Electric Fields and
Potential
Chapter 33
Electric Field Lines
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Electric fields have
both magnitude
and direction –
they are vectors
The direction of the
field lines is in the
direction of force
on a small positive
test charge.
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Electric Fields
What are the fields around the following objects?
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Electric Field Strength
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The electric field strength vector E at
any point in an electric field is
defined as the force per unit positive
(test) charge placed at that point in
the field
E=F/q
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SI Units : NC-1
Electric Shielding
Jeep Cherokee that was struck by lightning.
Electric Shielding
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If there is an electric field inside a
conductor then the free electrons
would distribute themselves to
cancel the field
There is no electric field in a
conductor – the charge is distributed
around the outside
Electric Potential Energy
To pull an electron away from an
atom requires energy. It has electric
potential energy.
 SI Units - Joules
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Electric Potential
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A measure of how much energy each
charge has.
Electric potential = electric potential energy
charge
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SI Unit = Joules/Coulomb = volts (V)
Allesandro Volta (1745-1827)
Electric Potential
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How much work is required to move an electron across an
electric potential of 100 volts?
Notice how small a number this is. When dealing with subatomic particles we normally use a much smaller unit of
energy.
One electron volt (1 eV) is the work done when an electron
moves through a potential difference of one volt.
1 eV = 1.6x10-19 J
Electric Potential Questions
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Explain why passengers in a metallic aeroplane are not affected by the
electric charge that builds up on its outside due to friction between air and
the metal
Does the Coulombic force exerted by one charge on another change if
other charges are brought into their vicinity? Explain your answer.
A proton is placed in the uniform field between two parallel oppositely
charged plates. It is allowed to move and so accelerates towards the
negative plate.
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Is the force on it constant during its movement?
Does the attraction force on the proton increase as it nears the negative plate?
Describe the energy changes that occur during the movement of the proton.
What experiences the equal and opposite force as required by Newton’s third law?
Calculate the force acting on an electron in a hair as it comes within 1mm
of a comb carry a static charge of 5.0x10-5C.
How far apart in a vacuum must two charges of 1C be placed in order to
C
produce a force of 1000N between them?
Three spheres, each carrying a charge of +1.00x10-11C are placed at the 10cm
corners of a right-angled isosceles triangle, with equal sides of 10.0cm, as
in the diagram. Find the resultant force acting on B.
A
10cm
B