# Lesson 27: Electric fields and Coulombs law

#### Aims

• Observe that, when rubbed, some materials experience a force that acts without the bodies touching and that this force can be attractive or repulsive depending on what materials are used.
• Understand how we can explain this force in terms of positive and negative charge.
• Use Coulombs law to calculate the force between two point charges.
• Define field strength as force per unit charge.
• Draw field lines for radial and uniform fields.
• Add the field strengths due to two or more bodies.

#### TOK

• Isn't it convenient that matter is made of + and - charge that add and cancel just like numbers. If there had been a 3rd type * and you had to have a +, - and a * to produce a neutral body then we couldn't use numbers to represent them. Is our number system the same as the charges by chance or is there something more deep rooted? Did we come up with the model of charge because our number system works like that or does our number system work like it does because our brains are made of two charges?
• The similarity between our model of electric and gravitational fields is worth mentioning.
• Could there be two masses in the same way as there are two charges?
• Why was field defined in terms of +ve charge?

#### Notes

If students have grasped the idea of gravitational fields then electric fields can be dealt with quite quickly. I always start by showing why scientists started to define electrical quantities with some simple electrostatics experiments. The sweater and balloon simulation from Phet is good here as it not only works every time but enables you to see the different charges. Without going into potential its difficult to explain how most of the classic electrostatics experiments work so it best to keep it simple.

#### Test

Electric fields

Electric fields mc test

Page 195 - 199

Problems 20-21

Attractive
Repulsive
Electrostatic

#### Resources

Static and Balloons

This is a great way of starting this part of the syllabus. You can show how the balloons attract to the sweater but repel from each other. if you put one balloon onto the sweater then the other balloon does not experience any force showing how charges cancel. I always ask my students "what has two types that add, subtract and cancel each other out?". The answer is numbers but they never get it.

Electric Fields

This applet can be used to map field lines for different arrangements of charge (circles, parallel plates and arrangement of points. The "show field lines" option doesn't show the field lines as we would like them to be draw, more line a series of arrows than continuous lines.

3D Field

Although difficult to see in the thumbnail this applet by Paul Falstad shows how the fields would look in 3D. You can also show how particles would accelerate if placed in the field. You can rotate the 3D view with your mouse (hand on SMART board) if you give it a flick it starts to rotate on its own. A cool trick on the board.

Electromagic