AQA Physics P3 - Higher

Moment = turning effect of a force

Moment (Nm) = force (N) x perpendicular distance between line of action and pivot (m)

Larger force = larger moment.

Larger perpendicular distance = larger moment.

Maximum moment = exert force at right angles (perpendicular) to object.

Point where whole mass of an object is considered to be concentrated.

Object will swing until centre of mass is directly below point of suspension.

Irreguar shape = suspend shape from plumb line, wait until it stops moving, draw line along plumb line, repeat with a different point. Centre of mass = where the two lines cross.

Regular shape = where lines of symmetry cross. 

Anticlockwise moments = clockwise moments, object won't move.

Total anticlockwise moments = total clockwise moments

Total anticlockwise and clockwise moments not equal = resultant moment.

Unstable = tips over easily.

Most stable = low centre of mass, wide base.

Centre of mass moves outside of base =  will tip over. 

Velocity = speed and direction.

Travelling in a circle = velocity changing, accelerating.

Centripetal force acting on centre of circle - car going around a bend = friction, bucket on rope = tension, planets orbiting sun = gravity.

Faster moving object = bigger centripetal force.

Heavier object = bigger centripetal force.

Smaller radius of circle = larger centripeal force. 

Gravity = force of attraction between masses - centripetal force keeping planets in orbit.

Larger masses = greater force. 

Orbit possible when balance between forward motion of object and gravitational force pulling inwards.

Planets orbit stars. Orbits = slightly elliptical. Further from sun = larger orbit.

Gravity decreases as distance increases - planets nearer sun have quicker orbit.

Geostationary = high orbits over equator, takes 24 hours. Stays above same point, Earth rotates with them.

Uses = communications as stay at same point.

Low polar orbit = sweeps over both poles, Earth rotates beneath it, takes 2-3 hours. Whole surface can be monitored.

Uses = weather, spying as low orbit. 

Real = can be formed on a screen. 

Virtual = light appears to be coming from different place, can't be formed on screen. Behind a mirror.

Describing images = size compared to object, upright or inverted, real or virtual.

Reflection on uneven surface = different angles = diffuse reflection.

Reflection on even surface = same angle = clear reflection.

Law of reflection - Angle of incidence = angle of reflection.

Normal at 90 degrees to mirror.

Refraction = light changes direction when entering a different medium - caused by change of speed in waves.

Plane mirror = image same size, same distance from mirror as object, virtual.

1. Draw virtual image - follow rules above.

2. Draw reflected ray - top of image to top of eye, dotted line behind mirror.

3. Draw incident ray - top of object to mirror, to where reflected ray meets mirror.

4. Repeat steps 2 and 3 for bottom of the eye.

Curved mirror - concave = converges, convex = diverges.

Like round portion of sphere - centre of sphere = centre of curvature, C.

Centre of mirror surface = vertex.

Halfway between C and vertex = focal point, F. 

C, Vertex and F lie on axis - middle of mirror.

C and F = in front of concave mirror, behind convex mirror.

Concave = ray parallel to axis reflected through F. Ray passing through F reflected parallel to axis.

1. Draw ray from top of object to mirror parallel to axis.

2. Draw ray from top of object to mirror passing through F.

3. Incident ray parallel to axis reflected through F - draw this line.

4. Incident ray passing through F reflected parallel to axis - draw this line.

5. Where reflected rays meet = top of image. If bottom of object is on axis, so will bottom of image. If not, repeat process for bottom of object.

Image:

Object at C = real, inverted, same size as object, same place/distance.

Between C and F = real, inverted, bigger, behind object.

In front of F = virtual, upright, bigger, behind mirror.

Convex = ray parallel to axis reflected as if it has come from F. Ray passing through F reflected parallel to axis.

1. Draw ray from top of object to mirror parallel to axis - dotted behind mirror.

2. Draw ray from top of object to mirror passing through F on other side - dotted behind mirror.

3. Incident ray parallel to axis reflected as if it starts at F - draw this line.

4. Incident ray passing through F reflected parallel to axis - draw this line - must meet incident ray.

5. Where reflected rays meet = top of image. If bottom of object is on axis, so will bottom of image. If not, repeat process for bottom of object.

Image:

Always virtual, upright, smaller than object, behind the mirror, and closer than F.

Further away the object = smaller the image - can see a wide area, so used on road corners.

Refraction = Entering denser medium = ray bends towards normal. Entering less dense medium = ray bends away from normal.

Different wavelengths refract by different amounts. Rectangular prism = parallel boundaries - white light emerges. Triangular prism = boundaries not parallel - rainbow effect.

Converging lens = convex, light converges to a focus. Focal point where rays hitting lens parallel to axis meet.

Diverging lens = concave, light diverges. Focal point where rays hitting lens parallel to axis appear to come from - can be dotted back to meet up.

Axis of lens - middle.  

Each lens has focal point in front of lens and behind. 

Rules = incident ray parallel to axis refracts through focal point. Incident ray through focal point refracts parallel to axis. Incident ray passing through centre of lens carries on.

1. Draw ray from top of object to lens parallel to axis.

2. Draw ray from top of object going through middle of lens.

3. Incident ray parallel to axis is refracted through the focal point. Draw this line.

4. Mark where rays meet = top of image. If bottom of object is on axis, so will bottom of image. If not, repeat process for bottom of object.

Image:

Object at 2F = real, inverted, same size as object, at 2F.

Between F and 2F = real, inverted, bigger than object, beyond 2F.

Nearer than F = virtual, upright, bigger than object, on same side of lens, further than 2F. 

Rules = incident ray parallel to axis refracts in line with focal point. Incident ray passing towards focal point refracts parallel to axis. Incident ray passing through centre of lens carries on.

1. Draw ray from top of object to lens parallel to axis.

2. Draw ray from top of object going through middle of lens.

3. Incident ray parallel to axis is refracted as if it has come through the focal point. Draw this line - dotted before it reaches lens.

4. Mark where rays meet = top of image. If bottom of object is on axis, so will bottom of image. If not, repeat process for bottom of object.

Image:

Always virtual, upright, smaller than object, on same side as lens as object. 

Caused by vibrating objects - passed through surrounding medium as longitudinal waves.

Denser medium = faster sound travels. Travels faster in solids than liquids, faster in liquids than gases.

Can be reflected by hard flat surfaces. Carpets/curtains absorb sound.

Also refract (change direction) when entering a different medium. 

Frequency = no of waves per second. Humans hear frequencies between 20 - 20,000 Hz.

Sound doesn't travel in vacuum - bell jar experiment = air sucked out, sound gets quieter. Bell mounted on foam to stop sound travelling through base and making base vibrate.

Greater amplitude = more energy = louder.

Higher frequency = shorter wavelength = higher pitch. 

Sine wave = clear, pure sound.

Sawtooth waveform = buzzy, brassy sound.

Rectangular waveform = thin, reedy sound.

Square waveform = hollow sound.

Triangle waveform = weaker, mellow sound.

Ultrasound = outside range of human hearing. 

Get partially reflected at a boundary between media.

Finding boundaries = Ultrasound pulse reflects off two separate boundaries, use seconds per division on CRO to work out time between pulses. Given speed of sound, work out distance between boundaries using d = v x t.

Industrial cleaning = ultrasound directed onto precise areas, vibrations knock dirt from equipment.

Quality control = pass through metal casting, some ultrasound reflects back when a boundary is reached - detected. Processed by a computer to give visual display of inside of object - cracks show up.

Pre-natal scanning = Ultrasound hits boundaries between amniotic fluid and body tissue of fetus. Reflected rays processed by computer, produces video image. Safer than X rays.  

Magnetic field = region where magnetic fields and wires carrying current experience force acting on them - north pole to south pole.

Current carrying wire = magnetic field made up of concentric circles - wire in centre.

Solenoid = magnetic field inside = strong. Outside coil = like a bar magnet. 

Direction of current reversed = poles reversed.

Increase strength of magnetic field = add soft iron core - makes an electromagnet.

Magnetically soft = magnetises and demagnetises easily.

Iron, steel and nickel = magnetic. 

Motor Effect:

Force = bigger if current or magnetic field increased.

Force on wire = 90 degrees to wire and magnetic field.

Full force = wire at 90 degrees to magnetic field.

Wire runs along magnetic field = no force.

Speed up motor = More current, more turns on coil, stronger magnetic field, soft iron core.

Coil on spindle = forces act one up, one down - rotates.

Split ring commutator swaps contacts every half turn - keep motor rotating in same direction.

Direction of motor reversed if polarity of DC supply or magnetic poles swapped. 

Creation of voltage (maybe current) in wire experiencing change in magnetic field.

Moving a magnet in a coil of wire = voltage. Side to side = 'blip' of current.

Move magnet in opposite direction = voltage/current reversed. Polarity reversed = voltage/current reversed.

Keep magnet moving backwards and forwards = AC current produced.

Turning magnet end to end in coil - magnetic field in coil changes, induces a voltage, makes current flow in wire. 

Direction of magnetic field reverses every half turn - voltage reverses, current flows in opposite direction - produces AC current. How generators work.

Bigger voltage and higher frequency = stronger magnet, larger area of coil, more turns on coil, faster movement.

AC generators = rotate a coil in magnetic field.

Kinetic energy to electrical energy. 

As coil spins, current induced in coil - changes direction every half turn. 

Slip rings and brushes - contacts don't swap.

Produce AC voltage.

Dynamos = rotate magnet, not coil - used on bike lights.

Step up = voltage up, more turns on secondary coil than primary.

Step down = voltage down, more turns to primary coil than secondary.

1. Primary coil produces magnetic field within iron core - passes through secondary coil.

2. AC in primary coil so magnetic field in iron core is alternating too.

3. Magnetic field lines cut by secondary coil.

4. Induces alternating voltage in secondary coil - same frequency as alternating current in primary = electromagnetic induction. 

But only works with AC - with DC supply, there would be magnetic field, but not constantly changing so no induction of voltage in secondary coil.

Iron core carries magnetic field NOT current. 

Primary voltage           =     Number of turns on primary

Secondary voltage             Number of turns on secondary 

Can be rearranged.

Transmit a lot of power = high voltage or current - P=VI

High current = loss of power due to resistance in cables.

Power loss formula = P=I^2R 

Cheaper to boost voltage and keep current low.

National Grid = step up transformer increases voltage at one, then use step down transformer to bring back down to safe levels before supplying to houses = more efficient.

Stars form from nebula = clouds of gas and dust. Spiral in due to gravitational attraction. Gravity compresses matter = intense heat, sets off nuclear fusion. Other lumps form = planets which orbit star.

Billions of stars in Milky Way galaxy.

Gravity keeps stars together in a galaxy. All galaxies rotate. 

Universe has billions of galaxies.

Early universe = only hydrogen - atoms clump to form stars.

In cores of stars, nuclear fusion takes place - hydrogen nuclei combine to form helium nuclei. 

Hydrogen runs out = helium nuclei fuse = form heavier elements = in red giant stars. Helium runs out = carbon, oxygen and neon combine to form silicon. Big stars = nuclear fusion until iron is formed.

End of lives = massive stars explode = all elements formed = supernova. Dust and gas from supernova can form new planets and stars. 

1. Form from nebula - force of gravity forces nebula to spiral in. Gravitational energy to heat energy.

2. Starts nuclear fusion - form helium nuclei, gives out heat and light. Long stable period when heat from nuclear fusion makes outward pressure - balances gravity pulling star inwards = main sequence. Lasts millions of years, sun in this period. 

3. Hydrogen runs out - star swells = red giant. Star cools.

4. Small star then cools, contracts = white dwarf. Light fades completely becomes black dwarf.

5. Big stars glow brightly again, undergo more fusion, expand and contract again, then explode in supernova.

6. Supernova throws out dust and gas, leaves dense core = neutron star, or if massive enough, black hole. 

7. Dust and gas can become second generation stars.

8. Matter from which stars are made of = million of times denser than matter on Earth as gravity so strong it crushes atoms.