Electric Charge: Crash Course Physics #25

Main Ideas

• Static electricity results from an imbalance of positive and negative charge. The imbalance tends toward equilibrium and can produce effects ranging from a small shock to lightning.
• Like charges repel; opposite charges attract. In the tape demonstration, two pieces removed separately acquire the same kind of charge and repel. When pieces are peeled apart after being stuck together, they acquire opposite charges and attract.
• Atoms and moving electrons: Atoms normally have equal numbers of positively charged protons and negatively charged electrons, so their net charge is zero. In solids, protons generally remain fixed, while some outer-shell electrons—called free electrons—can move.
• Conductors and insulators: Conductors, such as copper, allow free electrons to move readily. Insulators, such as wood, hold electrons more tightly and limit their movement.
• An object is negatively charged when it has extra electrons and positively charged when it has lost electrons. In the examples, charge changes when electrons move from one object or place to another.

Ways Charge Can Be Transferred or Rearranged

• Charging by friction: Rubbing two materials can transfer electrons between them. For example, rubbing a glass rod with a cloth transfers electrons to the cloth, leaving the rod positively charged. Which object gains electrons depends on the materials.
• Charging by contact: When a charged object touches a neutral conductor, electrons move between them. A positively charged rod touching a neutral rod draws some electrons from it; afterward, both rods are slightly positive.
• Polarization: A nearby charged object can rearrange electrons in a neutral conductor without touching it. For example, a positive rod attracts electrons toward the near side of a metal rod, leaving the far side relatively positive. The rod remains neutral overall as long as its charge is only redistributed.
• Charging by induction: If the polarized conductor is separated while the charge imbalance is present, its separated parts can retain opposite net charges. This creates charge without direct contact with the charged object.
• Grounding: Connecting a charged conductor to Earth gives electrons a path to or from the object. Because Earth is a very large conductor for this purpose, the object can lose most of its net charge and become neutral. In the example, a negatively charged rod repels electrons from a nearby neutral rod; grounding lets those electrons flow into Earth. Disconnecting the ground leaves the rod positively charged.

Conservation and Measurement of Charge

• Charge is conserved: Charging does not create new charge. It transfers charge between objects, so the total charge of the objects involved remains unchanged.
• Charge is measured in coulombs (C) and denoted by (q). A single electron has charge (-1.6 \times 10^{-19}\,\text{C}), and a proton has charge (+1.6 \times 10^{-19}\,\text{C}). This magnitude is the elementary charge, (e). About (6.24 \times 10^{18}) electrons make up one coulomb of negative charge.

Coulomb’s Law and Force

The electrostatic force between two point charges is described by Coulomb’s law:

[ F = k\frac{q_1q_2}{r^2} ]

Here, (q_1) and (q_2) are the charges, (r) is the distance between them, and (k) is the Coulomb constant. In air or a vacuum, (k) is approximately (9 \times 10^9\,\text{N·m}^2/\text{C}^2).

• The inverse-square relationship means that doubling the separation reduces the force to one quarter.
• The signs of the charges indicate the interaction: like charges repel, while opposite charges attract. The force acts along the line between the charges.
• Example: Two electrons 1 nanometer apart repel with a force of about (2.3 \times 10^{-10}\,\text{N}). A proton and an electron the same distance apart attract with the same magnitude.
• For multiple charges, calculate the force from each relevant pair and use vector addition to find the net force on a charge.

Speakers and Sources Featured

• Speaker: The episode’s narrator/host; the subtitles do not name the speaker.
• Sources and production credits: Crash Course Physics, PBS Digital Studios, and Thought Cafe (graphics). The narration also mentions PBS Idea Channel, Brain Craft, and Shank’s FX in a promotional plug.