SME
Grade 8 Science · Physical Science
Magnetic fields and electromagnets
Use this for “An electromagnet has 20 turns and a 1.5 A current. How strong is it, and how many clips can it lift?”
Diagram
The strength follows turns × current.
S = N × I = 20 × 1.5 A = 30 amp-turns
This is an example. Type your own values to start a new problem.
S (amp-turns)
=
N (turns)
×
I (A)
k
Paper clips picked up
worked out (not typed) · clips
Formulas
S
=
N
×
I
(strength
=
turns
of
wire
×
current)
30 = 20 × 1.5
k
=
S
÷
5
(paper
clips
picked
up
=
strength
÷
5)
6 = 30 ÷ 5
Assumptions
•
A current in a coil of wire makes a magnetic field. An iron nail inside makes it much stronger, and it switches off with the current.
•
More turns or more current makes a stronger electromagnet: strength follows turns × current, measured in amp-turns.
•
Clips picked up is how a class measures strength. The 5 amp-turns per clip here is this coil and nail’s result; yours will differ.
•
Turns come in tens because a class winds and counts them that way; the current is set in half amps.
Step-by-step
WE KNOW
N = 20 turns (turns of wire), I = 1.5 A (current)
FIND
strength (S), paper clips picked up (k)
Step 1 · Find strength (S)
Use S = N × I (strength = turns of wire × current)
Multiply the turns by the current: each turn carries the whole current round the nail once.
S
=
20
×
1.5
S
=
30
amp-turns
Step 2 · Find paper clips picked up (k)
Use k = S ÷ 5 (paper clips picked up = strength ÷ 5)
For this coil and nail, about one clip for every 5 amp-turns.
k
=
30
5
k
=
6
clips
(about)
Check
Put the numbers back in. Both sides should match.
30 = 20 × 1.5 ✓
6 = 30 ÷ 5 ✓
Topic check
See what you know: 5 questions on this topic
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