Electric Charge and Particles Overview

Electric Charge

• Amber, when rubbed, becomes charged, attracting small objects. This process transfers electric charge between the amber and the rubbing material.
• Electric charge is conserved—the net charge of a closed system remains constant.
• Electric forces can be attractive or repulsive. There are two types of charge: positive and negative.
• Net charge is the algebraic sum of all charges in a system.
• A neutral object has equal amounts of positive and negative charges, resulting in a net charge of zero.

Elementary Charge

• The magnitude of a proton's or electron's charge is the elementary charge (e).
• Any object's net charge is always a whole-number multiple of the elementary charge.

Proton, Electron, and Neutron

• | Particle | Mass (kg) | Electric Charge (C) |
• |---|---|---|
• | Proton | 1.673 × 10⁻²⁷ | +1.602 × 10⁻¹⁹ |
• | Electron | 9.109 × 10⁻³¹ | −1.602 × 10⁻¹⁹ |
• | Neutron | 1.675 × 10⁻²⁷ | 0 |

Charge Transfer Example

• A typical charge transferred when interacting (e.g., walking across a carpet) is about 1 nC.
• If transferred only by electrons, calculate the number of electrons transferred.
• Estimate the percentage of excess electrons in an object with a −1 nC net charge.

Attraction, Repulsion, and Polarization

• Like charges repel, unlike charges attract.
• Polarization is a separation of positive and negative charges within a neutral object, although the net charge remains zero.
• A polarized object can experience a force from an external electric field even if it's uncharged.

Induction and Polarization

• A neutral object can be polarized by an external charge. This polarization can create a net force if the external charge is present. This is called polarization by induction.

Polarized Molecules and Materials

• Some molecules, notably water, are intrinsically polarized.
• Materials differ in how easily charges move within them. Conductors allow easy charge movement, insulators resist charge movement, and semiconductors are intermediate.

Charging Insulators and Conductors

• Rubbing different insulators together can transfer electrons and charges, creating charged objects.
• Touching a charged insulator to a conductor can transfer charge, and this process can be repeated to build more charges.
• Grounding a conductor allows charge to flow to a large, neutral object (like the Earth), discharging the conductor.
• If a conductor is grounded near a charge, charge transfer may occur to or from the grounded object.

Charging Conductors by Induction

• An initially neutral conductor can be charged by grounding it while a charge is nearby.

Electroscope Example

• A negatively charged electroscope with separated leaves demonstrates the principles of charge interaction.
• Touching the electroscope's bulb and then bringing a positive charge near the bulb causes polarization of the electroscope.
• Adding the positive charge directly to the bulb increases the net positive charge causing greater foil separation.