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GRADE 8 · CHARGE AND ELECTRIC FIELD

Charging, conductors and dielectrics

Two signs of charge, the electroscope and charge redistribution.

20 minutes4 theory cards
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Understanding step by step

A rubbed balloon attracts paper that has not been rubbed. Must the paper have acquired an opposite charge? Begin by considering what can change inside a material.

A rubbed balloon attracts paperpaper is neutralNeutral paper is attracted too
A rubbed balloon attracts neutral pieces of paper. Negative charge is chosen for the balloon in this example; green arrows start at the pieces and show motion towards it. Zero total charge does not mean that paper has no charged particles inside.

Two signs of charge

Suspend two small charged objects and compare their effects: some combinations repel and others attract. Two signs of electric charge, positive and negative, describe this behaviour. Like charges repel; unlike charges attract. Charge is denoted q and measured in coulombs, Кл. It characterizes electric interaction but is not a force: force also depends on distance and the surrounding medium.

+++−RepulsionAttraction
Like charges repel and unlike charges attract. Arrows show forces acting on different objects.

Where does charge come from?

Rubbing does not add a new kind of matter to a material: it already contains positively and negatively charged particles. When materials touch and separate, some electrons can pass between them; rubbing brings many surface regions into contact. An electron carries negative charge. An initially neutral object that gains extra electrons therefore becomes negatively charged. Such transfer or redistribution of charge is called charging.

One electron transferred during contacte⁻+−lost an electrongained an electronThe pair's total charge remains zero
Both objects were initially neutral and exchanged no charge with their surroundings. During contact, one electron moved from left to right. After separation, the left object has positive net charge and the right has an equal-magnitude negative charge. The signs inside indicate the objects' final charges; e⁻ denotes an electron, whose own charge is negative.

Rubbing does not create charge

The other object has fewer electrons while its positive component remains. An initially neutral object that loses electrons therefore becomes positively charged. Follow both objects as an isolated pair: the negative charge gained by one equals that lost by the other. If both were neutral, their final charges have equal magnitudes and opposite signs, with a sum of zero. Exchange with a hand or the air would break this isolation condition.

Before contactAfter separationq = 0q = 0+q−qAn electron moves from the first object to the secondTotal charge of the pair: 0 → 0
Four positive and four negative charges of the pair are shown schematically. Before contact, each object has two plus and two minus signs. One electron moves from the first object to the second: positive charges stay in place while negative charges become one and three. Total counts are conserved and the pair's net charge remains zero.

Charge can move in a conductor

Why can a metal rod acquire charge at the end far from the contact point, while plastic behaves differently? Metal has mobile electrons that can move throughout it. Such a material is a conductor: when its parts are connected, charge redistributes quickly. A hand can also provide a path for charge to escape. An insulating handle hinders this exchange, letting us investigate charge on the rod instead of immediately transferring it to the surroundings.

Different charge mobility++++++ConductorDielectricMobile and bound charges
In the schematic metal region, positive parts of the material stay in place while electrons can move between them. The dielectric shows local neutral groups of bound charges. Each region contains equal numbers of plus and minus signs. Sizes and distances are schematic; circles on the right denote groups of particles.

A dielectric behaves differently

Under ordinary conditions, a dielectric has almost no charge carriers able to move freely through the entire object. It still contains charged particles. A nearby charge can slightly shift its positive and negative parts relative to one another. This separation is polarization. Total charge can remain zero even though its arrangement changes. Neutral therefore does not mean unable to interact electrically.

No external chargeNear ‘−’centres coincideclouds displacedThe dielectric's total charge is unchanged
In this schematic model, the positive centre and the centre of the negative electron cloud initially coincide. On the right, an external negative object shifts the clouds rightwards relative to the positive centres. Displacement is exaggerated for clarity. Each group remains neutral and each cloud stays within its own particle.

Why paper is attracted

Paper near a charged balloon becomes polarized: the oppositely charged side is slightly closer, and the like-charged side slightly farther away. Electric action weakens with distance, so the nearer attraction can exceed the farther repulsion. The paper is attracted without any change in its total charge. If the field acted identically on both sides, this imbalance would disappear: the variation with position matters.

Unequal forces produce attractionstrongerweakerpaper: q = 0The nearer side is attracted more strongly
The negative balloon polarizes the paper. Plus and minus schematically represent bound charges on its near and far sides; their displacement is greatly exaggerated. Attraction on the near side exceeds repulsion on the far side, so the resultant force points towards the balloon. The paper's total charge remains zero.

How an electroscope helps

An electroscope's metal leaves are connected to its upper rod. When charge is transferred to it, the charge spreads and the similarly charged leaves repel and separate. Separation alone does not reveal the sign without comparison with a known charge. A nearby charged object can also redistribute charges without touching: the leaves separate while the electroscope as a whole remains neutral. The instrument shows electric action on its leaves; interpretation requires knowing the conditions.

UnchargedExample: charge ‘−’leaves togetherleaves separatedWith ‘+’, the leaves separate too
The same instrument is shown uncharged and after receiving negative charge. The ball, rod and leaves are metal-connected; the rod passes through an insulating plug. Charge is marked on the metal parts themselves. Positively charged leaves also repel, so separation alone cannot identify the sign of charge.

Two metal objects

A neutral metal rod with an insulating handle touches a charged sphere. Why does the rod acquire charge too?

Before touchinginsulatorq₀ ≠ 0rod: q = 0sphereWhy will the rod also acquire charge?
Just before contact: the sphere has nonzero charge q₀; the metal rod is neutral and has an insulating handle. The arrow shows the rod approaching. Neither sign nor magnitude of q₀ is specified. In the following steps, the same rod touches the sphere.

Worked example · 1

On contact, the sphere and metal rod form one conductor. Mobile electrons can cross the contact point.

Metal touches metalinsulatormetalmetalcontactone conductorMobile electrons cross the contact
The same rod now touches the sphere: the metal parts form one conductor. Mobile electrons can pass through the contact. Since the initial charge sign is unspecified, no direction of net transfer is chosen here.

Worked example · 2

Redistribution continues until electrical equilibrium is reached. The insulating handle hinders charge leakage through the hand, so charge remains in this system.

Equilibrium reachedinsulatorq₁q₂the handle hinderscharge leakageq₁ + q₂ = q₀Directed transfer has stopped
The sphere, rod and handle are unchanged. Redistribution has reached equilibrium: directed charge transfer stops but electrons continue thermal motion. q₁ and q₂ denote charges on the parts. The insulator hinders leakage; neglecting exchange with the surroundings, q₁ + q₂ = q₀.

Worked example · 3

The individual charges need not become equal: equilibrium distribution depends on conductor shapes and sizes. Contact alone does not imply that charge divides in half.

Distribution depends on geometryinsulatorq₁q₂Not necessarily equalq₁ + q₂ = q₀The parts' shapes and sizes matter
The connected metals remain in the same equilibrium state, with the insulating handle. Without leakage, q₁ + q₂ is conserved, but equal shares do not follow from the conditions. Neither the initial sign nor specific shares on sphere and rod are assigned: the distribution depends on geometry.

Check the result

Mobile electrons allow charge to redistribute over both conductors; some charge ends up on the rod.

Key idea · 1

Charging involves transfer or rearrangement of charges already present.

Key idea · 2

In a conductor, carriers can move through the object; in a dielectric, bound charges can shift slightly relative to one another.

Key idea · 3

Neutral paper is attracted because of polarization and unequal forces on its near and far sides.

What comes next

To explain the sign of charge more precisely, next we will examine the nucleus, electrons and ion formation.

Basic Concepts

3 concepts

Charging

The process by which objects acquire or redistribute electric charge through contact, rubbing or induction.

Electrons can pass between materials when they touch and separate. An initially neutral object gaining extra electrons becomes negative; one losing electrons becomes positive. Two initially neutral objects acquire final charges of equal magnitude and opposite sign if the pair exchanges no charge with the surroundings.

Two signs of charge

Like electric charges repel, and opposite charges attract.

Positive and negative signs are conventional, but they describe electric interactions and electron transfer unambiguously.

Conductors and dielectrics

A conductor has mobile charge carriers able to move throughout the object. In a dielectric, charges are mostly bound and can shift slightly within particles of the material.

In metal, the mobile carriers are electrons, so the metal rod quickly redistributes charge. In a dielectric, polarization is possible without changing the total charge. The insulating handle makes it difficult to transfer charge through the hand.

Physical quantities

1 magnitude

Electric charge

q

A physical quantity characterizing an object's electric interaction.

Unit: coulomb · \mathrm{Кл}

Related formulas

The same formulas in the reference, with symbols and units.

Electric chargeq, Q · Кл

Problems on this topic

Statements, answer options and the walkthrough.

Charge and electric fieldGrade 8 · 27 problems