Force on a current-carrying conductor and Fleming’s left-hand rule

1. Definition

When a current-carrying conductor is placed in a magnetic field, it experiences a force if the current direction is not parallel to the magnetic field direction.

2. Key Ideas

• No force when current is parallel to the magnetic field.
• Maximum force when current is perpendicular to the magnetic field.
• The force direction is perpendicular to both current and field.
• Use Fleming’s left-hand rule to find the force direction.
• Reversing current or reversing field reverses the force direction (reversing both keeps it the same).
• In end-on diagrams, ⊙ means out of the page and ⊗ means into the page.

3. Detailed Explanations

A. When does the force act?

The force acts when current is not parallel to the field (i.e. there is a component of current across the field).

B. Direction of force (Fleming’s left-hand rule)

Hold your left hand with the three directions at right angles:

• first finger: magnetic field (N → S)
• second finger: current (conventional current)
• thumb: force (motion of the wire)

Scroll diagram horizontally to read all labels.

C. What happens when you reverse directions?

• Reverse the current → force reverses.
• Reverse the magnetic field → force reverses.
• Reverse both → force stays the same direction.

D. Force on a beam of charged particles (syllabus experiment)

An electron beam (for example, in an electron-beam tube) is a stream of moving negative charges. Conventional current is opposite to the direction of electron motion, so convert the electron direction before applying Fleming’s left-hand rule.

If the beam passes through a magnetic field:

• the beam is deflected (a magnetic force acts on the moving charges)
• reversing the magnetic field reverses the deflection direction
• reversing the beam direction would also reverse the deflection direction

4. Common Mistakes

• Using electron flow instead of conventional current in Fleming’s left-hand rule.
• Forgetting the condition for no force (current parallel to field).
• Mixing up right-hand grip rule (field around a wire) with left-hand rule (force on a wire).
• Treating electron motion as conventional current; the two directions are opposite.

5. Exam Tips

1. Start by drawing arrows for field and current.
2. Apply Fleming’s left-hand rule to get the force direction.
3. If the question mentions reversing current/field, state clearly what happens to the force direction.

6. Worked Examples

Example 1: Finding force directionCore

The magnetic field is into the page. Conventional current is to the right. What is the direction of the force on the wire?

Show Answer

Using Fleming’s left-hand rule, the force is upwards.

Example 2: Reversing currentCore

In Example 1, the current is reversed (to the left). What happens to the force direction?

Show Answer

The force reverses (it becomes downwards).

Example 3: No-force caseCore

A wire carries current upwards and is placed in a magnetic field that is also upwards (parallel). What is the magnetic force on the wire?

Show Answer

No force (0 N), because current is parallel to the magnetic field.

Example 4: Perpendicular case (maximum idea)Core

When is the magnetic force on a current-carrying conductor greatest: when current is parallel to the field or perpendicular to the field?

Show Answer

Greatest when the current is perpendicular to the magnetic field.

Example 5: Force direction checkCore

Magnetic field is from left to right. Conventional current is out of the page. What is the force direction on the conductor?

Show Answer

Using Fleming’s left-hand rule: field to the right (first finger), current out of the page (second finger), so the force is upwards (thumb).

7. Mind Stretchers

Mind stretcher 1: Reverse bothExtension

If both the current direction and the magnetic field direction are reversed at the same time, what happens to the force direction?

Show Answer

The force direction stays the same (reversing both cancels out).

Mind stretcher 2: Identify the ruleExtension

When should you use the right-hand grip rule, and when should you use Fleming’s left-hand rule?

Show Answer

Right-hand grip rule: to find the direction of the magnetic field around a current-carrying wire/solenoid.
Left-hand rule: to find the direction of the force on a current-carrying conductor in a magnetic field.

8. Practice and next step

Reverse current and field independently in the Motor–Generator–Transformer Lab, then apply the force pair to the D.C. Motor.