Static Electricity, Electric Charge, and Conservation
Electric charge is a fundamental property of matter that comes in two kinds, positive and negative; like charges repel, unlike charges attract, charge is quantized in units of e = 1.60 × 10−19 C, and the total charge of an isolated system is conserved. In brief: static electricity is the build-up of charge on an object, usually by rubbing two different insulating materials together so that electrons transfer from one to the other. The object that gains electrons becomes negative, and the one that loses electrons becomes positive. The SI unit of charge is the coulomb (C).
A balloon rubbed on hair sticks to a wall, and a spark can jump from a door handle in winter. These are effects of static charge. This guide explains what charge is and how objects become charged, states the law of conservation of charge, compares the types of charge in a table, works through calculations of the number of electrons and corrects common mistakes.
What is electric charge?
Atoms consist of a nucleus of positively charged protons and uncharged neutrons, surrounded by negatively charged electrons. An ordinary atom has equal numbers of protons and electrons, so it is electrically neutral. The proton and the electron carry equal and opposite charges of magnitude e = 1.60 × 10−19 C. Objects become charged when they have an unequal number of protons and electrons. The forces between charges:
- like charges (+ and +, or − and −) repel each other;
- unlike charges (+ and −) attract each other;
- a charged object also attracts neutral objects, by polarization: it shifts the charges in the neutral object slightly, so that the nearer side has the opposite charge.
The size of the force increases as the charges become larger and as they come closer together (Coulomb’s law).
How does static electricity arise?
When two different materials are rubbed together, electrons can be transferred from one material to the other, since the materials hold electrons with different strengths. The protons stay in the nuclei, and only electrons move. A glass rod rubbed with silk becomes positive (it loses electrons to the silk). A plastic rod rubbed with wool or fur becomes negative (it gains electrons). The charge stays on insulators because it cannot flow away. Examples: clothes sticking together in a dryer, hair standing up after combing, a balloon sticking to a wall, the shock from a car door, and lightning, which is a huge discharge between a cloud and the ground. Dry air makes static effects stronger, since humid air allows charge to leak away.
Static charge has uses (photocopiers, laser printers, electrostatic precipitators that remove smoke particles, spray painting) and dangers (sparks that can ignite fuel vapors, which is why fuel tankers are earthed and aircraft are grounded when refueling).
What is conservation and quantization of charge?
Conservation: charge cannot be created or destroyed; it can only be moved from one object to another. In rubbing a rod with a cloth, the cloth gains exactly as much charge as the rod loses (with the opposite sign), so the total remains zero. The total charge before and after any process in an isolated system is the same. This also holds in nuclear and particle processes: for example, in beta decay a neutron (0) becomes a proton (+e), an electron (−e) and an antineutrino (0).
Quantization: any charge is an integer multiple of e: Q = ne. The charge of quarks is ±e/3 or ±2e/3, but quarks are never found alone, so free charges are multiples of e. For ordinary objects n is so large that charge appears continuous.
Positive and negative charge compared
| Object | Electrons compared with protons | Net charge |
|---|---|---|
| Neutral | Equal numbers | Zero |
| Negatively charged | Excess of electrons | Negative |
| Positively charged | Deficit of electrons | Positive |
Worked example: counting electrons
- Number of electrons. A plastic rod has a charge of −1.0 × 10−9 C (−1.0 nC) after being rubbed. The number of excess electrons is n = Q ÷ e = 1.0 × 10−9 ÷ 1.60 × 10−19 = 6.3 × 109. This is a tiny fraction of the electrons in the rod (about 1024 or more).
- Charge of electrons. An object that loses 5.0 × 1012 electrons has a charge of +5.0 × 1012 × 1.60 × 10−19 = +8.0 × 10−7 C = +0.80 µC.
- Conservation. A glass rod rubbed with silk has a charge of +2.0 nC. The silk, initially neutral, therefore has a charge of −2.0 nC, so that the total charge remains zero.
- Charge on a lightning strike. A typical lightning flash transfers a charge of about 15 C (this varies widely). The number of electrons is 15 ÷ 1.60 × 10−19 = 9.4 × 1019.
- Impossible charge. A charge of 2.4 × 10−19 C cannot be carried by an isolated object: 2.4 ÷ 1.6 = 1.5 elementary charges, which is not an integer.
How was electric charge discovered?
Thales of Miletus (about 600 BCE) noticed that amber rubbed with fur attracts light objects: the Greek word for amber is elektron. Charles du Fay distinguished two kinds of electricity in 1733, and Benjamin Franklin named them positive and negative in the 1740s, and showed that lightning is electrical with his kite experiment (1752). Charles-Augustin de Coulomb measured the force between charges in 1785. J. J. Thomson discovered the electron in 1897, and Robert Millikan measured its charge in the oil-drop experiment (1909), showing that charge is quantized.
Which mistakes are common?
Mistake 1: Thinking that positive charges move in solids
In solids, it is the electrons that move. A positive charge results from a loss of electrons.
Mistake 2: Saying that rubbing creates charge
Rubbing separates charge. The total charge is unchanged.
Mistake 3: Believing that neutral objects have no charges
They contain equal amounts of positive and negative charge.
Mistake 4: Forgetting that charged objects attract neutral ones
A charged balloon attracts neutral paper by polarization.