OCR 21st Century Science - Physics P456

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P4 - Explaining Motion

  • Speed
    • Instantaneous speed - speed of an object at any given moment in time
    • Distance = speed * time
  • Speed, distance and velocity
    • Distance-time graphs - gradients = speed; curves = acceleration/deceleration
    • Displacement-time graphs - distance = +/-; distance of an object in a given direction
    • Velocity - describes both speed and direction (e.g. 30mph due north)
  • Acceleration
    • Change in velocity/speed in a certain amount of time (ms^-2)
  • Friction
    • When an object exerts force on another object, it always feels force in return (force pairs)
    • Reaction force - the equal and opposite force against an initial force
    • Friciton - reaction force, will match exertion force to a point, after which the object moves
    • Friction between gripping solid surfaces - e.g. walking, friction between shoes and floor
    • Flriction between sliding solid surfaces - e.g. in an engine, reduced w/ lubricant
    • Resistance (drag) from fluids - objects moving through fluid force themseleves past molecules, creating friction
  • Forces
    • Arrows show the size and direction of forces in diagrams
    • Resultant force is zero - steady speed (resultant force - overall force on an object)
    • Resultant forces change speed/direction (e.g. acceleration)
  • Momentum
    • Acceleration - unbalanced forces
  • Change in momentum
    • Resultant force - change of momentum in direction of the force
    • Car safety features - increasing time of impact reduces momentum; crumple zones, air bags, helmets, seat belts
  • Work
    • When a force moves an object it dos work, and energy is transferred
  • Kinetic energy
    • Anything that's moving has kinetic energy; the increase in an object's KE is slightly less than in theory due to heat waste
    • KE gained = GPE lost; GPE = weight * vertical height difference

P5 - Electric Circuits

  • Static electricity
    • Build of static electricty is caused by friction
    • When two insulating materials rub together, electrons move, leaving a charge on either one
  • Electric current
    • Flow of charge around a circuit
    • Metal conductors have free electrons for electricty to pass through; insulators do not
    • Current - will only flow if there's a voltage across the component (A)
    • Voltage - driving force that pushes current round (V)
    • Resistance - slows charge down (ohms)
    • Power - rate of energy transfer (W)
  • Electric currents
    • Potential difference - same as voltage; shows how much energy is transferred to/from each unit of charge
    • Battery transfers energy to the charge as it passes, 'pushing' it
    • The voltage of a battery shows how much work it will do to the charge passing through it
  • Series and parallel circuits
    • P.D. - series, shared (V=V1+V2); parallel, the same (V1=V2=V3)
    • Current - series, same (A1=A2=A3); parallel, shared ( A=A1+A2)
    • Resistance - series, adds (R=R1+R2+R3); parallel, lower than the branch with lowest resistance due to shared current
  • Mains electricity
    • 230v…


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