Chapter 20
Magnetic effects of current
Magnetic field around wires
When an electric current passes along a wire, a magnetic field is set up around the wire. These
are at right angles to the direction in which the electric current flows. The field is strongest near
the wire. The direction of the field is reversed if the direction of the current is reversed.
You can use your right-hand to determine the direction of the magnetic field with respect to
the direction of the current. If current is upwards, the magnetic field runs in anti-clockwise
direction and if the current is downwards, the field is in clockwise direction.
Right hand grip rule
Imagine gripping the wire with your right hand so that your thumb points in the direction of
current and the curve of your fingers shows the shape and direction of the field lines.
Magnetic field around a solenoid
A solenoid is a long coil of wire. When current is passed through a solenoid, a magnetic field
develops in and around the solenoid. Reversing the direction of the current reverses the
current of the magnetic field.
Outside the solenoid, the magnetic field pattern is that of a bar magnet. Inside the solenoid, the
magnetic field is uniform because the field has the same strength except at the ends where the
field gets weaker.
To work out the polarity of each end of the solenoid it needs to be viewed from
the end
o If the current is travelling around in a clockwise direction, then it is the south
pole
o If the current is travelling around in an anticlockwise direction, then it is the
north pole
If the current changes direction, then the north and south poles will be reversed
If there is no current flowing through the wire then there will be no magnetic
field produced around or through the solenoid
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A solenoid can be used as an electromagnet by adding a soft iron core
The iron core will become an induced magnet when current is flowing through the coils
The magnetic field produced from the solenoid and the iron core will create a
much stronger magnet overall
The magnetic field produced by the electromagnet can be switched on and off
When the current is flowing there will be a magnetic field produced around the
electromagnet
When the current is switched off there will be no magnetic field produced around the
electromagnet.
Changing the direction of the current also changes the direction of the magnetic field
produced by the iron core
Factors Affecting Magnetic Field Strength
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The strength of the magnetic field produced around a solenoid can be increased by:
o Increasing the size of the current which is flowing through the wire
o Increasing the number of coils
o Adding an iron core through the centre of the coils
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The strength of an electromagnet can be changed by:
o Increasing the current will increase the magnetic field produced around the
electromagnet
o Decreasing the current will decrease the magnetic field produced around the
electromagnet.
Applications of the magnetic effects
1. Relay
Electromagnets are commonly used in relay circuits
Relays are switches that open and close via the action of an electromagnet
A relay circuit consists of:
a. An electrical circuit containing an electromagnet
b. A second circuit with a switch which is near to the electromagnet in the first
circuit
When switch A is closed, a small current flow around the circuit through the coil of
the electromagnet. The electromagnet attracts the iron armature. It pushes the two
contacts at B together completing the second circuit.
2. A relay is used to start a car engine
When a driver turns the key to start a car, a small current flow to a relay in the engine
compartment. This closes a switch to complete the circuit, which brings a high current
to the starter motor from the battery.
Motor Effect
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The motor effect occurs:
When a wire with current flowing through it is placed in a magnetic field and experiences
a force
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This effect is a result of two interacting magnetic fields
o One is produced around the wire due to the current flowing through it
o The second is the magnetic field into which the wire is placed, for example,
between two magnets
As a result of the interactions of the two magnetic fields, the wire will experience a force
Force on a current carrying conductor
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A current-carrying conductor produces its own magnetic field
When interacting with an external magnetic field, it therefore will experience a force
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A current-carrying conductor will only experience a force if the current through it
is perpendicular to the direction of the magnetic field lines
Two ways to reverse the direction of the force are by reversing:
o The direction of the current
o The direction of the magnetic field.
Fleming’s left-hand rule
If the thumb and the first two fingers of the left hand are extended at right angles to
each other,
The first finger points in the direction of the field
The second finger in the direction of current
The thumb will indicate the direction of the force.
Applications of motor effect
1. Loud speaker
Loudspeakers convert electrical signals into sound. They work due to the motor
effect. A loudspeaker consists of a coil of wire which is wrapped around one pole
of a permanent magnet.
An alternating current pass through the coil of the loudspeaker. This creates a changing
magnetic field around the coil
As the current is constantly changing direction, the direction of the magnetic field will
be constantly changing
The magnetic field produced around the coil interacts with the field from the permanent
magnet
The interacting magnetic fields will exert a force on the coil. The direction of the force at
any instant can be determined using Fleming’s left-hand rule
As the magnetic field is constantly changing direction, the force exerted on the coil
will constantly change direction. This makes the coil oscillate
The oscillating coil causes the speaker cone to oscillate. This makes the air oscillate,
creating sound waves.
D C motors
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The motor effect can be used to create a simple d.c electric motor
The simple d.c. motor consists of a coil of wire (which is free to rotate) positioned in
a uniform magnetic field:
This causes the coil to rotate since it experiences a turning effect
The turning effect is increased by increasing:
o The number of turns on the coil
o The current
o The strength of the magnetic field
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When the current is flowing in the coil at 90o to the direction of the magnetic field:
o The current creates a magnetic field around the coil
o The magnetic field produced around the coil interacts with the field produced by
the magnets
o This results in a force being exerted on the coil
o The direction of the force can be determined using Fleming's left-hand rule
o As current will flow in opposite directions on each side of the coil, the force
produced from the magnetic field will push one side of the coil up and the other
side of the coil down
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This will cause the coil to rotate, and it will continue to rotate until it is in the vertical
position
o In the vertical position momentum keeps the coil turning until the magnetic
force takes over again
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The split ring commutator swaps the contacts of the coil
o This reverses the direction in which the current is flowing every half turn
o This keeps the current leaving the motor in the same direction (d.c)
Reversing the direction of the current will also reverse the direction in which the forces
are acting
o As a result, the coil will continue to rotate
The split-ring commutator reverses the direction of the current in the coil every half
turn
o This will keep the coil rotating continuously as long as the current is flowing
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Factors Affecting the D.C Motor
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The speed at which the coil rotates can be increased by:
o Increasing the current
o Use a stronger magnet
The direction of rotation of coil in the d.c motor can be changed by:
o Reversing the direction of the current
o Reversing the direction of the magnetic field by reversing the poles of the
magnet
The force supplied by the motor can be increased by:
o Increasing the current in the coil
o Increasing the strength of the magnetic field
o Adding more turns to the coil
Charged particles in a magnetic field
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When a current-carrying wire is placed in a magnetic field, it will experience a force if
the wire is perpendicular
o This is because the magnetic field exerts a force on each individual electron
flowing through the wire
Therefore, when a charged particle passes through a magnetic field, the field can exert
a force on the particle, causing it to deflect
o The force is always at 90 degrees to both the direction of travel and the
magnetic field lines
o The direction can be worked out by using Fleming's left-hand rule
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In the case of a electron in a magnetic field the second finger points in the opposite
direction to the direction of motion
o Conventional current is said to flow opposite to the direction of flow of electrons
o The finger represents current
o An alternative is to use the right hand to work out directions for charged
particles
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If the particle is travelling perpendicular to the field lines:
o It will experience the maximum force
If the particle is travelling parallel to the field lines:
o It will experience no force
If the particle is travelling at an angle to the field lines:
o It will experience a small force
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