Physics P3
Revision cards covering the raw skeleton of the AQA physics P3 units.
- Created by: melissa
- Created on: 08-06-09 09:21
Moments and stability
Moment = Turning effect of force.
- Force acts at right angles from spanner for maximum moment)
Moment (Nm) = Force (N) X Perpendicular distance from pivot (M)
Think of a seasaw.... If the seasaw is balanced - centre of mass is above pivot and..
Total clockwise moments = total anticlockwise moments
Otherwise the body will topple due to a resultant force
Balance and circular motion
Centre of mass = the point where the mass of the body is thought to be concentrated
To be most stable... Wide Base + low centre of mass
- Topples when centre of mass moves outside of base
- If weight doesn't act on line of pivot causes a resultant moment.
Circular Motion
- If moving on circular path muct constantly be unbalanced forces acting
- Constantly changing direction and accelerating
- Forces act towards centre of circle
- Force keeping it moving in a circle = CENTRIPETAL FORCE
Centripetal force
Centripetal force needed increases when:
- Mass increases
- Speed increases
- Radius decreases
Vehicle on roundabout- centripetal force due to friction between tyres and road
Circling aircraft- centripetal force due to combined effect of weight and lift acting upon it. centripetal force = result of these two forces
In planets and satellites centripedal force is due to gravity
Gravitational attraction
Gravity increases when:
- The greater the mass of two bodies
- Distance is closer between the two bodies
Orbit of planet is an ellipse with sun at focus
To remain in oribt a particular speed is need otherwise it will spiral into sun or fly off its orbit.
Orbits (of satellites)..
Geostationary- Communications, 24h orbit, high, equator
Low polar- weather, spying, over both poles, hours orbit
Images and light
Mirrors and lenses form images
Normal is a perpendicular line from surface at the point of incident.
- Normal bisects angle of reflected light
- Angle of incidence = Angle of reflection
In a plane mirror...
Object is as far behind mirror as object is in front
In a convex mirror- image is always...
- Virtual
- Upright
- Smaller than object
Further object from mirror, the smaller the image
Refraction
REFRACTION = change of direction of light rays
Convex lens- Converging
- Pass through focal point on other side (this ray was parralel to the axis)
- Ray passing through centre of lens continues in same direction
Concave lens- Diverging
- Incident ray parallel to axis travels through focal point
- Travelling through centre contiues in same direction
- Image is always -Virtual
- Right way up
- Smaller than object
- On same side of lens as object
Magnification(converging lense) must be closer to lens than Focal Point
Sound
Sound...:
- Is caused by vibrations
- LONGITUDINAL WAVES
- Cannot travel through a vacuum
- can reflect and refract
- human 20-20,000 Hz (beyound 20,000 is ULTRASOUND)
- pitch increase with frequency
- loundness increases with amplitude
Ultrasound:
Uses: Industry cleaning, quality control, medicine, prenatal scanning
- High frequencies
- Partically reflected an media boundaries (can detect distance)
Sound equations
Frequency (Hz) = . 1 .
Time period (s)
Speed (m/s) = Frequency (Hz) X wavelength (m)
Motors
Electirc currents create magnetic fields
MAGNETIC FEILD: region where magnetic materials and wires experience a force acting upon them
These forces can make things move (MOTOR EFFECT)
Electric current + magnetic field = kinetic energy
Force increased by:...
- Increasing magnet field strength
- Increasing size of current
(if either of the above are reversed sop is the direction of force)
Force on one side is in opposite direction to force on the other side
generator
Kinetic energy + Magnetic field = Electricle current
- Coil rotated in magnetic field
- as spins a current is induced in the coil
- this changes direction every half turn
- Produce AC voltage
- faster revs produce higher voltage
Direction of induced P.d. is reversed when either direction of motion or polarity of magnet is reversed
Induced P.d. increases when:
- Speed of movement increases
- strength of magnetic field increases
- number of turns on the coil increases
- area of coil is greater
Transformers
Step up (increase P.d.) or Step down (decrease P.d.)
More turns of Secondary coil More turns on primary coil
Must be A.C. current
- A.C. current creates changing magnetic field in the iron core
- hence induces a change of P.d. across Secondary coil
Used in National Grid:
- to transmit a lot of power you need high current or high voltage
- Cheaper to boost voltage
- transformers needed
- Less heat energy lost
Stars
Gravitational forces formed galaxies of stars
Sun radiates energy due to hydrogen fusion in core
ALL ELEMENTS MADE ORGINALLY FROM HYDROGEN DUE TO NUCLEAR FUSION (elements are distributed from stars by supernova at end of life)
Older stars- all hydrogen in core turns to hellium which fues into even heavier elements
Masses atracted together after explosions to create stars and planets.
Low mass stars: (PMRWB)
PROTOSTAR --> MAIN STAGE --> RED GIANT --> WHITE DWARF--> BLACK DWARF
High mass stars:(SAME UNTIL WHITE DWARF)
WHITE DWARF --> SUPERNOVA --> NEUTRON STAR ---> BLACK HOLE (sufficient mass)
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