Electric Charge Model

Types of Materials: Conductors and Insulators

Materials can be classified based on their ability to allow electric charges to move. This distinction is fundamental in understanding electric phenomena and devices.

• Conductors: Materials through or along which charges easily move. Examples include metals like copper and aluminum.

• Insulators: Materials on or in which charges are immobile. Examples include rubber, glass, and plastic.

Conductors vs. Insulators: Electroscope Demonstration

Behavior of Electroscope Components

An electroscope is a device used to detect electric charge. It consists of a metal post and vane (conductor) and other parts made of insulating material. The vane can rotate about the point of contact with the post.

• When a charged rod touches the metal sphere at the end of an insulating rod, nothing happens.

• When a charged rod touches the metal sphere at the end of a conducting rod, the vane rotates away from vertical.

Key Points:

• Charge flows from the sphere to the vane only when the conducting rod is used.

• Both the vane and the post can acquire excess charges in this process.

Atomic Structure and Electric Charge

Fundamental Particles and Charge

Atoms consist of a nucleus containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons that form an electron cloud.

• The charge of an electron or a proton is known as the elementary charge, .

• An atom is charged if it has an unequal number of protons and electrons.

• The process of removing or adding an electron from the atom creates ions:

  • Positive ion: Fewer electrons than protons.

  • Negative ion: More electrons than protons.

• Charge can be transferred by friction (e.g., rubbing a plastic rod with hair).

• Law of Conservation of Charge: Charge cannot be created or destroyed, only transferred. The total charge remains constant.

Fractional Electric Charge and Quarks

Search for Fractional Charges

Experiments have measured the charge of particles and found that most charges are integer multiples of . Quarks, however, have fractional charges:

• Up quark:

• Down quark:

Quarks combine in groups of two or three so that the total charge of observable particles (like protons and neutrons) is always an integer multiple of .

• Proton: 2 up quarks + 1 down quark total charge

• Neutron: 1 up quark + 2 down quarks total charge $0$

Properties of Conductors and Insulators

Electron Mobility

In insulators, electrons are tightly bound to nuclei and do not move freely. Rubbing an insulator can create molecular ions on the surface, but does not free electrons throughout the material.

• In conductors, outer atomic electrons become detached and move freely throughout the solid, forming a 'sea' of electrons.

• Metals are good conductors due to this electron mobility.

• Ionic solutions (e.g., salt water) are also good conductors.

Electroscope and Charge Transfer

Touching an Electroscope

When a person (assumed neutral) touches a positively charged electroscope, the following occurs:

• The excess charge on the electroscope decreases.

• The excess charge on the person increases.

• Charge is transferred between the electroscope and the person.

Grounding:

• Connecting an object to the earth allows charge to flow, neutralizing the object. This process is called grounding.

Polarization

Definition and Effects

Polarization is a slight separation of positive and negative charges within an object, often induced by an external electric field.

• Objects with separated charges form an electric dipole.

• Polarization can occur in both conductors and insulators.

Charged Rod and Electroscope Interaction

Induced Charge and Movement

When a negatively charged rod is brought close to the disk of an electroscope (without touching), the vane moves due to induced charge separation:

• The net charge on the system (disk, post, vane) remains zero.

• Charge redistributes: the disk may become negative, the vane positive or negative depending on configuration.

• Electrons are repelled from the negatively charged rod, moving to the far side, leaving an excess of positive charge near the rod.

Forces Due to Polarization

Attraction Between Charged and Neutral Objects

When a charged rod is brought near a neutral metal rod, polarization occurs:

• Positive charges accumulate near the charged rod, negative charges farther away.

• The force between the charged rod and the closer end of the metal rod (with opposite charge) is stronger than the force with the farther end.

• This results in net attraction.

Polarization in Insulators

Behavior of Insulating Materials

Insulators can also be polarized. When a charged rod is brought near an insulating rod (e.g., wood), atoms within the insulator polarize, causing attraction regardless of the sign of the charge.

• Polarization occurs in packing peanuts and other insulators, leading to attraction to charged objects.

• This explains phenomena such as static cling.

Summary Table: Conductors vs. Insulators

Key Equations

• Elementary charge:

• Charge conservation:

Example Application

When a plastic rod is rubbed with fur, electrons are transferred from the fur to the rod, making the rod negatively charged. This is an example of charge transfer by friction.

Additional info: Some explanations and table entries have been expanded for clarity and completeness based on standard physics curriculum.