Unit D Lesson 8: Ohm's Law

 Learning Targets

Big Question: What is Ohm's Law?
The relationship between current, voltage, and resistance in electric circuits is known as Ohm's Law.

At the end of this inquiry, you should be able to answer the following questions:

  • What are the three variables that are used in Ohm's law?
  • What factors affect the amount of resistance in a wire?
  • How is an ammeter different from a galvanometer?
  • What is the benefit of a multimeter?
  • Can I manipulate the variables in Ohm's law appropriately?

Pages 306 to 310 in your textbook will help you answer these questions about Ohm's law.


Introduction


Figure 1– Understanding Ohm's law is very important when designing electric circuits.
Current, Voltage, and Resistance and Ohm's law

Ohm’s law is very helpful in understanding the mathematical relationships among voltage, current, and resistance. 

Ohm’s law states: As long as the temperature and resistance stays the same, the voltage is directly proportional to the current.

This means that, if the voltage increases, then the current increases as well. Sometimes, we can keep the voltage the same and look at how resistance affects the current instead. If we increase the resistance, we are slowing the flow of electrons and, therefore, the current is decreased. Think of something that resists movement – for example when brakes are applied in a car – the applied resistance slows the car down.

Watch

Watch the video "Electronics - Ohm's Law" to gain a deeper understanding of Ohm's Law.


Try It!


Practice Worksheet: Ohm's Law

  1. DOWNLOAD this practice worksheet (S9_UD_S2_L8_ohms_law). If you prefer to use a Google Drive or PDF version of the worksheet, click here.

  2. Click here to open simulation website 1. Click here to open simulation website 2. You will need BOTH simulations to complete the questions on the worksheet. Switch between the two simulations to answer the worksheet questions.

  3.  When you are satisfied with your responses you can check your work by clicking on the "SUGGESTED ANSWERS" button below.

    Wait! Don't view the suggested answers first. This practice work is not for marks, it is meant to help you check your understanding. Check the answers AFTER doing the questions! Keep the practice worksheet for study purposes. If you don't understand something, contact your teacher!


  1. a.
    I.   V=  voltage measure in volts (V)
    II.  I=  current measured in amperes (Amps)
    III. R=  resistance measured in ohms (Ω)

    b. Conditions that maximize current are increasing voltage and decreasing resistance.

    c. Conditions that minimize current are decreasing voltage and increasing resistance.

  2. a. If the voltage is kept constant but the wire diameter is increased, the resistance goes down so therefore the current is increased.

    b. If the voltage is kept constant but the wire length is increased, the resistance increases so the current flow would slow down.

    c.
     I. Increasing voltage increases current.
     II. Decreasing the voltage decreases the current.

    d. If you change the wire from copper to iron, the resistance would increase and therefore the current would decrease.
Electricity Measuring Devices

We have learned in a previous section that a voltmeter measures the voltage drop across items in a circuit and that millivoltmeters measure small voltages. So let's summarize while adding a few more devices that are used to measure electricity in circuits:

  • ammeter: measures current
  • galvanometer: measures weak currents
  • voltmeter: measure voltage
  • millivoltmeter: measures weak voltages
  • ohmmeter: measures resistance
  • multimeter: measures all three variables; current, voltage, and resistance

Figure 2 – Multimeters are very convenient for measuring electricity.
Ohm's Law and Electricity Measurements

So how are electricity measurements used with Ohm's law? Well, that depends on what you have measured. Ohm's law allows you to calculate voltage, current, or resistance, as long as you have two out of the three variable. See Figure 3 to the right, it summarizes how to manipulate the Ohm's law formula in order to obtain the variable you seek.
Figure 3 – This chart helps to choose the proper manipulation of Ohm's law.


 Connections

Connections: Mathematics
>>The Math Behind Ohm's Law

Algebra is used to solve for certain variables when dealing with Ohm’s law.

  • To solve for resistance use: Resistance = VoltageCurrent

  • To solve for voltage use: Voltage = Current Ã— Resistance

  • To solve for current use: Current = VoltageResistance

Figure 4 – Electricity calculations... thankfully working with Ohm's law is easier than what's shown above!


Skill Builder

In each example, you will follow the four steps summarized here:

A.  Write what you know
B.  Choose and write the formula
C.  Substitute values into formula
D.  Solve and write answers in complete sentences.

Look at the examples, and then try the practice questions on your own.


Example 1
A 30 V  battery creates a current through a 15 Ω resistor. How much current is created?
Example 2
A motor has an internal resistance of 40.0 Ω.  The motor is in a circuit with a current of 4.00 A.  What is the voltage?
Example 3
What is the resistance of a vacuum cleaner with 9.0 A of current going through it from a 120-V outlet?

A.  Write out what you know.
Voltage = 30 V
Resistance = 15 Ω
Current = ?
A.  What do you know?
R = 40.0 Ω
I = 4.00 A
V = ?
A.  What do you know?
R = ?
I = 9.0 A
V = 120 V
B.  Write out and choose a formula.

V=IR
I=VR
R=VI
B.  Write out and choose formula.

V=IR
I=VR
R=VI
B.  Write out and choose formula

V=IR
I=VR
R=VI
To solve for current use: I=VR
To solve for voltage use: V=IR
To solve for resistance use: R=VI

C.  Substitute values into formula.

I=VRI=30 V15 Î©I=2.0 A
C.  Substitute values into formula.

V=IRV=(4.00 A)×(40.0 Î©)V=160 V
C.  Substitute values into formula.

R=VIR=120 V9.0 AR=13.33 Î©
D.  Write out answer with units in complete sentence.

There are 2.0 A of current in the resistor.
D.  Write final sentence.

There are 160 V of energy per charge in the motor.
D.  Write final sentence.

The resistance is 13 Ω.
Practice:  answer each question in the format as used in the examples above.

A.   Write what you know.
B.   Select and choose formula.
C.   Substitute into formula.
D.   Write final answer in sentence.
1. What is the voltage of a stove if the current flowing through the stove is 30.0 A and the resistance of the heating element in 10.0 Ω?

A.  Write what you know.

B.  Select and choose formula.

C.  Substitute into formula.

D.  Write final sentence.

1. What is the voltage of a stove if the current flowing through the stove is 30.0 A and the resistance of the heating element in 10.0 Ω?

A.  Write what you know.
I= 30.0 A
R= 10.0  Ω
V =?
B.  Select and choose formula.
V=IR
C. Substitute into formula.
V=IR
V = (30.0 A) x (10.0 Ω)
V = 300 V
D. Write final sentence.
The stove uses 300 V of energy per unit charge.
2.  What is the resistance of an electric heater if the heater draws 10 A of current from a 120-V outlet?


A.  Write what you know.

B.  Select and choose formula.

C.  Substitute into formula.

D.  Write final sentence.

2.  What is the resistance of an electric heater if the heater draws 10 A of current from a 120-V outlet?

A.  Write what you know.
V = 120 V
R=?
I = 10 A
B.  Select and choose formula.
R =V/I
C.  Substitute into formula.
R =V/I
R= 120V/10A
R= 12 Ω
D. Write final sentence.
The heater has a resistance of 12 Ω.
3.  What is the current in a hair dryer that is plugged into a 120-Volt socket and has a resistance of 4.0 Ω?


A.  Write what you know.

B.  Select and choose formula.

C.  Substitute into formula.

D.  Write final sentence.

3.  What is the current in a hair dryer that is plugged into a 120-Volt socket and has a resistance of 4.0 Ω?

A.  Write what you know.
V = 120 V
R= 4.0 Ω
I =?
B.  Select and choose formula.
I=V/R
C.  Substitute into formula.
I=V/R
I= 120 V/4 Ω
I = 30 A
D.  Write the final sentence.
The blow dryer has 30 A of current.