Force on a current-carrying rod in a magnetic field: rod on rails and Fleming's left-hand rule

A metal rod carrying current lies across two rails in a magnetic field. Change B, I and the length, reverse the current or the field, and watch the rod accelerate with F = BIL in the direction given by Fleming's left-hand rule.

Open this lab See plans

Also searched as: force on a current carrying conductor · force on a current carrying conductor · fleming left hand rule · force on a rod in a magnetic field · f = bil · motor effect

About this experiment

Subject	Physics
Class	12
Chapter	4. Moving Charges and Magnetism
NCERT section	4.6
Level	NCERT level
Time	about 15 minutes
Access	Included in a PractiSci plan

Aim

To show that a conductor carrying current in a magnetic field feels a force F = I L × B, to find its direction by Fleming's left-hand rule and to see it accelerate a rod on rails.

Theory

A moving charge q in a field B feels F = q v × B. A conductor of length L carrying current I has many moving charges, so the force on it is F = I L × B, where L is a vector along the current. The magnitude is F = B I L sin θ, largest when the wire is at right angles to the field.

The direction is at right angles to both L and B. Fleming's left-hand rule: stretch the thumb, the forefinger and the middle finger of the left hand at right angles to each other. If the forefinger points along the field and the middle finger along the current, the thumb gives the direction of the force (motion).

This is the principle of the electric motor, the moving coil galvanometer and the loudspeaker. Reversing the current or the field reverses the force; reversing both leaves it unchanged.

Formulas used

F = I L × B, F = B I L sin θ
force per unit length F/L = B I
a = (B I L − μ m g) / m
Fleming's left hand: forefinger = B, middle finger = I, thumb = F
v² = 2 a s

Procedure: what to do in the virtual lab

With B = 100 mT, I = 4 A, L = 50 cm, m = 50 g check that F = 0.20 N and a = 4 m/s² when there is no friction.
Reverse the current. The rod moves the other way. Reverse the field too.
Double the current and note that F doubles. Check this on the graph.
Increase the friction until the rod no longer moves: find the smallest force that moves it.
Use the left-hand rule to predict the direction before you let the rod go.

Observations to make

The force is at right angles to the rod and to the field.
F is proportional to B, I and L.
Reversing only the current or only the field reverses the force.

What you can change in this experiment

Magnetic field B (10 to 500 mT)
Current in the rod I (0 to 10 A)
Length of the rod in the field L (20 to 80 cm)
Mass of the rod (20 to 200 g)
Friction coefficient with the rails
Reverse the current (on / off)
Reverse the magnetic field (up / down) (on / off)

Viva questions on Force on a current-carrying rod in a magnetic field

Questions an examiner may ask in the Class 12 physics practical. Run the experiment in the lab to find the answers yourself, then check them in the Learn tab.

State Fleming's left-hand rule.
What is the force on a wire of length L carrying current I at an angle θ to a field B?
Why does the rod move if the current is reversed?
Which principle does a motor use?

Frequently asked questions

What is the aim of the Force on a current-carrying rod in a magnetic field experiment?

To show that a conductor carrying current in a magnetic field feels a force F = I L × B, to find its direction by Fleming's left-hand rule and to see it accelerate a rod on rails.

Which formulas are used in the Force on a current-carrying rod in a magnetic field experiment?

F = I L × B, F = B I L sin θ; force per unit length F/L = B I; a = (B I L − μ m g) / m; Fleming's left hand: forefinger = B, middle finger = I, thumb = F; v² = 2 a s

What viva questions are asked on Force on a current-carrying rod in a magnetic field?

State Fleming's left-hand rule. What is the force on a wire of length L carrying current I at an angle θ to a field B? Why does the rod move if the current is reversed? Which principle does a motor use?

How do I do the force on a current carrying conductor experiment for Class 12 online?

Open the Force on a current-carrying rod in a magnetic field virtual lab on PractiSci, follow the procedure above, change the values and read the live graph. It matches the Class 12 physics practical, with the aim, theory, observations and viva questions on this page.

How do you find the direction of the force on a current-carrying rod?

Use Fleming's left-hand rule: forefinger along B, middle finger along the current, and the thumb gives the force. F = I L × B.

What happens if both the current and the field are reversed?

The force stays in the same direction, because F = I L × B changes sign twice.

How fast does the rod accelerate on rails with friction?

a = (B I L − μ m g) / m once B I L is larger than the friction μ m g.

Can I do the Force on a current-carrying rod in a magnetic field experiment online?

Yes. Open it as a 3D virtual lab in your browser or on your phone, change the values and read the results from live graphs. It is part of a PractiSci plan.

Syllabus: Class XII Physics, Chapter 4 — Moving Charges and Magnetism, §4.6 (force on a current-carrying conductor in a uniform magnetic field)

What you do in the lab

Change the controls and watch the physics behave in real time in 3D. Live readouts and graphs show the numbers behind it, and the Learn tab has the theory, procedure, observations and viva questions. Works on a phone or a laptop, in English and Hindi.

Topics: force on a current carrying conductor · F = BIL · Fleming's left hand rule · motor principle · rod on rails · moving charges and magnetism

Read the full guide: Moving charges and magnetism.

Also try these physics experiments

Motional emf on a rod in magnetic field (Class 12)
Thomson's experiment: specific charge e/m of the electron (Class 12)
Heisenberg uncertainty principle with single-slit electron diffraction (Class 12)
X-rays: continuous spectrum, characteristic lines, Moseley and Bragg (Class 12)