Three Chapter 3

Transfer of Charge

• Solids are charged with an excess or deficit of electrons
• To charge a solid requires adding or taking away electrons
• Deficit of electrons is positive charge
• Excess of electrons is negative charge

• There are a number of common charging methods.

Charging by friction

• Different atoms have different abilities to hold onto electrons.
• When two substances made of different atoms are rubbed together, the one that holds onto electrons more strongly steals electrons from the other. The one that steals becomes negatively charged and the other positively charged.

E.g. Ebonite and fur:

Ebonite has stronger attraction so it becomes negative and the fur positive.

• Substances higher up when rubbed with substances lower down become negatively charged.

Charging by contact

• When a negatively charged object touches a neutral object, excess electrons on the negatively charged object are repelled off of the object onto the neutral object. Therefore, the neutral object becomes negatively charged.
• If a neutral object is touched by a positively charged object, electrons on the neutral object are attracted to the positively charged object. Some of these electrons will move to the positively charged object, leaving the neutral object with a deficit of electrons. Therefore, the neutral object becomes positively charged.

Charging by induction

• A negative rod held close to a metal leaf electroscope repels electrons to the leaves and therefore the leaves separate.
• There are two types of charging by induction.

• In both cases, the charged rod is held near the electroscope. The electroscope is touched by a finger. The finger is removed and then the rod is removed.
• In case A, electrons are repelled off the electroscope through the finger and the electroscope becomes positively charged.
• In case B, electrons are attracted onto the electroscope through the finger and the electroscope becomes negatively charged.
  • Rule for charging by induction – An object that is charged by induction has a charge opposite to that of the charging rod.

Induced charge separation

• Positive charges in solids cannot move about.
• Negative charges in solids can move about.
• Induced charge separation – The separation of charges on an object due to the presence of another charged object (electrostatic induction)
• The force between charged objects is inversely proportional to the square of the distance between objects. Therefore, charges that are close together experience greater forces between them than charges that are further apart.
• If a positively charged object is held close to a neutrally charged object, the electrons on the neutral object are attracted to the near (closer) side of the object to the positively charged object. Since unlike charges attract, and the average distance between unlike charges is less than the average distance between like charges, the neutrally charged object experiences a net force towards the positively charged object.
• If a negatively charged object is held close to a neutrally charged object, the electrons on the neutral object are repelled to the far side of the object away from the negatively charged object. Since unlike charges attract, and the average distance between unlike charges is less than the average distance between like charges, the neutrally charged object experiences a net force towards the negatively charged object.
• If the attractive force between the charged object and the neutral object is greater than the force due to gravity on the neutral object, the neutral object will be picked up.

December 6, 2013