Matter is made of charged particles — positively charged protons (in nuclei) and negatively charged electrons. When objects gain or lose electrons, they become electrically charged and exert forces on other charged objects. Electrostatics is the study of electric charges at rest.
6.1 Electric Charge
Matter contains two types of charge: POSITIVE charge (carried by protons in atomic nuclei — protons are fixed in the nucleus and do NOT move in a solid) and NEGATIVE charge (carried by electrons — electrons CAN move, especially in conductors). A neutral atom has equal numbers of protons and electrons. When electrons are transferred from one object to another (e.g. by friction), one object gains extra electrons (becomes negatively charged) and the other loses electrons (becomes positively charged).
Definition
Electric charge (Q)
Electric charge is a fundamental property of matter. It is measured in coulombs (C). The charge of a single electron is −1,6 × 10⁻¹⁹ C and the charge of a single proton is +1,6 × 10⁻¹⁹ C.
Note
SI UNIT: The SI unit of electric charge is the coulomb (C). Named after Charles-Augustin de Coulomb (1736–1806). One coulomb is a very large amount of charge — everyday electrostatic experiments typically involve microcoulombs (μC = 10⁻⁶ C).
Rules of electrostatic force (charge interactions)
- LIKE charges repel each other: + repels + ; − repels −
- UNLIKE (opposite) charges attract each other: + attracts −
HOW A CHARGED OBJECT ATTRACTS AN UNCHARGED OBJECT (polarisation): A positively charged rod brought near a neutral piece of paper. The electrons in the paper are ATTRACTED to the positive rod and move slightly toward it; the positive ions in the paper (which cannot move) stay put. The near side of the paper now has a slight negative charge (attracted to the rod), and the far side has a slight positive charge. Since the negative side is closer to the rod, the attractive force is stronger than the repulsive force — the paper is attracted overall. This is called POLARISATION by induction.
Definition
Principle of conservation of charge
Charge cannot be created or destroyed. The total charge in an isolated system remains constant. If two charged objects share charge, Q_total = Q₁ + Q₂ before = Q₁′ + Q₂′ after.
Definition
Principle of charge quantization
Charge exists only in integer multiples of the elementary charge e = 1,6 × 10⁻¹⁹ C. You cannot have half an electron's charge.
Worked Example
Sphere A has a charge of +8 μC and sphere B has a charge of −2 μC. They are brought into contact and then separated. What is the final charge on each sphere, assuming they are identical?
Given
- Q_A = +8 μC
- Q_B = −2 μC
Find
Final charge on each sphere
Solution
- 1Total charge = Q_A + Q_B = 8 + (−2) = +6 μC
- 2Charge is shared equally (identical spheres): each gets +6/2 = +3 μC
6.2 Electric Fields and Field Lines
Definition
Electric field
An electric field is a region in space in which an electric charge experiences a force. The direction of the electric field at a point is the direction of the force that would be exerted on a small positive test charge placed at that point.
ELECTRIC FIELD LINES are lines drawn to represent the direction and strength of the electric field. Rules for drawing electric field lines: 1. Field lines start on POSITIVE charges and end on NEGATIVE charges. 2. Field lines never cross each other. 3. The CLOSER together the lines, the STRONGER the field. 4. Field lines are always perpendicular to the surface of a conductor.
Practice Question
A small positive test charge placed at point X experiences a force directed to the right. (a) In what direction is the electric field at X? (b) Is there a positive or negative charge to the LEFT of X?
(4 marks)