Home » Handbook of O Level Physics » UNIT 17: STATIC ELECTRICITY

# UNIT 17: STATIC ELECTRICITY

• When two materials are rubbed,

• the material that gains electrons becomes negatively charged.

• the material that loses electrons becomes positively charged.

• Like charges repel and unlike charges attract each other.

• An atom with unequal number of electrons and protons is called an ion.

• Charge (Q) is measured in coulombs (C), e.g., Q = 5 C.

 Electrical insulators Electrical conductors Charges cannot flow easily. Charges can flow. In insulators, the electrons of an atom are firmly bound to the nucleus of that atom. In conductors, the outer (valence) electrons of an atom are ‘free electrons’. The valence electrons are loosely bound. After rubbing, the charge remains localized on the surface. After rubbing, the charges do not remain confined to the surface.
• Neutralizing / discharging

• Insulators discharge due to moisture in air or heating them

• Conductors can be discharged by earthing them.

• earth (as a verb): It means to connect an object to zero potential.

• earth (as a noun): Any conductor (e.g., human body or ground) that can take or give electrons without being noticeably charged itself.

• Electrostatic induction: It is the process in which an object is charged by bringing a charged object close to it.

• Example: If a negatively charged glass rod is brought close to a piece of paper then the
near-side of the paper becomes positively charged and the far-side of the paper becomes negatively charged.

Notes: If attraction between unlike charges is greater than the repulsion between like charges then the paper will be lifted (if the weight of the paper is overcome).

• Examination tips:

• When students are asked to show distribution of charges, the examiner may be interested in seeing equal number of positive and negative charges.

• The examiner may also be interested in the symmetry of charges.

• Electric field: It is the region of space where a unit positive charge experiences an electric force.

• Electric lines of force are drawn in the direction of force on a unit positive charge.

• Examples where charging could be a problem

• lightning

• charged body of an aircraft

• Examples where charging could be useful

• photocopier / laser printer

• electrostatic precipitator for flu-as removal in coal-fired power stations

• Van de Graaff generators are used in electrostatic experiments. You should understand its construction and working.