O Level – Combined Science

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  • #3795

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    Key Points – DC Circuits

    1. Total emf Et of n identical cells arranged in series
    Et = E1 + E2 + E3 …… + En

    2. Total emf Et of n identical cells arranged in parallel
    Et = E1 = E2 = E3 ……+ En

    3. At any point in the circuit, total incoming current is equal total outgoing current

    4. The resistance of thermistors decreases as temperature increases.

    5. The resistance of light-dependent resistors (LDRs) decreases as light intensity increases.

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    #3809

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    Key Points – Practical Electricity Part 1

    1. Electricity is essential in modern homes to power electrical appliances, which covert electrical energy into light, kinetic, sound and heat energy.

    2. Heating elements in household appliances such as electric kettle and heater convert electrical energy to thermal energy. The heating elements are usually made of nichrome because of its high resistance and high melting point.

    3. The amount of electrical energy E converted can be calculated by the following equations

    E = VIt
    E = (I^2)Rt
    E = (V^2/R)t

    V = Voltage (V)
    I = Current (A)
    t = Time (s)
    R = Resistance (Ohm)

    4. Power, P can be calculated by the following equations
    P = IV
    P = (I^2)R
    P = (V^2)/R

    5. Electrical energy consumption is measured in kilowatt hour (kWh).
    One kilowatt hour is the amount of electrical energy used by 1 kW appliance in 1 hr.

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    #3811

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    Petroleum

    1. Petroleum is a non-renewable resource and there is only a limited amount of petroleum in the Earth.

    2. Moet petroleum is used as fuel. Only a small amount is used as feedstock to make useful petrochemicals.

    3. Gasohol is an alternative energy sources used as a fuel in motor vehicles.

    4. Biogas is also used as an alternative energy source. It contain about 50% ethane.

    5. Hydrogen can also be used as a fuel for cars.

    If you need help in the Organic Chemistry, please contact Angie @96790479 or Mr Ong 98639633

    #3867

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    Key Points – Practical Electricity Part 2

    6. Fossil fuels and nuclear energy are non-renewable energy sources. Solar energy, wind energy and hydroelectric generation are renewable energy sources.

    7. Generally, non-renewable energy source produce electricity cheaper than
    renewable energy.

    8. A short circuit causes excessive current to pass through the circuit and produce a large amount of heat within a Short period of time.

    9. Electricity can be dangerous due to
    – damaged insulation which can cause electric shocks
    – overheating of cables which can cause fire.
    – damp conditions which can cause electric shocks.

    10. Live wire carry current into homes while neutral wires carry current out of homes. Earth wires are connected to the ground for safety purposes.

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    #3869

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    Important Definitions – Alkanes & Alkenes

    1. Saturated hydrocarbon are compounds containing only carbon and hydrogen and having single bonds between carbon atoms.

    2. Unsaturated hydrocarbon are compounds containing only carbon and hydrogen and having double bonds between carbon atoms.

    3. Alkane series has a general formula CnH2n+2

    4. Alkenes series has a general formula CnH2n

    5. Structural formula is the chemical formula that shows how the atoms are joined together in a molecue.

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    #4119

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    Important Definitions – Alkanes & Alkenes

    6. Isomerism is the existence of two or more organic compounds with the same molecular formula but different structural formulae. These different compound called isomers.

    7. A substitution reaction is a reaction in which an atom (or a group of atoms) in an organic molecule is replaced by another atom ( or group of atoms)

    8. An addition reaction is a reaction in which an unsaturated organic compound combines with another substance to form a single product.

    9. Cracking is a process in which a large hydrocarbon molecules are broken down into smaller molecules.

    10. Polyunsaturated compounds are organic molecules that contain more than c=c bond.

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    #4120

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    Key Points – Practical Electricity Part 3

    11. The earth wire provides a low resistance path for electric currents to flow from the metal parts of a faulty electrical appliance to the ground. Appliances with three-pin plugs are earthed.

    12. The fuse is a safety device that is connected to the live wire. It contains a metal wire that melts to to disconnect a circuit when excessive current flows through it. The current rating of a fuse is the minimum amount of current that will cause it to melt.

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    #4222

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    Key Points – Practical Electricity Part 4

    13. A circuit breaker is a safety device that automatically switches off a circuit when excessive current flows through it. This protect electrical appliances from damage arising from circuit faults.

    14. Double insulation is a safety feature that uses layers of insulation that keeps electricity within the circuit and protects the appliances’s internal components.

    15. Double insulated electrical appliances use two-pin plugs because that there is no need for the earth wire, and they normally do not have a metal casting.

    16. Switches, fuses and circuit breakers are connected to the live wires. When a circuit is disconnected by these safety devices, current is no longer supplied to the faculty circuit or electrical appliance.

    If you need help in the above topics, please contact Angie @96790479 or Mr Ong @98639633

    #4223

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    Alkanes

    1. Saturated hydrocarbon are hydrocarbons with only single bonds between carbon atoms.

    2. Unsaturated hydrocarbons are hydrocarbons with one or more double bond or triple bond between carbon atoms.

    3. The alkanes are saturated hydrocarbons with the general molecular formula CnH2n+1 where n = 1, 2, 3, ……

    4. Alkanes contain only single bonds between carbon atoms. Each carbon atom in an alkane molecule uses all its valence electrons in forming single bonds with four other atoms.

    5. First four members of the alkane
    Methane CH4
    Ethane C2H6
    Propane C3H8
    Butane C4H10

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    #4318

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    Alkenes

    1. Alkenes are unsaturated hydrocarbons that contain one or more carbon-carbon double bond

    2. The general formula of alkenes with one double bond is CnH2n

    3. Naming of alkenes
    Ethene C2H4
    Propene C3H6
    Butene C4H8

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    #4319

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    Key Points – Magnetism Part 1

    1. A magnetic field exists around a magnet.

    2. A magnetic field is a region in which another magnet or magnetic material experiences a magnetic force.

    3. This magnetic force is either repulsion or attraction.

    4. A magnet always has a north pole and south pole. The magnetic field of a magnet is the strongest at the poles.

    5. Magnetic materials are attracted to both ends of a magnet. Magnets do not affect non-magnetic materials.

    6. Like poles repel while unlike poles attract.

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    #4364

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    Isomerism

    1. Isomerism is where two or more organic molecules have the same molecular formula but different structural formula. These different molecule are called isomers.

    2. Isomers have different melting and boiling points

    3. Methane, ethane and propane do not have isomers.

    4. Butane has two isomers
    – Butane
    – Methylpropane

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    #4365

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    Key Points – Magnetism Part 2

    7. Only repulsion can differentiate magnets from other materials.

    8. Magnetisation is the process of aligning the magnetic domains in a magnetic material in order to convert it into a magnet.

    9. Demagnetisation is the process of removing the magnetic properties of a magnet.

    10. Soft magnetic materials are easily magnetised and lose their magnetic properties easily.

    11. Hard magnetic materials are difficult to be magnetised and retain their magnetic properties much longer.

    12. When a magnetic material is bought near a magnet, the magnetic material becomes an induced magnet. The magnetic material can also be magnetised by repeated stroking with a magnet or by putting it in a solenoid with a direct current.

    13. A magnet can be demagnetised by heating, subjecting it to heavy ;physical impact or by slowly withdrawing the magnet from a solenoid with an alternating current.

    14. The direction of the magnet field is indicated by the direction in which the compass needle points.

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    #4443

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    Chemical Properties of Alkenes Part 1

    1. The functional group of alkenes is the carbon-carbon double bond (C=C).

    2. Alkenes are more reactive than alkanes because of the presence of the double bond.

    3. The combustion of alkenes produces carbon dioxide and water

    4. Alkenes produce more soot than alkanes when they burn in air because a relatively higher percentage of carbon than the alkanes with the same number of carbon atoms.

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    #4445

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    Key Points – Electromagnetism Part 1

    1. A wore carrying an electric current induces a concentric circular magnetic field around it.

    2. According to the right hand grip rule, if the thumb points in the direction of the current flow, the fingers will indicate the direction of the magnetic field lines around a current-carrying conductor.

    3. Fleming’s Left-hand rule gives the direction of the magnetic force (represented by the thumb) which is 90 deg to the magnetic field (represented by the forefinger)and the conventional current current (represented by the second finger).

    4. The direction of motion of negatively charged particles is taken to the opposite of positively charged particles.

    5. A current-carrying conductor (or moving charged particle) will experience a magnetic force in a magnetic field unless the direction of current (or the motion of charged particle) and the magnetic field are parallel.

    6. Stationary charged particles do not experience magnetic force.

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