How do things move without contact?

In this area of study students examine the similarities and differences between three fields: gravitational, electric and magnetic. Field models are used to explain the motion of objects when there is no apparent contact. Students explore how positions in fields determine the potential energy of an object and the force on an object. They investigate how concepts related to field models can be applied to construct motors, maintain satellite orbits and to accelerate particles.

On completion of this unit the student should be able to analyse gravitational, electric and magnetic fields, and use these to explain the operation of motors and particle accelerators and the orbits of satellites.

Key knowledge

Fields and interactions

  • describe gravitation, magnetism and electricity using a field model
  • investigate and compare theoretically and practically gravitational, magnetic and electric fields, including directions and shapes of fields, attractive and repulsive fields, and the existence of dipoles and monopoles
  • investigate and compare theoretically and practically gravitational fields and electrical fields about a point mass or charge (positive or negative) with reference to:
    – the direction of the field
    – the shape of the field
    – the use of the inverse square law to determine the magnitude of the field
    – potential energy changes (qualitative) associated with a point mass or charge moving in the field
  • investigate and apply theoretically and practically a vector field model to magnetic phenomena, including shapes and directions of fields produced by bar magnets, and by current-carrying wires, loops and solenoids
  • identify fields as static or changing, and as uniform or non-uniform.

Effects of fields

  • analyse the use of an electric field to accelerate a charge, including:
    – electric field and electric force concepts: E=k\frac{Q}{r ^{2}}  and F=k\frac{q_1{q_2}}{r ^2}
    – potential energy changes in a uniform electric field: W=qV and E=\frac{V}{d}
    – the magnitude of the force on a charged particle due to a uniform electric field: F=qE
  • analyse the use of a magnetic field to change the path of a charged particle, including:
    – the magnitude and direction of the force applied to an electron beam by a magnetic field: F=qvB, in cases where the directions of v and B are perpendicular or parallel
    – the radius of the path followed by a low-velocity electron in a magnetic field: qvB=\frac{mv ^{2}}{r}
  • analyse the use of gravitational fields to accelerate mass, including:
    – gravitational field and gravitational force concepts: g=G\frac{M}{r ^{2}} and F_{g}=G\frac{m_1m_2}{r ^{2}}
    – potential energy changes in a uniform gravitational field: E_{g}=mg\Delta h
    – the change in gravitational potential energy from area under a force-distance graph and area under a field-distance graph multiplied by mass.

Application of field concepts

  • apply the concepts of force due to gravity, F_g, and normal reaction force, F_N, including satellites in orbit where the orbits are assumed to be uniform and circular
  • model satellite motion (artificial, Moon, planet) as uniform circular orbital motion:  a=\frac{v^2}{r} = \frac{4\pi^2r}{T^2}
  • describe the interaction of two fields, allowing that electric charges, magnetic poles and current carrying conductors can either attract or repel, whereas masses only attract each other
  • investigate and analyse theoretically and practically the force on a current carrying conductor due to an external magnetic field, F=nIlB, where the directions of I and B are either perpendicular or parallel to each other
  • investigate and analyse theoretically and practically the operation of simple DC motors consisting of one coil, containing a number of loops of wire, which is free to rotate about an axis in a uniform magnetic field and including the use of a split ring commutator
  • model the acceleration of particles in a particle accelerator (limited to linear acceleration by a uniform electric field and direction change by a uniform magnetic field).



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