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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
• •



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

���� =

���������������� ��������


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 ,

���� ����


���� ����



= the potential gradient

g) Explain the effect of a uniform electric field on the uniform electric field strength between charged parallel plates
• • • • A particle with charge q moves a distance d through a field with electric field strength E. The forceacting on the particle is Eq, therefore the work done is equal to Eqd. In a vacuum, its kinetic energy will rise by Eqd. Using other information, such as mass, it would be possible to find its acceleration, its velocity after travelling a particular distance and the time to travel that distance. Using the fact that in a field acting downwards, the horizontal component of a charge's velocity willnot change. As the force acting downwards is also constant, we can find the acceleration downwards and therefore the downwards velocity. Using both these components, the angle of deflection of the charge as it passes through the field can be found.

h) Describe the similarities and differences between the gravitational fields of point masses and the electric fields of point charges.