Fleming's Left-Hand Rule — Physics with Kate

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4.7.2.2

AQA GCSE Physics · Topic 4.7 Magnetism and Electromagnetism

6.11P · 6.13 · 6.14

Edexcel International GCSE Physics · Section 6 Magnetism and electromagnetism

Fleming's Left-Hand Rule

In this lesson you'll learn to: use Fleming's left-hand rule to find the direction of the force on a wire carrying a current in a magnetic field, read ⊙ and ⊗ and a cell's + and −, say what changes the size and direction of the force, and measure it with a balance. AQA only Calculate it with F = B I l. Edexcel only Find the force on a moving positive charge.

▶ Prefer to watch? Scroll to the bottom for the video on this topic ▶ A video walkthrough is coming soon — it will appear at the bottom of this page

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Start here — have a play

Hold up your left hand before you start. On tab 1, do the four workbook questions with your hand, then press New question as many times as you like: practice is the only way to get fast at this. Tab 2 measures the force with a balance. AQA only tab 3 is F = B I l. Edexcel only tab 4 is a proton moving through a field.

The force is

Left hand. Thumb, first finger and second finger all at right angles. First finger = magnetic Field (N → S) · seCond finger = Current (+ → −) · thuMb = force (Motion).

⊙ = coming out of the page, towards you (the point of an arrow) · ⊗ = going into the page (its feathers).

▲ Use your real left hand alongside the "your left hand" diagram: it builds up one finger at a time.

Got the hang of it? Now learn the rules for the exam.

Which finger is which, how to read ⊙, ⊗ and a cell, and what makes the force bigger, smaller or reverse.

Read the notes ↓

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Revision notes

The motor effect

A wire carrying a current in a magnetic field feels a force. The current makes its own magnetic field round the wire, and that field interacts with the magnet's field. This is the motor effect.

The force is biggest when the wire is at right angles to the magnetic field.
There is no force if the wire is parallel to the field.

Fleming's left-hand rule

Thumb, first finger and second finger all at right angles

Finger	Shows	Points
First finger	magnetic Field	from N to S
seCond finger	Current	from + to − (conventional current)
thuMb	force (Motion)	the way the wire is pushed

⭐ Reading the diagrams

⊙ = coming out of the page, towards you (the point of an arrow coming at you). ⊗ = going into the page (the feathers of an arrow going away).

On a cell, the long line is + and the short line is −. Current leaves + and goes round the circuit to −. Write + and − by the cell before you use your hand.

Workbook practice: which way is the force on the wire?

① field right, current out → up

② field down, current in → left

③ field left, current down → into the page

④ field up, current right → out of the page

The size of the force, and how to reverse it

Change	The force…
Bigger current	gets bigger
Stronger magnets (bigger magnetic flux density)	gets bigger
Longer length of wire in the field	gets bigger
Reverse the current	reverses
Reverse the field (swap N and S)	reverses
Reverse both	no change in direction

Measuring the force with a balance

Stand the magnets in a holder on a top-pan balance and zero it. Clamp a stiff wire between the poles so it can't move. Switch the current on and the reading changes.

Change the reading to kg (g ÷ 1000), then F = m g: that is the force on the magnets.
Newton's third law: if the wire pushes the magnets down, the magnets push the wire up with the same size force. So a reading that goes up means the force on the wire is upwards.
Use the g the question gives: AQA usually 9.8 N/kg (the workbook page uses 10), Edexcel 10 N/kg.

The reading was 0.26 g. Change…	New reading
current increased	more than 0.26 g
current decreased	less than 0.26 g
current reversed	−0.26 g
stronger magnets	more than 0.26 g
magnets swapped round	−0.26 g
current reversed and magnets swapped	0.26 g

Calculating the force: F = B I l AQA only · higher

=

F = B I l

force = magnetic flux density × current × length

F newtons (N) · B tesla (T) · I amps (A) · l metres (m)

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Only the wire in the field

l is the length of wire inside the magnetic field, not the whole wire. Change cm to m (÷ 100) and mA to A (÷ 1000) first.

It is on the equation sheet. Rearranged: B = F ÷ (I l) and I = F ÷ (B l). It applies when the wire is at right angles to the field.

The force on a moving charge Edexcel only · triple

There is a force on a charged particle when it moves in a magnetic field, as long as its motion is not parallel to the field. A moving proton (positive) is a current in the direction it moves, so use the left-hand rule with the second finger pointing the way it moves.

Field into the page, proton moving right → force up

⭐ Why it goes in a circle

The force is always at right angles to the way the proton is moving, so it keeps turning it: in a uniform field it moves in a circle, with the force pointing to the centre. A neutron has no charge, so it feels no force and goes straight on. A proton that is still, or moves parallel to the field, feels no force either.

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How to write it in the exam

The same three steps, every time

First finger along the magnetic field (N → S) → second finger along the current (+ → −) → thumb gives the force.

fieldcurrentforce

Exam-style question · AQA workbook page 8 · 8 marks

A metal rod rests on two metal rails between the poles of a magnet (shown end-on, current out of the page). Ignore friction.

(a) Field arrow [1]: pointing down, from N to S.
(b) Direction the rod moves [1]: to the right, along the rails.
(c) Two changes that make it move the opposite way [2]: reverse the current (turn the cell round); reverse the field (turn the magnet over). Doing both: no change.
(d) A bigger force [1]: a bigger current (more cells) or a stronger magnet.
(e) Acceleration, mass 4.0 g, force 0.012 N [3]: 4.0 g = 0.0040 kg; a = F ÷ m = 0.012 ÷ 0.0040 = 3.0 m/s².

Where marks go missing: using 4.0 instead of 0.0040 kg gives 0.003. Convert first.

F = B I l · AQA workbook page 9 · AQA only · higher

F = 0.20 × 3.0 × 0.050 = 0.030 N

B = 0.048 ÷ (4.0 × 0.12) = 0.10 T

I = 0.015 ÷ (0.25 × 0.20) = 0.30 A

Exam-style question (8 marks): magnets on a balance, a clamped wire between the poles; the reading is 204 mg when the current is on.

(a) 204 ÷ 1 000 000 = 2.04 × 10⁻⁴ kg [1]
(b) W = m g = 2.04 × 10⁻⁴ × 9.8 = 2.0 × 10⁻³ N [2]
(c) Newton's third law: 2.0 × 10⁻³ N upwards on the wire (equal and opposite) [1]
(d) l = 5.0 cm = 0.050 m; B = F ÷ (I l) = 0.0020 ÷ (2.0 × 0.050) = 0.020 T [3]
(e) current reversed: −204 mg (the force on the wire is now down, so the magnets are pulled up) [1]

Force on a moving charge · Edexcel workbook page 9 · Edexcel only · triple

① field out, moving down → left

② field in, moving left → down

③ field right, moving up → into the page

④ moving parallel to the field → no force

⑤ moving down, force right → field into the page

⑥ field out, force left → moving down

Exam-style question (9 marks): a proton moves in a circle at constant speed in a uniform magnetic field.

(a) An arrow from the proton pointing to the centre of the circle [1]
(b) Into the page: at the top the proton moves left and the force is down [1]
(c) r = 6.0 ÷ 2 = 3.0 cm = 0.030 m; v = 2πr ÷ T = 2 × π × 0.030 ÷ 8.7 × 10⁻⁶ = 22 000 m/s [3]
(d) a stronger magnetic field; make the proton move faster [2]
(e) a neutron goes in a straight line: it has no charge, so there is no force on it [2]

📘 Now do it in your workbook

Page 7: label the left hand and find the force in four situations. Page 8: measure the force with a balance, predict the readings and the rails question. Page 9 (higher): F = B I l with an exam question.

Page 8: label the left hand and find the force in four situations. Page 9 (triple only): the force on a moving proton, six to do, and an exam question.

AQA GCSE Physics Workbook · MAGNETISM AND ELECTROMAGNETISM · pages 7, 8 and 9 Edexcel International GCSE Physics Workbook · MAGNETISM AND ELECTROMAGNETISM · pages 8 and 9

⬇ Free: pages 7, 8 and 9 (PDF) 📄 Full workbook PDF · £2.99 Paperback on Amazon ⬇ Free: pages 8 and 9 (PDF) 📄 Full workbook PDF · £2.99 Paperback on Amazon

The full workbook covers the whole Magnetism and Electromagnetism topic, with exam-style questions and worked answers.