Electric Fields

Electric Field and Field Lines

By Rachel Wampler

Observing Electric Fields

• You can use a Test Charge to determine how the surrounding charges generate a force.

• A test charge has a small charge so that it doesn't change the surrounding charge orientation. ​

Multiple Choice

A test charge (q₀ = 1.0 x 10^-10 C) experiences a force of 2 x 10^-9 N when placed near a charged sphere. Determine the force per Coulomb that the charge experiences and predict the force on a 2 C charge.

• $$20\ \frac{N}{C}\ ,\ 40\ N$$  

• $$40\ \frac{N}{C}\ ,\ 20\ N$$  

• $$0.05\ \frac{N}{C}\ ,\ 0.1\ N$$  

• $$0.1\ \frac{N}{C}\ ,\ 0.05\ N$$  

• Electric field is defined as the electric force per unit charge.

  • ​Force per charge

• ​Vector

  • Same direction as the force on positive test charge​

  • REMEMBER to add them as vectors!

• Units: N/C​

Electric Field Definition

Equation Derivation

• Notice ​how the q₀ does not affect the electric field

Multiple Choice

There is a point charge of q = 2 x 10^-8 C. Determine the electric field at 0.50 m away using a test charge of 1 x 10^-10 C.

• 3.60 N/C

• 359.6 N/C

• 1.39 x 10^-3 N/C

• 719.2 N/C

Multiple Choice

There are two point charges of q₁ = 4C and q₂ = 8C and thet are 10 m apart. Find the point where E = 0 between them.

• 4.82 m

  from q₂ towards q₁

• 5.85 m

  from q₂ towards q₁

• 35.39 m

  from q₂ towards q₁

• 28.2 m

  from q₂ towards q₁

Field Lines

• Field lines map out the direction and strength of electric charges in space.

• Rules: ​

  • Lines begin at Positive charges only

  • Lines end at Negative charges only

  • The number of lines entering or leaving the charge is proportional to the size of the charge

  • Lines never cross each other

  • Lines leave surfaces at 90 degress​

​

Electric Field Lines Examples

​Positive Point Charge

​Negative Point Charge

Attractive Forces

Repulsion Forces

Multiple Select

What's wrong with this electric field diagram? Select all that apply.

• The four lines from +4q to -q are curved

• Lines on the left are straight

• Arrows go from positive to negative

• Lines cross at point P

Multiple Select

What's wrong with this field diagram? Select all that apply.

• All field lines are curved

• Number of lines are proportional to charges

• Charge arrows go from positive to negative

• Number of lines are not proportional to charges