mo

1
mo I1 / 2 p a
mo I2 / 2 p a
mo
mo I1I2 / 2 p a
mo I1I2 / 2 p a
They are action-reaction pair
2
Definition of the ampere (unit of current)
The ampere is the constant current which, flowing
in two infinite long, straight, parallel
conductors of negligible cross sectional area,
placed in vacuum 1 m apart, produces between them
a force of 2 x 10-7 N per meter.
Then, followed from the above definition mo 4 p
x 10-7 NA-1
3
Electrons inside a metal wire may probably flow
ONLY close to the centre region of the wire as
moving electrons attracts each other.
4
Absolute Measurement of current I
(1) I I1 I2
(2) F mg
a
F (mo I2 / 2 p a) L
L
I (2 p m g a / mo L)1/2
5
By means of an electronicbalance
L
a
(1) I I1 I2
(2) F mg (by balance)
F (mo I2 / 2 p a) L
I (2 p m g a / mo L)1/2
6
Example
0.1 m
(a) Bx mo Ix / 2 p a 4 x 10-6 T
(b) Force by X on Y Bx Iy L
2A
0.05 m
3A
(4 x 10-6) (3) (0.05) 6 x 10-7 N
X
Y