OCR 21st Century Science - Physics P456

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…