# A2 Physics-Electric and Magnetic Fields

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Unit 4-Physics On The Move
Topic 2-Electric and Magnetic Fields
Explain what is meant by an electric field and recognise and use
F
the expression for electric field strength E= Q
Charges and fields
Electric charge is one of the fundamental properties of nature. Two objects of the same charge will
push each other apart (repel) while objects of opposite charge pull each other together (attract).
Surrounding every charged object is an electric field-a region of space in which another charged
object will experience a force. The electric field is characterised by strength and direction. The
strength of the field at a point in space determines the size of the force that a charge will experience
if it is placed at that point. Electric field strength is a vector quantity. The direction of the field at a
point is defined to be the direction of the force acting on a small positive charge placed at that point.
When a field-line diagram is drawn to illustrate the nature of an electrical field, the arrows on the
field lines are always drawn to show the direction of the force on a positive charge.
The electric field strength, E, at a point is the force, F, acting per unit charge, q, at that point : E= F
Q
Draw and interpret diagrams using lines of force to describe
radial and uniform electric fields qualitatively
The field of a point charge
The simplest electric field belongs to a single point charge (that is, a charge that is very small
compared with any distance that we may be measuring), like an electron
The strength of the field is proportional to the size of the charge
It varies with distance from the charge
As the charge only occupies a point in space, the field must look the same along any line that is drawn
radially from that point. The field lines spread out as they get further from the charge, which
indicates that the field is getting weaker.
The field of one more than one charge
The total force exerted on a charge by a collection of other charges can be calculated by adding up
each of the `one-on-one' forces. This is because electric fields obey the principle of superposition.
The field at any point due to a collection of charges is the sum of the separate fields due to each
charge. In calculating the size of the field, or the force, one must remember that they are vector
quantities.

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Q1Q2 1
Use the expression F= r² where k= 4 and derive and use the
kQ
expression E= r² for the electric field due to a point charge
kQ1Q2
The electrostatic force between two charges spherical objects obeys an inverse square law: F= r²
, where Q and Q are the two charges, r is the separation of the centres of the two charged objects,
and k is a constant dependent upon the medium between the two charges.…read more

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Capacitance
Any arrangement of two conductors isolated from one another by an insulator will form a capacitor.
In the lab this may be two sheets of metal with small pieces of plastic between. In this case air forms
the insulator. A commercial capacitor can be made by using sheets of wax paper to separate two
sheets of thin metal foil. Very large areas of foil can be rolled up and contained in small cylinders.…read more

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Surprisingly, the uncharged capacitor does not resist the current arriving at one plate or
leaving the other. On a graph of capacitor charge against time, the initial current is the gradient of the
graph at the origin. As charge build up on the plates, it repels more charge than is arriving and the
current drops as the charge on the plates increases. Charging will stop when the p.d. between the
capacitor plates is equal to the e.m.f. of the cell.…read more

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Lines representing the magnetic field in a given region are called `lines of magnetic flux'. The number
of lines passing through a unit area perpendicular to the field represents the flux density, B, and is a
measure of the magnetic field strength.
The e.m.f. induced in a circuit can be calculated from Faraday's law of induction: e.m.f.= - rate of

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This is also the direction in which the magnetic force would act on a `free north pole' if
monopoles existed
In the case of the field round the long wire, the sense is determined by the direction of the
current in the wire
Force between two current-carrying wires
If the currents in two parallel wires are in the same direction, the force is attractive. If the currents
are in opposite directions, the force is repulsive.…read more

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The size of the torque on the coil can be
increased by
Increasing the area of the coil
Increasing the current in the coil
Using more turns of wire in the coil
Using a stronger magnetic field
Making the magnetic field radial rather than uniform
The electric motor
Means of improving an electric motor:
Using curved end pieces to the permanent magnet, helping make the field radial
Using a cylindrical soft iron armature (rotating part) to increase field strength, and help make

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° to the magnetic field lines, then the angle between the path and the field lines must also
be taken into account.
Magnetically induced electric currents
The magnetic force on a moving charge can be used as a means of generating an electrical current.
Any piece of conducting material contains charges, and simply moving the material through a
magnetic field will result in forces being exerted on those charges.…read more

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Very high-frequency transformers use cores
made from non-conducting but magnetic materials, such as ferrites. These materials are similar to
ceramics, and so can be quite brittle, and are far less easily magnetised than soft iron.
d(N )
Investigate, recognise and use the expression = - dt and explain
how it is a consequence of Faraday's and Lenz's laws
Lenz's law
The induced current is always in a direction that will help to counteract the change in flux that is