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Detection of Charges is taught in the note for Static Electricity. Read it below and look for the section on this topic.

Static Electricity

13 min read

Charging by rubbing

Everything is made of atoms, and atoms contain protons (positive), electrons (negative) and neutrons (no charge). Normally an object has equal positive and negative charge, so it is neutral. When two different materials are rubbed, electrons move from one to the other. The one that gains electrons becomes negatively charged; the one that loses them becomes positively charged.

Neutral and charged atomsThree atoms with 3 protons each. With 3 electrons the atom is neutral. With an extra electron it is negatively charged. With one electron removed it is positively charged.3+−−−Neutral3 protons = 3 electrons3+−−−−Negativegained an electron3+−−Positivelost an electron
An atom is neutral when it has as many electrons as protons. Gaining an electron makes it negative; losing one makes it positive.

Forces between charges

Like charges repel (+ and +, or − and −). Unlike charges attract (+ and −). The force is bigger when the charges are bigger and gets smaller quickly as they move apart.

Forces between chargesTwo positive charges repel, two negative charges repel, and a positive and a negative charge attract each other.++repel−−repel+−attractLike charges repel. Unlike charges attract.
Try itTwo charges

Change the charges and the distance between them. Watch the arrows: they point apart for like charges and together for unlike charges. The force falls fast with distance: halving the distance makes the force four times bigger.

Conductors, insulators and detecting charge

TypeCharge can flow?Examples
Conductoryes, easilymetals, graphite, the human body
Insulatornoplastic, glass, rubber, dry wood

A gold-leaf electroscope detects charge. When a charged rod touches or comes near the cap, the metal rod and the leaf get the same charge, so they repel and the leaf rises. The more charge, the more the leaf rises.

Gold-leaf electroscopeA glass-sided case with a metal rod through the top, a metal cap, and a thin gold leaf hanging from the rod. When charged, the rod and the leaf carry the same sign of charge, so the leaf is pushed away from the rod.−−−−metal capbrass rodgold leaf risesglass casechargedrod near
The leaf rises when the rod and leaf carry the same charge.
Worked exampleExplain the leaf

A negatively charged rod is brought near the cap of a neutral electroscope. Why does the leaf rise?

  1. The rod repels electrons in the cap, pushing them down to the rod and leaf.
  2. The rod and the leaf now have the same (negative) charge.
  3. Like charges repel, so the leaf is pushed away from the rod.

Answer: The leaf and rod carry the same charge, so they repel and the leaf rises.

Capacitors

A capacitor is a device that stores electric charge. In its simplest form it has two metal plates separated by a thin layer of insulator called the dielectric (air, paper, plastic or ceramic). In a circuit diagram it is drawn as two short parallel lines.

Capacitor: structure and symbolTwo parallel metal plates with a thin layer of insulator between them. The upper plate carries positive charge and the lower plate negative charge. Beside it is the circuit symbol: two short parallel lines.++++++−−−−−−dmetal platemetal plateinsulator (dielectric)wire to the cellcircuit symbolC = Q ÷ Vunit: farad (F)
The plates hold the charge. The insulator keeps the charge on the plates.

When a capacitor is connected to a cell, electrons are pushed off one plate and onto the other. The plate that loses electrons becomes positive and the plate that gains them becomes negative. No charge can cross the insulator, so the charge stays on the plates. The capacitor is charged when the voltage across it equals the voltage of the cell.

Charging a capacitorA cell, a closed switch and a capacitor in series. Electrons are pushed from the plate joined to the positive terminal round to the other plate, so one plate becomes positive and the other negative.+ + +− − −cellswitchcapacitorelectrons flow round this way
Watch the plates fill with charge and the voltage rise, then watch the stored charge run back through the lamp.
Capacitance
How much charge a capacitor stores for each volt across it. It is measured in farads (F). Real capacitors are much smaller than a farad, so we use microfarads (µF), where 1 µF = 0.000 001 F.
Capacitance = charge ÷ voltage. A bigger plate area, a thinner insulator or a better dielectric gives a bigger capacitance.
  • capacitance (farads, F)
  • charge stored (coulombs, C)
  • voltage across the capacitor (volts, V)
Worked exampleCharge stored

A 2 000 µF capacitor is charged to 9 V. How much charge does it store?

  1. Change microfarads to farads: 2 000 µF = 0.002 F.
  2. Use Q = C V.

Answer: The capacitor stores 0.018 C (18 millicoulombs).

UseWhy a capacitor is used
Camera flashIt stores charge slowly, then releases it in an instant to make a bright flash.
Radio tuningTogether with a coil it picks out one station.
Smoothing a power supplyIt evens out the bumps in the voltage.
Timing circuitsThe time to charge or discharge sets the delay.

Lightning conductors

Clouds become charged as ice and water droplets rub together, and a huge spark of lightning can jump to the ground. A lightning conductor is a pointed metal rod fixed on top of a building and connected to the ground by a thick copper strip buried in a metal plate. Charge from the cloud leaks away harmlessly through the points, or the lightning is led safely to earth instead of passing through the building.

Lightning conductorA house with a pointed metal rod on the roof joined by a thick copper strip down the wall to a metal plate buried in the ground. Charge from the storm cloud is led safely to earth through the rod.pointed metal rodthick copper stripmetal plate in the ground
Check yourself Which part of a capacitor stops charge crossing from one plate to the other?
Check yourself A rubbed rod attracts a small piece of paper. Which is certainly true?
Check yourself Two positively charged balloons are brought near each other. They will…
Check yourself A capacitor holds 0.05 C when the voltage across it is 10 V. What is its capacitance?

Key terms

Electron
A negatively charged particle in an atom that can move between objects.
Conductor
A material that lets charge flow.
Insulator
A material that does not let charge flow.
Capacitor
A device that stores electric charge.
Capacitance
The charge a capacitor stores per volt, in farads (F).
Dielectric
The insulator between the plates of a capacitor.

Summary

  • Rubbing moves electrons: the gainer is negative, the loser positive.
  • Like charges repel; unlike charges attract.
  • Conductors let charge flow; insulators do not.
  • An electroscope detects charge; a lightning conductor protects buildings.
  • A capacitor stores charge on two plates separated by an insulator: C = Q ÷ V, measured in farads.
Concept of Static Electricity Conductors and Insulators