1 Electricity Chapter 21

2 Charged objects Neutral Equal positive and negative charges Positive Fewer negative charges (lost electrons) Negative More negative charges (gained electrons)

3 Static electricity The accumulation of electric charges on an object.

4 Charges Like charges repel Opposite charges attract

5 Charging by induction A charged object causes the electrons on a neutral object to rearrange themselves. The charged object and the neutral object are then attracted to each other. See page 547

6 Electric field How an electron exerts a force on other particles. The field is strongest closest to the electron.

7 Conductor Material that allows electrons to move easily through it metals

8 Insulator A material that doesn’t allow electrons to move through it easily Plastic Wood Rubber Glass

9 Grounding Providing a path for electrons to reach the ground. A grounded object cannot accumulate excess charge.

10 Electroscopes Detect the presence of charges Contain two thin metal leaves that separate when a charged object is brought near

11 Lightning A large discharge of static electricity. Electrons accumulate on the bottom of a cloud. When there are too many, they are transferred to the ground. They can also go to another cloud. Causes air to expand rapidly thunder

12 Lightning rod Pointed metal rod on the highest point of a structure. Connected to the ground with a cable. Allow the electrons from lightning to travel safely to the ground.

13 discuss What is static electricity? What is the difference between a conductor and an insulator? What is an electric field? What is lightning? What is thunder?

14 Moving electrons Electrons move from areas of high potential energy to areas of low potential energy When the potential energies are equal, the electrons stop

15 Electric potential Potential energy difference divided by charge Measured in volts (V) Often called voltage Measured by a voltmeter

16 Circuit A closed path for electrons Electrons continue to flow as long as there is a potential difference Common source is a battery

17 Current The flow of electrons through a conductor Amount of current depends on number of electrons passing a point in a given time. Measured in amperes or amps (A) 1 ampere is 1 coulomb of charge per second 1 coulomb is the charge of 6.24 x electrons Measured with an ammeter

18 Dry cells AAA, AA, C, D, 9V, etc. Contain a carbon rod, a zinc plate, and a moist conducting paste. Have a potential difference between the positive and negative terminals Electrons are released in a chemical reaction As long as the reaction occurs, the battery works

19 Wet cells Car batteries Contain two conducting plates in an electrolyte solution A chemical reaction between a plate and the solution causes a potential difference

20 Resistance The tendency of a material to resist electron flow Measured in ohms (  ) Long wires have more resistance than short wires Thin wires have more resistance than thick wires

21 Ohm’s Law The current in a circuit depends on both voltage and resistance

22 Example Find the current flowing through a wire if its resistance is 20  and it is connected to a 12-V battery.

23 You try Find the current flowing through a wire if its resistance is 20  and it is connected to a 6-V battery.

24 Discuss What is current? Describe dry cell batteries. Describe wet cell batteries. What is resistance? What three quantities are related by Ohm’s law?

25 Series circuits The current only has one possible path. The current is the same everywhere. If one part goes out, the circuit is broken and it won’t work. Christmas lights

26 Parallel Circuits Contain separate branches for current to move through. The current is different in each branch. More current goes through branches with less resistance. The potential difference is the same in each branch. When one branch is opened, the current continues to flow through the other branches

27 Household circuits Mostly parallel circuits. Overheating protection is needed. Fuses Contain small pieces of metal that melt when the current gets too high Must be replaced when blown Circuit breakers Contain metal that bends when the current gets too high Can be reset by flipping a switch

28 Discuss The current splits up to move through separate branches in a(n) _____________ circuit. What kind of circuit is most of your home wiring connected in? What are two devices that can keep a circuit from overheating?

29 Power The rate at which work is done – ch 7 Electrical power – the rate at which electrical energy is converted to another form of energy

30 Power Expressed in watts (W) or kilowatts (kW) Found by multiplying current times voltage.

31 Example A lamp operates with a current of A and a potential difference of 120 V. How much power does it use?

32 You try A microwave oven uses 1000 W of power. The voltage source is 120 V. What is the current flowing through the microwave?

33 Electrical energy What you pay for Depends on power rating and time of use Multiply power times time

34 Kilowatt-hour Unit of electrical energy Equal to 1000 watts (1 kW) of power used for 1 hour Usually charged cents per kWh See table 21-2 on page 567

35 Example A 100-W light bulb is left on for 5.5 hours. How many kilowatt-hours of energy is used?

36 Example A 1500 W hair dryer is used for 6 minutes. How many kilowatt-hours of energy is used?

37 Discuss What is electrical power? What is the product of electrical power and time? What is a kilowatt-hour? SB on page 568