physics

• Pressure


  Determined by the area of an object and the force of an object
• Pressure in solids


  • The amount of pressure relates to the force of an object and its area
  • The larger the surface area the lower the pressure
  • This explains why football studs with a low surface area and higher pressure sink into the ground
  • Mountain bike tires with a large surface area and a low pressure don't sink into the mud
• Force (F)


  Measured in newtons (N)
• Area (A)


  Measured in m²
• Sample question


  • Jo weighs 500N and is wearing shoes which together have an area of 0.01m2. What pressure does she exert?
• Weight is a force. The weight of someone is a force.
• Pressure in liquids


  • Pressure acts in all directions
  • Pressure increases with depth
• Whales and sharks skeletons contain a lot of cartilage. Cartilage is more flexible than bone.
• Density


  Measures how much mass there is in a certain volume of a material
• An object will float on water if its density is lower than the density of water (which is 1g/cm3)
• Upthrust


  The upward force from a liquid or gas on an object
• If the upthrust on the object is more than its weight, then it will float
• If the upthrust on the object is less then it will sink
• Pressure in gases


  • Gas pressure is the force of gas particles colliding with the walls of the container
  • The more collisions the greater the gas (air) pressure
  • As the volume decreases the number of collisions between the molecules and the container increases, so the gas pressure increases
  • As the temperature increases particles have more kinetic energy and move faster, leading to an increase in the number of collisions, so the gas pressure increases
• Atmospheric pressure


  • Caused by the weight of the air acting down on a surface
  • The higher you go, the less atmospheric pressure, as there are less particles
• Stretch and compression


  • All materials can be squashed (by a compression force) or stretched (by a tension force) to some extent
  • When materials return to their original shape after the force is removed, it is called elastic behaviour
  • All elastic objects have an elastic limit, beyond which, if force is applied the object is deformed and will not return to its original shape
• Hooke's law


  • If you double the force the extension of the spring doubles when the object obeys Hooke's Law
  • An object will obey Hooke's law if the relationship on a graph between force and extension is a straight line
• The only metals which are magnetic are iron, cobalt and nickel. Steel is magnetic because steel has iron in it.
• Poles of a magnet

  The north pole and the south pole
• Magnetic forces are a non-contact force.
• The poles of a magnet are the places where the magnetic forces are strongest.
• Laws of magnetic attraction

  • When two like poles are put together, they repel each other
  • When two unlike poles are put together, they attract each other
• Magnetic field


  The region around a magnet where it has a magnetic effect
• Ways to map out the magnetic field around a magnet

  • Use iron filings
  • Use a compass
• Factors which affect the magnetic field

  • Distance - the greater the distance the weaker the magnetic field
  • Strength of the magnet - a stronger magnet will have a stronger magnetic field
• Magnetic field lines flow from north to south. The closer the field lines are the stronger the magnetic force.
• When current is passed through a coil of wire, the coil has a magnetic field around it. This is called an electromagnet.
• Key things needed to make an electromagnet

  • Coil of wire (called a solenoid)
  • Iron Core
  • Electricity supply
• Ways to increase the strength of an electromagnet

  • Increasing the voltage or the current
  • Increasing the number of coils (solenoid)
  • Moving the coils of wire closer to each other
• An electromagnet is powered by a battery. When the battery is reversed the current flows in the opposite direction so the magnetic field is reversed.