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OHM’s Law
© 2011 Project Lead The Way, Inc.Magic of Electrons
Ohm’s Law
• Georg Ohm• Ohm’s Law• Ohm’s Law Chart• Ohm’s Law Practice
This presentation is intended to be used with Activity 6.2.4 Ohm’s Law.
Georg Ohm
Georg Simon Ohm 1789-1854
German Physicist / School Teacher
Discovered the mathematical relationship between current, voltage, and resistance
Ohm’s Law
Quantities Abbreviations
Units Label
Voltage V or E Volts V
Current I Amperes A
Resistance R Ohms Ω
If you know 2 of the 3 quantities, you can solve for the third.
V=IR I=V/R R=V/I
Ohm’s Law Chart
V
I Rx
Cover the quantity that is unknown.
Solve for V
V=IR
Ohm’s Law Chart
V
I R
Cover the quantity that is unknown.
Solve for I
I=V/R
Ohm’s Law Chart
V
I R
Cover the quantity that is unknown.
Solve for R
R=V/I
New Prefixes
mA Milliamp = 0.001 amps
Kilo ohm = 1000 ohmsKΩ
When making circuit measurements, it is important to always work in Equivalent
Units.
KΩ V mA
The Equivalent Units are…
OR
Ω V A
For our lab, we will be using
V, KΩ and mA.
Your Turn!
VmA KΩ
VA Ω
V
I R
Remember our Multimeter?
Let’s look at how they are used to measure variables in Ohm’s Law.
Measuring Voltage
Light
Measure across a component.
Resistor
Battery
Switch
Measuring Current
Light
Circuit flow must go through the meter.
Resistor
Battery
Switch
Measuring Resistance
Light
Measure across the component being tested.Power must be off or removed from the circuit.
Resistor
Battery
Switch
Now Back to Ohm’s Law
Calculating Resistance• Power the circuit and use a multimeter to
measure the voltage drop across each fixed resistor. Add those values to get the total voltage. Record that value on your activity page, using volts for your units.
• Remove one of the connectors and measure the current with the power on. Record that value on your activity page, using mA (milliamps) for your unit.
Ohm’s Law Practice
DC
Create a series circuit with a DC source and the two supplied fixed resistors with the color bands visible.
Ohm’s Law Practice
DC
Calculating Resistance
• Use Ohm’s Law to calculate the total resistance delivered by the fixed resistors. Record that value on your activity page. KΩ will be your measurement unit.
• Remove the fixed resistors from the circuit and determine the value of each using the color bands. Add the values together. Record that total value on your activity page. KΩ will be your measurement unit.
How close were the calculated and color band resistance values?
Ohm’s Law PracticeCreate a series circuit with a DC source and fixed resistors V1 and V2.
DC
Calculating Voltage
• With the power off, use the multimeter to measure the resistance of each fixed resistor. Total resistance will be the sum of each of the measured resistances. Record the resistances, and their sum, on your activity page. KΩ will be your measurement unit.
• Remove one of the connectors and measure the current with the power on. Record that value on your activity page. mA will be your measurement unit.
Ohm’s Law Practice
DC
Calculating Voltage
• Use Ohm’s Law to calculate the voltage of the circuit. Record that value on your activity page. V (volts) will be your measurement unit.
• Power the circuit and use a multimeter to measure the voltage drop across each fixed resistor. Add those values to get the total voltage. Record the total voltage on your activity page. mA will be your measurement unit.
How close were the calculated and measured voltage values?
Ohm’s Law PracticeCreate a series circuit with a DC source and fixed resistors C1 and C2.
DC
Calculating Current
• With the power off, use the multimeter to measure the resistance of each fixed resistor. Total resistance will be the sum of each of the measured resistances. Record the resistances, and their sum, on your activity page. KΩ will be your measurement unit.
• Power the circuit and use a multimeter to measure the voltage drop across each fixed resistor. Add those values to get the total voltage. Record that value on your activity page. V (volts) will be your measurement unit.
Ohm’s Law Practice
DC
Calculating Current
• Use Ohm’s Law to calculate the current of the circuit. Record that value on your activity page. mA will be your measurement unit.
• Remove any one connection and measure the current. Record that value on your activity page. mA will be your measurement unit.
How close were the calculated and measured current values?
Answer the conclusion questions on your Activity
Sheet!
Image Resources
Microsoft, Inc. (2009). Clip Art. Retrieved January 13, 2009, from http://office.microsoft.com/en-us/clipart/default.aspx
