Physics paper 2

• 2 types of force fields:

  . electric fields

  . Gravitational fields

  Similarities:

  . both follow inverse square law

  . field lines represent strength of the force

  . equipotential surfaces

  Differences:

  . G fields - attractive , Whereas E fields attractive or repulsive.

  . G fields act on mass , E fields act on charge
• Uniform fields exert the same force everywhere on the field. Arrows show direction of the force that acts on a mass.

  Radial fields the force exerted depends on the position of the object. As an object moves further away from the centre, the magnitude of the force decreases.
• Gravitational potential energy is the work done to move an object from infinity to a point. The gravitational potential at infinity is 0.

  Gravitational potential difference is the energy needed to move a unit mass between 2 points.
• Equipotential surfaces are surfaces which are created through joining points of equal potential together. Therefore the potential is constant everywhere on an equipotential surface. No work is done when moving across an equipotential surface.
• When an object orbits a mass, it experiences a gravitational force towards the centre of mass and as the object is moving in a circle, this gravitational force acts as the centripetal force.
• Keplers 3rd law is that the square of the orbital period is directly proportional to the cube of the radius. T^2 proportional r^3
• Total energy of a satellite = Ke + Pe
• The escape velocity of an object is the minimum velocity it must travel at, in order to escape the gravitational field at the surface of mass.

  So equate Ke to Pe.
• Electric fields

  Coulombs law states that the magnitude of the force between 2 point charges in a vacuum is directly proportional to the product of their charges, and inversely proportional to the square of the distance between the charges.
• Electric field strength

  Uniform field is positive to negative plate, equal force everywhere in the field

  Radial field is positive charge means positive potential therefore its repulsive. Negative charge, negative potential therefore it is attractive.
• Absolute electric potential at a point is the potential energy per unit charge of a positive point charge at that point in the field.

  Electric potential difference is the energy needed to move a unit charge between 2 points.
• Capacitance is the charge stored by a capacitor per unit potential difference.

  A capacitor is made of 2 conducting parallel plates separated by an insulating material called dielectrics.

  Dielectrics have a property called permittivity which is a measure of the ability to store an electric field in a material. A dielectric is formed by polar molecules which are initially arranged randomly. But in an electric field the polar molecules align themselves with the charge plates.

  Each molecule has its own electric field, the strength of which depends on the dielectrics permittivity.
• Once a capacitor is connected to a power supply, current starts to flow and negative charge is built up on the negative plate. Electrons move to the positive terminal and an equal but opposite charge is formed on each plate. As the charge across the plates increase, the potential difference increases.

  Data loggers can be used to measure V, I and Q.
• When a current carrying wire is placed in a magnetic field, a force is exerted on the wire. However, if current is parallel to the magnetic field, the force is 0N. This is the motor effect.
• Flemings left hand rule:

  thumb - direction of force

  First finger - direction of field

  middle finger - direction of convectional current (positive to negative)
• A cyclotron is formed of 2 semi - circular electrodes with a uniform magnetic field acting perpendicular to the plane, high frequency alternating voltage between the electrodes.

  What happens:

  . Particle is accelerated by electric field

  . radius of circular path increases as they move through second electrode

  . Alternating field changes direction, causing particle to accelerate again

  .Accelerates several times until required speed is reached by particle and then leaves cyclotron.
• Magnetic flux is the magnetic field lines passing through a given area.

  Magnetic flux linkage is the magnetic flux multiplied by number of turns of a coil.

  Magnetic flux density is when a current passes through a wire, a magnetic field is induced. It is also a measure of the strength of the field and is measured in Tesla.
• Faradays law - the magnitude of induced emf is equal to the rate of change of flux linkage.
• Lenz law - the direction of induced current is such as to oppose the motion causing it.

  To demonstrate this law:

  . measure speed of a magnet falling through a coil of wire

  . and measure its speed falling from the same height without the coil
• Oscilloscope measures:

  . peak voltage

  . peak to peak voltage

  . root mean square voltage

  . Time period
• Transformers can be used with alternating currents to change the size of their voltage. Primary coil attached to input voltage. Secondary coil is attached to output voltage and iron core.

  Primary coil provides a changing magnetic field, which passes through the iron core and interacts with the secondary coil.
• 2 types of transformers:

  . Step - up transformers - increases input voltage by having more turns on the secondary coil than primary.

  . Step - down transformers - decreases input voltage by having less coils on secondary coil than primary coil.