Electricity

Solving Ohm’s Law

Extend your knowledge of the relationship between voltage, current, and resistance.

Grade

Duration

40 mins

Type

Class discussion

Overview

Your students extend their understanding of Ohm’s Law by doing calculations with voltage, current, and resistance data.

Instructions

What you'll need

Whiteboard or blackboard
"Solving Ohm's Law" slideshow
"Solving Ohm's Law" worksheet for each student

Ohm's Law triangle

Go through the slideshow with the class.
Show the students the triangle graphic from the slideshow. Ohm’s Law triangle illustrates the relationships between voltage (V), current (I) and resistance (R). Explain to the students how the triangle can be used to calculate unknown values. Simply cover the value you are looking for, and complete the related mathematical operation (either division or multiplication).
Write this example on the whiteboard:
- If a circuit has a current (I) of 3 A, and its resistance (R) is 4 Ω, what is the voltage (V) of the circuit? You know that I = 3 A and R = 4 Ω, but you don’t know the value of V. Cover the V in the graphic, replace the I with 3 A and the R with 4 Ω; then multiply: 3 x 4 = 12. The voltage (V) equals 12 V.
- To determine resistance when V = 15 V and I = 3 A, cover the R and replace the V with 15 and A with 3. Then divide: 15 / 3 = 5. The resistance is 5 Ω.
Work through these sample questions as a class:
- Find R when V = 9 V and I = 3 A.
- Find V when R = 100 Ω and I = 12 A.
- Find I when V = 12 V and R = 10 Ω.
Have students work individually or in small groups to complete the “Solving Ohm’s Law” worksheet.
As a class, review the Ohm’s Law triangle and answer any questions. Assist students as they complete the worksheet. Address any common misunderstandings or challenges with the problems.

Modify or extend this activity

Review your students’ conversion of mA to A. There are 1,000 mA in one A. A simple way to convert is to move the decimal three places to the left.
Check out "Exploring Ohm’s Law", "Introducing resistance" and "Investigating resistance" for more activities about resistance and Ohm’s Law.

Curriculum Fit

Grade 9 Science

Content

Voltage, current and resistance
Circuits

Curricular competencies

Questioning and predicting

Demonstrate a sustained intellectual curiosity about a scientific topic or problem of personal interest
Make observations aimed at identifying their own questions, including increasingly complex ones, about the natural world

Planning and conducting

Collaboratively and individually plan, select, and use appropriate investigation methods, including field work and lab experiments, to collect reliable data (qualitative and quantitative)
Select and use appropriate equipment, including digital technologies, to systematically and accurately collect and record data
Ensure that safety and ethical guidelines are followed in their investigations

Processing and analyzing data and information

Seek and analyze patterns, trends, and connections in data, including describing relationships between variables (dependent and independent) and identifying inconsistencies
Analyze cause-and-effect relationships

Evaluating

Evaluate their methods and experimental conditions, including identifying sources of error or uncertainty, confounding variables, and possible alternative explanations and conclusions
Describe specific ways to improve their investigation methods and the quality of the data

Applying and innovating

Transfer and apply learning to new situations
Generate and introduce new or refined ideas when problem solving

Communicating

Formulate physical or mental theoretical models to describe a phenomenon

Teaching Notes

Ohm’s Law represents the relationship between voltage and current. It is usually represented mathematically:

V = I x R

Where:

V is the voltage (volts or V)
I is the current (amperes or A)
R is the resistance (ohms or Ω). Resistance is the ratio of voltage to current: V/I

It’s necessary to convert milliamperes (mA) to amperes (A) when using Ohm’s Law. There are 1,000 milliamperes (mA) in one ampere (A).

Assessment

“Solving Ohm’s Law” worksheet answer key:

1. A smartphone with a resistance of 35 ohms has a current of 0.25 amps flowing through it. Sketch a circuit diagram and calculate how many volts supply the smart phone.