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Theme1 General Physics

Measure Instrument

- Vernier calipers + micrometer screw gauge

Pendulum – one oscillation

√

***even in a vacuum, T of different l is not the same

Speed: the distance moved per unit time

Velocity: the change in displacement per unit time

Displacement: distance in a specific direction

Acceleration: the change in velocity with time

Addition formula

( )

Air resistance: a frictional force

1. Apply on only moving objects

2. Air resistance↑ when Speed↑ surface area↑ density of air↑

Force: a push or pull that one object exerts on another

Scalar: only magnitude

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Vector: direction + magnitude

***For an object with constant velocity or zero acceleration, the

resultant force/net force is zero

Newton’s 1stlaw

‚Every object will continue in its state of rest or uniform motion in a

straight line unless a resultant force acts on it to change its state‛

Newton’s 2ndlaw

‚When a resultant force acts on an object of constant mass, the object

will accelerate and move in the direction of the resultant force. The

product of the mass and acceleration of the object is equal to the

resultant force.‛

Newton’s 3rdlaw

‚For every action, there is an equal and opposite reaction, and these

forces act on mutually opposite bodies.‛

Mass: a measure of the amount of matter or substance in a body

Weight: a force due to gravity

Gravitational field: the region surrounding the Earth where gravity is

experienced

Gravitational field strength: the gravitational force acting per unit

mass on an object

Inertia: the reluctance of the object to change its state of rest or

motion

***Inertia depends on only the mass of the object

Density: mass per unit volume

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Moment: the product of the force and the perpendicular distance from the

pivot to the line of action of the force

***Taking moments to the pivot, sum of anti-clockwise moment = sum of

clockwise moment

Principle of Moments

‚When a body is in equilibrium, the sum of clockwise moment about a

pivot is equal to the sum of anti-clockwise moment of the same pivot‛

Centre of Gravity: the point through which its whole weight appears to

act for any orientation of the object

*** A plumb line is used to find CG

Stability: the ability of an object to return to its original object

after it has been tiled slightly

Stable, Unstable and Neutral equilibrium (CG remains at the same level when it is tiled slightly) ***Low CG and wide base to increase stability

Energy: the capacity to do work

Kinetic Energy: the energy possessed by a body due to its virtual of

motion

Gravitational Potential Energy: the energy possessed by a body due to

virtual of its position

Principle of Conservation of Energy

‚Energy can neither be created nor destroyed in any process. It can be

converted from one form to another or transferred from one body to

another, but the total amount remains constant‛

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Work: the product of the force and the distance moved by the object in

the direction of the force

(1J=1Nm)

Power: the rate of woke done

(1W=1J/s)

Pressure: the force acting per unit area

,

*** Barometer is used to measure the atmospheric pressure

Theme2 Thermal Physics

Temperature: how hot or cold an object is

Heat: the amount of energy that is being transferred from a hotter to a

colder region

Thermocouple – electromotive force ( )

Factors affecting range, sensitivity and responsiveness

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State Arrangement Movement

s Closely packed in a regular

pattern

Vibrate about their fixed

position

l Closely packed in a disorderly

manner Sliding over each other

g spread far apart in a disorderly

manner Move rapidly at random

Brownian motion: the random, continuous and uneven movement of particles

suspended in a fluid

Boyle’s Law: pressure is inversely related to volume when the other

factors are constant

Overall formula

Key concepts

When the container is heated up (temperature increases); the particles

gain more kinetic energy and move faster randomly, the rate of collision

between the particles and the inner wall is more frequent, the total

force exerted to the inner wall increases, the pressure increases

When the volume is decreased; the space between the particles is

smaller, the number of molecules presented per unit volume increases,

the rate of collision between the particles and the inner wall

increases, the total force exerted to the inner wall increases, the

pressure increases

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Conduction: the process of thermal energy transfer without any flow of

the material medium

- Particle vibration; metal and non-metal

- Free electron diffusion; only metal

Convection: the transfer of thermal energy by mean of currents in a

fluid

Radiation: the continual emission of infrared waves from the surface of

all bodies, transmitted without the aid of a medium

***Dull, black surfaces are better emitters of infrared radiation than

shiny, white surfaces

Internal energy: The total kinetic and potential energy associated with

the motions and relative positions of the molecules of an object

Heat capacity(C): the amount of thermal energy required to raise the

temperature of a body by 1K (or 1˚C)

Specific heat capacity(c): the amount of thermal energy required to

raise the temperature of 1kg of a substance by 1K (or 1˚C)

Melting: the process of the change of solid state to liquid state

P b.p.

***increase pressure increase the melting point of water

Latent heat: the energy released or absorbed during a change of state

Latent heat of fusion: the amount of thermal energy required to change a

body from solid to liquid state, or vice versa, without a change in

temperature

Specific latent heat of fusion: the amount of thermal energy required to

change 1 kg of solid to liquid, or vice versa, without a change in

temperature

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Latent heat of vaporisation: the amount of thermal energy required to

change a body from liquid to vapour, or vice versa, without a change in

temperature

Specific latent heat of vaporisation: the amount of thermal energy

required to change 1 kg of liquid to vapour state, or vice versa,

without a change in temperature

,

Boiling: the process of the change of liquid state to gaseous state at a

fixed and constant temperature

Boiling Evaporation

1. Occurs at a fixed temperature 1. Occurs at any temperature

2. Quick process 2. Slow process

3. Takes place throughout the

liquid

3. Takes place only at the liquid

surface

4. Bubbles are formed in the liquid 4. No bubbles are formed in the

liquid

5. Thermal energy supplied by an

energy source

5. Thermal energy supplied by the

surroundings

Theme3 Light, Waves and Sound

Luminous and non-luminous object

Number of imaged formed – always round down

Refraction: Bending effect of light as it passes from one optical

material to another

Refractive index – Snell’s Law

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***light travels from lesser density to higher density

Critical angle: the angle of incidence in the optically denser medium

for which the angle of refraction in the less dense medium is 90°

Total internal reflection takes place only when

1. A ray of light travels from an optically denser to a less dense

medium

2. The angle of incidence in the optically denser medium is greater than

the critical angle

Optical fibres

_ can carry a much higher volume of information over long distances than

the electrical wires

_ lighter, thinner and cheaper for manufacture

_ high quality transmission of information over long distances with

negligible loss

Laws of Reflection1

1. Angle of incidence is equal to the angle of reflection

2. The incident ray, reflected ray and the normal at the point of

incidence all lie on the same plane

Lens

Periodic motion: motion repeated at regular intervals

***the source of any wave is a vibration or oscillation

***waves move/propagate through the medium

***In waves, energy is transferred without the medium being transferred

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Types of wave motion

1. Longitudinal waves: waves that travel in a direction parallel to the

direction of the vibration

2. Transverse waves: waves that travel in a direction perpendicular to

the direction of the vibration

Crests: the highest points of a transverse wave

Troughs: the lowest points of a transverse wave

***Compressions & rarefactions in longitudinal waves

Phase: any two points that move in the same direction, have the same

speed and same displacement from the rest position

Wavelength (λ): the shortest between two points that are in phase

Amplitude (A): the maximum displacement from the rest

Period (T): the time taken for one point on the wave to complete one

oscillation

Frequency (ƒ): the number of complete waves produced per second

ƒ

Wave speed (v)

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λ

ƒλ

Wave front: an imaginary line on a wave that joins all points that are

in the same phase

Refraction of waves

*** v & λ decrease from deep to shallow water

Reflection of waves

- i = r

- f, λ, v are the same

Electromagnetic waves – low f, high λ to high f, low λ

; Radio waves microwaves infrared visible light* ultraviolet

X-rays gamma rays

***red blue

Properties of Electromagnetic waves

1. They are transverse waves. They are electric (horizontal) and

magnetic (vertical) fields that oscillate at 90 to each other

2. They transfer energy from one place to another

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3. They can travel through vacuum, do not require any medium

4. Their speed in vacuum is 3.0108 m/s2

5. ƒλ

6. They obey the laws of reflection and refraction

7. They carry no electric charge

8. Their ƒ do not change when travel from one medium to another. Their ƒ

depend only on the source of the wave. Their v and λ change.

Ionisation: the process of ion formation

Ionising radiation: the rejection of one or more electrons from an atom

or molecule to produce a fragment with a net positive charge

Effects of ionising radiation

1. Molecular level: irradiation of human tissues, damage to proteins,

nucleic acid

2. Sub-cellular level: damage Chromosomes (DNA)

3. A pregnant woman: an abnormal pattern of cell division, leading to

cancers such as leukaemia

4. Organism level: premature aging and shortening of lifespan

***Sound is longitudinal wave

***Sound is produced by vibrating sources placed in a medium

***the series of compressions and rarefactions produced by the shifting

of air layers

Compressions – slightly higher pressure than the surrounding air

pressure

Rarefactions – slightly lower pressure than the surrounding air

pressure

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Wavelength of the sound (λ): the distance between two consecutive

compressions or rarefactions

Amplitude of the sound (A): the maximum pressure change

Medium of transmission of sound

V gas 5 =V liquid

V gas 15 =V solid

V solid > V liquid > V gas

√ , T=temperature

Reflection of sound

Echo: the repetition of the clap

***an echo is formed when a sound is reflected off hard, flat surfaces

Range of audibility: the range of frequencies which a person can hear

***human ears – 20 Hz-20,000 Hz

Ultrasound: sound with frequencies above the upper limit of the human

range of audibility (above 20,000 Hz)

Infrasound: sound with frequencies below the lower limit of the human

range of audibility (above 20 Hz)

Pitch – a music note or sound as ‘high’ or ‘low’

ƒ

Loudness: the volume of a sound related to the amplitude of a sound

Theme4 Electricity and Magnetism

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Electrostatics: the study of static electric charges

***same charges – repel (repulsive force)

***different charges – attract (attractive force)

The amount of charge an e-/P+ has is 1.610-19 C

6.2510-19 electrons = 1 C

Electrical insulators: materials where electrons are not free to move

about

***they are charged by friction

Electrical conductors: materials that allow electrons to move freely

within them

***they are charged by induction

Induction: the process of charging a conductor without any contact with

the charging body

Induction – 2 metals

1. Two conductors (metal spheres) on insulating stands are placed

touching each other

2. A negative charged rod is brought near sphere A. This causes the

electrons in the metal spheres to be replied to the far end of sphere B.

Sphere A can be seen to have excess positive charges, while sphere B has

excess positive charges.

3. Without removing the rod, separate spheres A and B.

4. Remove the charged rod. Spheres A and B now have equal amounts of

opposite charges. Spheres A and B have been charged by induction.

Induction – 1 metal

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1. Bring a positively charged glass rod near the metal conductor on an

insulating stand. The free electrons in the metal will be drawn towards

the side nearer the positively charged glass rod.

2. Without removing the glass rod, earth the positively charged side of

the metal conductor by touching in with your hand. The human body is a

relatively good conductor and will allow electrons to flow into the

conductor from the ground. This will neutralize the positive on this

side of the conductor.

3. With the glass rod still in place, remove your hand from the

conductor. This will stop the earthing process.

4. Remove the glass rod. The negative charges will be redistributed on

the surface of the conductor. The conductor is now negatively charged.

Electric force: a force experienced by charges

An electric field: a region where an electric charge experiences an

electric force

Electric lines of force: imaginary lines, showing the path a positive

charge would take if it was free to move.

The direction of the field: the direction of the force on a small

positive charge

***The strength of an electric field is indicated by how close the filed

lines are to each other

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An electric current is caused by a flow of electrons

Electron flow: movement of electrons from the negatively charged end to

the positively charged end

Convectional current flow: the assumption that an electric current

consist of positive charges flowing from the positively charged end to

the negatively charged end

An electric current (I): a measure of the rate of flow of electric

charge (Q) through a given cross section of a conductor

An electric circuit: a complete or close path through which charge can

flow from one terminal of an electrical source to the other terminal

The electromotive force (e.m.f.) of an electrical energy source: the work done by the source in driving a unit charge round a complete

circuit

Cells in series

‚The combined e.m.f. in increased because electric charges gain

electrical energy from each cell when they pass through them.‛

Cells in parallel

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‚The energy required to move electric charges through the load will be

contributed equally by each cell. Thus, each cell only needs to provide

half the energy to move the charges through the circuit.‛

Potential difference (p.d.) between two points in an electric circuit: the amount of electrical energy converted to other forms of energy when

one coulomb of positive charge passes between the two points

Resistance: a property of the material that restricts the movement of

free electrons in the material

The resistance (R) of a component: the ratio of the potential difference

(V) across it to the current (I) flowing through it

A resistor: a conductor in a circuit that has a known value of

resistance

Ohm’s Law

‚The current passing through a metallic conductor is directly

proportional to the potential difference across its ends, provided the

physical conditions are constant‛

***ohmic conductors – conductors that obey Ohm’s Law

*** I-V graph: a straight line passes through the origin

Non-ohmic conductors – I-V graph is not a straight line (

is not a

constant)

Ex; Filament lamp, thermistor, semiconductor diode

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Resistivity (ρ)

Series Circuits

‚When similar resistors are connected in series, the combined resistance is larger than the individual resistance of a single resistor. As a result, this cause the current in the circuit to be smaller if the e.m.f. supplied in the same.‛

‚In a series circuit, the sum of the potential across each component is

equal to the difference across the whole circuit‛

‚The combined resistance of resistors in series is the sum of all the

resistances.‛

Parallel Circuits

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‚In a parallel circuit, the sum of the individual currents in each of

the parallel branches is equal to the main current flowing into and out

of the parallel branches.‛

***Bulbs connected in parallel will glow more brightly than when

connected in series

***Another advantage of connecting bulbs in parallel is that when one of

the light bulbs blows, the other light bulb will continue to glow –

there is still a complete circuit through the other parallel branch for the current to flow

≠

***When one of the bulbs in series blows, the entire circuit will be open and the other bulb will not light up

Potential Divider: A circuit with resistors arranged in series

(

)

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Transducers: electric or electronic devices that convert energy from one

form to another – they respond to physical quantities such as temperature and light

Input transducers – convert non-electrical energy to electrical energy

Output transducers – convert electrical energy to non-electrical energy

Ex; Thermistors, Light-dependent resistor (LDR)

Thermistors –

Light-dependent resistor (LDR) –

Electric heating

_ Usually made up of nichrome wire; because of its high resistivity and

ability to withstand high temperature;

_ As it has high resistivity, the electric current is decreased and

hence the temperature cannot reach the m.p. and b.p.

_ Thermal energy is generated when an electric current passes through

the heating element

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Electric lighting – filament & fluorescent lamp

Filament lamp

_ Filament is made of a tungsten coil; tungsten has high resistivity and

m.p. (3400°C)

_ Filament is thin (small cross-sectional area – A)

***↑ρ ↑l ↓A ↑R ↓I temperature cannot reach the m.p.

_ contains Argon and Nitrogen to prevent the tungsten to burnt

Fluorescent lamp

_ more efficient than filament lamps (3000 hours vs 1000 hours)

_ use less energy than filament lamps

_ light produced when passing electric charges between two electrodes

_the mercury vapour contained in the glass tube emits ultraviolet light

with invisible light which is converted to visible light by fluorescent

powder coated on the inner wall

Advantages Disadvantages

Filament Give cosy and relaxed

atmoshere

10% light

90% heat

Fluorescent Energy efficient Costly & toxic

Electric motors

_ work on the principles of the magnetic effects of a current

_ electric energy rotational kinetic energy

Power: the rate of woke done or energy converted

Dangers of Electricity

Damage insulation

_ electrical insulation crack and break; exposing the conducting wires

inside

_ cause severe electric shock if it is touched

Overheating of cables

_ an unusually large current flows through the conducting wires

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_ the higher resistance of thinner wires will produce more thermal heat

that will damage the insulation and may cause a fire

***thin wires are used for appliances that need less power and vice

versa

Damp conditions

_ as our human body can only withstand a current of about 50mA, the

large current will electrocute the person

_ R of human body is low; ↓R ↑I ***

Safe use of electricity at home

_ electricity is supplied by a cable containing 2 wires – live wire (L)

and neutral wire (N)

live wire (L) – 240V

neutral wire (N) – 0V

These 2 wires are connected to a main fuse box, an electricity meter and

a consumer unit.

The consumer unit: the distribution point for the household’s

electricity supply. – consists of a main switch and circuit breakers

1. Circuit breakers: safety devices that can switch off the electrical

supply in a circuit when there is an overflow of current

1.1 Miniature Circuit Breaker (MCB): when the current exceeds the

current values labeled, the circuit breaker will trip.

1.2 Earth Leakage Circuit Breaker (ELCB): detects the small current

leakages from the live wire to the earth wire. When this happens, the

current in the live wire will be greater than the neutral wire, causing

the ELCB to trip.

2. Fuses: safety devices included in an electrical circuit to prevent

excessive current flow

_ same function as the MCB

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_ A fuse consists of a short thin piece of wire which becomes hot and

melts when the current flowing through it is greater than its rated

value.

a. Fuses should have a current rating just slightly higher than the

current an electrical appliance will use under normal

b. A fuse should be connected to the live wire so that the appliance

will not become charged after the fuse has melted due to the over flow

of current

c. Before you charge a fuse, always switch off the mains power supply

3. Switches

_ they break or complete an electrical circuit

‚If the switch id fitted onto the neutral wire, the appliance will be

‘live’ even though the switch is ‘off’. Anybody who touches the

metal casing the appliance would experience an electric shock.‛ –

wrong

_ Switches must be fitted onto the live wire so that switching off dis

connects the high voltage from an appliance – correct

4. Plugs and sockets

_ a cartridge fuse protects the appliance from excessive current flow

5. Earthing

_ earth wire (E) – green & yellow

_ live wire (L) – brown

_ neutral wire (N) – blue

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The earth wire: a low-resistance wire which usually connected to the

metal casing of the appliance

‚If there is a fault – the live wire touches the metal casing of the

appliance – the user could get an electric shock.‛

_ the earth wire connected to the metal casing diverts the large current

due to the electrical fault to the ground

6. Double insulation

_ a safety feature which provides 2 levels of insulation in an

electrical appliance that can substitute the earth wire

6.1. The electric cable is insulated from the internal components of the

appliance

6.2. The internal components are also insulated from the external casing

Magnetite: a naturally occurring iron oxide mineral

Magnetic/ferromagnetic materials: the materials that are attracted by a

natural magnet

A permanent magnet: a material that retains its magnetism for a long

time

Properties of magnets

1. The poles are where the magnetic effects are the strongest.

2. When we suspend a bar magnet freely, the north-seeking pole will

point to the North Pole and the south-seeking pole will point to the

South Pole.

3. Law of magnetism

‚Like poles repel, unlike poles attract‛

Repulsion: the only test to confirm that an object is a magnet

Magnetic Induction: the process where ferromagnetic materials become

magnetised when they are near or in contact with a permanent magnet

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Theory of magnetism

‚If we take a bar magnet and cut it into three smaller pieces, we will

notice that every piece becomes a magnet itself with an N pole and an S

ploe.‛

A magnetic domain: a group of atomic magnets pointing in the same

direction

***In a permanent bar – the magnetic domains point in the same

direction

***In an unmagnetised bar – the magnetic domains point in random

direction; the magnetic effects of the atomic magnets cancel out so

there is no resultant magnetic effect

Phenomena

1. Magnetic saturation

‚Every magnet has a maximum strength when all the magnetic domains are

pointing in the same direction‛ – the magnet is magnetically saturated

2. Demagnetisation of magnets

Demagnetisation: the process of removing magnetism from a magnet

Ex; heating, hammering

‚They cause the atoms of the magnet to vibrate vigorously, mixing up

the directions of the magnetic domains.‛

3. Storage of magnets using soft iron keepers

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‚If we store magnets side by side, the magnets become weaker after some

time as ‘free’ poles near the ends of the magnet will repel one

another. The magnetic domains will be altered, weakening the magnets.‛

‚We store bar magnets in pairs by using soft iron keepers across the

ends of the bar magnets. The poles of the atomic magnets are in closed

loops with on ‘free’ poles to weaken the magnetic domains.‛

Ways of making magnets

1. Stroking method

***precaution is that the stroking magnet must br lifted sufficiently

high above the steel bar between successive stroke.

2. Electrical method using a direct current

‚When an electric current flows through the solenoid, it produces a

strong magnetic field which magnetizes the steel bar.‛

***The poles of the magnet is determined by the right-hand grip rule

Ways to demagnetising magnets

1. Heating

‚The atoms of the magnet vibrate vigorously when heated, causing the

magnetic domains to lose their alignment.‛

2. Hammering

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‚Hammering alters the alignment of the magnetic domains, causing the

magnet to lose its magnetism.‛

3. Electrical method using an alternating current

‚An alternating current is an electric current which varies its

direction many times per second. The magnet is then slowly withdrawn in

the East-West direction with the alternating current still flowing in

the solenoid.‛

A magnetic field: a region in which a magnetic object, placed within the

influence of the field, experiences a magnetic force

Magnetic field lines: invisible lines of force which we assume are

emerging from the North Pole and entering the South pole of the magnet

_ Magnetic field lines do not cross or intersect one another

_ the field lines drawn closer together represent strong magnetic fields

_ the field lines drawn further apart together represent weak magnetic

fields

***the point between two N poles is the neutral point

The earth’s magnetic field

A large imaginary magnet within the earth is believed to be caused by

convection currents inside the earth’s molten outer core

The imaginary ‘S’ pole is at the geographic north pole

The imaginary ‘N’ pole is at the geographic south pole

Magnetic shielding: a method of creating a region or space that is free

of magnetic fields by means of a closed loop of soft magnetic materials

_ use thin sheets of soft magnetic materials; they work by diverting the

magnetic fields

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‚Magnetic field lines tend to pass through magnetic materials easily.‛

Iron –soft magnetic material

_ gain and lose magnetism easily; strong induced magnet

_ induced magnetism occurs instantaneously either induced by another

magnet, or by a solenoid conducting electricity

_ loses its induced magnetism when the inducing magnet is removed

_ a temporary magnet; does not retain its magnetism

Steel –hard magnetic material

_ difficult to gain and lose magnetism; weak induced magnet

_ induced magnetism occurs slowly

_ does not lose its induced magnetism easily once steel is magnetised

_ a permanent magnet; retain its magnetism

‚A current-carrying conductor produces a magnetic field around it.‛

***using right-hand grip rule to find the direction of the magnetic

field around the wire

***the magnetic field of a long, straight current-carrying wire is

stronger when it is closer to the wire or when a large current flows

through the wire

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To increase the magnetic field strength at the centre of the flat coil

1. increase the current

2. increase the number of turns of the coil

To increase the magnetic field strength in a solenoid

1. increase the current

2. increase the number of turns per unit length of the solenoid

3. place a soft iron core within the solenoid; the soft iron core

concentrates the magnetic field lines, thereby increasing the magnetic

field strength

Uses of electromagnets

Circuit breaker

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_ When the current in a circuit increases, the strength of the

electromagnet will increase in accordance; this will pull the soft iron

armature towards the electromagnet.

_ As a result, the spring pulls apart the contact and disconnects the

circuit immediately, and the current stop to flow.

_ We can reconnect the circuit by using the reset button. The reset

button can be pushed to bring the contact back to its original position

to reconnect the circuit.

Motor effect

‚The force on the current-carrying conductor in a magnet field acts

perpendicular to both the direction of the current and the direction of

the magnetic field.‛

‚The force is reversed when we reverse the direction of the current or

magnetic field.‛

Fleming’s Left-Hand Rule

_ Thumb – motion

_ Forefinger – field

_ Second finger – current

Combined magnetic field when the wire is placed between the poles of the

magnet

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_ the combined field lines acting in the same direction gives a stronger

field than the combined field lines acting in the different direction

force

Forces between two parallel current-carrying wires

‚Currents in opposing directions cause repulsion. Currents in similar

directions cause attraction.‛

To increase the turning effect on the wire coil in a magnetic field

1. increase the number of turns on the wire coil

2. increase the current in the coil – lower the resistance / increase

the voltage supply

3. insert a soft iron core into the coil to concentrate the magnetic

field lines

4. use stronger permanent magnet

The D.C. motor

_ electrical energy mechanical energy

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Components

1. Rectangular coil connected in series to a battery and rheostat

2. Permanent magnets

3. Split-ring commutator

4. two carbon brushes

Split-ring commutator

; to reverse the direction of the current in the loop (coil every half a

revolution) whenever the commutator changes contact from one brush to

the other

Carbon brushes

; to conduct current to flow into and out of the coil

Electromagnetic induction: the phenomenon of inducing on electromotive

force (e.m.f.) in a circuit due to a changing magnetic field

The magnitude of this induced e.m.f. depends on;

1. the number of turns in the solenoid

2. the strength of the magnet

3. the speed inserting the magnet or withdrawing from the solenoid

Faraday’s law of electromagnetic induction

‚The e.m.f. induced in a conductor is proportional to the rate of

change of magnetic lines of force linking the circuit.‛

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Lenz’s law

‚The direction of the induced e.m.f., and hence the induced current in

a circuit, is always such that its magnetic effect opposes the motion or

change producing it.‛

***There is no e.m.f. generated when bar magnet is stationary

An A.C. generator: a device that uses the principle of electromagnetic

induction to transform mechanical energy into electrical energy

Alternation current generators (A.C. current)

_ the slip rings ensures that the direction of the induced current

flowing in the external circuit changes every half revolution

‚The induced e.m.f. is maximum when the coil in parallel to the

magnetic lines of force. The coil experiences the greastest changes in

magnetic field.‛

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‚The induced e.m.f. is zero when the coil in perpendicular to the

magnetic lines of force as the coil is not cutting through the magnetic

field lines. The coil experiences no changes in magnetic field.‛

To increase the induced e.m.f.

1. increase the numbers of turns on the coil

2. increase the frequency of rotation of the coil

3. use stronger permanent magnets

4. insert a soft iron core into the coil to concentrate the magnetic

field lines

The fixed coil A.C. generator is preferred over the simple A.C.

generator

1. Carbon brushes wear and tear easily

2. The connection with the slip ring becomes loose when the carbon brush

is eroded increase the resistance at the connecting point lesser

current is generated as it causes unnecessary thermal energy

3. The fixed coil A.C. generator design is more compact and space-saving

A transformer: a device that changes a high alternating voltage (at low

current) to a low alternating voltage (at high current), and vice versa

_ coil A induces e.m.f. in coil B, using A.C. current

_ this e.m.f. in turn drives an induced current to flow in coil B

Function of a transformer

1. Electrical power transmission

2. Regulating voltages for proper operation of electrical appliances

A closed-core transformer

34 | P h y s i c

_ the lamination of the soft iron core reduces heat loss due to induced

eddy currents

A step-up transformer – Vs > Vp

A step-down transformer – Vs < Vp

***100% efficiency

Causes of power loss

1. heat loss due to the resistance of the coils

2. leakage of magnetic field lines between the primary and secondary

coils

3. heat loss due to eddy currents induced in the iron core

4. hysteresis loss caused by the flipping of magnetic dipoles in the

iron core due to A.C.

Reduce heat loss due to resistance

1. use thicker cables

2. reduce the current I, using a step-up transformer

(

)

***Power can be transmitted more efficiently at higher voltages and

lower currents

Converting A.C. to D.C. – diodes

The diode: a semiconductor device that allows a current to flow easily

in one direction only

35 | P h y s i c

Rectification: the conversion of A.C. into D.C.

Half-wave rectification

Full-wave rectification – a bridge rectifier

36 | P h y s i c

Cathode-Ray Oscilloscope – C.R.O.

_ The electron gun emits a beam of electrons (thermonic emission) – a

cathode ray

_ The fluorescent screen is coated with Zinc sulphide

_ the Y-plates – vary the vertical position

_ the Y-plates – sweep the electron beam horizontally

Y-gain

_ amplifies the Y-deflection so that the small input voltages are

amplified before they are applied to the Y-plates

Time-base

_ controls the speed, at which the electron beam sweeps across the

screen horizontally from left to right– by the X-plates

_ sawtooth voltage applied to the X-plates

Physics Formula

Topic Formula SI unit Final unit

2.1: Kinematics DistanceSpeed

Time

Distance (m) Time (sec)

m/s

DisplacementVelocity

Time ;

sv

t

Displacement (m) Time (sec)

m/s

Diff. in VelocityAcceleration

Time

Condition: Used only when acceleration is constant.

Velocity (m/s) Time (sec)

m/s2

2.2 Dynamics Resultant Force Mass Acceleration

F ma

Force (N) Mass (kg) Acceleration (m/s

2)

Newton (N)

2.3 Mass Weight Density

W mg Mass (kg) g = 10 N/kg

Newton (N)

(Density)m

V ;.

Mass (g/kg) Volume (cm

3/m

3)

g/cm3 or

kg/m3

2.4 Turning Effect of Forces

Moments Fd Force (N) Perpendicular Distance (m)

Newton metre (Nm)

Note: Perpendicular Distance is not always the length of the rod. 2.5 Pressure

Solids: Force

PressureArea

F

A

Force (N) Area (m

2)

N/m2 , Pa

Liquids: Pressure h g h (m): Depth of Liquid (kg/m

3): Density of liquid

g: 10N/kg

N/m2 , Pa

Gases (when temp. is constant)

1 1 2 2PV PV

P (Pa): Pressure V (m

3): Volume

NA

2.6 Energy, Work, power

(Work Done)W Fd F (N): Force d (Perpendicular dist): m

J

21. . Kinetic Energy

2K E mv

m (kg): Mass v (m/s): Velocity

J

. . Potential EnergyPE mgh m (kg): Mass g: 10N/kg h (m): Height

J

X or Energy change

PowerTime

WP

Energy change /Work done(J) Time (s)

J/s, W (watt)

3.1 Principles of Thermometry

0

100 0

X X

X X

(For Celsius scale only) Theta: Unknown temperature X0: “ice point”, X100: Steam pt

oC

3.2 Thermal Properties of

Matter

(heat energy)Q C C: Heat capacity

J

Q mc m: mass c: Specific Heat Capacity

J

fQ ml fl : Latent heat of fusion J

vQ ml vl : Latent heat of vapourisation J

4.1: General Wave Properties

1fT

f: Frequency t (sec): Time

Hz

v f

v (m/s): Velocity

(m): Wavelength

f(1/t): Frequency

m/s

4.2: Light Snell’s Law:

sin

sin

in

r

n = refractive index (ratio) i/r (

o): angle of

incidence/refraction *Set calculator in degree mode.

NA. Ratio.

Condition: The angle of incidence must be in the less dense medium; angle r must be in the denser medium.

4.2: Light Ht of imageReal depth

Apparent depth Ht of object

cn

v

c (m/s): Speed of light in vaccum (3x10

8 m/s)

v (m/s): Speed of light in medium.

NA. Ratio.

1sinc n

c (o): Critical angle.

o

5.1: Current Electricity

QI

t

I: Current (A) Q: Charge (Columb) t: Time (sec)

Coloumb, C

W

Q

: E.m.f. (Volts – V)

W: Work done/energy of circuit (J) Q: Charge (Columb)

V, J/C

WV

Q

V: Potential Diff. (V) W: Work done/energy across circuit component Q: Amount of charge

V, J/C

Ohm’s Law: V IR

Condition: Only for ohmic conductors.

R: Resistance ( ) V

lR

A

m)

L: Length A: Cross-sectional Area

5.2: Practical Electricity

22 V t

E VIt I RTR

J

22 V

P VI I RR

W

5.3: Electromagnetic

Induction

ps s

p p s

IV N

V N I