Coulomb's Law Experiment

Experiment #2

COULOMB’S LAW References th

Elements of Electromagnetism, 5  Edition, M.N.O. Sadiku rd Introduction to Electrodynamics, 3  Edition, David J. Griffiths

Introduction In the original experiments used by Charles Cou lomb to establish a relationship between force and charge (published in 1785) 178 5) he used small spheres with defined relative charges and defined distances of separation in conjunction with a torsion balance. The result that has come to be known as Coulomb's Law states that the force, F, between two point charges, of charge q1 and q2, a distance r from each other is given by

⃗    ̂

(1) 9

2

2

̂

where k is a proportionality (Coulomb) constant equal to 8.9876 x 10  Nm /C  and is a unit vector pointing from the charge causing the force to the charge the force is acting upon. From this equation it is clear that the magnitude of the force acting on a charged particle due to another charged particle is proportional to the charge of the particle experiencing the force, the charge of the particle causing the force, and the inverse square of the distance between the charges. In the present experiment the PASCO torsion balance shown in figures 1 and a nd 2 is used to verify the inverse square relationship and the charge dependence of Coulomb's Law. The torsion  balance consists of a conductive sphere is mounted at one end of a rigid insulating rod of length d which is counterbalanced, and suspended from a thin torsion wire. The rod can rotate around a  point P when a torque is applied to the sphere and as the sphere rotates the wire is twisted. The sphere can then be returned to its equilibrium position by twisting the torsion dial on top of the apparatus to counteract the initial torque. The torq ue the wire applies to the sphere,  is given  by the equation

  

 

(2)

where  is a proportionality constant (the torsion co nstant) and  is the twist angle required to return the sphere to equilibrium.