Episode 405: Preparation for electric fields topic

There are some tricky experiments in this topic, and it is well worth practising them in advance.

Some equipment to look out for:

• A van de Graaff generator is a most useful accessory. Learn to operate it without giving yourself (or your students) electric shocks
• The flame probe is a useful device for exploring potential in electric fields, but you need to know how to set it up, calibrate it and use it, refer to the flame probe construction notes found on:

  Field strength and potential energy

Try to establish in your mind the similarities and differences between the three types of field that you will be teaching about: electric, magnetic and gravitational. Watch out for confusions among your students. One common problem that can occur is confusing electric charge with magnetic poles. This more normally happens when students are studying magnetic fields, when they talk about magnets being charged up, or the field emanating from the positive end of a magnet, for example. It is best when studying electric fields to be aware of this confusion, and any time it crops up (rare in this part of the course) to try to nip it in the bud as soon as possible.

Main aims of this topic

Electrical fields

Students will:

• Understand the concept of an electric field and explain how a field might arise
• Interpret diagrams showing field lines and equipotentials
• Understand the meaning of the term field strength
• Understand and use Coulomb’s law, and the meaning of the term permittivity
• Understand and use uniform field equations
• Understand the difference between the terms energy and potential, and relate potential to field strength
• Draw parallels between electric and gravitational fields

Prior knowledge

Your students should have covered the following concepts:

• Basic definition of a field
• Law of force between charges
• Charging by friction and induction
• Work
• Energy in kinetic stores
• Newton’s laws of motion

We have assumed that students have already studied gravitational fields. Hence, they should have covered the following concepts:

• Inverse-square field
• Field lines
• Equipotentials
• Gravitational field-strength
• Energy in gravitational stores and potential
• Zero of potential at infinity

Where this leads

These sections will consolidate the work on fields that was started with gravitational fields. With the occurrence of repulsions, students will learn to think carefully about the sign of an energy change and what this means, linking into the core ideas of change of energy and field strength.

For those who go on to study beyond post-16 level, the firm grounding in field ideas they’ve had here, coupled with those from magnetic fields, will lead them into one of the great pillars of classical physics – electromagnetism. Indeed difficulties in electromagnetism led the way for relativity and the more modern field theories of today.