Exam answers paper 2

• Terminal velocity - vertical

  1. Initially weight > air resistance
  2. Resultant force = accelerate downwards
  3. As speed increases so does air resistance
  4. Eventually, weight = air resistance
  5. No overall force so no more acceleration
  6. Terminal velocity
• Terminal velocity - horizontal

  1. Initially driving force > air resistance
  2. Resultant force = accelerate forwards
  3. As speed increases so does air resistance
  4. Eventually, driving force = air resistance
  5. No overall force so no more acceleration
  6. Terminal velocity
• Why does terminal velocity affect air resistance?

  • Larger surface area = larger air resistance
  • Air resistance catches up with weight quickly
  • Less time to accelerate
  • Lower terminal velocity
• What is thinking distance?

  Distance travelled between seeing the hazard and pressing on the brake
• What is stopping distance?

  The distance a travelled from applying the brakes until the vehicle comes to a complete stop.
• Factors affecting stopping distance

  Thinking distance: tiredness, alcohol, drugs, caffeine

  
  

  Braking distance: poor tyres or brakes, weather (icy/wet road conditions), speed
• Inelastic deformation

  Application of a force that causes a change of shape. When removed, the object cannot go back to its original shape
• Elastic deformation

  Application of a force that causes a change of shape. When removed, the object goes back to its original shape
• Three ways a spring can be deformed

  • Bend
  • Stretch
  • Compress
• Stiffer vs flexible springs

  • stiffer springs have larger spring constants
  • stiffer springs extend or deform less than flexible springs
• Springs RP (required practical)

  1. Attach a spring to a clamp
  2. Measure the natural length of the spring (starting length)
  3. Add masses to increase the force on the spring
  4. Record new length and calculate extension
  5. Keep adding masses and repeat steps 3 and 4
  6. Plot data on a graph
  7. Gradient = F/e = (spring constant, k)
• Electromagnetism

  • Current flows in a metal wire
  • Creating a magnetic field
• Right hand grip rule (magnetic fields)

  Reverse current = reversed magnetic field
• What do you call a wire that is coiled?


  Solenoid
• What happens if you reverse the current in a solenoid?

  The magnetic poles swap
• 3 ways to increase magnetic field strength

  • Increase current
  • Increase number of coils
  • Use an iron core inside solenoid (becomes electromagnet)
• What is the motor effect?

  1. Current flows through a wire
  2. Creates a magnetic field in the wire
  3. This field interacts with field of permanent magnet
  4. Creates a Force on the wire, which makes it move
• Left-hand rule

  Thumb - Direction

  First finger - Field

  Second finger - Current
• How does a DC motor work?

  1. Coil is in a magnetic field
  2. Forces act on each arm of the coil
  3. Current is flowing in opposite directions in each arm
  4. Each arm feels opposite force (one UP other DOWN)
  5. Split-ring commutator swaps contacts every half turn
  6. This swaps current in each arm, ensuring motor keeps turning in same direction.
• Calculating speed of water ripples RP (required practical)

  Set up the ripple tank as shown in the diagram with about 5 cm depth of water.

  1. Adjust the height of the wooden rod so that it just touches the surface of the water.
  2. Switch on the lamp and motor and adjust until low frequency waves can be clearly observed.
  3. Measure the length of a number of waves then divide by the number of waves to record wavelength. It may be more practical to take a photograph of the card with the ruler and take measurements from the still picture.
  4. Count the number of waves passing a point in ten seconds then divide by ten to record frequency.
  5. Calculate the speed of the waves using: wave speed = frequency × wavelength.
• Low of reflection

  Angle of incidence = angle of reflection
• What is refraction?

  When a wave passes through a medium and bends
• Do waves travel faster or slower in denser mediums?


  Slower
• What happens when a wave travels into a more dense medium?

  Frequency increases and wavelength decreases
• What kind of materials are good at absorbing and emitting infrared radiation?

  Black and matte objects
• What happens when infrared meets the surface of a bright shiny object?

  Bright and shiny objects are poor absorbers of infrared so it will get reflected back
• Leslie cube BBR practical

  1. Pour hot water inside a Leslie cube
  2. Measure the temperature outside each surface - should be the same
  3. Use an IR detector to measure IR from emitted from each surface
  4. Use a stopwatch to record IR readings every minute for 10 minutes.
  5. Repeat experiment for all surfaces on Leslie cube
  6. Make sure the detector is held from the same distance each time
  7. Compare results to show how IR readings changed over time
• Electromagnetic spectrum

  Radio

  Microwave

  Infrared

  Visible light

  UltraViolet

  X-Ray

  Gamma
• Similarities between EM waves

  transverse waves

  same speed in a vacuum

  can all travel in a vacuum
• Differences between EM waves

  Wavelengths

  Frequency

  Release of energy
• Microwaves example

  used for communication or cooking
• Infrared example

  Used in thermal cameras or remote controls
• What is equilibrium in forces?

  When forces are balanced so there is no resultant force
• Fluid pressure summary

  • When an object is submerged fluid exerts pressure from all sides
  • This creates an overall force upwards (upthrust)
  • If weight > upthrust, it will sink
  • If upthrust > weight, it will float
  • If upthrust = weight, it was stay suspended
• How does a microphone work?

  1. Sound waves hit the diaphragm of the microphone.
  2. The diaphragm is attached to a coil of wire.
  3. Behind the coil is a permanent magnet.
  4. The vibrating diaphragm moves the coil in the magnetic field.
  5. This induces a voltage (electromagnetic induction).
  6. The voltage produces a current matching the sound.
• How does a loudspeaker work?

  1. An (AC) flows through the coil in the loudspeaker.
  2. This coil is positioned in a magnetic field
  3. The current creates a magnetic field around the coil.
  4. The coil experiences a force due to the motor effect.
  5. The coil moves back and forth as the current changes direction.
  6. The cone moves with the coil, creating sound waves by vibrating the air.
• Dynamo summary

  1. Coil is attached to a turbine
  2. Coil rotates inside Magnetic Field
  3. Rotating coil experiences a "changing magnetic field"
  4. Current is induced inside coil (Electromagnetic Induction)
  5. Current swaps direction every half turn
  6. Split ring commutator swaps contacts every half turn
  7. This creates a direct current (D.C) in the main circuit
• Alternator summary

  1. Coil is attached to a turbine
  2. Coil rotates inside Magnetic Field
  3. Rotating coil experiences a "changing magnetic field"
  4. Current is induced inside coil (Electromagnetic Induction)
  5. Current swaps direction every half turn
  6. Slip-rings and brushes ensure contacts DON'T swap
  7. This creates an alternating current (A.C) in the main circuit
• Life cycle of a star

  • Nebula (cloud of dust) is pulled together by gravity → Protostar
  • Hydrogen to fuse into Helium (Nuclear Fusion) = Main sequence star
  • Hydrogen runs out causing Helium to fuse
  • Red Giant (Helium fuses into elements up to Iron)
  • Red Giant ejects outer layers forming white dwarf

  Very big stars:

  
  

  • Red Super Giant (Helium fuses into elements up to Iron)
  • Supernova (Elements heavier than Iron are produced)
  • Supernova → Neutron Star or Black Hole
• What is red-shift?

  High wavelength and low frequencies shows galaxies are moving away from us