simple phenomena of magnetism

1) Describe the forces between Magnets, and between Magnets and Magnetic Materials
Magnets:
  • Have a magnetic field around them. The 2 opposite poles (North and South) exert forces on other magnets. Like poles repel and unlike poles attract. This is caused by the interaction of magnetic fields.

Magnetic materials:
  • Are materials that are attracted to magnets and can be magnetised, eg. iron, nickel, cobalt.
  • Can be magnetised by inducing (permanent or temporary) magnetism in them.
​
Non-magnetic materials:
  • ​Non-magnetic materials are materials that are not attracted to magnets and cannot be magnetised eg. plastic, wood, rubber.​​​
2) Describe Induced Magnetism 
  • Magnets attract materials by inducing magnetism in them; the material becomes a magnet as well.
  • The side of the material facing the magnet will become the opposite pole as the magnet.
3) Describe Methods of Magnetisation
Methods of inducing magnetism:
  • Placed near another magnet, but its magnetism is usually weak.
  • Stroking it with one end of a magnet.
  • Place inside a solenoid and pass a direct current (d.c.) through the coil.
 
Methods of demagnetisation:
  • Hammering: if a magnet is hammered, its atomic magnets are thrown out of line and it becomes demagnetized.
  • Heating a magnet to a high temperature also demagnetize it.
  • Place magnet inside a solenoid and pass a alternating current (a.c.) through the coil.
4) Distinguish between the magnetic properties of Soft Iron and Steel
Soft Iron
Steel
  • Gets magnetized faster but loses its magnetism as soon as inducing magnet is removed.
  • High susceptibility but low retentivity
  • Use: core in the transformer
  • Slow to be magnetized but retains acquired magnetism for a long time.
  • Low susceptibility but high retentivity.
  • Use: making magnets.
5) Distinguish between the design and use of Permanent Magnets and Electromagnets
Permanent Magnet
Electromagnet
  • Are hard magnetic material that has been permanently magnetised.
  • Use: for applications where magnetism is needed over long periods eg. fridge doors
  • Consists of coil of wire (solenoid) around a magnetically soft core and can be turned on or off.
  • Use: for applications where magnetic field needs to be turned on & off eg. moving scrap metal.

electric quantities

6) Describe Electric Charges
  • There are 2 types of charges: positive and negative.
  • Unlike charges attract and like charges repel.
  • The SI unit of charge is the Coulomb (C).

Charge can be detected using a gold leaf electroscope:
  •  If a positively charged rod is brought close to the disc on top of the electroscope, electrons are attracted to the top of the disc, away from the bottom of the metal stem and the gold leaf.
  • The gold leaf will then be repelled from the metal stem because they both become positively charged.
  • If someone then touches the disc, electrons flow from the ground into the disc as they are attracted to the rod, and the electroscope now contains a net negative charge. This is called charging by induction.
7) State that charging a body involves the addition or removal of electrons
  • When insulating materials rub against each other, they may become electrically charged.
  • Electrons, which are negatively charged, may be ‘rubbed off’ one material and on to the other.
  • The material that gains electrons becomes negatively charged. The material that loses electrons is left with a positive charge.
8) Describe a simple experiment to show the production and detection of electrostatic charges by Friction
  • Suspend one of the insulating materials using a cradle and a length of string so that the material can rotate freely.
  • Rub the rod using a cloth (in order to give it a charge).
  • Hold the charged rod close to the cloth.
  • They should attract because now they have opposite charges.
9) Describe an Electric Field
  • Region in which electric charge experiences a force.
  • The direction of an electric field at a point is the direction of the force on a positive charge at that point.
  • Electric field lines point away from positive charges and towards negative charges.
10) Distinguish between Electrical Conductors and Insulators
Conductors:
  • Materials that let electrons pass through them. Metals are the best electrical conductors as they have free electrons, e.g. copper.
  • Used as wires in circuits.
 
Insulators:
  • Materials that impede the flow of electrons, so cannot conduct electricity.
  • Their electrons are tightly held to atoms and hardly move, but they can be transferred by rubbing, eg. rubber.
11) Describe Current, Potential Difference, E.M.F., and Resistance
Current:
  • Is a rate of flow of charge, the SI unit is the Ampere (A).
  • An ammeter measures the current in a circuit and is connected in series.
  • In metals, current is caused by a flow of electrons.
  • Current follows path of least resistance.
  • Conventional current flows in the direction opposite to that which electrons flow in.
\[Current=\frac{Charge}{Time}\]
\[I=\frac{Q}{t}\]
Potential Difference
  • Potential difference, or PD for short, is also known as voltage. 
  • Voltage is the amount of energy the cell gives the electrons it pushes out. Voltage is measured in volts (V) and is measured by a voltmeter (connected in parallel). If a cell has 1 Volt, it delivers 1 Joule of energy to each coulomb of charge (J/C).
\[Voltage=\frac{Energy}{Charge}\]
\[V=\frac{E}{C}\]
Electromotive Force (EMF):
  • The energy supplied by the source in driving a unit charge around a circuit. 
  • The maximum voltage a cell can produce is called the electromotive force (EMF), measured in volts.
  • When a current is being supplied, the voltage is lower because of the energy wastage inside the cell.
  • A cell produces its maximum PD when not in a circuit and not supplying current.
Resistance:
  • SI unit ohms (Ω)
\[Resistance=\frac{Voltage}{Current}\]
\[R=\frac{V}{I}\]
Factors affecting resistance:
  • Length ∝ resistance: the electrons have to travel a longer length and thus encounter more resistance.\
  • Cross-sectional area ∝ 1 / resistance: more electrons can flow per unit time, increasing the current and therefore decreasing the resistance.
  • Material: better conductor = less resistance.
Current-voltage characteristic of an ohmic conductor and a non-ohmic conductor (eg. filament lamp):
Ohmic Conductor
  • ​Ohm’s law states that voltage across a resistor is directly proportional to the current through it, constant resistance.​
Non-Ohmic Conductor (Filament Bulb)
  • The resistance of a lamp increases as the temperature of its filament increases.
  • The current flowing through a filament lamp is not directly proportional to the voltage across it.