1. Book Reference : Pages 76-78 1.To examine the concept of Electric field strength (two ways) 2.To examine the factors which affect the nature of an electric field

2. Previously we saw that a field line in an electric field is the path a small positive charge would take if it were free to move. If this charge is moved by the field then it must be experiencing a force The electric field strength, E at a point in an electric field is defined as the force per unit charge on a positive test charge placed at that point

3. E = F / Q Where E is the electric field strength (N/C * ), F is the Force experienced(N) and Q is the charge in Coulombs (C) Note that E is a vector quantity in the identical direction to F and is the same as the direction of the field line for a positive charge (opposite direction for a negative charge) * Other units to follow

4. Under normal conditions air is a good insulator. However, if the electric field strength is high enough then the air becomes ionised (electrons ripped from the molecules). During a thunderstorm this happens suddenly and lightning strikes

5. + + + + + + + - - - - - - - +QF d V +- Consider the force F experienced by a small charge +Q in the uniform electric field between a pair plates separated by d metres which have a potential difference (Voltage V) between them. Note the field lines are : Parallel to each other, perpendicular to the plates and go from the +ve plate to the –ve plate

6. By rearranging our field strength equation (E=F/Q) we can show that the force on Q is: F = QE If this charge is moved from one plate to another, a distance of d metres by the force F then work is done (W = f x d) W = QE x d

7. Definition : The potential difference V between two points is the work done per unit charge when a small charge is moved through the potential difference V = W/Q  V = W/Q = QEd/Q = Ed (rearrange) E = V/d [Note : alternative units for E: V/m)

8. Any object with a charge will have an electric field. As the charge increases, so does the strength of the electric field The more concentrated the charge the greater the field strength So far for parallel plates we’ve seen the charge spread evenly across parallel plates

9. + + + + + + + V +- - - - - - With a V shaped plate the field lines are concentrated at the tip since this is where the charge is most concentrated

10. + + + + + + + - - - - - - - V +- For a uniform field between parallel plates the charge is spread evenly over the surface Experimentally it is possible to show that the electric field strength is proportional to the charge per unit area on the surface

11. E  Q/A Like all proportionalities we can turn this into an equation by introducing a suitable constant of proportionality....  0 E = Q/A  0 is called “epsilon nought” & has the value 8.85 x10 -12 Farads/Metre (F/m)

12. We’ll cover this later.... (Use of  0 is required for A2 but further knowledge is not) A Farad is the unit of capacitance and is 1 Coulomb per Volt “Epsilon nought” is the permittivity of free space and is the charge per unit volume on a surface in a vacuum which results in a field strength of 1 volt per metre between the plates