Electric fields

What is an electric field?

The region around the electric charge in which an electric force acts.

Define an Electric Field Strength in words

The force experienced per unit positive charge

Define an Electric Field Strength as an equation

E = F / Q

What are the units of electric field strength?

• NC^-1

• Newtons per Coulomb

Is electric field strength a scalar or a vector?

Vector

How is electric field strength different to gravitational field strength?

Electric field strength is force per unit charge, whereas gravitational field strength is force per unit mass

Draw a diagram for a uniform electric field

Draw a diagram for a spherical field around a positive charge

What are the rules for drawing Electric field patterns?

• Each field line has an arrow, which shows the direction of the field.

• Electric field lines are always at right angles to the surface of a conductor.

• Equally spaced, parallel electric field lines represent a uniform field - one in which the electric field strength is the same everywhere.

• Closer electric field lines represent greater electric field strength.

Draw a diagram of the field lines between a positive and a negative charge

Draw a diagram of the field lines between two positive charges

Should be symmetrical but same general shape as image

Draw a diagram of the field lines between two negative charges

to be completed

What is the force between charges dependent on?

• The size of the two charges, Q and q. F∝Q, F∝q

• How close together the charges are, r. F ∝ 1/r²

• Combining these gives F∝Qq/r²

State Coulomb’s Law in words

Any two point charges exert an electrostatic (electrical) force on each other that is directly proportional to the product of their charges and inversely proportional to the square of their separation

What is the equation for Coulomb’s Law?

• F = Qq / (4πε₀r²)

• Q and q are the magnitude of the charges

• ε₀ is the permitivity of free space (a constant)

• r is the seperation between the charges

What is the constant of proportionality equal to in Coulomb’s Law?

1 / 4πε₀

How do you derive the equation for radial electric field strength from Coulomb’s law and E=F/Q?

E = F / q and F = Qq / (4πε₀r²)

E = Qq / (4πε₀r²) × 1/Q = q / (4πε₀r²)

What are the similarities between gravitational and electric fields?

• Both gravitational and electric field strength defined as force per unit…

• F∝Mm for gravitational field, F∝Qq for electric field

• F∝1/r² for both

What are the differences between gravitational and electric fields?

• A graviational fields is always attractive, but a electric field can be attractive or repulsive

• Mass creates a graviational field, whereas charge creates an electric field

What is the work done moving an electron across a potential difference V?

W = Vq

What is the force on an electron in a uniform electric field strength of E?

F = E / e

What is the equation for the Electric field strength between 2 plates at a potential difference V and a distance d apart?

V = Ve / d

Sketch a graph of how the force varies with distance from a point charge.

What does the area under the force-distance graph of an electric field represent?

The work done

What is the equation for Electric Potential Energy?

Energy = Vq = Qq / (4πε₀r)

Define electric potential in words

• The work done by an external force per unit positive charge to bring a charge from infinity to a point in an electric field

• Unit volt or JC^-1

What is the equation for electric potential?

V = Q / 4πε₀r

How is Electric Potential Difference defined?

The work done by an external force per unit positive charge to move a charge between two points in an electric field