Magnetism and electromagnetism

• Magnetism

  it describes the ability of magnets to attract and repel other magnets without touching them.
• Repulsion

  Like poles on a magnet repel
• Attraction


  Opposite poles on a magnet attract
• Magnetic Materials


  Iron, nickel, steel and cobalt are the only pure metals that can be turned into a magnet.
• Hard magnet


  Steel
• Soft magnet


  Iron
• Magnetically soft material


  They lose their magnetism easily and are also easily magnetised.
• Magnetically hard material


  They are difficult to magnetise and they do not lose their magnetism.
• Permanent magnets produce their own magnetic field.
• Induced magnetism


  they are magnetic materials that become magnets when they are in a magnetic field.

  When moved away from the permanent magnet, the magnetic material will stop being a magnet.
• (induced magnet)

  The permanent magnet induces (creates) temporary poles in the magnetic material and these align so that the magnetic material is attracted to the permanent magnet.

  If the north pole of the magnet is next to the magnetic material, then a south pole will be induced in the part of the material closest to the magnet.
• There is always an attractive force between an induced magnet (magnetic material) and a permanent magnet.
• Magnetic forces (the forced between magnets)


  Are caused by invisible magnetic fields
• A magnetic field


  it is the area around a magnet where another magnet or magnetic material (iron, nickel, cobalt and steel) feels a force.
• Strength of magnetic field

  The strength of a magnetic field depends on the distance from the magnet. The magnetic field is strongest at the magnet's poles.
• When the magnetic fields of two magnets overlap, the fields cause the magnets to push or pull each other.
• Magnetic field patterns


  - A magnetic compass contains a small bar magnet. The needle of the compass points in the direction of the Earth's magnetic field. This shows that the Earth's core is magnetic.

  - The arrows on magnetic field lines show the direction of a magnetic field. The arrows always point from the north pole of the magnet to the south pole.

  - The strength of the magnetic field at any point is indicated by the density of the field lines at that point.

  - We can use iron filings to observe this magnetic field pattern.

  - We can create a uniform magnetic field by aligning the north pole of one bar magnet with the south pole of another.
• When do magnets attract or repel each other?

  When their magnetic fields overlap
• The needle of the compass points in the direction of the Earth's magnetic field.
• How can a solenoid be made into an electromagnet?

  What materials should be used?

  - The solenoid should be wrapped around a core.

  - For a strong electromagnet, the core should be made of iron.
• Uniform magnetic field


  A field that has a series of straight lines that are evenly spaced.

  The magnetic field lines are the same distance apart between the gaps of the poles to indicate that the field strength is the same at every point between the poles.
• Magnetic field lines show:

  - The shape of the magnetic field

  - The direction of the magnetic force - the field lines "travel" from north to south

  - The strength of the magnetic field - the field lines are closest together where the magnetic field is strongest
• The Magnetic Effect of a Current


  Any electric current produces a magnetic field.
• Strength of magnetic field

  The strength of the field depends on the size of the current and the distance away from the wire.
• Right hand grip rule


  The direction of the field lines depends on the direction of the current.
• - changing the direction of the current changes the direction of the magnetic field

  - the direction of the magnetic field depends upon the direction of current
• Solenoid


  it is a coil of wire coiled into a cylindrical shape.
• The magnetic field inside a solenoid is strong and uniform.
• Increasing the size of the current increases the strength of the magnetic field produced by a solenoid.
• Motor effect


  When a wire with a current flowing through it (a current carrying wire) is placed in a magnetic field, the conductor will experience a force.
• Conductor


  material that allows electricity to flow through it
• The motor effect


  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 one is the magnetic field into which the wire is place (ex: between two magnets)

  - as a result of the interactions of the two magnetic fields, the wire will experience a force.
• Cause of the motor effect

  - The field created by the electric current interacts with the magnetic field that the wire is placed inside. This causes the force on the wire.

  - A moving charge (current) will experience a force as long as the direction of motion is not parallel to the magnetic field.
• Direction of motor effect

  The direction of the force on the wire depends on the direction of the magnetic field and the direction of the current flow. The direction of the force is determined by Fleming's left hand rule.
• When a current carrying wire is placed in a magnetic field, it will experience a force.
• Catapult experiment


  The catapult experiment shows that a wire carrying a current in a magnetic field experiences a force.

  When the power supply is switched on, a current flows in the free wire and so this wire feels a force which causes it to move.

  Swapping the north and south poles of the magnet will reverse the wire's direction of movement.

  Swapping the wires connecting to the power supply reverses the direction of the current.

  Reversing the direction of the current will reverse the direction that the wire moves.
• Electromagnetic induction


  A current is a flow of charge. A potential difference (p.d) is needed for a current to flow and the p.d drives a current round a circuit.
• A p.d can be induced in two ways:

  - If a wire (or any electrical conductor) moves near a magnet, it induces a p.d in the conductor.

  - A changing magnetic field is a magnetic field with changing strength.

  If there is magnetic field through the middle of a coil of wire, it is 'linked' with the coil.

  A changing magnetic field that is linked with a coil of wire induces a p.d in that wire.
• Induced Potential Difference


  We can induce a bigger potential difference in an electrical conductor (coil of wire) during electromagnetic induction in 2 main ways:

  - increasing the number of turns of coils of wire

  - making the magnetic field change more rapidly by moving the wire or the magnet more quickly
• Alternating Current - The Rotating-Coil Generator


  - Alternating current (AC) is generated by a rotating-coil generator, where a coil of wire is rotated inside a fixed magnetic field.

  - When the coil rotates, it cuts across the magnetic field lines and a p.d is induced.

  - The induced p.d causes a current to flow in the coil of wire.

  - The faster the coil cuts across lines, the greater the magnitude (size) of the induced p.d.