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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).
€
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
€
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.