Physics notes

• Properties of magnets:

  • Magnetic poles refer to the ends of a magnet where the power of attraction or repulsion is greatest
  • The force of attraction of a magnet is greatest at its poles, reducing away from poles
  • Directional property of a magnet: it rotates and finally rests in the North-south direction when suspended by a thread
  • Repulsion is the only sure test for polarity of a magnet
• Magnetic and non-magnetic materials:

  • Magnetic materials can be attracted by magnets (e.g. Iron, Nickel, Cobalt)
  • Non-magnetic materials cannot be attracted by magnets (e.g. Copper, Brass, Aluminium)
  • Ferromagnetic materials are strongly attracted by magnets (e.g. soft iron)
• Basic Law of Magnetism:

  • Like poles of magnets repel while unlike poles attract
• Magnetic field:

  • The space around a magnet where the magnetic influence is felt
  • Magnetic field lines start from the North Pole and end at the South Pole
  • Characteristics of magnetic field lines: they do not intersect, repel each other sideways, and are closer together where the field is stronger
• Magnetization:

  • Process of making a magnet from magnetic material
  • Methods of magnetization: Induction, Stroking, Hammering, Electrical method using direct current
  • Electrical method: polarities of the magnet depend on the direction of the electric current
• Demagnetization:

  • Process by which a magnet loses magnetism
  • Methods of demagnetization: Heating, Hammering in east-west position, Dropping on a hard surface, Electrical method using alternating current
• Ways to demagnetize a magnet:

  • Dropping a magnet on a hard surface floor several times makes it lose most of the magnetism
  • Heating a magnet until red hot and cooling it suddenly when resting in the east-west direction makes it lose its magnetism
  • Placing a magnet in a solenoid placed in east-west direction and passing an alternating current demagnetizes it because alternating current reverses many times per second, disorienting the magnetic dipoles
• Types of magnetic materials:

  • Soft magnetic materials: magnetized easily but do not retain their magnetism for long. Examples include iron and alloy of iron and nickel
  • Applications of soft magnetic materials: making electromagnets, making transformer cores, used for magnetic shielding
  • Hard magnetic materials: difficult to magnetize but once magnetized they retain their magnetism for a long time. Example: steel
  • Application of hard magnetic materials: used in making permanent magnets
• Storing magnets:

  • Bar magnets are stored in pairs with soft iron keepers placed across their ends to prevent self-demagnetization
  • Unlike poles of the magnets are placed close to each other
  • Magnets magnetize soft iron keepers through induction, forming a closed loop or chain round the magnets and the keepers, with no free poles
• Uses of magnets:

  • Used in hospitals for removing pieces of iron from the eyes of patients
  • Used in industries as stirrers, lifting iron scrap metals
  • Used in weather stations for resetting six’s minimum and maximum thermometer
  • Used in navigation for showing direction as in compass needles
  • Magnetic tapes used in audio and video recorders
• Vernier Calipers:

  • Used to measure length using two scales: main scale and Vernier scale
  • Inside jaws used to measure internal diameters and outside jaws used to measure external diameters
  • Vernier calipers have a least count of 0.01cm
• Micrometer Screw Gauge:

  • Used to measure very small lengths such as the diameter of a thin wire
  • Consists of a thimble with a circular rotating scale, a spindle, a sleeve with a linear scale, and a ratchet
  • Least count of the screw gauge is 0.01mm
• Significant Figures:

  • Refer to the number of digits used to specify the accuracy of a value
  • The number of significant figures is determined by counting the number of digits from the first significant figure on the left
• The number of significant figures is determined by counting the number of digits from the first significant figure on the left
• Zero may be significant or not depending on the position of the digit
• If zero occurs between non-zero digits, it is significant
• When zero occurs at the left end of a number, it is not significant
• If the zero occurs at the right-hand end of an integer, it may or may not be significant
• If zero occurs at the right-hand end after the decimal point, it is always significant
• Standard form is a way of writing a number with only one integer before the decimal point
• A positive number in standard form is written as AX10^n, where A is such that 1≤A<10 and the index n is an integer
• If the number lies between zero and 1, the index n becomes negative
• Decimal places refer to the number of digits to the right of the decimal point
• The volume of a cube with a side of 2.22 cm is calculated to 3 decimal places
• Standard prefixes used with SI units include deci, centi, milli, micro, nano, pico, femto, atto, deca, hecto, kilo, mega, giga, tera, peta, exa
• The Oil Drop Experiment is used to estimate the diameter/size/thickness of a molecule
• Functions of lycopodium powder:

  • It breaks surface tension
  • It clearly shows the extent of spread of the oil drop
• Assumptions made in the Oil Drop Experiment:

  1. The oil drop is perfectly spherical
  2. The oil patch is perfectly cylindrical
  3. The oil patch is one molecule thick
• Possible sources of error in the Oil Drop Experiment:

  1. Error in measuring the diameter or volume of the oil drop
  2. Error in measuring the diameter of the oil patch
• Moment of a force refers to the turning effect of the force
• SI unit of moment of a force is the newton meter (Nm)
• Factors affecting moment of a force:

  1. Amount of force – directly proportional to the force applied

  2. Perpendicular distance between line of action of force and point of support – directly proportional to the distance
• Examples of activities where force produces a turning effect:

  • Opening and closing a door
  • Closing a lid of a container
  • A pair of scissors or garden shears in use
  • Children playing on a see-saw
  • A wheelbarrow lifting heavy loads
  • A screwdriver tightening or loosening a screw
  • Beam balance in use
• The Principle of Moments states that for a system in equilibrium, the sum of clockwise moments about a point must be equal to the sum of anticlockwise moments about the same point
• Parallel forces:

  • The sum of forces acting on one side of the system is equal to the sum of forces acting on the opposite direction
  • The sum of clockwise moments is equal to the sum of anticlockwise moments
• A uniform metal rod of length 80cm and mass 3.2kg is supported horizontally by two vertical spring balances C and D
• Balance C is supported from one end while balance D is 30cm from the other end
• To find the reading on each balance, calculate the moments of the forces acting on the rod
• Anti-parallel forces (Couples)
• Anti-parallel forces or a couple refers to equal, parallel but opposite forces