Two parallel currents
"Two parallel currents" is a free, interactive Class 12 Physics simulation for NEET and JEE, built on the NCERT section "Force between parallel currents" in Moving Charges and Magnetism. Two long parallel wires: each one's field at the other and the equal, opposite forces per metre. NEET has asked about this idea 2 times in 10 years of papers. JEE Main asked 56 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson Force between parallel currents in Moving Charges and Magnetism. Change the values and watch what happens.
What it shows
NCERT §4.8
Two long parallel wires: each one's field at the other and the equal, opposite forces per metre.
Controls
3 to try
- SliderI_a −10 to 10 A
- SliderI_b −10 to 10 A
- Sliderseparation 1–50 cm
Try this
Make the wires repel with 2.0 × 10⁻⁵ N per metre at 10 cm separation (e.g. 5 A and −2 A)
Common mistake
Expecting like currents to repel, as like charges do.
In NEET
10 years of papers
Asked 2 times in 10 years of NEET.
See the past questions: Ampere's Circuital Law, Moving Charges and Magnetism (chapter)
In JEE Main
42 shifts analysed
56 questions from Moving Charges and Magnetism across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT §4.8
- Wire a makes a field B_a = μ₀I_a/2πd all along a parallel wire b a distance d away. Wire b, carrying I_b, feels a sideways force I_b L B_a on a length L.
- So the force per unit length is f = μ₀I_aI_b/2πd, equal in size on both wires and opposite in direction, as Newton's third law needs for steady currents.
- Currents in the same direction attract; currents in opposite directions repel. This is the reverse of charges, where like repels like.
- Definition of the ampere: the steady current that, flowing in two very long, straight, thin parallel wires 1 m apart in vacuum, makes each feel 2 × 10⁻⁷ N per metre. This definition dates from 1946.
- In practice the earth's field and stray fields must be removed, long wires are replaced by coils, and a current balance measures the force. One coulomb is then the charge carried past a point in 1 s by 1 A.
- Example 4.9: in a horizontal earth's field of 3.0 × 10⁻⁵ T pointing south to north, a wire with 1 A running east to west feels IB = 3 × 10⁻⁵ N per metre (downwards), far more than 2 × 10⁻⁷ N/m; running south to north it feels nothing.
Questions
5 answers
- What does the "Two parallel currents" simulation show?
- Two long parallel wires: each one's field at the other and the equal, opposite forces per metre.
- What mistake does the "Two parallel currents" simulation fix?
- A common mistake it clears up: expecting like currents to repel, as like charges do.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 2 times in 10 years of papers.
- Does JEE Main ask questions from Moving Charges and Magnetism?
- Yes. JEE Main asked 56 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Force between parallel currents" in Moving Charges and Magnetism (Class 12 Physics).
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