CHAPTER 20 NOTES
A. Electric Charge
a. Electric force – the attraction or repulsion between electrically charged objects
1. Like charges repel, and opposite charges attract.
2. Protons have a positive charge and electrons have a negative charge.
a. Static electricity – the study of the behavior of electric charges, including how charge is
transferred between objects.
b. Charge can be transferred by friction, by contact, and by induction.
1. example: Friction from rubbing a balloon on your hair causes electrons
to move from your hair to the balloon because atoms in the rubber
balloon have a greater attraction for electrons. This causes the balloon
to gain negative charge and because your hair loses electrons, it
becomes positively charged. Attraction results.
2. Induction – a transfer of charge without contact between materials
Example: You can pick up extra electrons by walking across a
carpet and your hand becomes negatively charge. This repels
electrons in a metal doorknob so they move to the back part of
the doorknob, leaving a net positive charge in the part of the
doorknob closest to the hand. Overall, the doorknob is still
neutral, but charge has moved within it.
c. Static discharge occurs when a pathway suddenly forms through which charges can move.
1. A shock and a bolt of lightning are both results of a sudden pathway
forming through which electrons travel. For these to occur, there has to
be a difference in charge that is either very great (lots of electrons) or is
very close together (like your finger just millimeters from a doorknob).
Section 20.2 Notes
B. Electric Current – the continuous flow of electric charge. The metric unit is the ampere (A) or amp.
1. Direct current (DC) – charge that flows only in one direction
a. Battery-operated devices use direct current
2. Alternating current (AC) – a flow of electric charge that regularly reverses its
direction
a. Electric current in buildings is mostly AC
3. Electrical conductor – a material through which charge can flow easily
a. Metals like copper and silver are good electrical conductors
4. Electrical insulator – a material through which charge cannot flow easily
a. Wood, plastic, rubber, and air are good electrical insulators
5. Resistance – the opposition to the flow of charges in a material (current)
a. Resistance is lower in thicker wires because more electrons can flow
b. Resistance is greater in a longer wire because the charges travel farther
c. As temperature increases, a metal’s resistance increases because
electrons collide more often.
6. Battery – a device that converts chemical energy to electrical energy
a. Batteries have different voltages, depending on the difference in energy
at the two terminals
b. Increasing the voltage increases the current
Section 20.3 Notes
C. Electric Circuits – a complete path through which charge can flow
1. Circuit diagrams use symbols to represent parts of a circuit, including a source
of electrical energy and devices that are run by the electrical energy.
2. Series circuit – an electric circuit where charge has only one path through
which it can flow
a. If one element stops functioning in a series circuit, none of the elements
can operate.
b. Adding bulbs to a series circuit increases the resistance so the current
decreases and each bulb shines less brightly.
3. Parallel circuit – an electric circuit with two or more paths through which
charges can flow
a. If one element stops functioning in a parallel circuit, the rest of the
elements still can operate