O/A Level Physics Simplified

Frequently asked questions and their answers from IGCSE P1
Composed and compiled by Rafquat Sir
Topic: Electricity

Q: Explain how a fuse along with an earth wire provides safety to the user.
A: Earth wires are connected to the metal part of an appliance. When there is a fault in the appliance and a large current flows through the wire, the fuse blows preventing any further flow of current in the circuit.

Q: Explain why electricity is transmitted using high voltages.
A: According to P=VI, since the power remains the same, using high voltage means current can be kept low. Current has a heating effect. Using low current means less energy wasted in the form of heat which in turn increases efficiency.

Q: An appliance requires a current of 4.9A for operating normally. Explain why a fuse rated 5A is not suitable for this appliance.
A: Even a small fluctuation in current will cause the fuse to blow and it has to replaced frequently.

Q: Some appliances have three pin plugs and others have two pin plugs. Explain why?
A: Three pin plugs have an earth connection and are required when the appliance has a metal casing. Appliances with two pin plugs have double insulation. This means that their casings are made from insulating materials such as plastic or rubber. Appliances with a double insulation do not require an earth connection.

Q: Explain why mains electricity is transmitted using AC.
A: Only AC electricity can be stepped up and stepped down.

Q: Explain the advantages of electrostatic painting
A: The paint droplets are given the same charge. Since like charges repel each other, they spread out and cover a wide area. The object being painted is given an opposite charge. Since unlike charges attract each other, the paint droplets stick to the object and reduce wastage

Q: Briefly describe the functions of an electroscope.
A: An UNCHARGED electroscope is used to detect whether an object is charged or uncharged. If a charged object is brought near an electroscope, irrespective of the charge, the electroscope will show a deflection. However, to detect the type of charge on an object, the electroscope must have a known charge. Depending on the type of charge, the deflection of the leaf will either increase or decrease

Q: Briefly describe the advantages and disadvantages of series and parallel circuits.
A: In series, the advantage is less wiring is required to set up a circuit but the disadvantage is that if one component is damaged, the entire circuit is shut down.
In parallel, the advantages are all components connected in parallel can be operated individually and even if one component gets damaged, the entire circuit is not affected.

Q: Explain why a voltmeter has a very high resistance and is always connected in parallel
A: Voltmeters measure the voltage across a component. Since voltage remains same in parallel, they should be connected in parallel.
They must have a very high resistance so that no current flows through them.

P. S conversely you may be asked why an ammeter is always connected in series and has a very low resistance

Frequently asked question and their answers from IGCSE P1
Composed and compiled by Rafquat Sir

TOPIC: MECHANICS
Q: A car travelling in a circle with a steady speed is actually accelerating. Explain why?
A: Velocity depends on both speed and direction. Even though the speed is constant, the direction is constantly changing causing the velocity to change. A change in velocity means the car must be accelerating.

Q: Explain why an object falling in air initially accelerates but then reaches a steady speed (terminal velocity)
A: As the speed increases, the air resistance also increases. When the air resistance becomes equal to the weight, the forces on the object are balanced OR resultant force becomes zero

Q: State the factors affecting thinking distance and braking distance for a vehicle.
A: Thinking distance - speed, reaction time
Braking distance - speed, mass of the vehicle, condition of the brakes and condition of the road
P.S you should also remember how these factors affect the braking/thinking distance. For instance if a question asks how the braking distance would be affected due to icy road conditions, you should explain how friction would be less causing braking distance to increase.

Q: Explain why a gun recoils when it is fired and why is its velocity much smaller than that of the bullet.
A: The initial momentum before the shot is fired is zero. According to the law of conservation of momentum, the momentum after the shot fired must also be zero. To conserve momentum the gun recoils in the opposite direction with the same momentum as the gun. Even though they both have the same (value of) momentum, since the mass of the gun is more, its velocity is much smaller

Q: Explain how crumple zones/seat belts reduce the chances of injury during collisions.
A: Crumple zones/seat belts increase the time of impact during collision. The same momentum changes over a larger time. This reduces the impact force and therefore reduces injury
P.S. The same theory applies to why a fielder moves his hand backward when the ball comes into contact with his hand

Q: Explain how a smaller force can be applied to open a tin can using a spoon.
A: Moment = force X perpendicular distance. If a longer spoon is used, the perpendicular distance increases. Therefore a smaller force can be used to produce the same moment.
P.S. The same theory applies to how a tight screw/nut can be opened by applying a smaller force

Q: Describe how the orbit of comets differ from that of a planet (e.g earth)
A: The orbit of comets are elliptical with the sun at one focus. The orbit of planets are somewhat circular. Comets and planets have different time periods and different orbital radii.

Q: Explain how two astronomical objects (such as moons, planets) can have the same speed even if they have different orbital radii.
A: Orbital speed = 2πr/T. Therefore to keep the speed constant, if one object has a greater radius, it must have a greater time period.
P.S Conversely a question might also ask how two astronomical objects can have the same time period even if they have different orbital radii. Try to work it out from the equation above keeping T constant.

Q: Explain why a comet speeds up when it approaches the sun.
A: The gravitational force on the comet increases, when the distance between the sun and the comet decreases. Greater force means greater acceleration and therefore greater speed

Registration Deadline: 25th January
Orientation and routine discussion: 29th January
Class starts from February
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IGCSE JAN 2016 MOCK SCHEDULE
Mock 1: 10th November
Mock 2: 17th November
Mock 3: 24th November
Mock 4: 1st December
Mock 5: 8th December
Mock 6: 15th December
Mock 7: 22nd December

Paper 1: 12:30 to 2:30PM
Paper 2: 2:30 to 3:30PM

If you're thinking that the upward force from the blower is balancing out the weight of the ball, you're gravely mistaken!

Defying Gravity Making a 45g metal golf ball float in air stream using a 1000W air blower

What if I told you both the sides of the cube are of the same colour? Cover the red dots with your index finger and see it for yourself.

Last minute revision tips for Physics UNIT 2
ELECTRICITY
● Always construct answers involving potential divider from the context of voltage, not current!
● Questions involving resistance and dimensions of a material should always be answered using the intrinsic equation of resistance (R=ρl/A)
● The y intercept gives the emf and the negative of the gradient gives the internal resistance for the V-I graph
● In a series circuit, the higher the resistance of a component, the greater the P.D across that component
● To answer questions that ask why the resistance of certain materials (metals/ thermistors) change with temperature, always use the transport equation, I = nAve
● While analysing graphs of electricity, make sure you have assessed the axes carefully i.e whether it's a V-I graph or an I-V graph
● If identical resistors are connected in parallel, the total resistance given by an individual resistance divided by number of resistors. for example, if three 9 Ohm resistors are connected in parallel, the total resistance is 9/3 = 3 Ohm!
● If two cells are connected + to - , the resultant emf is the summation of the two emfs. If two cells are connected + to +, the resultant emf is given by the difference between the two emfs
● Some questions will try to trick you into opting for a wrong method. Always try to link a question with either the definition or the equation. For instance, if a question asks you to find the rate at which energy is being transferred, it's basically asking you to find the power!
● While calculating values using gradient from graphs, always remember to draw a tangent if it's a curved line! Local values don't give the gradient if it's a curved line.

Since you're still online moments before your exam, you might as well make the most of it! Check out these last minute revision tips for PHYSICS UNIT 2
WAVES
● Try to figure out the numerical link between path difference and phase difference
● In calculations, substitute the term path difference with difference in path lengths in your mind and you'll find that this approach is way easier
● Never confuse the terms plane and direction while answering questions about polarisation. In plane polarised light, OSCILLATIONS are in a single plane which includes direction of MOTION of the wave. NOTE: This plane is NOT PERPENDICULAR to the direction of motion of the wave
● Doppler effect is only eligible when there is a RELATIVE motion between the source and observer. For instance. If the source and observer are both moving in the same direction with the same speed, there will be no doppler shift.
● Every time you come across the concept of echo make sure you recall the correct equation v =2d/t and not just d/t
● In pulse echo techniques, the smaller the wavelength, the better the resolution/quality of the image
● The two phenomena, interference pattern and standing wave both rely on the principle of superposition.
● While answering qualitative questions, analyze the factors that determine the properties of a wave. For instance the speed of a wave depends on the medium and the frequency of a wave depends on the source. Therefore even after refraction, the frequency remains unchanged
● The critical angle of glass is about 42 degree and the refractive index is about 1.5. Values that differ too much from these are most likely to be wrong.
● The particle nature of light is demonstrated by photoelectric effect and the wave nature of electrons is demonstrated by electron diffraction. Never mix up the two concepts
● For standing waves in stringed instruments, there is always a node at each end. For wind instruments, there is always an antinode at the open end and a node at the closed end
Keep an eye on the page for the revision tips for electricity and nature of light

LAST MINUTE REVISION TIPS FOR PAPER 2

● Check the command words (state, describe, explain, suggest) carefully before starting to answer a question.

● In experimental questions, always remember you repeat an experiment to ensure reliability NOT accuracy!

● A fair test is ensured by keeping a variable constant that would otherwise affect the other quantities. For instance in an experiment to demonstrate how volume changes with pressure, the temperature must be kept constant. because a change in temperature would affect both volume and pressure

● There are several types of variables –
A variable which can have ANY numerical value is called a continuous variable. Examples include time, length mass etc
A variable which can only have whole numbers is called a discrete variable. Examples include number of people, numbers on shoes (shoe sizes) etc
Variables can be related. If one variable depends on the other, it is called the dependent variable. The one on which it depends is called the independent variable.
Always draw a BAR CHART while dealing with a discrete variable!

● Anomalous values are discarded and the experiment is repeated

● While answering questions that link Force and change in momentum, always construct your answer based on TIME. For instance, wearing seatbelts increases the time it takes for momentum to change, in turn reducing the impact force.

● While describing the different types of waves never confused the following terms with one another vibration/oscillation AND propagation/motion

● When a question asks you to use the graph to find a quantity, you MUST use the graph and not an equation.

● Using graphs automatically averages values, helps to identify anomalous results and increases reliability

● Recall the arrangement and motion of particles in the three states of matter
● Recall how gas molecules exert a pressure on the walls of their container
● In calculations associated with gas laws ALWAYS convert the temperature to Kelvin scale!
● While describing an experimental method never mix up the two keywords - measure and calculate. For example, in an experiment to find the terminal velocity of a falling object, you MEASURE the distance and the time but CALCULATE the velocity

● Recall the advantages of digital signals over analog signals
● In electromagnetism and electromagnetic induction, never confuse the MOTOR effect with the GENERATOR effect.

● Flemings left hand rule only helps you to identify the DIRECTIONS of the quantities in concern. The quantities themselves are not produces because of FLHR

● While doing trigonometric calculations, always make sure the mode on your calculator is in D and not R!

● When describing the harmful effects of radiation, just don't write "causes cancer"!

● FINALLY, go through the specification and check if you have covered all the topics

GOOD LUCK!

Registration is now open for IGCSE May 16 Physics candidates
Registration deadline: June, 2015
Class starts from July

Last Minute Revision Tips for IGCSE Paper 1
● The equation s=d/t is only used to calculate constant speed or average speed. For objects undergoing acceleration, always use the equations of motion
● An object accelerates when the R.F acts in the same direction and decelerates when R.F acts in the opposite to its motion
● In questions associated with energy, never use the term TRANSFERRED to describe the lost energy. Use converted, wasted or dissipated instead.
● If by any chance you get the value of efficiency greater than 100%, cross it out and star over again.
● In Sankey diagrams, the straight arrow always represents the useful energy; the deviated ones are used to show the lost energy
● In electricity be very careful while writing these terms - current through and voltage across. Interchanging the terms, for instance current across and voltage through makes the answer invalid
● In questions associated with gas laws, always remember that when you're dealing with two quantities i.e trying to identify the relationship between the two quantities, the third one must be kept constant. For example in Boyle's law, Pressure is inversely proportional to Volume ONLY if the Temperature is kept constant.
For all the gas laws, the common quantity that always has to be kept constant is the mass/quantity of the gas
● In experimental questions, you repeat an experiment to ensure RELIABILITY not ACCURACY!
● Once you find an anomalous value, it has to be discarded and disregarded in any further calculations
● Fleming's Left hand rule tells you the direction of the quantities in concern. The quantities are not generated according to this rule. Students often mistakenly write "According to Fleming's left hand rule there is a force on the wire." This is WRONG!
● The golden rule of studying electromagnetism and electromagnetic induction is to separate the two concepts! Easiest way to remember is to recall the two effects produced by them - motor effect and generator effect.
● In questions associated with radioactivity, remember carbon dating and dating rocks are completely different concepts! Carbon dating is done to estimate the age of fossils i.e matter that was once alive
● If a question asks you why is ionising radiation harmful, just saying it causes cancer won't get you all the points.
● The pe*******on power of ionizing radiation depends on both the mass and charge of the ionising particles
BEST OF LUCK!