DSE Physics - Magnetic Field and the Motor Effect Prep Guide (for tutors) | F equals BIL, coil torque, back emf and inrush current

DSE Physics – Magnetic Field and the Motor Effect Prep Guide (for tutors) | F equals BIL, coil torque, back emf and inrush current

2026-11-05

Tutor lesson-prep guides · Secondary Physics

Students can recite force equals B I L, then explain a large starting current by saying the motor has not turned yet. This lesson links force, torque and back emf with three numbers so every formula has a physical cause.

Objectives — Students can: (1) compute the 0.5 N force on a 0.25 m conductor carrying 4 A in a 0.5 T field and give its direction with the left-hand rule; (2) use turns, field, current and area to get a maximum torque of 0.48 N m and an input power of 75.39822368615503 W at 1,500 rpm; (3) explain a back emf of 226.0 V from a 230 V supply and 2.0 ohm coil, and show the 115 A starting current is 58 times the running current.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Current and fields	Does a current-carrying wire create a magnetic field?	Yes, with direction given by the right-hand grip rule
Components	At thirty degrees, is the effective component sine or cosine?	Sine, so multiply by sin thirty
Power	What are the units of torque times angular velocity?	Newton metres times radians per second, which is watts

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Prior knowledge check	Ask the three prior questions and write B, I and L on the board	Answer orally and copy any wrong item once
5-17 min	Act one: three letters, one force	Show 0.5 times 4 times 0.25 and fix the direction with the left-hand rule	Copy once, then reverse the current to see the force turn
17-30 min	Act two: one turn becomes torque	Build from 50 turns and 0.012 m2 to 0.48 N m	Compute once, then find the torque at thirty degrees
30-40 min	Act three: why starting current is huge	Go from 230 V minus 226.0 V back emf to the current	Compute the inrush current and state one consequence
40-43 min	One-sentence summary	Ask for one sentence on where back emf comes from	Answer orally; tutor corrects only wording
43-45 min	Exit check	Hand out three items and collect them	Work alone with books closed

03 Teaching script (read it out)

Scene 1: Act one: the force is a product, the direction is separate

Force equals B I L only when the conductor is perpendicular to the field; at thirty degrees multiply by sin thirty, which halves it.
With 0.5 T, 4 A and 0.25 m the force is 0.5 N, or 2.0 N per metre.
Direction uses the left-hand rule: field into the index finger, current along the middle finger, force out of the thumb. Reverse any one finger and the force reverses.
One line to remember: reverse the current and the force reverses; reverse the field and it reverses too; reverse both and nothing changes.

Ask students to repeat: size from B I L, direction from the left hand, and two reversals cancel.

Scene 2: Act two: one turn is weak, fifty turns are not

A single loop feels two opposing forces forming a couple; torque equals N times B times I times A, so only area matters, not shape.
With 50 turns, 0.4 T, 2 A and 0.012 m2 the maximum torque is 0.48 N m, reached when the coil plane is parallel to the field.
At thirty degrees it falls to 0.23999999999999996 N m, and at ninety degrees it is zero, which is why real motors use a commutator or alternating current to keep reversing.
Input power is torque times angular velocity: 1,500 rpm is 157.08 radians per second, giving 75.39822368615503 W.

Ask students to repeat: torque depends on area and turns, dies at ninety degrees, so the current must be reversed.

Scene 3: Act three: back emf is the motor braking itself

A turning coil cuts field lines and generates an emf opposing the supply. Running current is (supply minus back emf) divided by resistance, which is (230 minus 226.0) divided by 2.0, or 2 A.
At rest there is no back emf, so current is 230 divided by 2.0, or 115 A, which is 58 times the running value and squares the heating.
That is why large motors use reduced-voltage starting or a starting resistor, and why a sudden load slows the coil, shrinks the back emf and lets the current rise on its own.

Ask students to repeat: no back emf at start means huge current, and falling speed raises current.

04 Question types and worked steps

Type	Example	Steps	Answer
Force calculation	0.5 T, 4 A, 0.25 m	Check perpendicularity, then multiply B, I and L	0.5 N
Direction question	Current east to west, field pointing north	Left-hand rule with three mutually perpendicular fingers	Force up or down depending on charge sign
Torque calculation	50 turns, 0.4 T, 2 A, area 0.012 m2	N B I A, then multiply by the sine of the angle	0.48 N m
Inrush current	230 V supply, 2.0 ohm coil, running at 2 A	Set back emf to zero at start	115 A

Sanity check (teach this): Three read-backs: (1) tesla times ampere times metre is a newton, so a newton metre answer means an angle or a torque crept in; (2) torque scales with area, so doubling the radius quadruples it; (3) efficiency must stay below one hundred, and 60 over 75.39822368615503 gives 79.6 percent, so anything above one hundred means input and output were swapped.

05 Independent practice (3 questions)

Question	Answer	What to watch
0.20 T, 3 A, 0.50 m, perpendicular. Find the force.	0.30 N	Check that all three quantities were multiplied
Same conductor at thirty degrees to the field. Find the force.	0.15 N	Check for the sine of thirty degrees
Why is the starting current so much larger?	With no back emf at rest, the whole supply sits across the resistance	Check whether back emf is named rather than warming up

06 Common mistakes and what to say

Mistake	Student writes	Say this
Turns left out	Torque reported as 0.0096 N m	One conductor force must be multiplied by 50 turns.
Angle ignored	Torque stays at its maximum at thirty degrees	Only the parallel position is maximum; otherwise multiply by sine.
Starting current treated as normal	The starting current equals the running current	At rest back emf is zero, so the current is 115 A.
Hand rule memorised wrongly	The left-hand rule is described as the right-hand rule	Field to index, current to middle, force to thumb; the right hand is the generator rule.

07 Homework (paper, section and question numbers)

Redo this chapter — Redo the three items, formula and units first
Waves — Compare the units of wave speed and rotational speed in three points
Physics chapter index — Pick one more chapter and draw the field lines only

08 Exit check (last 5 minutes)

Question	Answer	Decision
0.5 T, 4 A, 0.25 m, perpendicular: force?	0.5 N	Error: one factor dropped
What is the back emf at the instant of starting?	Zero, so the current is largest	Error: answering that it equals the supply
Torque when the coil plane is perpendicular to the field?	Zero	Error: answering maximum

09 Tutor FAQ

Which year is this for?

Mainly S4, and a starting point for S5 electromagnetism.

Is 45 minutes enough for three formulas?

Yes, because all three sit on one causal chain and only one data set is computed in full.

Is there a companion worksheet?

Three practice items with answers on the page, plus the waves chapter.

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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.