Question: (i) State what is meant by a field of force. (H2 2012 P3 Q7)
Possible Answer: A field of force is a region in space where a particle experiences a force.
Further details: Strictly speaking, a field is not just a region in space. There is also no agreement on how the field of force should be defined. For example, one may consider the field to be a medium in space instead of simply space. Furthermore, a textbook author may state that a field of force exists in a region in space where a particle experiences a force (Hecht, 2003). However, one may analyse the definition of a field from the following perspectives: (1) a region in space (condition); (2) a particle (object); (3) experiences a force (effect).
Question: (ii) Define electric field strength
Possible answer: Electric field strength is the electric force per unit charge acting on a test charge that is stationary.
Further details: An alternative definition of the electric field strength is the electric force per unit charge experienced by an infinitely small test charge placed in a vacuum having an electric field. Mathematically, it is given by the equation E = F/q, where F is the electric force experienced and q is the infinitely small test charge. This is because a large test charge will influence the charge carriers in the surrounding and distort the electric fields. (Note that physics educators may prefer the term charge carrier as compared to charge because charge is an attribute of an object.)
Question: (iii) Suggest why, when defining electric field strength, the test particle must be stationary.
Possible answer: In a region in space, it is possible to have gravitational fields, electric fields, and magnetic fields. Thus, we should specify the condition that the test particle is stationary because the measured force could include magnetic force for the moving test particle in addition to the electric force.
Further details: Some physics teachers explain that a moving charge produces electromagnetic waves which are perpendicular electric and magnetic fields, and hence it will distort the original field in the region. Physics students should question these teachers on the source of wave energy. Strictly speaking, it is the accelerating charge carrier that produces electromagnetic waves.
Reference:
Hecht, E. (2003). Physics: Algebra/Trigonometry (3rd ed.). Pacific Grove, California: Brooks/Cole Publishing.