Fields & Static

Electric Charge and Static Electricity

Electric charge is a fundamental property of matter. Objects can have a positive or negative charge. Like charges repel each other, while opposite charges attract.

Static electricity occurs when electric charge builds up on the surface of an object. This usually happens when two different materials are rubbed together, causing electrons to be transferred from one material to the other. This process is called charging by friction.

For example, when a balloon is rubbed on a jumper, electrons move from the jumper to the balloon. The balloon becomes negatively charged, and the jumper becomes positively charged.

Materials that allow electric charge to move freely through them are called conductors (e.g. metals like copper and aluminium). Materials that do not allow charge to move freely are called insulators (e.g. plastic, rubber, glass).

Because charges can move easily in conductors, static charge usually does not build up on them. Instead, insulators are more likely to become statically charged.

The attraction and repulsion of charged objects can be demonstrated using simple experiments:

• Two objects with the same charge repel each other.
• Two objects with opposite charges attract each other.
• A charged object can attract a neutral object by causing charges within it to rearrange (induced charge).

For instance, if you bring a negatively charged balloon near small pieces of paper, the paper is attracted because the balloon induces a positive charge on the paper's surface.

Example: When a plastic rod is rubbed with a cloth, it gains a negative charge by gaining electrons from the cloth. The rod will then repel another negatively charged rod but attract a positively charged one.

Electric Fields

An electric field is the region around a charged object where other charged objects experience a force.

Electric fields are represented by field lines:

• Field lines point away from positive charges and towards negative charges.
• The closer the field lines, the stronger the electric field.
• Field lines never cross.

The force on a charged object in an electric field depends on the strength of the field and the amount of charge on the object.

Electric field strength decreases with distance from the charged object. This means the force on a charged object is stronger when it is closer to the source charge.

Example: A small positive charge placed near a large positive charge will be repelled because the electric field points away from the large positive charge. If the small charge moves twice as far away, the force it experiences will be weaker.

For example, if a charge of $1.0 \times 10^{-6}$ C is placed in an electric field of strength $500 \, \text{N/C}$, the force on the charge is $F = Q \times E = (1.0 \times 10^{-6}) \times 500 = 5.0 \times 10^{-4} \, \text{N}$.

Charging and Discharging Objects

Objects become charged by the transfer of electrons. When two insulators are rubbed together, electrons move from one to the other, leaving one object positively charged (loss of electrons) and the other negatively charged (gain of electrons).

Discharging occurs when the excess charge on an object is suddenly neutralised by electrons moving back, often through the air. This can cause a spark, which is a visible and sometimes audible discharge of static electricity.

Sparks happen because the electric field between the charged object and the Earth becomes strong enough to ionise air particles, making the air conductive and allowing charge to flow.

Earthing (or grounding) is a safety method used to prevent dangerous static build-up. It provides a path for excess charge to flow safely into the Earth, neutralising the object.

For example, fuel tankers often have a wire connected to the Earth while refuelling to prevent static sparks that could ignite fuel vapour.

Example: When a person walks across a carpet and touches a metal doorknob, they might feel a small shock. This happens because the person has built up static charge by friction with the carpet. Touching the doorknob allows the charge to discharge safely through the metal.