# Electric field strength at a point between charges

Find the electric field strength at point B between two charges shown below:  Given/Known Values
q1 = 4.0×10-6 C
r1 = 40 cm = 0.4 m (Distance from q1 to point B)
q2 = -1.0×10-6 C
r2 = 30 cm = 0.3 m (Distance from q2 to point B)
k = 9.0×109 Nm2/C2

Equations
Electric Force:
FE = (k⋅q1⋅q2)/r2
FE = q⋅E​

Electric Field:
E = (k⋅q)/r2

My thought process was that I could find the electric field strengths of both charges, and then find the difference between them, since point B is between the two charges.

E1 = (k⋅q1)/r12
= (9.0×109⋅4.0×10-6)/0.42
= 225000 N/C

E2 = (k⋅q2)/r22
= (9.0×109⋅-1.0×10-6)/0.32
= -100000 N/C

ΔE = EB = E1 - E2
= 225000 - (-100000)
= 325000 N/C

I'm not really sure if this is the correct way of finding the electric field strength of point B. Any help would be appreciated! #### Attachments

• 18.4 KB Views: 58

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Find the electric field strength at point B between two charges shown below:  Given/Known Values
q1 = 4.0×10-6 C
r1 = 40 cm = 0.4 m (Distance from q1 to point B)
q2 = -1.0×10-6 C
r2 = 30 cm = 0.3 m (Distance from q2 to point B)
k = 9.0×109 Nm2/C2

Equations
Electric Force:
FE = (k⋅q1⋅q2)/r2
FE = q⋅E​
Electric Field:
E = (k⋅q)/r2

My thought process was that I could find the electric field strengths of both charges, and then find the difference between them, since point B is between the two charges.

E1 = (k⋅q1)/r12
= (9.0×109⋅4.0×10-6)/0.42
= 225000 N/C

E2 = (k⋅q2)/r22
= (9.0×109⋅-1.0×10-6)/0.32
= -100000 N/C

ΔE = EB = E1 - E2
= 225000 - (-100000)
= 325000 N/C

I'm not really sure if this is the correct way of finding the electric field strength of point B. Any help would be appreciated! I get the same numerical answer. To complete the problem, you need to specify the direction of the $\vec E$ vector.

#### Attachments

• 18.4 KB Views: 317
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