1. A –Level Physics: Electric Fields

2. Objectives:

3. Starter Activity
FLASHBACK: [8 marks] explain how you could work out resistivity of a wire

4. FLASHBACK: How does an x-ray machine function?
Electric Fields
Many machines function through the use of fast moving charged particles. They rely on an electric field which would cause any charged particle to accelerate
A charged particle will feel a force when it enters an electric field. All field line directions show the journey a positive particle would take (i.e. + to -).
The closer the lines are, the stronger the field.
The Force that a particle feels is the electric field strength (E) multiplied by the charge in coulombs
F=𝐸 ×𝑄
So what would the equation for acceleration be?
A=EQ/m (as F=ma)
FLASHBACK: How does an x-ray machine function?

5. Practice
What force will an electron feel when it is in an X-ray machine’s electric field which has a strength of 4.5x105Vm-1?
NB: CHARGE of an electron= 1.6x10-19C
How fast will the electron be travelling if in this field it uniformly accelerates from rest and is at a distance of 10cm.
Hint: The distance can be considered as the ‘displacement’…
See pg 34/35

6. Electric Potential E= 𝑉 𝑑
The kinetic energy that particles gain is provided by the particle’s location within the electric field. Every location is equivalent to a specific potential energy for each unit of charge. This is…electric potential!
The electric potential is equal to the potential difference through which the particle moves multiplied by the charge on the particle
E𝑝=𝑉 ×𝑄
The strength of the electric field is a measure of how quickly it changes potential over a distance (d):
E= 𝑉 𝑑
Just like with gravitational potential, equipotentials exist in an electric field. These equipotentials are drawn perpendicular to the field lines.
The units are NC-1 and Vm-1 for E (the first as E=F/Q)
An x-ray machine has a potential difference of 45kV between the two electrodes which are 10cm apart. What is the electrical field strength between them? How about if they were moved an extra 15cm away from one another?

7. Independent Research
Utilise the worksheet provided to first research Millikan’s oil drop experiment and then complete the following:
Draw a diagram of the set up (no…you can’t just cut it out)
Number annotate the diagram to explain the process
Explain how he discovered (in equation terms) the charge on each droplet

8. Extension
Complete the questions on the board and then if complete, prepare notes on radial electric fields
(see page 97 in revision guide or L3 in SOW).