Static Electricity & Electric Fields
The Structure of the Atom and Electric Charge
All matter is made up of atoms. Each atom has a tiny, dense nucleus at its centre, made up of positively charged protons and uncharged neutrons. Orbiting the nucleus at set distances are negatively charged electrons.
The nuclear radius is much smaller than the radius of the atom as a whole — roughly 10,000 times smaller. Almost all of the atom’s mass is concentrated in the nucleus. In a neutral atom, the number of protons equals the number of electrons, so the overall charge is zero.
Proton: relative charge +1, relative mass 1. Neutron: relative charge 0, relative mass 1. Electron: relative charge −1, relative mass negligible (about 1/1836). In a neutral atom, the number of protons equals the number of electrons.
How Objects Become Charged
Objects become electrically charged when electrons are transferred between them. It is always electrons that move — protons are locked inside the nucleus and cannot transfer between objects. When two insulating materials are rubbed together, friction causes electrons to move from one surface to the other.
The material that gains electrons becomes negatively charged (it now has more electrons than protons). The material that loses electrons becomes positively charged (it now has more protons than electrons). The total charge is conserved — no charge is created or destroyed, only transferred.
For example, rubbing a polythene rod with a cloth causes electrons to transfer from the cloth to the rod. The rod gains electrons and becomes negatively charged. The cloth loses electrons and becomes positively charged. Rubbing an acetate rod with a cloth has the opposite effect — electrons move from the rod to the cloth, leaving the rod positively charged.
Protons are bound tightly inside the nucleus by the strong nuclear force and cannot be removed by rubbing. Electrons, however, orbit the nucleus in shells and the outermost electrons are held relatively weakly. Friction provides enough energy to dislodge these outer electrons and transfer them to another material.
Attraction and Repulsion
Charged objects exert forces on each other without touching — this is a non-contact force. The rule is straightforward:
Like charges repel — two positive objects push each other apart, as do two negative objects. Unlike charges attract — a positive and a negative object pull towards each other.
A charged object can also attract an uncharged object. This happens because the charge on the first object causes the electrons in the neutral object to redistribute: the opposite charge moves closer and the like charge moves further away, creating a net attractive force. This is called electrostatic induction.
Like charges repel, unlike charges attract. A charged object can also attract a neutral object through induction. These electrostatic forces are non-contact forces.
Electric Fields
An electric field is the region around a charged object where another charged object would experience a force. Every charged object creates an electric field in the space surrounding it. The field is strongest close to the charge and gets weaker with distance.
Electric field lines are used to represent the field. They always point from positive to negative. For a single positive charge, the field lines point outwards (away from the charge). For a single negative charge, the field lines point inwards (towards the charge). Between two oppositely charged parallel plates, the field lines are straight, evenly spaced, and point from the positive plate to the negative plate — this is a uniform electric field.
The closer together the field lines, the stronger the field at that point. Near a point charge, lines are bunched closely together; far away, they spread out, indicating a weaker field.
Objects can interact at a distance without contact through three types of field: gravitational fields (between masses), magnetic fields (between magnets or current-carrying conductors), and electrostatic fields (between charges). All three are non-contact forces. In each case, the force is transmitted through the field that surrounds the object.
Ions and Electron Transfer
When atoms lose or gain electrons, they become ions. An atom that loses one or more electrons becomes a positive ion (it has more protons than electrons). An atom that gains one or more electrons becomes a negative ion (it has more electrons than protons).
This process of electron transfer is fundamental to understanding both static electricity and the formation of ionic compounds. In metals, the outer electrons are free to move — this is what allows metals to conduct electricity. In insulators, electrons are fixed in position, so charge tends to build up on the surface rather than flowing away.
Static charge causes many familiar effects. Walking across a carpet in socks can cause electrons to transfer to your body, and touching a metal door handle creates a small spark as charge flows to earth. Lightning occurs when charge builds up in clouds through friction between ice particles and water droplets. Electrostatic charge is also used in practical applications such as spray painting (charged paint droplets are attracted to a grounded metal surface, giving an even coat) and photocopiers.
In conductors (metals), outer electrons are free to move, allowing charge to flow. In insulators, electrons are fixed in position, so charge builds up on the surface. Static charge is caused by the transfer of electrons between materials by friction.
When explaining how an object becomes charged, always say that electrons are transferred — never say “charge is transferred” without specifying which type. Also be clear about the direction: state which material gains electrons (becomes negative) and which loses them (becomes positive).
Summary
Atoms contain protons (+1), neutrons (0), and electrons (−1). Objects become charged by the transfer of electrons, not protons. Like charges repel and unlike charges attract. Electric fields surround charged objects — field lines point from positive to negative, and closer lines mean a stronger field. Conductors allow charge to flow; insulators allow charge to build up on their surface.