Daily SprintSTART

1. Turn in homework

2. Copy the objective

3. Describe what happens to a light if the resistance in a circuit increases.

MSA Prep

Give the missing exponent.

38,700,000 = 3.87 x 10?

 

A. 7

B. 5

C. -5

D. -7

Ohm’s Law

Measuring Resistance•Remove the resistor from the circuit.•Touch the ends of the probe to the wires. • If your multimeter requires you to adjust the range manually:

– Start with largest resistance value and adjust it lower until you get a reading of 1.

– Go back one position and use that setting.

In your engineering notebook, record the values for each of the resistors you measured.

Did you have the resistors in the right order?

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 4.2.4 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 Symbols

Voltage V 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

Using the Law

1. If a circuit has total voltage of 9V and resistance of 3Ω, what is the current in the circuit?

2. If a circuit has 2A of current running through it and the total resistance in the circuit is 19Ω, then what is the total voltage?

Using the Law

3. A circuit is supplied with 15V of voltage and has current of 3A. What is the total resistance in the circuit?

4. A house is supplied with voltage of 120V. If the total resistance is 60Ω how much current is running through the wires?

Using the Law

5. If a computer uses 5A of current and is supplied with 120V of voltage, then what is the total resistance of the computer?

6. A circuit uses 0.5A of current and has total resistance of 18Ω. How much voltage is the circuit supplied with?

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

Tip: Convert all values to Amps, Ohms, and Volts to work with Ohm’s Law. Use appropriate prefixes such as K (kilo) and m (milli) after you have calculated the answers.

• Adjust a variable resistor to somewhere in the middle of the range.

• Create a series circuit with a DC source and the variable resistor that you just adjusted.

Ohm’s Law Practice

Calculating Resistance• Power the circuit and use a multimeter to

measure the voltage drop across the variable resistor. Record that value in your engineering notebook.

• Remove one of the connectors and measure the current. Record that value in your engineering notebook.

• Use Ohm’s Law to calculate the resistance delivered by the variable resistor. Record that value in your engineering notebook.

• Remove the variable resistor from the circuit (leaving it in the same position) and measure its value. Record that value in your engineering notebook.

Ohm’s Law Practice

How close were the calculated and measured resistance values?

Ohm’s Law PracticeCreate a series circuit with a DC source and two fixed resistors.

DC

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 in your engineering notebook.

• Remove one of the connectors and measure the current. Record that value in your engineering notebook.

Ohm’s Law Practice

DC

Calculating Resistance

• Use Ohm’s Law to calculate the resistance delivered by the fixed resistor. Record that value in your engineering notebook.

• Remove the fixed resistors from the circuit and determine the value of each using the color bands. Add the values together. Record that value in your engineering notebook.

How close were the calculated and measured resistance values?

Ohm’s Law PracticeCreate a series circuit with a DC source and two different fixed resistors.

DC

Calculating Voltage

• Remove the fixed resistors from the circuit and determine the value of each using the color bands. Add the values together. Record that value in your engineering notebook.

• Remove one of the connectors and measure the current. Record that value in your engineering notebook.

Ohm’s Law Practice

DC

Calculating Voltage

• Use Ohm’s Law to calculate the voltage of the circuit. Record that value in your engineering notebook.

• 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 in your engineering notebook.

How close were the calculated and measured voltage values?

Ohm’s Law PracticeCreate a series circuit with a DC source and two different fixed resistors.

DC

Calculating Current

• Remove the fixed resistors from the circuit and determine the value of each using the color bands. Add the values together. Record that value in your engineering notebook.

• 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 in your engineering notebook.

Ohm’s Law Practice

DC

Calculating Current

•Use Ohm’s Law to calculate the current of the circuit. Record that value in your engineering notebook.

•Remove any one connection and measure the current. Record that value in your engineering notebook.

How close were the calculated and measured current values?

Image Resources

Microsoft, Inc. (2009). Clip Art. Retrieved January 13, 2009, from http://office.microsoft.com/en-us/clipart/default.aspx