Static Electricity MCQ 1

Electric Charge

• Electric charge (Q or q) indicates if a body has more or less electrons than protons.
• The unit of charge is coulomb (C).

Atomic Structure

• An atom is made up of three particles: protons, neutrons, and electrons.
• Protons and neutrons are located in the centre of the atom, and electrons orbit the nucleus.

Charge Interactions

• Like charges repel each other, unlike charges attract each other.
• Rubbing cloth and polythene causes the polythene to become negatively charged, and rubbing cloth against perplex causes the perplex to become positively charged.
• This demonstrates forces between charges.

Conductors and Insulators

• A conductor is any substance through which electric charge can flow.
• An insulator is any substance through which electric charge cannot flow.
• All static charges reside on the outside of a conductor.
• Static charge on a conductor tends to accumulate where the conductor is most pointed.
• Point discharge occurs at a very sharp point due to charge density, resulting in a strong electric field being created.

Ions and Neutrality

• Ions are both attracted and repelled, neutralising the object.
• This can be demonstrated by showing the effect of the ‘electric wind’ created on a naked flame.

Gold Leaf Electroscope

• The gold leaf electroscope is used to detect small charges, estimate the size of a charge, identify the sign of the charge, and distinguish between a conductor and insulator.
• It consists of multiple components, most importantly a thin gold leaf placed close to a metal rod.
• If these are both charged with the same sign, they repel each other.

Coulomb's Law

• The force between two point charges is proportional to the product of their charges and inversely proportional to the square of the distance between them.
• Unit = newton (N)
• This is an example of an inverse square law.
• The size of the force is the same on each body, and the force is along the line between them.

Factors Affecting Electrostatic Force

• One of the factors affecting the size of the electrostatic force is the permittivity (ε) of the medium surrounding the charges.
• If the permittivity is low, the force is big; permittivity and force are inversely proportional.
• The force is greatest when the charges are in a vacuum.

Electric Field

• An electric field is any region of space where a static electric charge experiences a force other than the force of gravity.
• An electric field line is the line along which a positive charge would move if placed in the electric field.
• The electric field strength (E) at a point in an electric field is the force per unit positive charge at that point.
• S.I. Unit = newton per coulomb (N C-1) or volt per meter (V/m)