Experiment shows that the electric force between two charges is proportional to the product of the charges and inversely proportional to the distance between them. Coulomb’s law gives the magnitude of the force. The force is along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if they are the same. Unit of charge: coulomb, C The proportionality constant k can also be written in terms of the permittivity of free space. Coulomb’s law strictly applies only to point charges. Superposition: for multiple point charges, the forces on each charge from every other charge can be calculated and then added as vectors.
In 1820 Oersted arranged in his home a science demonstration to friends and students. He planned to demonstrate the heating of a wire by an electric current, and also to carry out demonstrations of magnetism, for which he provided a compass needle mounted on a wooden stand. While performing his electric demonstration, Oersted noted to his surprise that every time the electric current was switched on, the compass needle moved. He kept quiet and finished the demonstrations, but in the months that followed worked hard trying to make sense out of the new phenomenon. But he couldn't! The needle was neither attracted to the wire nor repelled from it. Instead, it tended to stand at right angles (see drawing below). In the end he published his findings (in Latin!) without any explanation.
MSSE 580 Screen Cast
Differences Between Electric and Magnetic Forces Screen Cast for MSSE 580
Electric Force Coulomb’s Law permittivity of free space Charles-Augustin de Coulomb (1736–1806)
Electric Field Electric Field Electric field is Electric force per unit charge – It is a vector quantity Space around every charged body is filled with an electric field This distortion of space is what causes the electric force, rather than what is normally considered to be a contact force. The electric field can be represented by field lines. These lines start on a positive charge and end on a negative charge The number of field lines starting or ending from a charge is proportional to the magnitude of the charge dipole
Magnetic Force <ul><li>Magnetic Force is due to the motion of a charged particle </li></ul><ul><li>Moving charge particles are normally electrons </li></ul><ul><li>A stationary charged particle does not interact with a static magnetic field </li></ul><ul><li>A moving charge has both a magnetic and electric field surrounding it </li></ul><ul><li>A charge moving through a magnetic field experiences a force - This force acts perpendicularly to both the magnetic field and the direction of the particle </li></ul>
Magnetic Field Magnets exert a magnetic field around them The direction of the field is from the north to south pole Where the field lines are closer the field is stronger A charge surrounded by electric field is also surrounded by a magnetic field if it is moving Magnetic field lines are closed loops Electric sources are inherently monopole, i.e., point sources Magnetic sources are inherently dipole, i.e., poles cannot be isolated
Electric & Magnetic Force/Field Electric force is in the direction of the electric field if the charge, q is positive Electric Force = q E The magnetic force on the other hand is given by the right hand rule and is perpendicular to the magnetic field Magnetic Force = q v B
Ø rsted’s Observation Hans Christian Ørsted (1777-1851) A current of charges generates a magnetic field