you do?
Either, multiply the absolute (number) uncertainties by the same factor.
Or you can keep the percentage ones the same.
How do you deal with an uncertainty with a power
Multiply the uncertainty with the power.
To get the gravitational field strength at the midpoint between 2 masses of equal mass
It is always 0.
To calculate the gravitational field strength between 2 points
The vector sum
In between 2 parallel circular plates, the electric field strength is
The same everywhere.
How do you measure the frequency
Using an oscilloscope, and measure` the time period and then inverse that.
Or, use data from a motion sensor
In SHM, for small angles, does changing the amplitude change the time period?
No, the time period stays pretty much constant for small angles.
This also means that the frequency stays the same as well.
When does Newton’s third law apply?
Only to forces of the same type, and to forces acting on different objects.
Why does the pressure decrease if the temperature decreases?
Because when the temperature decreases, the total kinetic energy of the atoms decreases. This means that the total pressure also decreases, as the atoms collide less often with the walls of the container.
OR
Because the temperature is proportional to the pressure, FOR A CONSTANT N AND VOLUME.
Why may a capacitor take a long time to discharge?
It may be discharging through a voltmeter, which has VERY high resistance, therefore having a very big time constant.
What to do when relating current to capacitance?
Remember that Q=CV, and Q=IT, therefore, CV=IT, and go from there.
How do you know that a graph is an exponential?
The voltage halves at each half life.
Assumptions etc. for the time constant in a capacitor
The time must not be comparable to the time constant.
Capacitors - calculating the change in energy from one V to another?
Use E= 0.5 V2 C, but find the energy for one V, take away the energy for another V.
What happens when a wire is clamped?
If you have a wire clamped on the ends, with a mass balance under it, if the force on the wire is up, then due to Newton’s 3rd law, there will be a force on the wire that’s in the opposite direction. It is possible for a mass balance to show negative answers.
Explain how a motor works
When a current is flowing in a wire, there is a magnetic field induced around that wire. Then, when you put this current-carrying wire in an external magnetic field, the 2 magnetic fields interact, producing forces on the coil that are perpendicular to the magnetic field, which makes the coil turn. The coil rotates as the forces provide a momentum. This forms a motor (but you will need a split ring commutator to keep the coil spinning)
Explain how a generator works
When you spin a coil in a magnetic field, the coil cuts the field lines, producing a rate of change of magnetic flux. This induces an EMF in the coil and when the coil is connected to a circuit, a current is induced as well.
You will need slip rings to enable the generator to continue working.
How does a transformer work
There is an AC Current in the primary coil. This AC current induces an alternating magnetic flux in the primary coil. As the wire cuts the magnetic flux, there is a rate of change of magnetic flux, and it induces an EMF in the primary coil. The soft iron core connects this magnetic flux to the secondary coil. Again, there is an EMF induced in the secondary coil, but it may be to a different degree depending on the number of coils in the secondary coil.
Why are non-invasive techniques good?
Less chance of infections
What is the use of medical tracers?
It is injected into the body. It is then absorbed by the body or a certain organ, and then a gamma or a beta camera is used to detect the amount of radiation coming from the tracer.
