TJC 14 Electric Field

Unit 14: Electric Field LEARNING OUTCOMES Candidates should be able to: (a) recall and use Coulomb's law in the form 1 2 24 o Q QF r for the electric force between two point charges in free space or air. (b) recall and use 24 o QE r for the field strength due to a point charge, in free space or air, to solve problems. (c) define electric potential at a point as the work done per unit charge by an external force in bringing a small positive test charge from infinity to that point. (d) use the equation 4 o QV r for the electric potential in the field due to a point charge, in free space or air. (e) show an understanding that the electric potential energy of a system of two point charges is E o QQU r 1 21 4 . (f) recall that electric field strength at a point is equal to the negative potential gradient at that point and use this to solve problems. (g) calculate the field strength of the uniform electric field between charged parallel plates in terms of the potential difference and plate separation. (h) calculate the forces on a charge in a uniform electric field. (i) describe the effect of a uniform electric field on the motion of a charged particle. (j) define capacitance as the ratio of the charge stored to the potential difference and use QC V to solve problems. (k) recall that the electric potential energy stored in a capacitor is given by the area under the graph of potential difference against charge stored, and use this and the equations U QV1 2 , QU C 21 2 and U CV 21 2 to solve problems.

Unit 14: Electric Fields 2026 Temasek Junior College 2 1. 2 TYPES OF ELECTRIC CHARGES There are 2 types of electric charge – positive charge and negative charge. Using a simple model, we can consider matter to be made up of three types of particles: electrons (which have negative charge), protons (positive) and neutrons (neutral). An uncharged object has equal numbers of protons and electrons, net charge is zero. When one material is rubbed against another, there is friction between them, and electrons may be rubbed off one material onto the other. The material that has gained electrons is now negatively charged, and the other material is positively charged. It was observed that like charges repel and unlike charges attract each other. This shows that there exists an electric force of repulsion or attraction between two charges. The French physicist, Charles Coulomb (1738-1806), using a torsion balance of his own invention shown in the figure below , confirmed the existence of an inverse square law of the electric force. A Coulomb torsion balance On the basis of his experiments, he concluded that 1. the force F between two point charges Q1 and Q2 was directly proportional to the product of the two point charges. F ∝ Q1 x Q2 2. the force between