momentum motors transformers generators

• formula for momentum


  p = mv

  momentum (kgm/s) = mass (kg) x velocity (m/s)
• what quantity is momentum and why


  vector because it has both direction and magnitude
• using light gates


  - starts when the trolley blocks the light beam

  - stops when the trolley moves out of the way and light shines on the sensor again

  - during this time the distance the trolley has travelled is equal to the length of the card
• conculsion of momentum


  momentum is conserved in collisions or explosions unless an unbalanced or external force acts

  this means that

  total momentum before collision = total momentum after ollsion
• elastic and inelastic collisions

  - an elastic collision is one where there is no loss of kinetic energy

  - an inelastic collision occurs when the colliding objects have less kinetic energy after the collision than before the collision
• newton's third law and conservation of momentum

  newton's third law states that whenever two bodies interact the forces they exert on each other are equal and opposite. the force of the gun on the bullet is equal in size and opposite in direction to the force of the bullet on the gun

  - these equal and opposite forces cause equal and opposite changes in momentum
• equation linking force and momentum

  force = change in momentum / time

  F = mv - mu/t
• Why do you bend your knees when you jump?

  - same change of momentum as momentum is conserved

  - if you increase the time taken for your feet the reach the floor , it decreases maximum force to prevent injury
• crumple zones


  - if both cars travel at the same speed and stop they experience the same momentum

  - if the time for the collision to occur is increased

  - the rate of change of momentum decreases

  - so the peak force is reduced a nd the passenger is less likely to be injured
• car safety features

  
  

  - why is it important to wear a helmet

  rear crumple zone, airbags, front crumple zone, collapsible steering wheel, front seat belts, side impact bars, rear seat belts

  
  

  - material of helment should be tough so it does not break and also absorb the shock but it is important that the helment should be deforemed by the force of the impact so that the head is brought more slowly to a stop , this means the change of momentum occurs in a longer period of time so that the force on the head is less
• the motor effect


  - there is a magnetic field around a current in a wire

  - this interacts with the magnetic field from the permanent magnet

  - the combined field is sometimes called a catapult field . This results in a force on the wire
• size of the force : how can it be increased

  direction of the force : how can it be changed

  when is the force zero

  the direction of the force can be changed by

  - reversing the direction of the current ( turn battery round )

  - turning the magnet round ( swap north and south poles)

  
  

  the size of the force can be increased by

  - increasing the current

  - stronger magnetic field

  - wrapping the wire into a coil of wire

  
  

  - the force is zero when the wire is parallel to the magnetic field
• fleming's left hand rule


  - the direction of the motion can be worked out using fleming's left hand rule

  - conventional current flows from positive to negative

  - magnetic field lines go from north to south

  - thumb = motion

  - first finger - magnetic field

  - second finger - current
• what do crossed circles and dotted circles represent

  crossed circles = magnetic field into plane of paper

  dotted circles = magnetic field out of plane of paper
• what do long lines and short lines on a battery symbol represent

  long line = positive short line = negative
• thermionic emission


  the filament becomes hot when a current is passed through it . As a result, some of the electrons gain enough kinetic energy to leave the filament . This is called thermionic emission. The larger the current, the hotter it will get, so more electrons are released
• deflection tube


  1- thermionic emission : current makes the filament hot giving some electrons enough KE to leave the filament

  2- a high voltage between the anode and the cathode attracts the electrons causing them to accelerate

  3- the vacuum inside the tube prevents ionisation so the electrons reach the screen

  4- electrons are invisible. When they hit the fluoroscent screen energy is absorbed and released as visible light
• force on a charged particle in a magnetic field


  - there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field. - The direction of the force can be worked out using Flemings left hand rule. Conventional current flows from positive to negative so it is the direction that a positive charge would flow. If a charged particle is negative, the direction of current is in the opposite direction to the motion of the negative particle.
• how does a motor work

  - there is a current in coil

  - which creates a magnetic field around the coil

  - this interacts with the field from the permanent magnet

  - resulting on an upwards force on one side of the coil

  = and a downwards force on the other side

  - the split ring commutator reverses the direction of the current every half turn to keep the motor spinning in the same direction

  - the direction of the force can be worked out using fleming's left hand rule
• motors


  - current flowing through the coil, create a magnetic field around the coil

  - this field interacts with a magnetic field produced by the permanent magnets

  - there is a downward force on one side of the coil and an upward force on the other side of the quote for the coil turns around the axle
• split ring commutators


  - when the coil is vertical the pressures are not in contact with the commutator and the current stops

  - the coil keeps moving past the vertical until brushes make contact again on the opposite side so the current in the coil is reversed every half turn and the coil continues to turn in the same direction
• speed and direction of a motor

  - how to increase the speed and to change the direction of the motor

  to increase the speed of the motor increase the

  1- increase the strength of the magnet

  2- increase the current

  3- increase the number of turns of the coil

  
  

  to change the direction of the motor

  - swap the poles over ( change direction of magnetic field)

  - turn the current round ( changes the direction of the current )
• learn graphs of power supply between 3v ac power supply and 6v ac from power supplu
• Mains frequency in the UK is...

  50 Hz
• direct current dc


  Electric current that flows in only one direction
• alternating current ac


  - mains electricity supplies alternating current where the current constantly changes direction

  - mains electricity has a voltage of 230 v , frequency of 50Hz
• electromagnetic induction


  - if a conductor is moved at right angles to a magnetic field a voltage is induced across it

  - the wire cuts through the magnetic flux ( field lines)

  - if the conductor is a part of a complete circuit a current flows

  - this is called electromagnetic induction

  - no voltage is induced if the wire is moved parallel to the field
• electromagnetic induction - conclusion

  - the magnetic flux cuts thorough the coil

  - this induces a voltage in the coil, because the magnetic field ( flux) through the coil changes

  - the voltage causes a current to flow

  - no voltage is induced if there is no movement because there is no change in the magnetic flux
• how to increase size of induced voltage and how to reverse direction of induced voltage

  to increase size of the induced voltage

  - faster movement

  - stronger magnet

  - wrapping a wire into a coil/ using a coil with more turns

  - larger cross sectional area of coil

  
  

  the direction of the induced voltage can be reversed by

  - changing the direction of movement

  - turning the magnet round to change the direction of the current
• difference between motors and generators

  motors - split ring commutator - electric current in, motion out

  generator - slip ring, turn it and we get voltage out
• how do you make a bulb brighter

  - increasing the water flow - increase voltage

  - use stronger magnet

  -increasing number of turns of the coil
• generators


  in a generator a rotating coil cuts through the magnetic field lines

  - this induces a voltage across the coil

  - the voltage causes a current to flow

  - one side of the coil moves up through the field and then down again and so on so the current direction reverses every half turn, producing an alternating current

  - electrical connections are made by brushes which slide over slip rings

  - these prevent the wires getting tangled
• generators ( bicycle dynamo )

  - a strong bar magnet is spun by the wheel of a bike. this results in a changing magnetic field in the coil which induces a voltage in the coil
• learn circuit symbols for power supply dc and ac and transformer
• How does a transformer work?

  → an alternating current is passed through the primary coil

  → the alternating current produces a continuously changing magnetic field around the primary coil

  -> the magnetic flux cuts through the secondary coil

  -> an alternating voltage is induced in the secondary coil

  
  

  transformers only work with alternating current because they need a changing magnetic field

  - the iron core increases the strength of the magnetic field
• transformer experiment

  - what happens if there are more turns in the secondary coil that the primary

  - what happens to the voltage if there are double the number of coils in the secondary

  - what happens if there are more turns on the primary coil than the secondary


  - the secondary voltage is higher than the primary voltage

  - the secondary voltage is double the primary voltage

  - the primary voltage is higher than the secondary voltage
• transformer voltage equation


  input (primary voltage)/ output ( secondary voltage ) = primary turns/ secondary turns = Vp/Vs =Np/Ns
• transformers and current - if a transformer is 100% efficient


  input power = output power

  Vp x Ip = Vs x Is
• conclusion - transformers

  - step up, step down, large current


  a step up transformer has more turns on the secondary coil and so increasing voltage and decreasing the current

  - a step down transformer has less turns on the secondary coil and so decreasing voltage and increasing the current

  - a large current has a heating effect
• national grid


  - the national grid is a network of cables and transformers used to distribute energy

  - to reduce the energy lost as heat during the transmission of electricity , it is more efficient to use a higher voltage and lower current