1. Chapter 20: Electricity
Section Electric Charge and Static Electricity
Section Electric Current and Ohm’s Law
Section Electric Currents

2. Section 20.1 Electric Charge and Static Electricity
Lightning and static cling result from the movement of electric charges.
Electric Charge
Def.-a property that causes subatomic particles such as protons and electrons to attract or repel each other
Two types of electric charge: positive and negative
Protons (+ charge) and electrons (- charge)

3. Section 20.1 Electric Charge and Static Electricity
Examples of movement: lightening bolts
Examples of attraction: clothes taken out of the dryer
Almost everything in our day to day lives is affected some way by charges.

4. Section 20.1 Electric Charge and Static Electricity
Cloud of (–) electrons surround (+) nucleus
Atom is neutral b/c #protons=#electrons
Atom gains 1 or more electrons becomes a negatively charged ion
Atom loses electrons become a positively charged ion
Key Concept: An excess or shortage or electrons produce a net electric charge.

5. Section 20.1 Electric Charge and Static Electricity
Electric Charges
SI Unit of electric charge= C or coulomb
Takes 6.24 x 1018 electrons to produce a single coulomb

6. Section 20.1 Electric Charge and Static Electricity
Electric Forces
Key Concept: Like charges repel, and opposite charges attract.
Electric force-the force of attraction or repulsion between electrically charged objects
Charles-Augustin de Coulomb ( )-discovered that electric forces obey laws similar to the law of universal gravitation

7. Section 20.1 Electric Charge and Static Electricity
Electric Forces
Electric force b/t 2 objects directly proportional to net charge on each object and inversely proportional to the square of the distance between them.
Doubling net charge on one object doubles the electric force
Doubling distance between objects: electric force is one fourth as strong
Electric forces inside atoms are much stronger than gravitational forces

8. Section 20.1 Electric Charge and Static Electricity
Electric Fields
Def.-the effect an electric charge has on other charges in the space around it
Key Concept: The strength of an electric field depends on the amount of charge that produces the field and on the distance from the charge.

9. Figure 4

10. Section 20.1 Electric Charge and Static Electricity
Electric Fields
Exert forces on any charged object placed in the field
Force depends on: net charge in the object, strength and direction of the field at the object’s position
The greater the net charge an object has, the greater the force on it.

11. Section 20.1 Electric Charge and Static Electricity
Static Electricity and Charging
Static electricity-the study of the behavior of electric charges, including how charge is transferred between objects
Key Concept: Charge can be transferred by friction, by contact, and by induction.
When charge transfer occurs, the total charge is the same before and after the transfer (law of conservation of charge)

12. Section 20.1 Electric Charge and Static Electricity
Charging by Friction
Ex. Balloon attracts hair
Electrons move from hair to balloon making balloon negative and hair becomes positive
Ex. Walking across a carpet

13. Section 20.1 Electric Charge and Static Electricity
Charging by Contact
Hair standing on end
Person acquires a charge large enough where hairs have like charges
Like charges repel

14. Section 20.1 Electric Charge and Static Electricity
Charging by Induction
Ex. Walk across carpet and reach for doorknob
You pick up electrons from the carpet: hand is negatively charged
Net negative charge of hand repels the electrons in the metal doorknob
Electrons move to base of doorknob=doorknob is positively charged in part
Doorknob is still neutral but charge moved into it
Induction-a transfer of charge without contact between materials

15. Section 20.1 Electric Charge and Static Electricity
Static Discharge
Shock from doorknobs or other objects=result of static discharge
Key Concept: Static discharge occurs when a pathway through which charges move forms suddenly.
Air becomes charged suddenly when the gap b/t your finger and the doorknob is small. Air provide path for electrons to flow from your hand to the doorknob. Spark can be seen in the dark.

16. Section 20.1 Electric Charge and Static Electricity
Static Discharge
Lightning-more dramatic discharge
Friction between moving air masses causes charge to build up in storm clouds
Negative charge in lower part of cloud induces a positive charge in the ground below it
Amt. of charge in cloud ↑, attraction b/t charges in cloud and ground ↑
Air becomes charged (forms pathway for electrons to flow from cloud to the ground)

17. Section 20.2 Electric Current and Ohm’s Law
Def.-the continuous flow of electric charge
SI Unit of electric current-ampere (A); equals 1 coulomb/second
Key Concept: The two types of current are direct current and alternating current.
Direct current (DC)-charge flows in one direction
Flashlight; most battery-operated devices

18. Section 20.2 Electric Current and Ohm’s Law
Alternating current (AC)-a flow of electric charge that regularly reverses its direction
Houses and schools

19. Figure 8
A Flashlight

20. Section 20.2 Electric Current and Ohm’s Law
Conductors and Insulators
Electrical conductor-a material through which charge can flow easily
Electrical insulators-a material through which charge cannot flow easily
Metals-have ions in a lattice that don’t move, but have some electrons that a not bound to the lattice =they can conduct charge
**Most materials don’t conduct charge b/c they have no free electrons
Metal such as copper and silver are good electrical conductors. Wood, plastics, rubber and air are good electrical insulators.

21. Section 20. 2 Electrical current and Ohm’s Law
Resistance
Def.-opposition to the flow of charges in a material (SI unit=ohm)
Key Concept: A material’s thickness, length, and temperature affect its resistance.
Resistance > in long wire b/c charges have to travel farther; Temp. >=more resistance b/c electrons collide more often

22. Section 20.2 Electric Current and Ohm’s Law
Resistance
Can resistance be 0?
Superconductor-material that has almost zero resistance when it is cooled to low temperature.

23. Section 20.2 Electric Current and Ohm’s Law
Voltage
Key Concept: In order for charge to flow in a conducting wire, the wire must be connected in a complete loop that includes a source of electrical energy.
Potential Difference
Objects at greater height have more potential than those at a lower height. So those objects fall from higher to a lower potential energy. This is true of charges. Charges flow spontaneously from a higher to lower potential energy.

24. Section 20.2 Electric Current and Ohm’s Law
Voltage (Potential Difference)
Potential difference of a charge depends on its position in an electric field.
Def.-the difference in electrical potential energy between two places in an electric field
Measured in Joules/Coulombs (Volts) aka Voltage
Voltage Sources
Batteries, solar cells, and generators.
Have terminals (positive and negative) that connect to wires in a circuit
Battery-a device that converts chemical energy to electrical energy.

25. Section 20.2 Electric Current and Ohm’s Law
Ohm unit-German scientist-Georg Ohm ( )
1st determined how resistance and current affect voltage.
Discovered voltage is not the same everywhere in a circuit; Hypothesized that resistance reduces voltage.
Found a relationship b/t voltage, current, and resistance

26. Section 20.2 Electric Current and Ohm’s Law
Voltage (V) in a circuit equals the product of the current (I) and the resistance (R).
V=I x R or I=V/R
If current in amps and resistance in ohms, voltage in volts
Key Concept: Increasing the voltage increases the current. Keeping the same voltage and increasing the resistance decreases the current.

27. Section 20.3 Electric Currents
Circuit Diagrams
Electric current-a complete path through which charge can flow; wires in houses, etc. have complex networks of circuits
Electricians use circuit diagrams to monitor how elements in a circuit are connected.
Key Concept: Circuit diagrams use symbols to represent parts of a circuit, including a source of electrical energy and devices that run by the electrical energy

28. Section 20.3 Electric Circuits
Circuit Diagrams
Show one or more complete paths where charge can flow
Switches show where circuit can open; if open circuit not in a complete loop so current stops=open circuit
If switch closed, the circuit is complete and charge can flow=closed circuit

29. Section 20.3 Electric Circuits
Electrons in a wire flow in the opposite direction.
Series Circuits
**Charge has only one path through which it can flow
Key Concept: If one element stops functioning in a series circuit, none of the elements can operate.
One light bulb blows it becomes an open circuit.
Bulbs are a source of resistance; the more present the more resistance there is.
>resistance=<current;decrease brightness of bulbs

30. Section 20.3 Electric Currents
Parallel Circuits
Circuits in homes are mostly parallel.
Def.-an electric circuit with two or more paths through which charges can flow
Key Concept: If one element stops functioning in a parallel circuit, the rest of the elements still can operate.
Allows independent operation of devices.

31. Series and Parallel Circuits
Figure 12
Series and Parallel Circuits

32. Section 20.3 Electric Circuits
Power and Energy Calculations
Electric power-the rate at which electrical energy is converted to another form of energy
Units of joules per second (watt, W). Power measured in watts or kilowatts (kW)
Key Concept: Electric power can be calculated by multiplying voltage by current.
P (watts)=I (amps) x V (volts)
**Appliances vary in the amount of power they use.

33. Section 20.3

34. Section 20.3 Electric Currents
Electrical Safety
Inspectors check all new homes to make sure electrical wiring is installed safely.
Key Concept: Correct wiring, fuses, circuit breakers, insulation, and grounded plugs help make electrical energy safe to use.
The amount of current in a circuit depends on how many devices in the circuit. > # of devices turned on the > the current
If current exceeds the circuits safety limit, wire may overheat and start a fire.

35. Section 20.3 Electric Currents
Home Safety
Fuses prevent current overload in a circuit; wire in the center of a fuse melts if there is too much current passing through it “blowing a fuse”. Has to be replaced with a new one to use circuit
Most homes use circuit breakers (a switch that opens when current in a circuit is too high); has to be reset for circuit to be used again

36. Section 20.3 Electric Currents
Personal Safety
Electrical wiring is insulated to protect from shock
Don’t touch electrical devices with wet hands
Insulation prevents short circuits; three pronged plugs help prevent shocks from short circuits
Circular prong connects to ground; current takes easier path to ground instead of entering your body
Grounding-the transfer of excess charge through a conductor to Earth