Electric Current and Ohm’s Law

Chapter 20.2

• Key Concepts:– What are two types of current?– What are some examples of conductors and insulators?– What factors affect electrical resistance?– What causes an electric current?– How are voltage, current, and resistance related?

• Vocabulary:– Electric Current– Resistance– Direct Current– Superconductor– Alternating Current– Potential Difference– Electrical Conductor– Voltage– Battery– Ohm’s Law

Electricity: The Mouse and Cheese Analogy

• All matter is made up of positive charges and negative charges. – The positives have mass and are not usually

free to move. – The negatives have no mass and are free to

move through some materials (conductors).

Mouse and Cheese Analogy• Negative charges are attracted to

positive charges the same way mice are attracted to cheese. – Any time there is a natural attraction between two

things we can use it to make the objects do work. – The negative charges (mice) will gladly do work in

order to get to the positive charges (cheese).

Current (I)• Continuous flow of electric charge.– SI unit of electric current is the AMPERE (A) or amp.– 1 amp = 1 coulomb per second.

• Two types of electric current:– Direct Current (DC)– Alternating Current (AC)

Mouse and Cheese Analogy• Current: –The number of charges (mice) passing a point

per second. The rate of flow of charges.

Direct Current• DC• Charges flow in

one direction.–Most battery

operated items use DC.

http://www.geocities.com/capecanaveral/hall/6645/electmag/electricmotor.htm

Alternating Current• Flow of electric charge that regularly

reverses its direction.Most electric current at home and school.

Conductor and Insulator• Electrical Conductor– Material through which charge

can flow easily.• Copper and Silver

• Electrical Insulator– Coating around a wire that

controls the current and keeps it where its needed.• Wood, plastic, air Rubber

Insulating Material(plastic)

Conducting Wire(copper)

Superconductor• Material that has

almost zero resistance when cooled to low temperatures.– Best superconductors

must be cooled to 138K.

Resistance• Opposition to the flow of charges in a

material.– SI unit of resistance is an ohm (Ω).

• Four factors affect the resistance in a wire:– Material type– Thickness– Length– Temperature

Mouse and Cheese Analogy• Resistance: • The opposition to the flow of charge. • Any appliance that asks the charge (mouse) to

do work will slow it down.

Voltage• The difference of electrical

potential between two points of an electrical circuit.• Also called the potential difference.

• Voltage: –The amount of work that each charge

(mouse) will do as it goes through the circuit. –Can also be thought of as

the amount of push on the charges or how hungry the mice are.

Mouse and Cheese Analogy

Voltage• SI unit for voltage is a VOLT (V)• One volt = one joule per coulomb

•V = J/C

Voltage and Current Video

• Link:– Click on the batter below to view video.– http://www.youtube.com/watch?v=1xPjES-

sHwg&feature=related.

Voltage Sources• Three common voltage sources:–Battery–Solar Cell–Generator

Battery• Device that converts chemical

energy to electrical energy

Questions• Page 607• Questions 1 - 4

Ohm’s Law• Georg Ohm (1789-1854)– Found a relationship between voltage, current,

and resistance.

• Ohm’s Law:– The voltage (V) in a circuit equals the product of

the current (I) and the resistance (R).– V = I x R I = V R = V

R I

Ohm’s Law• Calculate the potential difference

(voltage) when the current is 4.0 amps and the resistance is 3.0 ohms.

Ohm’s Law• Calculate the current when the

resistance is 6.0 Ω and the potential difference is 12 V.

Ohm’s Law• What is the resistance of a wire if the

potential difference is 24 V and the current is 4 A?

• Key Concepts:– What are two types of current?– What are some examples of conductors and insulators?– What factors affect electrical resistance?– What causes an electric current?– How are voltage, current, and resistance related?

• Vocabulary:– Electric Current– Resistance– Direct Current– Superconductor– Alternating Current– Potential Difference– Electrical Conductor– Voltage– Battery– Ohm’s Law