Have you ever experienced a “snap” when touching a doorknob in winter? This unit explores the electrical foundations of the substances around us.
Basic Knowledge
All substances are made of atoms. An atom consists of a central nucleus (protons carrying positive charge and neutrons carrying no charge) and electrons (negative charge) moving around it.
Normally, the numbers of protons and electrons are equal, so the atom is electrically neutral. However, when materials are rubbed together (friction), electrons move from one material to the other. One becomes negatively charged, and the other becomes positively charged. This is static electricity.
Static electricity has two basic laws:
・Charges with the same sign repel each other
・Charges with different signs attract each other
Atom (the smallest particle that makes up matter. Consists of a nucleus and electrons)
Proton (a positively charged particle in the nucleus)
Electron (a negatively charged particle moving around the nucleus)
Static electricity (electricity stored on an object. Produced by friction and other causes)
Discharge (the sudden flow of stored electricity. Lightning is one example)
Cathode rays (a flow of electrons emitted from the cathode during a vacuum discharge. Key to the discovery of electrons)
Further Exploration (Background and Significance)
The voltage of static electricity is surprisingly high. Static electricity accumulated in the human body can reach several thousand to several tens of thousands of volts. Nevertheless, it does not cause fatal electric shock because the current is extremely small and flows for only a very short time.
In 1897, J.J. Thomson discovered the electron through experiments with cathode rays (Crookes tube). This revealed that atoms were not the smallest units and had even smaller components, opening the door to modern physics.
Static electricity can cause problems but also has practical uses:
・Problems: damage to electronic devices, ignition and explosions (at gas stations, etc.), and discomfort
・Uses: copy machines and laser printers (attracting toner with static electricity), electrostatic precipitators (collecting dust in air purifiers), and the mechanism that makes plastic food wrap stick
Lightning is a phenomenon in which static electricity accumulated between clouds and the ground is discharged all at once, reaching approximately 1億V and several tens of thousands of A.
- Atom = nucleus (protons + neutrons) + electrons
- Electrons carry negative electric charge
- Electrons move through friction → static electricity
- Same signs repel; different signs attract
- Static electricity has high voltage but small current
- Thomson discovered the electron in 1897
- Uses: copy machines and air purifiers / Danger: ignition and explosions
Points to Note (Easily Confused)
① The nucleus consists of protons + neutrons, while a nucleus + electrons makes an atom. A nucleus and an atom are different things. ② Electrons are negative. Confusing this also reverses the direction of current. ③ Static electricity has high voltage but small current. Therefore, injury is rare even after receiving a shock. ④ Cathode rays are a flow of electrons. This is frequently tested at the middle-school level.
Practice
- What two things make up an atom?
- Do electrons carry positive or negative electric charge?
- Do charges with the same sign attract or repel each other?