D.3.1 Magnetic fields
Magnetic and Electric Fields
Electric field | Magnetic field | |
Symbol | E | B |
Caused by | Charges | Magnets (or electric currents) |
Affects | Charges | Magnets (or electric currents) |
Two types of | Charge: Positive and negative | Pole: North and South |
Simple force rule | Like charges repel, unlike charges attract | Like poles repel, unlike poles attract |
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In order to visualize a magnetic field, we should once again use the concept of field lines
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This time the field lines are lines of magnetic field- also called flux lines
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If a test magnetic north pole is placed in a magnetic field, it will feel a force
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The direction of the force is shown by the direction of the field lines
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The strength of the force is shown by how close the lines are to one another
D.3.1-1 Magnetic Field around magnets
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An electric current can also cause a magnetic field
D.3.1-2 Magnetic Field around right hand grip rule
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The field lines are circular around the current
D.3.1-3 Magnetic Field around the circular magnet
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The direction of the field lines can be remembered with the right-hand grip rule as shown in the figure above
D.3.2 Magnetic force on a current
Flemming’s Left Hand Rule
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When a current carrying wire is placed in a magnetic field the magnetic interaction between the two results in a force
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This is known as motor effect
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The direction of this force is at right angles to the plane that contains the field and the current
D.3.2-1 Diagram of Magnetic field with FLH Rule effect
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The direction of force, field and current could be determined by using Fleming’s left-hand rule
D.3.2-2 Diagram of FLH Rule
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The experiments has show that the force is proportional to the :
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The magnitude of the magnetic field B
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The magnitude of the current I
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The length of the current L that is in the magnetic field
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The sine of the angle, theta, between the field and current
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The magnetic field strength, B is defined as follows :
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Where the unit of B is the tesla
D.3.3 Magnetic force on a moving charge
Charge and Magnetic Field
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A single charge moving through a magnetic field also feels a force in exactly the same way that a current feels a force
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In this case the force on a moving charge is proportional to :
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The magnitude of the magnetic field, B
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The magnitude of the charge, q
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The velocity of the charge, v
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The sine of the angle, theta, between the velocity of the charge and the field
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Using these information and relationship, we can give an alternative definition of the magnetic field strength B
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Which is mathematically :