A rectangular coil of turns has the sides and each long

PHYSICS · Multi-part · AP · Physics 2: Algebra-based · Magnetism and Electromagnetism · Magnetism and Electromagnetism

Problem

A rectangular coil

PQRS

50

turns has the sides

PQ

and

RS

each

5.0\,\text{cm}

long, while the sides

QR

and

SP

are

4.0\,\text{cm}

long. The coil is mounted so that it can rotate freely about a central horizontal axis perpendicular to

PQ

and

RS

. It is placed in a uniform magnetic field of

0.40\,\text{T}

, the field lines being horizontal and perpendicular to

PQ

and

RS

. A current of

2\,\text{A}

is passed through the coil in the direction

P\rightarrow Q\rightarrow R\rightarrow S\rightarrow P

. (i) Determine the direction of the force acting on the sides

PQ

and

RS

. (ii) Calculate the maximum force acting on each of these sides. (iii) Explain why the coil starts rotating and how the split-ring commutator helps the coil to continue rotating in the same sense during successive half turns.

Related theory

Electromechanical Devices
Magnetic Forces
Torque on current loop
DC electric motor
Force on a current-carrying conductor in a magnetic field
Magnetic Effects of Electric Current

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