Physics · 9th Grade · Electricity and Magnetism · Weeks 19-27

Current, Resistance, and Ohm's Law

Analyzing the flow of charge through conductors and resistors.

• Common Core State Standards
• HS-PS3-3
• HS-PS3-5

About This Topic

Electric current is the rate of charge flow through a conductor, measured in amperes. Resistance is the property of a material or component that opposes the flow of charge, measured in ohms. Ohm's law states that for many materials over a range of conditions, the current through a conductor is proportional to the voltage across it: V = IR. These relationships address HS-PS3-3 and HS-PS3-5 in the US NGSS framework and underpin every circuit analysis problem students will encounter in physics and engineering.

Why certain materials conduct well while others strongly resist comes down to atomic structure. In metals, valence electrons are loosely bound and move freely through the lattice, carrying charge with low resistance. In semiconductors like silicon, conductivity is intermediate and can be controlled by adding impurities, which is the basis of all transistors and integrated circuits. Insulators like rubber and glass have electrons tightly bound to individual atoms, preventing flow. Resistors exploit this property deliberately to control current and protect sensitive components in circuits.

Active learning is essential for Ohm's law because students need to distinguish between its role as a measurement tool and its status as a relationship that holds for some materials but not others. Building circuits with multiple resistors, measuring voltage and current at each component, and checking V/I ratios directly from their own data gives students evidence-based understanding. Investigating non-ohmic components, such as light bulbs where resistance increases with temperature, adds important nuance and prevents overgeneralization.

Key Questions

1. What determines how much current will flow through a specific material?
2. How do resistors protect sensitive electronic components from damage?
3. Why are some materials better conductors of electricity than others?

Learning Objectives

• Calculate the current, voltage, or resistance in a simple circuit using Ohm's Law (V=IR).
• Compare the electrical resistance of different materials (e.g., copper, rubber, silicon) based on their atomic structure and conductivity.
• Explain the function of a resistor in a circuit, including how it protects sensitive components.
• Analyze simple series and parallel circuits to determine voltage drops and current flow through individual components.
• Critique the applicability of Ohm's Law to both ohmic and non-ohmic components.

Before You Start

Electric Charge and Electric Fields

Why: Students need to understand the concept of electric charge and its properties to grasp the flow of charge that constitutes current.

Voltage and Potential Difference

Why: Understanding voltage as the 'push' or potential energy difference that drives charge flow is essential for applying Ohm's Law.

Basic Circuit Components (Battery, Wire)

Why: Familiarity with simple circuit elements provides a foundation for introducing resistors and analyzing more complex circuits.

Key Vocabulary

Electric Current	The rate at which electric charge flows past a point in a circuit, measured in amperes (A).
Resistance	A measure of how difficult it is for electric current to flow through a material or component, measured in ohms (Ω).
Ohm's Law	A fundamental relationship stating that the voltage across a conductor is directly proportional to the current flowing through it and the resistance of the conductor (V=IR).
Ohmic Component	A component for which the resistance remains constant over a range of applied voltages and currents, obeying Ohm's Law.
Non-Ohmic Component	A component whose resistance changes with applied voltage or current, such as a light bulb filament whose resistance increases with temperature.

Watch Out for These Misconceptions

Common Misconception

Ohm's law is a universal law that all electrical components obey.

What to Teach Instead

Ohm's law describes a linear V-I relationship that holds for ohmic materials under stable conditions. Diodes, transistors, and light bulbs are non-ohmic: their resistance changes with voltage, current, or temperature. Measuring V-I curves for both ohmic and non-ohmic components side by side gives students direct evidence that Ohm's law is a material property with defined conditions, not a universal rule.

Common Misconception

Current is used up as it flows through resistors, so less current reaches the end of the circuit.

What to Teach Instead

Current is not consumed; charge is conserved. The same current that enters a single resistor exits it. What changes across a resistor is the electrical potential energy per charge (voltage), not the amount of charge flowing per second. Building a simple series circuit and measuring current at multiple points, finding the same value throughout, directly counters this misconception.

Active Learning Ideas

Hands-On Lab: Verifying Ohm's Law with Resistors

Pairs build a simple circuit with a variable power supply, a known resistor, an ammeter, and a voltmeter. They systematically vary the supply voltage in five steps, record current at each step, and plot current versus voltage. They determine whether their data fits a linear relationship and calculate resistance from the slope, comparing it to the labeled value.

40 min·Pairs

Comparative Investigation: Ohmic vs. Non-Ohmic Materials

Small groups measure V and I for a standard resistor, a light bulb, and an LED at several voltages and plot all three on the same axes. They identify which components are ohmic, explain why the light bulb's resistance increases at higher temperatures, and discuss the implications for circuit design.

45 min·Small Groups

Think-Pair-Share: Conductor vs. Insulator Mechanism

Students receive atomic model diagrams of a metal, semiconductor, and insulator and answer: what enables charge flow in each, and why does resistance vary so dramatically between them? Pairs develop an explanation before a class discussion that builds a consensus model of why resistance depends on material structure.

20 min·Pairs

Gallery Walk: Real-World Resistor Circuits

Stations feature real-world scenarios: selecting a resistor to protect an LED, calculating the fuse rating for a household appliance, determining the heating element resistance in a toaster. Student groups rotate through stations, solve each problem, and check their reasoning against the answer posted on the back of each card.

30 min·Small Groups

Real-World Connections

• Electrical engineers designing smartphone circuits use resistors to limit current flow to sensitive microprocessors and LEDs, preventing them from overheating and failing.
• Automotive technicians diagnose electrical issues by measuring voltage and resistance in various car systems, using Ohm's Law to identify faulty components like spark plugs or sensors.
• Power grid operators manage the flow of electricity across vast distances, understanding how resistance in transmission lines affects voltage drop and energy loss.

Assessment Ideas

Quick Check

Provide students with a circuit diagram containing a battery, a resistor, and an ammeter. Ask them to calculate the current flowing through the circuit if the battery provides 12V and the resistor is 4Ω. Then, ask them to predict what would happen to the current if the resistance was doubled.

Discussion Prompt

Pose the question: 'Why do we need resistors in electronic devices?' Facilitate a discussion where students explain the role of resistors in controlling current and protecting components, referencing specific examples like a simple LED circuit.

Exit Ticket

On an index card, have students write down one material that is a good conductor and one that is a good insulator. Then, ask them to explain in one sentence why their chosen conductor allows charge to flow easily, referencing atomic structure.

Frequently Asked Questions

What is Ohm's Law?

Ohm's law states that for ohmic materials, the voltage across a conductor is proportional to the current through it: V = IR, where V is in volts, I is in amperes, and R is in ohms. Doubling the voltage doubles the current for a fixed resistance, and increasing the resistance at fixed voltage decreases the current. It holds well for metal resistors at constant temperature but not for all components in all conditions.

Why are some materials better conductors of electricity than others?

Electrical conductivity depends on the availability of free electrons in the material. Metals have loosely bound outer electrons that move freely through the crystal lattice, carrying charge efficiently. Insulators have electrons tightly bound to atoms with no free electrons available to carry current. Semiconductors sit in between, with conductivity that can be dramatically altered by temperature or deliberately added impurities, which is the principle behind all transistors.

How do resistors protect sensitive electronic components?

By limiting current to a safe level for a given voltage. Ohm's law shows that for a fixed supply voltage, higher resistance means lower current. A current-limiting resistor in series with an LED prevents the current from exceeding the LED's rated maximum by ensuring the excess voltage is dropped across the resistor rather than the LED. Without it, even a modest supply voltage can destroy the component instantly.

What is the best active learning approach for teaching Ohm's law?

Students understand Ohm's law most reliably when they measure their own V-I data, plot it, and determine resistance from the slope rather than from the labeled component value. This experimental approach shows them that Ohm's law is an empirical observation, not a definition imposed from above. Following up with a non-ohmic component like a light bulb, where the slope changes, helps students identify the law's boundaries of applicability, which is a more accurate and sophisticated understanding.