Moving charges and magnetism (Class 12): force, field and the right-hand rule
A moving charge makes a magnetic field, and a magnetic field pushes on moving charges. These labs let you reverse the current or the field and see the direction of the force, then measure the field around a wire.
NCERT Class 12 Physics, Chapter 4 (Moving Charges and Magnetism). Every lab below runs in 3D in your browser or on a phone, in English and Hindi.
Try the labs
Force on a current-carrying rod in a magnetic field: rod on rails and Fleming's left-hand rule · plan · Chapter 4
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.
Magnetic field of a straight wire: right-hand rule, compass needles and force between wires · free · Chapter 4
To see that a long straight current produces a magnetic field in circles around it, to find its direction by the right-hand rule and its size from B = μ₀I/2πr, and to find the force between two…
Force between parallel current-carrying wires · plan · Chapter 4
To study the magnetic force between two parallel current-carrying conductors and the definition of the ampere.
Biot–Savart law: field of a straight wire, circular loop and solenoid · plan · Chapter 4
To calculate and visualise the magnetic field produced by steady currents using the Biot–Savart law, and compare with the standard results for a wire, a coil and a solenoid.
Charged particle in uniform E and B fields · plan · Chapter 4
To study the motion of a charged particle in uniform magnetic and electric fields and to relate the radius, period and pitch of its path to q, m, v and B.
Torque on a current loop and magnetic moment · plan · Chapter 4
To study the torque on a current-carrying loop in a uniform magnetic field, its magnetic moment and potential energy.
Cyclotron · plan · Chapter 4
To understand the working of a cyclotron and the resonance condition between the oscillating voltage and the orbital motion.
Galvanometer to ammeter and voltmeter · plan · Chapter 4
To convert a moving-coil galvanometer into an ammeter of a given range using a shunt, and into a voltmeter using a high series resistance.
Hall effect: carrier sign, density and mobility · plan · Chapter 4
To study the Hall effect: find the sign and density of the charge carriers in a conductor or semiconductor from the Hall voltage.
Helmholtz coils: a uniform magnetic field · plan · Chapter 4
To understand the Helmholtz coil arrangement that produces a nearly uniform magnetic field and compare with other coil separations.
Moving coil galvanometer, ammeter and voltmeter conversion · plan · Chapter 4
To study the moving coil galvanometer, its sensitivity, and to convert it into an ammeter and a voltmeter.
Magnetic field of a solenoid and a toroid · plan · Chapter 4
To study the magnetic field of a solenoid and a toroid using Ampere's circuital law.
Tangent galvanometer: I = K tan(theta) · plan · Chapter 4
To study the tangent galvanometer: relation tanθ = B_coil/B_H, the reduction factor K and the measurement of a current.
Velocity selector and mass spectrometer · plan · Chapter 4
To understand the velocity selector and the mass spectrometer, and separate isotopes by their mass.
Force on a current-carrying rod
A rod of length L carrying a current I in a field B feels the force F = I L × B, with magnitude B I L sin θ. Use Fleming's left-hand rule: forefinger along the field, middle finger along the current, and the thumb shows the force. Reversing either the current or the field reverses the force, and reversing both leaves it unchanged.
On rails, the rod accelerates with a = (B I L − μ m g) / m once the force beats friction.
F = I L × BF = B I L sin θa = (B I L − μ m g) / m
Magnetic field of a long straight wire
Grip the wire with the right hand, the thumb along the current: the fingers curl in the direction of the field. The magnitude at a distance r is B = μ₀ I / (2π r), so the field falls as 1 / r. Compass needles around the wire line up along circles, longer needles showing a stronger field.
B = μ₀ I / (2π r)μ₀ = 4π × 10⁻⁷ T m / A
Force between two parallel wires
Each wire sits in the field of the other, so each feels a force. Currents in the same direction attract, opposite currents repel. The force per unit length is μ₀ I₁ I₂ / (2π d). This is how the ampere is defined: 1 A in each of two long wires one metre apart gives 2 × 10⁻⁷ N per metre.
F / L = μ₀ I₁ I₂ / (2π d)
Frequently asked questions
What is Fleming's left-hand rule?
Stretch the forefinger, middle finger and thumb at right angles. If the forefinger points along B and the middle finger along the current, the thumb points along the force on the conductor.
What is the force on a current-carrying conductor in a magnetic field?
F = I L × B, with magnitude B I L sin θ. It is zero when the conductor is parallel to the field and largest when it is perpendicular.
How does the magnetic field of a straight wire depend on distance?
B = μ₀ I / (2π r): it is inversely proportional to the distance r from the wire and directly proportional to the current.
Do two parallel wires attract or repel?
They attract if the currents flow in the same direction and repel if the currents are opposite.
How is the ampere defined?
By the force between two long parallel wires: a current that gives 2 × 10⁻⁷ newton per metre between wires one metre apart is 1 ampere.