Page 372 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
Living in a field
The scientist in the photograph (Figure 23.1) is using
a detector to measure the electric field produced by
a mobile phone mast. People often worry that the electric field produced by a mobile phone transmitter may be harmful, but detailed studies have yet to show any evidence for this. If you hold a mobile phone close to your ear, the field strength will be far greater than that produced by a nearby mast.
Figure 23.1 Mobile phone masts produce weak electric fields – this scientist is using a small antenna to detect and measure the field of a nearby mast to ensure that it is within safe limits.
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Any two point charges exert an electrical force on each other that is proportional to the product of their charges and inversely proportional to the square of the distance between them.
Electric fields
In Chapter 8, we presented some fundamental ideas about electric fields:
■■ An electric field is a field of force and can be represented by field lines.
■■ The electric field strength at a point is the force per unit positive charge that acts on a stationary charge:
field strength = force E = F
■■ There is a uniform field between charged parallel plates: field strength = potential difference E = V separation d
In this chapter, we will extend these ideas to consider how electric fields arise from electric charges. We will also compare electric fields with gravitational fields (Chapter 18).
Coulomb’s law
Any electrically charged object produces an electric field in the space around it. It could be something as small as an electron or a proton, or as large as a planet or star. To say that it produces an electric field means that it will exert a force on any other charged object which is in the field. How can we determine the size of such a force?
The answer to this was first discovered by Charles Coulomb, a French physicist. He realised that it was important to think in terms of point charges; that is,
electrical charges which are infinitesimally small so that we need not worry about their shapes. In 1785 Coulomb proposed a law that describes the force that one charged particle exerts on another. This law is remarkably similar in form to Newton’s law of gravitation (page 273).
A statement of Coulomb’s law is as follows:
We consider two point charges Q1 and Q2 separated by a distance r (Figure 23.2). The force each charge exerts on the other is F. According to Newton’s third law of motion, the point charges interact with each other and therefore exert equal but opposite forces on each other.
charge
Q
F Q1
Figure 23.2 The variables involved in Coulomb’s law.
Q2 F
r