Chapter 34

Electric Current

The Big IdeaElectric current is related to the voltage

that produces it and the resistance that opposes it.

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V = IRVoltage equals the current times the resistance

Ohm’s Law

Basic Facts

• We use the letter I to represent current.• Current is measured in Amperes.• Ampere or Amps or the Letter A.

• We use the letter R to represent resistance.• Resistance is measured in Ohms.• Ohms or the Greek letter omega .

Some Examples with

1. A Battery with 10Volts is hooked up in a circuit with a resistor at 100. Calculate the current that flows through the circuit.

2. You want 1 A to flow through a resistor of 1,000. What Voltage battery will you need?

3. You have a battery with 20V. You have 3 different resistors, 10 , 100 , 1000 . Which resistor will give the largest current? Which will give the smallest current?

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V = IR

Power!

• To calculate the power used in a circuit is very similar to Ohm’s Law.

• Units for power are Watts (W).

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V = IR

P = IV or P = I2R

Circuit

• A Complete Path

We hook the battery up and make a circuit.

+

_

How to Draw a Battery

Resistors

• Resists the flow of electrons.

• They get warm when they do this.

• Ex. A Light bulb.

Series and Parallel

• Series– More then one component in a row

• Parallel– One component, multiple paths.

Equivalent Resistance forSeries & Parallel

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1

Rtotal=

1

R1

+1

R2€

Rtotal = R1 + R2Series

Parallel

Remember Charge.

• Battery has a positive (+) and a negative (-) side.

• The plus (+) side does not have enough electrons.

• The minus (-) side has too many electrons.• This difference is called a difference in

electric potential or more commonly a Voltage.

Why Electrons MoveElectric current is the flow of electrons.

Remember: Opposites attract. 1. Negative Charges want to move towards positive charges.

2. Electrons move freely through conductors.

3. If there are too many electrons in a place and they have a path to where there aren’t as many electrons, they will move towards that place.

AC/DC• Direct Current (DC) Vs. Alternating

Current (AC)

Direct current stays the same while alternating current alternates with a given frequency.

Capacitors

• Another component of electric circuit is a capacitor.

• Commonly called a cap.

• They store charge and can be deadly!

• The units for caps are called Farad (F).

A capacitor is just two conductors separated by an insulator.

Old Style Cap.Modern capacitors

Transistors

• Transistors are used in circuits in two ways.

• 1. As an amplifier. – Transistors can amplify signals.

• 2. As a switch. Transistors can act just like your light switch. – On, current flows,– Off, current does not flow.