F. Electricity and Magnetism · Electricity and Magnetism

Build, Test and Refine an Electromagnetic Device

F2.8 F3.6 A1.1 A1.5 A1.6 A1.10About 75 minutes

Learning goals

I can build a working prototype of an electromagnetic device, such as a simple motor.
I can explain how each part of my device works using the motor principle and the hand rules.
I can plan a fair test of one design change, collect data and draw a conclusion.
I can suggest and justify improvements to my design.

Words to know

prototype
A first working model of a device, built to test an idea.

design cycle
Define the problem → research → design → build → test → evaluate → refine, and repeat.

independent variable
The one factor you deliberately change in a test.

dependent variable
The factor you measure to see the effect.

controlled variables
Factors kept the same so the test is fair.

make-and-break circuit
A circuit that repeatedly switches itself on and off, as in an electric bell or buzzer.

enamelled (magnet) wire
Copper wire with a very thin coat of clear insulation, used for coils.

Engineers build it, then make it better

You have learned the motor principle and the hand rules. Now you will put them to work by building a prototype: a simple electric motor that you can hold in one hand. Then you will think like an engineer: test it, measure how well it works, change one thing, and test again.

The curriculum names several devices you could build: an electric bell, a loudspeaker, an ammeter, a motor or a generator. This lesson gives full instructions for the motor, which can be built at home, and explains the bell so you can build or describe one too.

The design cycle

Define the problem and criteria. For example: "Build a motor that runs on one D cell and spins steadily for at least 30 s."
Research and design. Sketch your device, label every part and predict how it will work.
Build the prototype.
Test. Does it meet the criteria? Measure something (speed, current, time running).
Evaluate and refine. Change one variable at a time, test again, and compare.

Keep a design log: sketches, measurements, what you changed and why.

A simple paper-clip motor. The coil rests on two paper-clip supports connected to the ends of a D cell, above a magnet. One lead has its insulation scraped off only on one half, which acts as a simple commutator.

How the paper-clip motor works

Current flows from the battery, up one paper clip, through the coil and down the other paper clip.
The coil sits in the magnet's field, so by the motor principle, its sides feel forces in opposite directions. That makes a torque that turns the coil.
There is no split ring. Instead, one lead has its insulation removed on one half only. For half of each turn, bare wire touches the clip and current flows, pushing the coil around. For the other half, the insulated side touches the clip, the current switches off, and the coil coasts on its momentum.
Without that trick, the current would push the coil forward for half a turn and then backward for the next half, and the coil would just rock back and forth. A real motor's split-ring commutator reverses the current instead of switching it off, so it gets a push during both halves of each turn.

An electric bell is a make-and-break circuit. The electromagnet pulls the armature, so the hammer hits the gong and the contact opens. With no current the magnet switches off, the spring returns the armature, the contact closes again, and the cycle repeats.

Other devices you could build

Electric bell or buzzer: an electromagnet, a soft-iron armature on a springy strip and a contact screw, wired in a make-and-break circuit (diagram above).
Loudspeaker: a coil of fine wire glued to a paper cone or plate, sitting near a strong magnet. An alternating signal current makes the coil and cone vibrate (motor principle).
Galvanometer: a coil around a compass. The compass needle deflects more as the current increases (Oersted's principle). With a scale, it becomes a simple ammeter.
Generator: spin a strong magnet quickly near a coil connected to a sensitive meter. A changing field induces a current. You will learn why in the next lesson, Electromagnetic Induction.

Worked example

Example 1: Explaining the half-scraped lead

A student strips the insulation all the way around both leads of her coil. The coil twitches but will not spin. Explain, and give the fix.

With both leads fully bare, current flows during the whole turn.
During the first half-turn, the forces turn the coil one way. After half a turn the sides of the coil have swapped places, but the current through each side is in the same direction as before.
So the forces now produce torque in the opposite sense, slowing and reversing the coil. It rocks back and forth.
Fix: on one lead, scrape the insulation from only the top half (with the coil held vertical, as in many instructions). The current then switches off for the half-turn when the torque would be backwards, and the coil coasts round.

Current all the time makes the torque reverse every half-turn; scraping only half of one lead acts as a simple commutator.

Worked example

Example 2: Planning a fair test

A student wants to find out whether more turns make the motor spin faster. Plan the test.

Question: How does the number of turns in the coil affect the motor's rotation speed?
Independent variable: number of turns (for example 5, 10, 15, 20).
Dependent variable: rotation speed, in revolutions per minute (rpm). Measure it by filming in slow motion with a phone and counting turns in a known time.
Controlled variables: the same battery (or a fresh one of the same type each time), the same magnet and its distance from the coil, the same wire, the same coil diameter, the same scraping method and the same supports.
Prediction (hypothesis): more turns give more torque, so faster spinning, up to a point, because extra turns also add mass and more resistance (longer wire).

One independent variable (turns), one dependent variable (rpm), everything else controlled.

Worked example

Example 3: Analysing sample data

Sample data from one student: in a slow-motion video, the 10-turn coil completes 15 revolutions in 2.0 s, and the 20-turn coil completes 21 revolutions in 2.0 s. Find each speed in rpm and the percent increase.

10 turns: 15 rev ÷ 2.0 s = 7.5 rev/s; × 60 s/min = 450 rpm.
20 turns: 21 rev ÷ 2.0 s = 10.5 rev/s; × 60 = 630 rpm.
Percent increase: (630 − 450) ÷ 450 × 100% = 40%.
Conclusion: in this test, doubling the turns increased the speed by 40%, not 100%. The extra wire also adds mass and resistance, so the gain is less than proportional. More trials would make the conclusion stronger.

450 rpm and 630 rpm, a 40% increase.

Just for fun

The device often called the first electric motor, shown by Michael Faraday in 1821, did not power anything at all. It just made a hanging wire swing round and round a magnet standing in a cup of mercury. Your paper-clip motor does the same job and, thankfully, skips the mercury.

Hands-on activity

Build a paper-clip motor

You need: about 1 m of enamelled (magnet) copper wire (from a hobby or electronics store, or a science kit), fine sandpaper, one D cell, two large metal paper clips, a strong flat magnet (a ceramic or fridge-strength disc magnet; a neodymium disc works even better), two elastic bands or tape, and a marker. Grown-up note: the coil makes a near short circuit, so the wire and battery get hot: run the motor for only 20 to 30 s at a time, let it cool, and never use a rechargeable lithium battery. Keep strong magnets away from young children. Take care with the sharp wire ends.

Wind the wire about 10 times around the D cell to make a neat coil. Slide it off and wrap each loose end two or three times around the coil to hold it together, so the two leads stick straight out on opposite sides.
On one lead, sand off all the enamel for about 2 cm. On the other lead, hold the coil vertical and sand only the top half of the enamel (colour the bottom half with marker to see it).
Bend the paper clips into supports with a small loop at the top. Fix them to the ends of the battery with elastic bands or tape so the loops are level.
Place the magnet on the battery under the loops. Rest the coil leads in the loops, so the coil can spin just above the magnet.
Give the coil a gentle flick. Adjust until it spins on its own.
Refine: test one change at a time (number of turns, magnet strength, coil shape) and record the speed with a phone slow-motion video.

What to look for: a coil that spins steadily in one direction. If it only twitches, check the scraped lead (Example 1), make the coil balanced, and make sure the leads are straight and level. Flipping the magnet over or turning the battery around reverses the direction of spin, as the motor rule predicts.

Your turn

What does the half-scraped lead in a paper-clip motor do?

Why does the coil keep moving during the half-turn when no current flows?

Name the independent, dependent and one controlled variable for a test of how magnet strength affects motor speed.

In an electric bell, what makes the hammer move back after hitting the gong?

Your motor spins clockwise. Give two ways to make it spin counterclockwise.

A coil makes 12 revolutions in 1.5 s. Find its speed in rpm.

More practice: warm-up

What is a prototype?

List the steps of the design cycle.

Why should you change only one variable at a time?

Why does the paper-clip motor's wire get hot?

Which principle explains a homemade loudspeaker?

More practice: on your own

A student's motor runs well for 10 s, then slows and stops. Give two likely reasons.

Explain why making a coil of 40 turns instead of 10 might not make the motor faster.

Describe how a coil wrapped around a compass can work as a simple current meter.

A student tests motor speed with 1, 2 and 3 stacked magnets and gets 300, 520 and 610 rpm. Describe the trend and suggest why it levels off.

Write a testable hypothesis about the effect of coil diameter on the speed of a paper-clip motor.

Challenge

A real DC motor gets a push during both halves of each turn, but the paper-clip motor gets a push during only one half. Explain the difference and why the real motor runs more smoothly.

Design a refinement that would let you measure the current your paper-clip motor draws without stopping it. Where must the meter go, and what range should it have?

A student claims her motor's speed is proportional to the number of turns. Her data: 5 turns, 240 rpm; 10 turns, 330 rpm; 15 turns, 400 rpm. Do the data support her claim? Explain using ratios.

Check your understanding

Quiz

In a paper-clip motor, why is the insulation scraped off only half of one lead?

Which variable should be controlled when testing how the number of turns affects motor speed?

A coil makes 20 revolutions in 4.0 s. What is its speed?

What makes an electric bell ring repeatedly?

Which change would reverse the direction of a motor's rotation?

Why do the coil and battery of a paper-clip motor become hot?

For parents and teachers

Big idea: Expectation F2.8 asks students to build, test and refine a prototype electromagnetic device. The paper-clip motor is inexpensive and teaches the motor principle (F3.6) and the design cycle, including fair testing and data analysis (A1.1, A1.5, A1.6, A1.10).

Materials: enamelled (magnet) wire is sold at electronics and hobby stores and in inexpensive science kits. A single D cell and a strong disc magnet are enough.

Safety: please supervise. The coil is nearly a short circuit, so the wire and battery get hot within a minute: run it in short bursts. Use only a single alkaline D or AA cell. Keep strong magnets away from young children.

Common difficulties:

Scraping both leads completely (the coil only rocks back and forth).
An unbalanced coil or bent leads that rub on the supports.
Changing several things at once while "improving" the motor, which makes it impossible to tell what helped. Encourage one change at a time, with a design log.