Electrostatic force

Electrostatic force is the push or pull exerted by a charged object on another object — whether that second object is charged or completely neutral. It belongs to the family of non-contact forces: unlike a push on a door or a pull on a rope, the two objects never need to touch for the force to act.

How charge appears — charging by friction. Ordinary objects are electrically neutral; rubbing two different materials together transfers a little charge from one surface to the other. A plastic pen rubbed on a piece of cloth, a balloon rubbed on a wool sweater, a glass rod rubbed with silk — in every case, friction leaves both objects carrying small, opposite electric charges. The charge is called "static" because it stays put on the surface for a while rather than flowing away immediately, which is why the whole phenomenon is known as static electricity.

Two kinds of charge, one simple rule. Everything electrically charged carries one of exactly two kinds of charge: positive or negative. How two charged objects behave toward each other follows a single rule, echoed later in magnetism's like-poles-repel pattern:

• Like charges repel — two positively charged objects push apart, and two negatively charged objects also push apart.
• Unlike charges attract — a positive charge and a negative charge pull toward each other.

Since the rule only ever needs the kind of charge, not any measured amount, it can be stated and used at this level without a single number.

Attracting the uncharged. A charged object can also pull in objects that carry no charge at all, such as tiny paper bits or a thin trickle of water from a tap. This is why a charged comb or balloon can lift uncharged paper scraps even though nobody deliberately charged the paper — the paper's own charges shift slightly in response to the nearby charged object, and the resulting attraction is strong enough to lift very light objects against gravity.

At this level, the electrostatic force is taught as a rule you observe and apply, not as a formula you calculate — there is no charge quantity, no distance term, and no Coulomb's law until much later grades. Still, it helps to see why the rule is reasonable rather than just memorising it.

Why rubbing creates charge. Every material's surface holds tiny particles of matter that carry electric charge. Different materials hold onto these charge-carrying particles with different strengths. When two different materials are rubbed together, some of these particles transfer from the surface that holds them more loosely to the surface that holds them more tightly. The material that loses particles is left with one kind of charge, and the material that gains them is left with the opposite kind — this is exactly why rubbing plastic on wool leaves the plastic and the wool with opposite charges, and why the specific pair of materials rubbed together decides which one ends up positive and which ends up negative.

Why like charges repel and unlike charges attract. This is stated as a basic rule of nature at this level, verified by countless simple experiments: bring two similarly charged pith balls near each other and they swing apart every time; bring oppositely charged ones near each other and they swing together every time. The rule is consistent and predictable, which is exactly why it is safe to use it to reason out new situations, as in the worked example below.

Why a charged object can attract something with no charge. An uncharged object still contains charge internally — it just has equal amounts of positive and negative charge spread through it, so its outside appears neutral overall. When a charged object comes close, it very slightly rearranges the charge already present inside the neutral object, pulling the opposite kind of charge a little closer to itself. Because the opposite charge is now nearer than the like charge, the net effect is attraction — enough to lift light objects such as paper bits, even though the paper was never charged on its own.

Question. A student rubs a plastic pen with a piece of cloth and brings it close to a small pile of tiny paper bits. The bits jump up and stick to the pen; after a few seconds, some fall away on their own. Separately, two identical balloons are each rubbed on the same wool sweater and hung close together by threads — as they are brought near each other, they swing apart rather than together. Explain both observations using electrostatic force.

Step 1. Rubbing the plastic pen with the cloth is charging by friction: it transfers charge between the two surfaces, leaving the pen with a static electric charge while it remains neutral only in the sense that the paper bits themselves are untouched and uncharged.

Step 2. The paper bits are uncharged, but a charged object can still exert an electrostatic force of attraction on an uncharged object — the pen's charge slightly rearranges the charge inside each paper bit, pulling it in.

Step 3. This attraction is strong enough to lift the light paper bits upward against their own weight, which is why they visibly "jump up" and stick to the pen.

Step 4. Over the following seconds, some of the charge on the pen leaks away into the surrounding air or is neutralised by contact with the paper bits, so the electrostatic force weakens; once it becomes too weak to hold them up, the paper bits fall away on their own.

Step 5. For the balloons: rubbing each balloon on the same wool sweater, by the same rubbing process, gives both balloons the same kind of charge — say, both become negatively charged.

Step 6. Since the two balloons carry like charges, the rule for electrostatic force says they repel. This is why the two balloons swing apart when brought close, while the charged pen can still attract the fully uncharged paper bits — two different situations governed by the same single rule of like-repel, unlike-attract, plus the separate fact that charged objects can also pull in neutral ones.

• Thinking a charged object can only attract, never repel — two objects carrying the same kind of charge always repel each other.
• Forgetting that a charged object can still attract a completely uncharged object, such as paper bits or a thin water stream — attraction is not limited to two oppositely charged objects.
• Mixing up electrostatic force with magnetic force — both follow a similar like-repel, unlike-attract pattern, but electrostatic force involves electric charge (positive and negative), while magnetic force involves magnetic poles (north and south); they are different forces entirely.
• Assuming rubbing always gives the same result regardless of material — the specific pair of materials rubbed together (plastic and wool versus glass and silk) determines which object ends up positively charged and which ends up negatively charged.

Ask the tutor: Curious why the static shocks and clingy clothes you notice are worse in dry winter weather than in humid weather?