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

Electric Motors and Transformers

Applying electromagnetism to the design of industrial and household technology.

• Common Core State Standards
• HS-PS2-5
• HS-ETS1-2

About This Topic

Electric motors and transformers are the most widespread applications of electromagnetism in everyday life. A DC motor operates because a current-carrying coil in a magnetic field experiences a torque (from F = IL × B), causing it to rotate. A commutator reverses the current direction each half-turn to sustain continuous rotation. US students encounter motors in everything from fans and dryers to electric vehicles.

Transformers use mutual induction to step voltage up or down between two coils wound on a shared iron core. The voltage ratio equals the turns ratio (V_s/V_p = N_s/N_p), and because power is approximately conserved in an ideal transformer, stepping up voltage steps down current proportionally. This is why the US power grid transmits electricity at hundreds of kilovolts: high voltage means low current, and lower current means far less power lost as heat in transmission lines.

Active learning is effective here because motors and transformers both involve multiple interacting principles that students must synthesize. Tear-down activities (examining salvaged motors) and transformer-ratio experiments connect theory to the physical objects students see and use daily.

Key Questions

1. How does a DC motor convert electrical energy into rotation?
2. Why is high voltage used for long-distance power transmission?
3. How do transformers allow us to use different voltages for different appliances?

Learning Objectives

• Calculate the torque on a current-carrying coil in a magnetic field, predicting motor rotation.
• Compare and contrast the function of a commutator in a DC motor with the continuous voltage change in AC transmission.
• Analyze the relationship between voltage, current, and turns ratio in a transformer using experimental data.
• Design a simple circuit demonstrating how a transformer can step voltage up or down for a specific application.
• Explain the necessity of high voltage transmission for minimizing energy loss in the power grid.

Before You Start

Electromagnetism: Force on Current-Carrying Wires

Why: Students must understand that a current-carrying wire in a magnetic field experiences a force (F=ILB) to grasp the torque in a motor.

Faraday's Law of Induction

Why: Understanding how a changing magnetic flux induces an electromotive force (voltage) is essential for comprehending transformer operation.

Electrical Power and Energy Conservation

Why: Students need to know the relationship between power, voltage, and current (P=IV) to understand why high voltage transmission minimizes energy loss.

Key Vocabulary

Torque	A twisting or turning force that causes rotation, experienced by a current-carrying loop in a magnetic field.
Commutator	A component in a DC motor that reverses the direction of current in the coil every half rotation, ensuring continuous movement.
Mutual Induction	The process where a changing magnetic field in one coil induces a voltage in a nearby coil, fundamental to transformer operation.
Turns Ratio	The ratio of the number of turns of wire in the secondary coil to the number of turns in the primary coil of a transformer, determining voltage change.
Power Grid	The interconnected network for delivering electricity from producers to consumers, utilizing transformers for efficient voltage management.

Watch Out for These Misconceptions

Common Misconception

Transformers can work with direct current.

What to Teach Instead

Transformers require a changing magnetic flux to induce a voltage in the secondary coil. DC produces a constant field with no change in flux, so no voltage is induced. AC continuously changes direction, creating the changing flux needed. This is a core historical reason why AC won the 'War of Currents' for power distribution.

Common Misconception

A step-up transformer creates energy because it increases voltage.

What to Teach Instead

A step-up transformer increases voltage while decreasing current by the same ratio, keeping power approximately constant (P = IV). Energy is conserved; only the voltage-current ratio changes. Students often confuse 'higher voltage' with 'more energy per second' without accounting for the current decrease.

Common Misconception

Electric motors simply spin because magnets attract.

What to Teach Instead

Continuous rotation requires the current direction to reverse every half-turn via the commutator. Without reversal, the coil would snap to an aligned position and stop. The commutator is what transforms an oscillating force into continuous rotation. Seeing a motor run and then removing the commutator makes the function of that part clear.

Active Learning Ideas

Motor Teardown: Identify the Parts

Provide groups with a salvaged or inexpensive DC motor. Students disassemble it, sketch each part, and label the armature, field magnets, commutator, and brushes. Each group explains to the class how their assigned component contributes to continuous rotation, with the teacher connecting explanations to the underlying force law.

40 min·Small Groups

Transformer Ratio Lab

Students wind two coils with different numbers of turns on a shared iron core and connect the primary to a low-voltage AC source. They measure input and output voltages, calculate the turns ratio, and compare it to the voltage ratio. A second trial uses a reversed coil assignment to see step-up vs. step-down behavior.

45 min·Pairs

Think-Pair-Share: Why Transmit at High Voltage?

Ask: if power equals IV, and we transmit 100MW, what current flows at 100kV versus at 100V? Students calculate both scenarios, compare the power lost in a transmission wire of fixed resistance using P = I²R, and share their conclusions. The dramatic numerical difference makes the engineering choice obvious.

20 min·Pairs

Gallery Walk: From Power Plant to Outlet

Set up six stations showing each stage of power delivery: generator, step-up transformer, transmission line, step-down substation, local distribution transformer, and household panel. Student groups annotate each station with the voltage level, reason for that voltage, and which principle (generation, transformation, or distribution) applies.

30 min·Small Groups

Real-World Connections

• Engineers at Tesla utilize principles of electric motors and transformers to design efficient power systems for electric vehicles, optimizing battery usage and motor performance.
• Power line technicians work with high-voltage transmission lines and substations, where transformers are critical for stepping down voltage to safe levels for homes and businesses.
• Appliance designers at Whirlpool or GE incorporate small transformers within devices like microwave ovens and phone chargers to provide the specific voltages required for their internal components.

Assessment Ideas

Quick Check

Present students with a diagram of a simple DC motor. Ask them to label the magnetic field, current direction, and the point where the commutator reverses current. Then, ask: 'What would happen to the motor's rotation if the commutator failed?'

Discussion Prompt

Pose the question: 'Imagine you are designing a power system for a remote village. Why would you choose to transmit electricity at a very high voltage and then step it down locally, rather than transmitting it at the voltage used by appliances?' Facilitate a discussion focusing on energy loss.

Exit Ticket

Give students a transformer with a primary coil of 100 turns and a secondary coil of 500 turns. If the input voltage is 120V, ask them to calculate the output voltage. Add the question: 'What happens to the current as the voltage is stepped up?'

Frequently Asked Questions

How does a DC electric motor work?

A DC motor places a current-carrying coil in a magnetic field. The field exerts a force on each current-carrying segment of the coil (F = IL × B), creating a torque that spins the coil. A commutator reverses the current direction each half-turn, so the torque always acts in the same rotational direction, producing continuous spin.

Why does the power grid transmit electricity at high voltage?

Transmission line resistance causes power loss equal to I²R. Transmitting at high voltage means low current for the same power, which dramatically reduces I²R losses. A step-up transformer near the power plant raises voltage to hundreds of kilovolts, and step-down transformers reduce it to safe levels before it reaches homes and businesses.

How do transformers change voltage?

A transformer has two coils wound on an iron core. Alternating current in the primary coil creates a changing magnetic field in the core, which induces a voltage in the secondary coil. The output voltage equals the input voltage multiplied by the ratio of secondary turns to primary turns. More secondary turns means higher output voltage.

How does active learning support understanding of motors and transformers?

These technologies involve multiple simultaneous principles (forces, induction, energy conservation) that are hard to hold together through lecture alone. Teardown activities and transformer labs let students handle physical components, measure real voltages, and reconcile observations with theory, building a coherent mental model through direct experience rather than passive description.