Secondary Four Physics

THE REPUBLISHED 2012 ED. Secondary Four Physics Lim Ting Jie VS Class of 2011

1. WAVE MOTION TOPIC 1: GENERAL WAVE PROPERTIES A wave is the collective motion of many particles. When energy is transferred through a medium, it causes the molecules of the medium to move in a specific manner giving rise to a wave. However, the medium does not move about. 2. DEFINITIONS AND WAVEFRONTS Wavelength Shortest distance between any two points in phase Period The time taken to produce one complete wave, 1/T Velocity Distance per unit time, hence Wavelength (m)/period (s), Frequency x Wavelength Frequency The number of complete waves produced per second, 1/P Amplitude Maximum displacement (of crest or rarefaction) from the rest position Wavefront An imaginary line on a wave that joins all wave crests 3. TRANSVERSE AND LONGITUDINAL WAVES Transverse waves Waves that travel in a direction perpendicular to the direction of vibration. Crests (amplitude) and troughs (minimum displacement) indicate presence of a transverse wave. Longitudinal waves Waves that travel in a direction parallel to the direction of vibration Rarefactions (amplitude) and compressions (minimum displacement) indicate presence of a transverse wave. 4. RIPPLE TANK Deeper water Shallower water Wavelength Increases Decreases Velocity Increases Decreases Frequency Remains the same (Velocity/Wavelength) Remains the same (Velocity/Wavelength) Direction Away from the normal Towards the normal Wavefront Perpendicular to direction of wave Perpendicular to direction of wave 1. PROPERTIES OF EM WAVES TOPIC 2: ELECTROMAGNETIC WAVES Abrv Property Description Electro Electric No electric charge is carried through EM waves Magnetic Medium No medium is required and the wave and travel through vacuum Senior Speed 3 x 10 8 ms -1 in vacuum, slowing down in matter Leader. Laws They obey the laws of reflection and refraction Veloci-T Velocity Decreases from optically less dense to denser For Frequency Remains the same all the time The Type Transverse waves» Electric and magnetic fields that oscillate 90 o to each other Win! Wavelength Decreases from optically less dense to denser

Decreases downwards 2. COMPONENTS OF THE EM SPECTRUM Component Frequency Applications Wavelength Radio and television communications Radio waves 1 10^ 8 Longer wavelengths» Able to go around obstructions Microwave oven Satellite television Microwaves 1 10^ 10 Infra-red 1 10^ 12 Red Violet Water molecules vibrate millions of times a second to create heat from friction Remote controllers Can penetrate haze, light rain, snow, clouds and smoke with proper alignment Intruder alarms The alarm rings when it receives infra-red radiation an intruding humans give out Light 5 10^ 14 Medical optical fibres Telecommunications Ultra-violet 3 10^ 16 Xrays 3 10^ 18 Gamma rays 3 10^ 20 Sunbeds Shorter frequency UVA» Artificial tanning Sterilisation Longer frequency UVB/C» Germicidal lamps Diagnose fractures (imagery) and Airport scanners Can penetrate through all materials other than lead Cancer treatment High energy» Kill cancer cells in cancerous tumours 3. EFFECTS OF ABSORBING EM WAVES EM Wave Effect Permanent human damage Infra-red Human skin absorbs infrared waves from BBQ pits to make humans feel warm. - Frequencies greater than light X-rays Ionisation, the process of forming an ion, which is an electrically charged atom or molecule. This includes living matter. Exposure of developing fetus to X-ray imagery may result in a deformed baby. Damage to DNA-containing chromosomes and proteins. Overexposure leads to premature ageing and lifespan shortening. Abnormal cell division and leukaemia TOPIC 3: SOUND 1. PRODUCTION AND NATURE OF SOUND WAVES 2. SPEED OF SOUND Sound waves are produced when vibrating sources are placed in a medium. They are longitudinal waves. The energy transmitted by sound waves depends on its frequency and amplitude. 3. INTENSITY OF SOUND Medium Approx speed of sound in medium/ms -1 Air 330 Water 1500 Iron 5000 Steel 6000 Frequency Pitch Loudness Relative effect Increases Increases Remains the same Amplitude Pitch Loudness Relative effect Increases Remains the same Increases

4. ECHOES An echo is sound is reflected off hard and flat surfaces (e.g. a large wall or distant cliff). 5. ULTRASOUND Application d d Speed of sound = 2d/t» d = 1500t/2 = 750t Where: t: time taken seabed Ultrasound refers to sound above 20 khz frequency. It refers to sound waves with frequencies above the upper limit of the human range. lower limit of human audibility range Organism Bats Humans Elephants Dogs Range of audibility Within ultrasound range 20 Hz to 20 khz Within infrasound range Within both ultrasound and infrasound range 1. CHARGES AND ELECTRONS TOPIC 4: STATIC ELECTRICITY Charge is measured in coulombs. One negative electron is charged with 1.6 x 10-19 coulombs. Electrons flow from a region of lower potential (i.e. greater negative charges) to a region of higher potential (i.e. greater positive charges). Positive charge Positive charge Negative charge Negative charge Positive charge Negative charge Repel Repel Attract + charged object More + charges than - charges - charged object More - charges than + charges Neutral object - charges = + charges Energy is needed to remove two objects with opposing charges to overcome the attractive forces between the objects. Insulators Conductors Process of charging By friction By induction (to charge negatively) How it happens *The positively charged rod induces the charges in the conductor, attracting the negative charges to the end of conductor nearest to the rod. Negative charges are transferred from one object to another when rubbed. The following states the *COMMON conditions and movement: Positive charge acquired Transparent object Smooth and soft ratio object Negative charge acquired Smooth and soft object Opaque or translucent object *refer to Textbook page 316 for examples 1. Bring positively charged rod near the conductor. 2. *The positively charged rod leaves behind an excess of positive charges in the conductor. 3. Earth the end of the conductor with the positive charges. 4. Negative charges will move up from the Earth and neutralise the positive charges. 5. Remove the earth, then remove the rod. Presence of electrons Very few free electrons Many free electrons Diagram - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Process of discharging By heating» By increasing humidity» Surrounding air ionises» ions in air neutralise excess charges on object High amounts of water vapour» Greater ions» neutralise excess charges By earthing» By making contact wit the object using your finger, electrons can flow to Earth or out of Earth to neutralise excess charges contained in the object.

2. ELECTRIC FIELD AND ITS PATTERN Positive charge Negative charge Negative-Negative Positive-Negative Arrow direction reverses between two positive charges Positive-Negative parallel arrangement + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - 3. ELECTROSTATIC CHARGING OF TWO CONDUCTING SPHERES (POSITIVE AND NEGATIVE) # Summary Description 1 Contact Let the two conductors touch each other. 2 Neg. Rod Bring negatively charged rod near the conductor on the left. 3 Induction The negatively charged rod induces the charges in the two conductors, repelling the negative charges to the furthest end of the conductor on the right, leaving excess positive charges at the end of conductor on the left nearest to the rod. 4 Separation Separate the two conductors far from each other. 5 Rod Removal Remove the rod. 6 Outcome The conductor on the left will be positively charged while the other on the right will be negatively charged. + Diagram What would happen The smaller ball is initially attracted to the larger ball. Explain. Both balls are conductors of electricity. As the larger ball is positively charged, it induces charges in the smaller ball. Negative charges attracted to the positive charges in the larger ball, moving to the left side. Excess positive charges will be left on the right side. Since unlike charges attract, the smaller ball moves and sticks to the larger ball. Summary of explanations The smaller ball repels the larger ball after some time. Explain. The larger ball induces charges in the smaller ball. The negative charges travel to the larger ball, neutralising the postive charges. The larger ball become negatively charged and the smaller ball becomes positively charged. Since like charges repel, the smaller ball repels and moves away from the larger ball. When a neutral object is brought close to a positively charged conductor, [1] the charges in the neutral object will be induced. [2] The negative charges will be brought closer to the positively charged conductor and positive charges left further away. [3] Unlike charges attract.

TOPIC 5: CURRENT ELECTRICITY, DC CIRCUITS 1. CHARGE, CURRENT, ELECTROMOTIVE FORCE, POTENTIAL DIFFERENCE, RESISTANCE, OHM S LAW Term Definition Related formulas Charge When an object is charged, it is electrified. Q = ti (C, coulombs) Current EMF Potential Difference Resistance A measure of the rate of flow of electric charge through a cross section of a conductor. Ammeter is connected in series. Work done by an electrical source in driving a unit charge round a complete circuit. Amount of energy converted to other forms of energy when one coulomb of positive charge passes between the two reference points. Voltmeter is connected across two point parallel. Ratio of the potential difference across a component to the current flowing through it. Length l, Cross sectional area A, Type of material are factors which affect resistance. Resistivity A property of the material of a wire. ρ = Ohm s Law Potential divider Current passing through a metallic conductor is directly proportional to the potential difference across its ends, provided the physical conditions are constant. Used to create a sub circuit for potential difference I = Q (A, amperes) (Cs -1, coulombs per second) t Є = V = W (V, volts) (JC -1, joules per coulomb) Q W (V, volts) (JC -1, joules per coulomb) Q R = V (Ω, ohm) I (VA -1 ) RA l and V = RI (Ω m, ohm metres) I α V, where constant refers to R V out = R 1 V R 1 + R 2 2. CIRCUIT DIAGRAMS Device Symbol Device Symbol Device Symbol Cell Battery Fixed resistor longer and shorter line, with the longer representing + charge series of the longer and shorter line symbol rectangular box Switch small circle, line upwards, small circle Ammeter Lamp a cross in a big circle Voltmeter Rheostat arrow northeast in rectangular box Power supply A in big circle V in big circle switch, no line but gap Element Parallel circuit Series circuit Current Addition Equal Potential difference Equal Addition Resistance Power negative, i.e. R = (R 1-1 + R 2-1 + R 3-1 ) -1 Addition 1. POWER, ENERGY AND COST TOPIC 6: PRACTICAL ELECTRICITY P = VI P = RI 2 P = V2 E = Pt R

Earth Leakage Circuit Breaker Miniature Circuit Breaker Switches 2. HAZARDS OF ELECTRICITY Hazard Damaged insulation Overheating of cables Damp conditions Dangers Electrical insulation can crack or break when the applicance is not used carefully. If one touches the exposed live wires, it can cause severe electric shock, injury and death. If too many electrical appliances are used at the same time, the total power drawn by them through the electric cable from the mains supply may be very large. If the manufacturer uses thin wires for a high power electrical appliance, resistance produced would be very high, producing more thermal heat. Cable becomes overloaded and overheated, which may result in a fire Water can provide a conducting path for a large current to flow, which can electrocute a human body. We can only withstand a maximum current of 50 ma. 3. FUSES AND CIRCUIT BREAKERS 3rd pin of 3-pin plug, lower resistance than human body resistance Live wire Brown Usually zero volt Neutral wire Blue Low resistance Earth wire Green Yellow Delivers electrical energy to appliance» enables the appliance Forms a path back to the supply» Completes the circuit Earths the metallic parts of the appliance continuously» Prevents live wire touching the metal casing from causing electric shock Fuse Circuit breakers Purpose Usage Adaptations Safety device that protects electrical appliances from damage when excessive current flows through. When live wire has an electrical fault and earth wire is used to prevent electric shocks, a large current that flows from the live wire to the metal casing will blow the fuse, cutting supply. Used to turn electrical appliances off in order to save energy consumption. Safety devices that can switch off electrical supply in a circuit when there is overflow of current. Prevents excessive current flow through the circuit by tripping or breaking it. Monitors the amount of current flowing from the live wire to the earth wire. Usually a short piece of wire, a lead and tin alloy of low melting point, that melts and breaks when a current flowing through the circuit is higher than its rating. Always installed in the live wire Fuse ratings are annotated on the fuse Choose a fuse which can take a current slightly larger than what the appliance will draw in normal circumstances. Switches and fuses must always be wired into the live wire of the household circuit. When the switch is off or fuse has blown, the exposed live wire of the socket can be safely touched as the flow of current has been cut off. The MCB trips when there is a fault in the circuit. The MCB can be reset by switching it back on after the faulty appliance is removed or repaired. If an earth leakage current of more than 25 ma occurs, it will switch off all the circuits in the house in less than 25 ms.

4. DOUBLE INSULATION Some electrical appliances (e.g.hair dryers and television sets) connected to power circuits are not earthed by having only a 2-pin plug (live and neutral wires). Hence double insulation is used. First insulation - Electric cable insulated from internal components Second insulation - Internal components insulated from external components The accessible metal parts cannot become live unless two independent layers of insulation fail. 1. PROPERTIES OF MAGNETS TOPIC 7: MAGNETISM Magnetic objects Iron Steel Nickel Cobalt Properties 1. They have magnetic poles, where the magnetic effects are strongest. 2. They align themselves to the north and south poles of the Earth when suspended freely. 3. They repel from another magnet with like poles and attracts magnets with unlike poles. 4. They can only be identified by repulsion. 2. MAGNETIC INDUCTION Magnetic induction is the process where ferromagnetic materials (materials that are attracted by magnets) become magnetised when they are near or in contact with a permanent magnet. The magnetic field from the magnet/electromagnet with align the domains of the ferromagnetic material to cause it to be magnetised. 3. MAGNETISATION AND DEMAGNETISATION Magetisation Demagetisation Stroking Method How Reason Electrical method with direct current Heating Unmagnetised magnetic object is stroked several times with the same pole of a permanent magnet from one end to the other in one direction. The magnet must be lifted high enough after each stroke. Magnetic object is placed in a solenoid, a cylindrical coil of insulated copper wires carrying currents. Heat the magnet strongly with a Bunsen flame. Magnetic induction. The pole the stroking magnet starts is the same as the pole the object where stroking starts. A strong magnetic field is produced when direct electric current, DC, flows through the solenoid. Atoms of the magnet vibrate vigorously. Magnetic domains lose alignment. Hammering Hammer magnet several times. Magnetic domains lose alignment. Electrical method with alternating current Magnetic object is placed in a solenoid, a cylindrical coil of insulated copper wires carrying currents. AC is electric current which varies its direction many times per second. The magnet is withdrawn in the East West direction when AC flows through the solenoid. 4. TEMPORARY AND PERMANENT MAGNETS Iron (Soft magnetic material) Easily magnetised Do not easily retain magnetism Used in Electromagnets, Transformer cores, Shielding Steel (Hard magnetic material) Hard to magnetise Easily retains magnetism Permanent magnets

5. MAGNETIC FIELD PATTERNS The magnetic field of a magnet is the region around the magnet where the magnetic force can be detected. (North to South) 6. PLOTTING MAGNETIC FIELD LINES WITH A COMPASS i. Place a bar magnet on the centre of a sheet of paper. ii. Draw the outline of the bar magnet. iii. Place the plotting compass near the north pole of the magnet. iv. Mark with dots, the position of the south and north poles of the compass. v. Bring the end of the compass to the dot marked as the initial north pole of the compass such it replaces the new south pole. Repeat until the compass reaches the south pole of the magnet. vi. Join all the dots to form a smooth curve. vii. Place the compass at other positions around the north pole and plot more field lines around the magnet. 1. CIRCUIT BREAKERS TOPIC 8: ELECTROMAGNETISM What happens The current is within the limit. The solenoid magnetic field is not strong enough to attract the soft iron latch. The interrupt point remains closed and current flows normally through the circuit. Diagram Refer to Figure 21.12(a) in your Physics Textbook There is sudden surge in current such as a short circuit or overloading. Solenoid gains magnetism and becomes strong electromagnet due to larger current. It is able to attract the soft iron latch and release the spring. The safety bar is pushed outward. The interrupt point opens and current is cut off. Refer to Figure 21.12(b) in your Physics Textbook Button revealed may be pushed back to reset circuit breaker.

current outward current inwards 2. MAGNETIC FIELD PATTERNS Right hand grip rule Opposite of magnetism Fleming s Left Hand DC Rule S N F F unlike directions repel F (Force) B (Field from N (A) to S (B) ) N S F like directions attract I (Current) Explaining the Left Hand Rule: The magnetic field is directed from A to B. From the top, the current flows inwards, causing a clockwise field due to current. To the right of the field + + - - direction, field by the current x x adds on to field by the magnets as they travel in the same Up Down Up direction of A to B. To the left of the field dir n, field by the current opposes the field by the magnets as they travel in opposing direction. The magnetic force to the right of the field direction is stronger than than to the left of the field direction. Therefore, the overall force is to the right of the field direction. 3. INCREASING TURNING EFFECT EXPERIENCED BY A COIL Increase current in coil Why? Increase number of turns of coil Why? Insert soft iron core in the coil Why? A larger current will produce a greater concentration of field lines A strong field will lead to a larger force. Each loop of wires produces its own magnetic field. Since the magnetic field strength is the sum of the field lines, more lines will produce stronger field and hence greater force. The iron core becomes a magnet within the field lines, increasing overall concentration of magnetic field lines, hence greater force produced. 4. SPLIT-RING COMMUTATOR (DC MOTOR) Use left hand rule for DC! Discontinuous: It is split up, allowing current to be cut off at a point and current to continue its flow at another point, which ensures flow of current can be reversed. 1. Rectangular coil 2. Connected to a battery 3. Connected to a rheostat 4. Permanent magnet 5. Split-ring commutator 6. Two carbon brushes Purpose: Reverses the direction of current in the coil every half a revolution, when the coil passes the vertical position, switching to the other brush, so that it continues to turn in the same direction.

TOPIC 9: ELECTROMAGNETIC INDUCTION 1. FARADAY S LAW AND LENZ S LAW Electromagnetic induction is a phenomenon of inducing an emf in a closed circuit due to a varying magnetic field linking the circuit. It means that whenever there is a relative movement the magnet and the coil changes, magnetic field changes, thus emf is induced and hence a current is produced. When there is no movement, the magnetic field is steady and thus, no emf is induced and no current induced. Faraday s law states that the e.m.f. generated in a conductor is proportional to the rate of change of the magnetic lines of force linking with the circuit. Lenz s law states that the direction of the induced e.m.f and hence the induced current in a closed circuit is always such as to oppose the change in the applied magnetic field. Larger current is produced (greater deflection on galvanometer) when: Magnet is moved at a faster rate in and out of a coil A stronger magnet is used The number of coil is increases clockwise repel anticlockwise attract Magnet direction N Into coil N Out of coil Emf then current Opposite direction of the magnet motion North pole of the coil South pole of the coil Galvanometer Tries to repel the north pole of the magnet Invalid Invalid Tries to attract the north pole of the magnet Deflects right (as illustrated above) Deflects left (as illustrated above) 2. AC GENERATOR N S slip rings brush contacts Use right hand rule! Why are the slip rings continuous? Ensures continuous contact with the carbon brushes when the coil is rotating. This ensures that the alternating current induced in the coil is transferred to the external circuit. Why is the induced emf maximum when the coil is horizontal? The rectangular coil, when viewed from the side, is parallel to the magnetic lines of force. It is the point where both sides of the coil cuts through the magnetic field lines at the greatest rate. On the graph, when emf is read as minimum, it simply means that current is travelling in different direction (alternated) Why is induced emf zero when the coil is vertical? The rectangular coil is perpendicular to the magnetic lines of force and moves parallel to magnetic field, not cutting through the magnetic field lines.

3. WAIT A MINUTE... SO WHEN DO I USE LEFT HAND RULE AND RIGHT HAND RULE? Use the left hand rule (1 st one you learnt) when you are finding direction of force with the direction of magnetic field and direction of current. Use the right hand rule (2 nd one you learnt) when you are finding direction of current with the direction of magnetic field and direction of force. 4. GRAPH PLOTTING Number of coils doubles Speed of rotation doubles Strength of magnet doubles Graph of induced emf against time Induced emf of AC Wavelength halfs Frequency doubles generator (amplitude) doubles 5. TRANSFORMERS A device that changes a high alternating voltage at low current to a low alternating voltage at high current or vice-versa. It does so due to electromagnetic induction between the two coils to transfer electrical energy supplied from the primary coil to the secondary coil. Primary coil Secondary coil Electrical source Connected to A.C. Supply Connected to Electrical Load Transfers To create a continuously changing magnetic flux linkage with the secondary solenoid Applied alternating voltage sets up a changing magnetic field which passes through the soft core to the secondary coil to induce emf and hence current in secondary solenoid. An induced e.m.f. is produced Step-up Less coils More coils Step-down More coils Less coils More coils cause less current to be induced and more emf dissipated & vice versa The efficiency of a transformer is lower than 100% due to heat loss from: Resistance of coils Eddy currents induced in the soft iron core 6. EQUATIONS V S I S = V P I P V S I P = = N S V P I S N P V S N p = N S V P Turns ratio = N S N P Turns ratio of more than 1 is Step Up. Efficiency = V S I S V P I P 100% 7. ENERGY LOSS AND HIGH VOLTAGE TRANSMISSION Effect Power Loss Equation Resistance Lower Lower P = I 2 R Usage Use thicker cables for less power loss P 2 out R High voltage transmission in cables Voltage Greater Lower P loss = V 2 will result in less power loss Current Greater Greater P = I 2 R Disadvantage of usage May be expensive

TERMINOLOGY AND FORMULAE FOR PRELIM 1 1) Wave [1] 2) Wavefront [1] 3) Transverse wave [1] 4) Longitudinal wave [1] 5) Sound wave [1] 6) Echo [1] 7) Ultrasound [1] 8) Charge [1] 9) Current [1] 10) Electromotive force [1] 11) Potential difference [1] 12) Resistance [1] 13) Resistivity [1] 14) Ohm s law [1] 15) Live [1] 16) Neutral [1] 17) Earth [1] 18) Magnetic field [1] OTHER FORMULAE FOR PRELIM 1 1) State the formula to find velocity of a wave. 2) Find a formula that links distance between a navigator ship sailing in the middle of the Pacific Ocean and the seabed, D, and time taken for the ship to receive an echo, T, when it releases an ultrasound wave perpendicular to the seabed. 3) Find a formula that links distance between your hands and a wall, D, and time taken for you to hear an echo, T, when you clap your hands once. GENERAL QUESTIONS FOR PRELIM 1 1) How is a wave produced in general? [1] 2) How is a sound wave produced? [1] 3) What is the difference between a transverse and longitudinal wave? [2] 4) State the changes, if any, in frequency, wavelength, velocity and direction when water waves travel from shallow water to deeper water. [4] 5) State the properties of EM waves. [7] 6) Arrange the components of the EM spectrum in order of increasing wavelength. [1] 7) Arrange the components of the EM spectrum in order of decreasing frequency. [1] 8) State and describe one use of each of the components of the EM spectrum. [7] 9) How does infrared waves affect humans? [1] 10) How do rays of the EM spectrum result in permanent human damage. [2] 11) Why do hospitals try to use ultrasound waves instead of X-rays to scan an image of a developing baby in mothers wombs. [2] 12) What is the speed of sound in air? [1] 13) Arrange the following in order of higher transmission of sound waves to lower transmission of sound waves: Solids, Liquids and Gases. [1] 14) What effect does changes in frequency, wavelength and amplitude have on sound waves? [3] 15) What is ultrasound? Why can t humans hear ultrasound? [1] 16) Name an organism with that is unable to hear: i. ultrasound but able to hear infrasound [1] ii. infrasound but able to hear ultrasound [1] iii. both infrasound and ultrasound [1] 17) Describe the nature of the flow of electrons. [1] 18) State if repulsion or attraction happens for like and unlike charges. [1] 19) Explain how an object can be charged by friction and how such objects can be discharged. [2] 20) Explain how an object can be charged by induction and how such objects can be discharged. [2] 21) Draw the electric field between a positive and a negative charge. [1]

22) You are given only two conductors, two insulating stands that can be attached to the conductors, and a negatively charged rod. Explain how you can leave one of the conductors with an excess of positively charges and the other with an excess of negative charges simultaneously. [3] 23) What happens when a neutral object is brought close to a positively charged conductor? [2] 24) Decribe some hazards of electricity. [2] 25) Why are switches, fuses and circuit breakers wired into the live conductors? [1] 26) Explain the purpose of double insulation. [2] 27) Describe methods of magnetisation. [2] 28) Describe methods of demagnetisation. [3] 29) State Fleming s left hand and right hand rule. [1] 30) How can turning effect experienced by a coil be increased. [3] 31) What is the purpose of the carbon brush in a dc motor? [1] -END-