Static Electricity

Causes of Static Electricity

Static electricity is caused by the transfer of electrons between objects. When two objects are rubbed together, electrons can move from one object to the other. This transfer changes the electrical charge of each object:

• The object losing electrons becomes positively charged (more protons than electrons).
• The object gaining electrons becomes negatively charged (more electrons than protons).

This process is called charging by friction. It mainly happens between insulators, materials that do not allow electrons to move freely, such as plastic, rubber, or dry cloth. Conductors, like metals, allow electrons to move freely, so static charges do not build up easily on them.

For example, rubbing a balloon on your jumper transfers electrons from the jumper to the balloon, making the balloon negatively charged and the jumper positively charged.

Electric Charge and Charging

Electric charge is a property of particles such as electrons and protons. Electrons carry a negative charge, and protons carry a positive charge. The unit of charge is the coulomb (C).

Charging by friction happens when electrons are transferred between two insulators rubbed together, as explained above.

Charging by induction is different. It happens without direct contact:

• If a charged object is brought near a neutral conductor, it causes electrons in the conductor to move.
• This creates a separation of charges inside the conductor: one side becomes more positive, the other more negative.
• If the conductor is then earthed (connected to the ground), electrons can flow in or out, leaving the conductor charged when the earthing is removed.

The conservation of charge means that the total charge before and after any process remains the same. Charges are not created or destroyed, only transferred.

For instance, if a negatively charged rod is brought near a neutral metal sphere and the sphere is earthed, electrons will flow out of the sphere. When the earth connection is removed and the rod taken away, the sphere is left positively charged.

Example: If a plastic rod gains 3.2 × 10⁻⁶ C of negative charge by rubbing, how many extra electrons does it have?

The charge of one electron is $-1.6 \times 10^{-19}$ C. The number of electrons, $n$, is:

$$n = \frac{\text{total charge}}{\text{charge per electron}} = \frac{3.2 \times 10^{-6}}{1.6 \times 10^{-19}} = 2.0 \times 10^{13}$$

So, the rod has gained 20 trillion extra electrons.

For instance, if a charged object with charge $+Q$ and another with charge $+q$ are close, they repel each other with a force that depends on the size of the charges and the distance between them. This is described by Coulomb's law, which states that the force $F$ is proportional to the product of the charges and inversely proportional to the square of the distance between them:

$$F \propto \frac{Q \times q}{r^2}$$

Electric Fields and Forces

An electric field is the region around a charged object where other charged objects experience a force. The strength of the electric field depends on the amount of charge and the distance from the charged object.

Electric field lines are used to represent the field:

• They point away from positive charges and towards negative charges.
• The closer the lines, the stronger the electric field.

Charged objects exert forces on each other:

• Objects with opposite charges attract.
• Objects with the same charge repel.

Example: Two small spheres carry charges of +4 μC and −6 μC and are 0.5 m apart. Describe the force between them.

Since the charges are opposite, the spheres attract each other. The force acts along the line joining the two spheres, pulling them together.

Applications and Effects of Static Electricity

Static shocks happen when a charged object suddenly discharges, allowing electrons to jump across a gap. This can be felt when touching a metal object after walking on a carpet.

Electrostatic precipitators are devices used in factories to remove smoke and dust particles from waste gases. The particles are given a charge and then attracted to oppositely charged plates, cleaning the air.

Spray painting uses static electricity to give paint droplets a charge. The object to be painted is given the opposite charge, so the paint is attracted evenly, reducing waste and improving coverage.

Lightning is a natural static discharge. During storms, charges build up in clouds by friction between ice particles. When the charge difference between cloud and ground becomes large enough, a huge spark (lightning) occurs.

For instance, if a plastic rod gains a charge of $-2.4 \times 10^{-6}$ C, the number of extra electrons gained can be calculated by dividing the total charge by the charge of one electron.