# Physique

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Fields, Particles and Frontiers of Physics
Module 1: Electric and magnetic fields
• 5.1.1 Electric fields • 5.1.2 Magnetic fields • 5.1.3 Electromagnetism

Module 2: Capacitors and exponential decay
• 5.2.1 Capacitors

Module 3: Nuclear physics • 5.3.1 The nuclear atom

• 5.3.2 Fundamental particles • 5.3.3 Radioactivity • 5.3.4Nuclear fission and fusion

Module 4: Medical imaging

• 5.4.1 X-rays • 5.4.2 Diagnostic methods in medicine • 5.4.3 Ultrasound

Module 5: Modelling the universe

• 5.5.1 Structure of the universe • 5.5.2 The evolution of the universe

5.1.1 Electric Fields
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a) State that electric field are created by electric charges
All fields are regions where a force is exerted on an object at a distance Electric fields are created by an electric charge and so exerts a force on any charged object within the field ���� =

b) Define electric field strength as force per unit positive charge

���� ����

; where E is the electric fieldstrength, F the force and Q the positive charge

Note: the word positive had been used to indicate the direction of the field is in the direction in which a force acts on a positive charge, the field is away from a positive charge and towards a negative charge

c) Describe how electric field lines represent an electric field
• • • An electric field always starts on a positive charge and finisheson a negative charge The stronger the field, the higher the line density When a charge is stationary on a surface, the field must be at a right-angle to the surface, otherwise it would not be stationary

d) Select and use Coulomb's law in the form:

F = Force between electric charges q and Q Q and q = Two charges -12 2 -1 ε = Permittivity of free space = 8.85 x 10 C N r = Distance betweenthem

���� =

���������������� ��������

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e) Select and apply ����
• • • ���� =
���� ����

The force between two electric charges q and Q is proportional to the product of q and Q and inversely proportional to the square of the distance r between them

f) Select and use: ���� = parallel plates
• •

���� = ��������, so by equating with Coulomb's Law, ���� ���� = ���� ,so
���������������� ����

=
���� ����

���������������� ��������

����

for the electric field strength of a point charge

for the magnitude of the uniform electric field strength between charged

���� = ���� =

Consider two parallel plates a distance d apart with a potential difference V across them, with a charge q moving between them. ���� As potential difference V ischarge and work done on the charge is Fd, As ���� =
���� ���� �������� ���� ���� ����

OR ,

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=

���� ����

,

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