Electromagnetism

What happens when an electric current is put in a conductor?

Magnetic field of a wire carrying current

• When a current flows through a conducting wire a magnetic field is produced around the wire

• The shape and direction of the magnetic field can be investigated using plotting compasses

Diagram showing the magnetic field around a current-carrying wire

• The magnetic field is made up of concentric circles

  • A circular field pattern indicates that the magnetic field around a current-carrying wire has no poles

• As the distance from the wire increases the circles get further apart

  • This shows that the magnetic field is strongest closest to the wire and gets weaker as the distance from the wire increases

• The right-hand thumb rule can be used to work out the direction of the magnetic field

The right-hand thumb rule shows the direction of current flow through a wire and the direction of the magnetic field around the wire

• Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field

Side and top view of the current flowing through a wire and the magnetic field produced

• If there is no current flowing through the conductor there will be no magnetic field

• Increasing the amount of current flowing through the wire will increase the strength of the magnetic field

  • This means the field lines will become closer together

What makes up an electromagnet?

An electromagnet has a soft iron core (or other magnetic material) wrapped in wire. When current flows through the coil of wire it becomes magnetic.

How do you draw magnetic field patterns for a straight wire, a flat circular coil and a solenoid when each is carrying a current?

Magnetic field patterns

• Magnetic field line patterns are all slightly different around:

  • Straight wires

  • Flat circular coils

  • Solenoids

Magnetic field in a straight wire

• When a current flows through a conducting wire a magnetic field is produced around the wire

• The shape and direction of the magnetic field can be investigated using plotting compasses

• The magnetic field is made up of concentric circles

  • A circular field pattern indicates that the magnetic field around a current-carrying wire has no poles

• As the distance from the wire increases the circles get further apart

  • This shows that the magnetic field is strongest closest to the wire and gets weaker as the distance from the wire increases

• The right-hand thumb rule can be used to work out the direction of the magnetic field

• Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field

• If there is no current flowing through the conductor there will be no magnetic field

• Increasing the amount of current flowing through the wire will increase the strength of the magnetic field

  • This means the field lines will become closer together

Magnetic field in a flat circular coil

• When a wire is looped into a coil, the magnetic field lines circle around each part of the coil, passing through the centre of it

The magnetic field around a flat circular coil

• To increase the strength of the magnetic field around the wire it should be coiled to form a solenoid

• The magnetic field around the solenoid is similar to that of a bar magnet

• Using this, we can draw the pattern of magnetic field lines of a current carrying solenoid

Magnetic field around and through a solenoid. This is similar to the field of a bar magnet.

Magnetic field in a solenoid

• The magnetic field inside the solenoid is strong and uniform

• Inside a solenoid (an example of an electromagnet) the fields from individual coils

  • Add together to form a very strong almost uniform field along the centre of the solenoid

  • Cancel to give a weaker field outside the solenoid

• One end of the solenoid behaves like the north pole of a magnet; the other side behaves like the south pole

  • To work out the polarity of each end of the solenoid it needs to be viewed from the end

  • If the current is travelling around in a clockwise direction then it is the south pole

  • 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

Poles of a solenoid. The right hand rule can be adapted for this situation, with fingers following the direction of current and the thumb pointing in the direction of the central magnetic field lines.

Factors affecting magnetic field strength of a solenoid

• The strength of the magnetic field produced around a solenoid can be increased by:

  • Increasing the size of the current which is flowing through the wire

  • Increasing the number of coils

  • Adding an iron core through the centre of the coils

• 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

Factors affecting field strength

• The strength of the magnetic fields field depends on:

  • The size of the current

  • The distance from the long straight conductor (such as a wire)

• A larger current will produce a larger magnetic field and vice versa

• The greater the distance from the conductor, the weaker the magnetic field and vice versa

The greater the current, the stronger the magnetic field. This is shown by more concentrated field lines

What are the effects of magnetic force on a current-carrying wire, and how do D.C motors and loudspeakers work?

Magnetic force on a current-carrying wire

• The motor effect occurs when:

A wire with current flowing through it is placed in a magnetic field and experiences a force

• This effect is a result of two interacting magnetic fields

  • One is produced around the wire due to the current flowing through it

  • 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

• When no current is passed through a conductor in a magnetic field, however, it will experience no force

The motor effect is a result of two magnetic fields interacting to produce a force on the wire

The D.C. motor

• The motor effect can be used to create a simple d.c. electric motor

  • The force on a current-carrying coil is used to make it rotate in a single direction

• The simple D.C. motor consists of a coil of wire (which is free to rotate) positioned in a uniform magnetic field

• The coil of wire, when horizontal, forms a complete circuit with a cell

  • The coil is attached to a split ring (a circular tube of metal split in two)

  • This split ring is connected in a circuit with the cell via contact with conducting carbon brushes

Forces on the horizontal coil in a D.C. motor

Forces acting in opposite directions on each side of the coil, causing it to rotate. The split ring connects the coil to the flow of current

• Current flowing through the coil produces a magnetic field

  • This magnetic field interacts with the uniform external field, so a force is exerted on the wire

• Forces act in opposite directions on each side of the coil, causing it to rotate:

  • On the blue side of the coil, current travels towards the cell so the force acts upwards (using Fleming's left-hand rule)

  • On the black side, current flows away from the cell so the force acts downwards

• Once the coil has rotated 90°, the split ring is no longer in contact with the brushes

  • No current flows through the coil so no forces act

Coil in the vertical position

No force acts on the coil when vertical, as the split ring is not in contact with the brushes

• Even though no force acts, the momentum of the coil causes the coil to continue to rotate slightly

• The split ring reconnects with the carbon brushes and current flows through the coil again

  • Now the blue side is on the right and the black side is on the left

• Current still flows toward the cell on the left and away from the cell on the right, even though the coil has flipped

  • The black side of the coil experiences an upward force on the left and the blue side experiences a downward force on the right

  • The coil continues to rotate in the same direction, forming a continuously spinning motor

Forces on the coil when rotated 180°

Even though the coil has flipped, current still flows anticlockwise and the forces still cause rotation in the same direction

Factors affecting the D.C. motor

• The speed at which the coil rotates can be increased by:

  • Increasing the current

  • Increasing the strength of the magnetic field

• The direction of rotation of coil in the D.C. motor can be changed by:

  • Reversing the direction of the current

  • Reversing the direction of the magnetic field by reversing the poles of the magnet

• The force supplied by the motor can be increased by:

  • Increasing the current in the coil

  • Increasing the strength of the magnetic field

  • Adding more turns to the coil

Loudspeakers

• Loudspeakers and headphones convert electrical signals into sound

  • They work due to the motor effect

• They work in the opposite way to microphones

• A loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet

Diagram showing a cross-section of a loudspeaker

• An alternating current passes 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

How does the force on a current-carrying conductor in a magnetic field change with the magnitude and direction of the field and current?

If you increase the magnitude of the current through a wire or the size of the magnet being used, you increase the force on the wire.

If you change the direction of the current or reverse the poles of the magnet, you change the direction of the force on the wire