Charged Particle
The Basics of Electric Charge
At the heart of the topic are the tiny particles that make up atoms. An atom consists of a nucleus, containing protons (positively charged) and neutrons (neutral), surrounded by electrons (negatively charged). Normally, an atom has an equal number of protons and electrons, making it electrically neutral. A charged particle is created when this balance is upset.
An object becomes positively charged when it loses electrons. For example, when you rub a balloon on your hair, electrons are transferred from your hair to the balloon. Your hair, now with fewer electrons, gains a net positive charge. The balloon, with extra electrons, gains a net negative charge. This is why your positively charged hair strands are attracted to the negatively charged balloon and why they might even repel each other because they all have the same type of charge.
The fundamental unit of electric charge is the Coulomb (C)[1]. A single proton has a charge of approximately $+1.602 \times 10^{-19}$ C, and an electron has a charge of $-1.602 \times 10^{-19}$ C. The neutron has zero charge.
Methods of Charging
There are three primary ways an object can acquire a net electrical charge, each involving the movement of electrons.
| Method | Process | Example |
|---|---|---|
| Friction | Electrons are physically rubbed off one material and onto another. | Rubbing a plastic rod with wool. The rod becomes negative, the wool positive. |
| Conduction (Contact) | Charging by direct contact. A charged object touches a neutral one, and charge is shared. | Touching a negatively charged balloon to a neutral wall. The wall's surface becomes negative. |
| Induction | Charging without direct contact. A charged object is brought near a neutral one, causing a temporary charge separation. | A negatively charged balloon brought near a soda can will repel electrons to the far side, making the near side positive. |
Forces and Fields: The Guides for Motion
Charged particles do not move randomly; they are guided by invisible influences called fields. An electric field is a region of space around a charged object where a force would be exerted on other charged objects. The strength of the electric field ($E$) is defined as the force ($F$) per unit charge ($q$):
$E = \frac{F}{q}$
The direction of the electric field is defined as the direction a positive test charge would move. Therefore, a positive charge placed in an electric field will move in the direction of the field lines, while a negative charge will move in the opposite direction.
Charged Particles in Action: From Lightning to Screens
The motion of charged particles is not just a theoretical concept; it's the basis for countless phenomena and technologies we encounter daily.
Static Electricity and Lightning: When you walk across a carpet in dry weather, your body can pick up extra electrons from the carpet through friction. When you then reach for a metal doorknob, the electrons jump from your finger to the knob, creating a tiny spark. Lightning is a massive version of this. In a thundercloud, air currents cause ice crystals and water droplets to collide, separating charge. The ground below becomes positively charged by induction. When the electric field becomes strong enough, the air, which is normally an insulator, breaks down, and a massive flow of electrons (a lightning bolt) travels between the cloud and the ground.
Cathode Ray Tubes (CRTs)[2]: Old television and computer monitors used CRTs. Inside a vacuum tube, a heated cathode emits a beam of electrons (cathode rays). These negatively charged particles are then accelerated and steered by electric and magnetic fields to hit a phosphorescent screen at the front, creating a glowing picture. This is a perfect example of controlling the motion of charged particles to perform a useful task.
Inkjet Printers: In a common type of inkjet printer, tiny droplets of ink are given a precise electrical charge as they are ejected from the print head. These charged droplets then pass between two metal plates that have a strong electric field between them. By controlling the charge on each droplet, the printer can use the electric field to steer the droplet to a specific spot on the paper.
Common Mistakes and Important Questions
Q: Can an object have a charge without being touched by another charged object?
Yes, through the process of induction. Bringing a charged object near a neutral one will cause the charges within the neutral object to separate. If the neutral object is then connected to the ground (a process called grounding) while the charged object is nearby, it can become permanently charged. This is how electroscopes, devices that detect charge, are often charged.
Q: Do charged particles always move in a straight line?
No, not at all. A charged particle will only move in a straight line if the electric field is uniform and there are no other forces. If it enters a magnetic field, its path will be bent into a circle or a spiral. This is because the magnetic force always acts at a right angle to the particle's direction of motion, causing it to constantly change direction. The Large Hadron Collider (LHC) uses incredibly powerful magnets to bend beams of protons, which are positively charged particles, in a circular path.
Q: Is it protons or electrons that move when an object is charged?
In most everyday situations involving static electricity, it is almost always electrons that move. Protons are bound very tightly inside the nucleus of an atom and do not move around freely in solids. When you rub a balloon on your hair, it is electrons that are transferred, not protons. In solutions and plasmas, however, both positive and negative ions (atoms or molecules that have lost or gained electrons) can move.
Footnote
[1] Coulomb (C): The SI unit of electric charge. It is defined as the charge transported by a constant current of one ampere in one second.
[2] Cathode Ray Tube (CRT): A vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen.