Home PHYSICS TOPIC 2: ELECTROMAGNETISM | PHYSICS FORM 4

TOPIC 2: ELECTROMAGNETISM | PHYSICS FORM 4

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Magnetic Fields due to a Current-carrying Conductor
How Electric Current Produce a Magnetic Field
Explain how electric current produces a magnetic field
Electromagnetism is the effect produced by the interaction of an electric current with a magnetic field. The interaction can result in a force causing the conductor carrying the current.
If, on the other hand, a force is applied to a conductor (with no current) in a magnetic field the resulting movement can result in a current being noticed in the conductor.
When the switch is closed an electric current flows through the conductor.
The electric current generates magnetic field around the conductor. This will cause a deflection on the compass needle. The magnetic field around a current-carrying conductor can be shown by means of magnetic field lines.
The Pattern of the Magnetic Field Lines around a Straight Conductor
Identify the pattern of the magnetic field lines around a straight conductor
The
magnetic field pattern is usually given in a plan view. In the plan
view, the conductor is represented by a circle. A dot in circle shows
that the current is coming out of the plane. A cross the circle shows
that the current is moving into the plane.
The
strength of the magnetic field on the magnitude of the electric
current. The higher the current, the stronger the magnetic field, and
therefore the greater the deflection. The strength of the magnetic field
decreases as you move further from the conductor. There will be less
deflection as the compass is drawn from the current-carrying conductor.
The Direction of Magnetic Field around a Current-Carrying Conductor
Determine the direction of magnetic field around a current-carrying conductor
The direction of the field is determined by applying two rules, these are:
  1. Right-hand Grip Rule
  2. Maxwell’s cork screw rule
Right-hand Grip Rule
The
Right-hand Grip Rule can be applied to a straight conductor or a
solenoid-carrying an electric current. For a straight conductor, the
Right-hand Grip Rule can be stated as:
“Imagine
the wire carrying the current is gripped by the right hand with the
thumb pointing in the direction of the conventional current (from
positive to negative), the fingers will curl around the wire pointing in
the direction of the magnetic field.”
For a solenoid, the Right-hand Grip Rule states that:
“When
you wrap your right hand around a solenoid with your fingers pointing
in the direction of convectional current, your thumb point in the
direction of the magnetic North pole.”
A solenoid is a long coil containing a large number of close turns of illustrated copper wire.
Maxwell’s –Right –hand screw rule states that:
“If
a right-hand screw advances in the direction of the current, then the
direction of rotation of the screw represents the direction of the
magnetic field due to the current.”
The Presence and Direction of a Force on a Current carrying Conductor in a Magnetic Field
Determine the presence and direction of a force on a current carrying Conductor in a magnetic field
The
direction of the force on a current-carrying conductor in a magnetic
field can be determined using Fleming’s Left –Hand Rule.
Fleming’s Left –Hand Rule states that:
“If
you hold the index finger, the middle finger and the thumb of your left
hand mutually perpendicular to each other so that the index finger
points in the direction of the magnetic field and the middle finger
points in the direction of current in the conductor, then the thumb will
point in the direction of the force acting on the conductor.”
The Direction of Force due to two Current carrying Conductors when the Current Flowing in the Same or Opposite Direction
Determine
the direction of force due to two current ’82air-carrying conductors
when the current flowing in the same or opposite direction
If
two current-carrying conductors are placed side by side close to one
another, the currents in the conductors will interact with the magnetic
fields produced by the two conductors. A force may result depending on
the direction of the two currents.
When
the currents are flowing in opposite directions, the conductors repel
one another. When the currents are flowing in the same direction, the
conductors attract one another, the conductors attract each other.
When
the currents flow in the same direction, the magnetic field between the
conductors cancel out, thus reducing the net field. However, on the
outside, the magnetic fields add up, thus increasing the net field.
Therefore, the magnetic field is weaker between the conductors that on
the outside. The resultant force pushes the conductor towards each
other.
When
the currents are in the opposite directions, the fields between the
conductors add up, while they cancel out on the outside. The field
between them is stronger than on the outside. The resultant force is
toward the outside of each conductor, hence repulsion.

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