ELECTRICITYChapter 7

Positive and Negative Charges Protons = positive charge Electrons = negative charge

Most atoms have the same number of protons and electrons and thus will have no net charge. Atoms become charged when they gain or

lose electrons.

Static Electricity The accumulation of excess electric

charges on an object

Electricity obeys rules??? Law of conservation of charges - Charges

can be transferred from object to object, but cannot be created or destroyed.

Opposite charges attract, like charges repel.

electrons move easier through conductors ex. metals

Electrons do not move easy through insulators ex. plastic, wood, rubber, and glass

Transferring Electric Charge

Charging by contact process of transferring charge by touching or

rubbing Ex. Static electricity from rubbing your feet on

carpet Charge by induction

the rearrangement of electrons on a neutral object caused by a nearby charged object Ex. a negatively charged balloon near your

sleeve causes an area of your sleeve to become positively charged

Transferring Electric Charge

Static discharge A transfer of charge through the air b/w two

objects because of a buildup of static electricity

Ex. Lightning

Read Page 197 Write and Answer the following questions1. When the warm humid air rises to meet

the cold air, what causes the air masses to churn together?

2. What electric property causes the negative charges in the cloud to be attracted to the positive charges in the ground?

3. Why does the ground below a cloud have a concentration of positive charges?

ELECTRIC CURRENTChapter 7 Sec 2

Electric current The flow of charges through a conductor

(like a wire) is called electric current. Usually the flow of electrons Measured in Amperes (A) Flow from high to low voltage.

A voltage difference is the push that causes charges to move.

Voltage difference is measured in volts (V).

Circuit For charges to flow, the wire or conductor

must always be connected in a closed path called a circuit.

Sources of Electricity Dry Cell Battery

Wet Cell Battery

Wall Socket

Sources of Electricity Dry cell battery - produces voltage

difference b/w its zinc container and its carbon suspension rod, causing current to flow b/w them

Wet cell battery – contains two connected plates made of different metals in a conducting solution.

Wall sockets – have a voltage difference across the two holes of an electrical outlet, and a generator at a power plant provides this voltage difference

Resistance The tendency for a material to oppose

the flow of electrons, changing electrical energy into thermal energy and light. All materials have some electrical resistance.

Measured in ohms (Ω) Making wires thinner, longer, or hotter

increases the resistance.

Ohms Law

current (in amperes) = voltage difference (in volts)

resistance (in ohms)

I = V/R

Ohms LawExample Problem #1: What is the current in a

30V circuit if the resistance is 6Ω?

current (in amperes) = voltage difference (in volts) resistance (in ohms)

I = ?V = 30VR = 6Ω

Ohms LawExample Problem #2: An Ipod uses a standard 3.7

V battery. How much resistance is in the circuit if it uses a current of 0.025 A?

current (in amperes) = voltage difference (in volts) resistance (in ohms)

I = V/R

ELECTRICAL CIRCUITS

Chapter 7 Section 3

Series Circuit Series circuit - The

current only has one loop to flow through Parts of the circuit

are wired one after another, so the amount of current is the same through every part

Ex. String of holiday lights

Parallel Circuit Parallel circuit –

contains two or more branches for current to move through Parts can be

turned off without affecting the entire circuit

Ex. the electrical system in a house

Household Circuits Parallel circuits

connected in a logical network.

Electric energy enters your home at the circuit breaker or fuse box and branches out to wall sockets, lights, and major appliances

Guards against overheating Electric fuse Circuit breaker

Electrical Energy Electrical energy is easily converted to

thermal, radiant or mechanical energy. Electrical power – the rate at which electrical

energy is converted to another form of energy Electrical power is expressed in watts (W). Power = current X voltage difference P(watts) = I (amperes) X V (volts)

Example Problem Your microwave runs at a current of 10

amps. A standard plug in your house has a voltage difference of 120 volts. How much electrical power does it take to run this appliance?

P = I x V

Example problem 2 If it takes 1750 watts of power to run my

hairdryer, and we know that it is plugged up to 120 volt outlet what must be the current?

P = I x V