S3 Science | Magnets and Electromagnets: Field Lines, Ampere-Turns and Current

2026-11-12

Most students have played with magnets but cannot explain why a current creates magnetism. This lesson starts with one iron nail and a coil of enamel wire and links fields, ampere-turns and switching into a single chain of cause and effect.

Objectives — By the end of 45 minutes students can draw field lines around a bar magnet and a current-carrying coil, state that like poles repel and unlike poles attract, compare electromagnet strength using turns multiplied by current, and explain why an electromagnet can be switched on and off while a permanent magnet cannot.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Pole experience	What happens when the north poles of two magnets are brought together?	They say it pushes away but not that like poles repel
Circuit basics	A 6 V supply across 12 ohms: what current flows?	Often cannot answer; revise I = V/R first
Direction	Do field lines leave the north pole or enter it?	Commonly reversed

02 Lesson flow (45)

Time	Stage	Tutor does	Student does
0-5 min	Two magnets	Bring N to N and then N to S and ask what happens each time	State repel and attract aloud
5-14 min	Drawing field lines	Field lines leave N and enter S, are closer where the field is stronger, and never cross	Draw the field of a bar magnet with arrows
14-25 min	Current makes magnetism	Wind enamel wire round an iron nail, connect a cell and pick up paper clips; disconnect and they fall	Wire the coil, switch on, count the clips
25-35 min	Ampere-turns and three factors	200 turns at 0.5 A gives 100 ampere-turns; 400 turns doubles it; 300 turns at 0.3 A gives 90, which is weaker	Calculate ampere-turns and compare
35-41 min	Practice and application	Three items: draw field lines, calculate ampere-turns, explain how a doorbell or scrap-yard magnet switches	Work independently and justify
41-45 min	Exit check	Three quick questions: field direction, ampere-turn formula, can an electromagnet be switched	Answer orally

03 Teaching script (read it out)

Scene 1: Minutes 5-14 | Field lines

Draw a bar magnet with N on the left and S on the right. Ask where a compass placed at the top right would point, then draw the field lines one by one: out of N, round to S, with arrows from N to S outside the magnet. Give three rules: lines never cross because a point has only one field direction, lines are denser where the field is stronger near the poles, and lines form closed loops returning through the magnet. Finally draw two magnets with like poles facing: the lines push apart between them, which is the picture of repulsion.

Students draw a bar magnet field with correct arrows and explain that denser lines mean a stronger field

Scene 2: Minutes 14-25 | The current-carrying coil

Give each pair enamel wire, an iron nail and a cell. Wind 200 turns, scrape the enamel off both ends, connect and pick up paper clips, counting them. Then disconnect and the clips fall at once: this is the key image of the lesson, magnetism that can be switched. Ask what happens if the cell is reversed, and show that the poles swap while the strength stays. Add a safety note: coils heat up, so never keep the current on for more than ten seconds.

Students wire the coil, count the clips and explain that switching off removes the magnetism

Scene 3: Minutes 25-35 | Ampere-turns

Write that strength is proportional to turns times current, measured in ampere-turns. Compare three cases: 200 turns at 0.5 A gives 100; 400 turns at 0.5 A gives 200, twice as strong; 300 turns at 0.3 A gives 90, weaker than the first. Add a fourth: the same 200-turn coil with 6 V across 12 ohms draws 0.5 A, but across 6 ohms draws 1 A, so ampere-turns double. Then answer the design question: an electromagnet can be switched, reversed and tuned, which a permanent magnet cannot.

Students compute ampere-turns from turns and current and compare two electromagnets numerically

Scene 4: Minutes 35-45 | Practice and wrap-up

Set three items: draw the field of a current-carrying coil using the right-hand grip rule, calculate ampere-turns, and explain how a scrap-yard magnet lifts metal and then drops it. While circulating, ask only which concept each item tests. Close with one sentence: a permanent magnet relies on its material, an electromagnet relies on current, and that is why only one of them can be turned off.

Students complete all three items and justify the scrap-yard answer with switching

04 Question types and worked steps

Type	Example	Steps	Answer
Field direction	Two compasses near a bar magnet: mark their directions	1. Locate N and S. 2. Lines leave N and enter S. 3. The compass north points along the line.	Compass north points away from the N pole along the line
Ampere-turn comparison	200 turns at 0.5 A versus 300 turns at 0.3 A: which is stronger?	1. 100. 2. 90. 3. Compare and state the reason.	The first, 100 against 90 ampere-turns
Current and strength	A 6 V cell across a 12 ohm coil of 200 turns: find current and ampere-turns	1. I = V/R = 0.5 A. 2. 200 x 0.5 = 100 ampere-turns.	I = 0.5 A, 100 ampere-turns
Application	Why can a scrap-yard electromagnet pick up iron and then release it?	1. Current gives magnetism. 2. Switching off removes it. 3. The iron falls.	It can be switched off, so the load is released

Sanity check (teach this): Three sanity checks: ampere-turns are turns multiplied by amperes, so turns alone never settle a comparison; outside a magnet, field lines run from N to S, and reversed arrows are wrong; a larger current also means more heating, so any answer that simply adds cells should mention the heat limit.

05 Independent practice (3 questions)

Question	Answer	What to watch
A coil of 500 turns carries 0.2 A. Find the ampere-turns and compare with 200 turns at 0.4 A.	100 ampere-turns, the same as the second	Did they compare turns only
Draw the field of a current-carrying coil and mark which end is N	Right-hand grip rule: fingers follow the current, thumb points to N	Did they label the thumb end as S
Why can an electromagnet pick something up and put it down while a permanent magnet cannot?	Switching off removes the field; a permanent magnet cannot be switched	Did they answer only that it is stronger

06 Common mistakes and what to say

Mistake	Student writes	Say this
Reversed field lines	Drawing lines leaving S and entering N	Outside a magnet the lines always leave N and enter S. What you drew is the direction inside the magnet, and the two must be kept apart.
Turns only	Assuming 300 turns always beats 200 turns	Multiply by the current: 300 turns at 0.3 A gives 90, while 200 turns at 0.5 A gives 100, so the second is stronger.
More cells is always better	Adding cells without limit to get more strength	A bigger current heats the coil, and melted enamel causes a short circuit. Real electromagnets are designed around heat.
Confusing the two kinds of magnet	Saying the nail itself is a magnet	The nail is not magnetic by itself; the current makes it temporarily magnetic and the magnetism disappears when the current stops.

07 Homework (paper, section and question numbers)

Secondary Science hub — Other chapters in the same subject; finish the three practice items first
Same subject: S2 electricity and circuits — Current calculations use I = V/R, which this chapter depends on
Same subject: S3 acids, alkalis and neutralisation — Another experiment-led lesson; compare how observations are recorded

08 Exit check (last 5 minutes)

Question	Answer	Decision
Outside a magnet, where do field lines start?	At the north pole, ending at the south pole	If reversed, redraw once
How do you calculate ampere-turns?	Turns multiplied by current in amperes	If they answer turns only, redo the 90 against 100 comparison
The biggest difference between the two magnet types?	An electromagnet can be switched and tuned by current	If unclear, repeat the switch-off demonstration

09 Tutor FAQ

Is the experiment essential?

Strongly preferred: the moment the clips fall on switching off is what students remember. Without apparatus, use an animation but keep the on and off contrast.

Should the right-hand grip rule be taught?

Yes at S3, but only for coils, not for derivations. If students cannot recall it, fall back on drawing current direction and field lines.

Do we need Faraday law here?

No. S3 only needs current producing magnetism; induction belongs to S5 physics.

Find a tutor — free Tutor Registration →

A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.