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WBJEE 2019 Physics Solved Paper
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© examsnet.com
Question : 19
Total: 40
Eleven equal point charges, all of them having a charge +Q, are placed at all the hour positions of a circular clock of radius r, except at the 10 hour position. What is the electric field strength at the centre of the clock ?
Q
4
π
ε
0
r
2
from the centre towards the mark 10
Q
4
π
ε
0
r
2
from the mark 10 towards the centre
Q
4
π
ε
0
r
2
from the centre towards the mark 6
Zero
Validate
Solution:
The clock diagram is as shown below
In the above diagram, charge
+
Q
is not placed at
10
h
position.
So, net electric field strength at centre
C
,
E
net
=
E
1
+
E
2
+
E
3
+
E
4
+
E
5
+
E
6
+
E
7
+
E
8
+
E
9
+
E
11
+
E
12
⇒
E
net
=
E
1
+
E
2
+
E
3
+
E
4
+
E
5
+
E
6
+
(
−
E
1
−
E
2
−
E
3
−
E
5
−
E
6
)
⇒
E
net
=
E
4
=
Q
4
π
ε
0
r
2
,
from centre towards the mark 10.
© examsnet.com
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