Question

A current of `10 A` flows around a closed path in a circuit which is int eh horizontal plne as shown in the figure. The circuit consists oif eight alternating arcs of radii `r_1=0.08m` and `r_2 =0.12m`. Each subtends the same angle at the centre.

a. Find the magnetic field produced by this circuit at the centre.
b. An infinitely long straight wire carryin as current of `10 A` is passing through the centre of the above circuit vertically with the direction of the current being into the pane of the circuit. what is the force acting on the wire at the centredue to the curren in the circuit? What is the force acting on the arc `AC` and the straight segment `CD` due to the current at the centre?

Answer 1

a. Given i=10A, `r_1=0.08` and `r_2=0.12m.` Straight portion i.e. `CD` etc, will produce zero magnetic fidl at the centre. Rest eight arcs will produce the masgnetic field at the centre in the same direction i.e. perpendicular to the paper outwards or vertically upwards and its magnitude is
`B=B_("inner arcs")+B_("outer arcs")`
`=1/2{mu_0i)/(2r_1)+1/2{(mu_0i)/(2r_2)}`
`=((mu_0)/(4pi))(pii) ((r_1+r_2)/(r_1r_2))`
substituting the values we have
`B=((10^-7)(3.14)(10)(0.08+0.12))/((0.08xx0.12))`
`B=6.54xx10^-5T`
(vertically upward or outward normal to the paper)
b. Force on `AC`
Force on circular portions of the circuit i.e. `AC` etc, due to the wire at the centre will be zero because magnetic field due to the central wire at these arcs will be tangential `(theta=180^@)`.
force on `CD`
Current in central wire is also is `i=10A`. Magnetic field at distance `x` due to central wire
`B=mu_0/(2pi).i/x`
`:.` Magnetic force on element `dx` due to this magnetic field
`dF=(i)(mu_0)/(2pi).(i/x).dx=((mu)/(2pi))i^2(dx)/x` `[F=ilBsin90^@]`
Therefore net force on `CD` is
`F=int_(x=r_1)^(x=r_2) dF=(m_0i^2)/(2pi) int_(0.08)^(0.12) (dx)/xmu_0/(2pi)i^21n(3/2)`
substituting the values `F=(2xx10^-7)(10)^2ln(1.5)`
or `F=8.1xx10^-6N ("inwards")`
force on wire at the centre
Net magnetic field at the centre due to the circuit is in vertical direction and current in the wire in centre is also in vertical direction. Therefore, net force on the wire at the centre will be zero. `(theta=180^0)`. Hence
i. force acting on the wire at the centre is zero.
ii. force on arc `AC=0`
Force on segment `CD` is `8.1xx10^-6N("inwards")`