Electric Charge

Question 1

What are electrical conductors and insulators?

Answer 1

• Conductors: Materials that allow electric charge to flow easily due to free electrons.
• Examples: Metals (copper, aluminium) – used in electrical wiring because they have low resistance.
• Insulators: Materials that do not allow charge to flow easily due to lack of free-moving electrons.
• Examples: Plastics, rubber, glass, wood – used to insulate wires and prevent electric shocks.

Question 2

How can insulating materials be charged by friction?

Answer 2

• Equipment: Two insulating rods (acetate & polythene), dry cloth, small paper pieces, gold-leaf electroscope.
• Method:
  
  • Rub acetate rod → Loses electrons → Becomes positively charged.
  • Rub polythene rod → Gains electrons → Becomes negatively charged.
  • Attracted to small paper pieces and causes electroscope leaves to diverge.
• Conclusion: Rubbing causes electron transfer, creating static charge.

Question 3

How can conductors be charged by induction?

Answer 3

• Equipment: Metal sphere, charged insulating rod, earth wire, electroscope.
• Method:
  
  • Bring a negatively charged rod close to a neutral metal sphere (no touching).
  • Electrons move away, leaving the near side positively charged.
  • Connect an earth wire, allowing excess electrons to escape.
  • Remove earth wire first, then remove the rod → Sphere remains positively charged.
• Conclusion: Conductors can be charged without contact by redistributing electrons.

Question 4

How can a gold-leaf electroscope be used to detect charge?

Answer 4

• What is it? A device used to detect and measure electrostatic charge.
• Structure: Metal cap, metal rod, two thin gold leaves in an insulating case.
• Method:
  
  • Bring a charged rod near the metal cap → Electrons shift → Gold leaves diverge.
  • If a positive rod is used, electrons are attracted to the cap, making leaves positively charged.
  • If a negative rod is used, electrons are repelled, causing repulsion of leaves.
• Conclusion: The greater the charge, the more the leaves diverge.

Question 5

What is an electric field?

Answer 5

• A region where electric forces act on charged objects.
• Field lines show direction:
  
  • Point away from positive charges.
  • Point towards negative charges.
• Closer lines = Stronger field.
• Strong electric fields can cause sparks, as seen in lightning.

Question 6

What are the dangers of electrostatic charges?

Answer 6

• Can cause sparks, leading to fires or explosions.
• Fueling aircraft/tankers:
  
  • Fuel flow builds up static charge due to friction.
  • A spark could ignite fuel, causing an explosion.
  • Solution: Earth cables safely discharge excess charge before fueling begins.
• Factories handling flammable gases/powders must prevent static build-up to avoid fires.

Question 7

How are electrostatic charges used in photocopiers?

Answer 7

• The image drum is positively charged.
• A bright light shines on the document; the reflected light removes charge from lit areas.
• Negatively charged toner particles stick to the remaining charged areas.
• The toner is transferred to paper and heated, producing a permanent copy.

Question 8

How are electrostatic charges used in inkjet printers?

Answer 8

• Tiny ink droplets are charged as they leave the nozzle.
• Electrically charged plates create an electric field that deflects the ink.
• Charge control ensures accurate color placement.
• Inkjet printers work without physical contact, ensuring precise, high-speed printing.