Chapter 1 - Measurements

1.

Describe an experiment to show how the period of pendulum can be determined

  • The simple pendulum is made to oscillate at a small angle.
  • When the oscillation is steady, the time taken for 20 oscillations is measured using a stopwatch.
  • The measurement is repeated and the average is calculated.
  • The period of the pendulum is given by average time for 20 oscillations divided by 20.

State the precautions you would take for the experiment.

  • Ensure that the oscillation is steady before starting to time.
  • Ensure that the angular displacement of the bob is less than 10 degrees.

2.

Describe how you would obtain experimentally accurate values for internal (external) diameter of a thick metal pipe.

  • Use internal (external) jaws of vernier calipers to measure the internal (external) diameters of the pipe at three different positions.
  • Take the average values of the measurements.

Why is there a need to obtain the average of several readings?

  • To reduce random errors (the positive errors and negative errors will cancel out) 

Chapter 2 - Kinematics

1.

A car travels round a circular track at a constant speed of 40 km/h. Explain why the car’s velocity is not constant

  • Velocity is a vector quantity that depends on both direction and magnitude.
  • Since the car’s direction changes continuously as it travels around a circular track, its velocity changes all the time.

 Is the car accelerating? Explain your answer.

  • Acceleration is defined as the rate of change of velocity with time.
  • Since velocity is changing, the car is accelerating.

 2.

An athlete initially at rest, runs a 100 m race and completed it with an average speed of 13.2 m/s. Explain why at some point in time his instantaneous speed must have exceeded the average value.

  • Since the athlete starts from rest, he must have had a speed of less than 13.2 m/s during his acceleration.
  • Therefore, his speed at other parts of the race must exceed 13.2 m/sin order for the average to be obtained as 13.2 m/s

Chapter 3 - Forces

1.

Describe ways in which a force might change the motion of a body.

  • A force is may make

o   stationary bodies move.

o   moving bodies speed up, slow down or stop.

o   moving bodies change direction.

 2.

Describe how an object released from rest (subjected to air resistance) will eventually fall with constant velocity.

  • Immediately upon release, gravitational force acting on the object causes it to accelerate at 10 m/s2.
  • As the object gains speed, the upward air resistance increases. This will cancel out the downward gravitational force and decrease the acceleration.
  • Eventually the upward air resistance increases to a point where it equals the downward gravitational force.
  • The resultant force acting on the object is now zero. Therefore the acceleration is zero and it continues falling with constant velocity (known as terminal velocity).

 3.

Suggest and explain why a car moves at constant velocity after some time even though the car engine continues providing the same amount of forward driving force (thrust).

  • During acceleration, the car gains speed and the backward resistive forces (air resistance and contact friction with road) increases.
  • It comes to a point where backward resistive forces equals the forward driving force (thrust)
  • The resultant force acting on the object is now zero. Therefore the acceleration is zero and it continues moving with constant velocity (known as terminal velocity).

Chapter 4 - Mass, Weight and Density

1.

Explain why a beam balance will give the same value of mass at different places for the same object.

  • Bean balance measures the mass of the object and mass is constant and does not change from place to place.

 2.

Explain why a spring balance may give different readings at different places when weighing the same object.

  • Spring balance measures the weight of an object which depends on the different gravitational field strength at different places (usual referring to different planets).

 3.

Distinguish between mass and inertia. State how they are related to each other.

  • Mass is the measure of amount of matter in a body.
  • Inertia refers to the reluctance of a body to change its state of motion.
  • The inertia of an object increases with its mass.

4.    Describe how you would find the density of a small stone with irregular shape.

  • Place a certain volume of water in a measuring cylinder and record the volume, V1.
  • Lower the stone into the measuring cylinder of water with a string until it is fully submerged and record the new volume, V2.
  • The volume of the stone, V is given by V2 – V1.
  • Measure the mass of the stone, m, using an electronic balance.
  • The density of the stone can be calculated using the formula, density = mass / volume.

 5.

Describe how you would measure the volume of a piece of floating object by means of a measuring cylinder, a thread, a sinker, and water.

  • Lower a sinker into a measuring cylinder containing some water until it is fully submerged and records the volume, V1.
  • Attach the floating object to the sinker and lower both into the cylinder again and note the volume, V2.
  • The volume V of the floating object is given by V2 – V1.

State two precautions that should be taken when using the measuring cylinder.

  • The measuring cylinder should be placed on a flat surface when reading the volume.
  • The volume of the liquid should be read from the base of the meniscus.

 6.

Describe an experiment to find the density of a liquid. State how the measurements are taken, and show how the final results are calculated.

  • Find the mass of a clean dry beaker, m1, using an electronic balance.
  • Pour a volume, V, of the liquid from a burette (or pipette) into the beaker.
  • Find the mass of the beaker and liquid, m2, using an electronic balance.
  • The mass of the liquid m is given by m1 - m2.
  • The density of the liquid can be calculated using the formula, density = mass / volume.

Chapter 5 - Moments

1.

Explain briefly how the use of a screwdriver enables the lid of a paint can to be opened with a relative small force.

  • Moment is the product of force and the perpendicular distance between the force and the pivot.
  • When a force is applied at the far end of the screw driver, the perpendicular distance between the pivot and line of action of applied force is large. This will then produce a large moment to open the paint can.

 2.

Describe an experiment to locate the centre of gravity of an irregular shaped lamina.

  • Make 3 small holes, as far apart as possible, near the edge of the card.
  • Suspend the card through one of the holes using a pin.
  • Hang a plumbline from the pin in front of the card.
  • When the plumbline is steady, draw the vertical line on the cardboard as indicated by the plumbline.
  • Repeat the above for two other holes.
  • The point of intersection of the three lines is the position of the centre of gravity of the cardboard (lamina).

3.

For any free hanging object, the centre of gravity must be vertically below the pivot. Explain.

  • When the CG is vertically below the pivot, the perpendicular distance between the line of action of the weight to the pivot is zero.
  • Since the perpendicular distance between the force and the pivot is zero, the resultant moment will also be zero and the object does not turn.

 4.

How can an object be made more stable?

  • Lower its centre of gravity by making the base heavy.
  • Increase its base area.

Chapter 6 - Work, Energy and Power

1.

Describe the energy changes that take place when a ball is being thrown upwards by a hand. (Neglecting air resistance)

  • As the ball just leaves the hand, it possesses maximum amount of KE. Its GPE is zero due to the zero height from the hand.
  • As it moves upwards, it loses speed. The KE is converted to GPE as the ball looses speed and gain height.
  • At the highest point from the ground, the stone is momentarily at rest. The KE is zero while the GPE reaches a maximum. All the KE have been converted to GPE. The GPE at the top equals the KE as the ball just left the hand (at the bottom).
  • As the object moves downwards, its speed increases. GPE is converted back to KE.
  • Just before hitting the hand, the KE reaches maximum and GPE is zero.
  • According the Principal of Conservation of Energy, the sum of KE and GPE is always constant at any point in time.

 

2.

Explain why in practical, the actual useful work done by a motor is less than the calculated value.

  • Some mechanical energy is converted to thermal energy as it is used to overcome frictional force.
  • Some energy is also converted to sound energy.
  • The thermal energy and sound energy is lost to the surroundings.
  • Therefore, the actual useful work done is always less than the calculated value.

Chapter 7 - Pressure

1.

Explain why a sharp knife cuts better than a blunt one.

  • A sharp knife has a smaller area of contact with the object to be cut.
  • For the same force exerted, a smaller area gives greater pressure as indicated by Pressure = Force / Area

2.

*Explain why the height of the mercury in a barometer remains unchanged when the diameter of the inverted tube is changed.

  • Since Pressure = (height of liquid column) x (density of liquid) x (gravitational field strength), then the height = pressure / [(density of liquid) x (gravitational field strength)]. It means that the height of the mercury in the barometer depends only on the pressure and gravitational field strength.

3.

*Explain why the atmospheric pressure decreases with height

  • The pressure at any given altitude in the atmosphere is due to the weight of the air above.
  • As altitude increases, there will be less air above. The density of air also decreases.
  • Since Pressure = (height of liquid column) x (density of liquid) x (gravitational field strength), a decrease in height and density of the air column (or any other fluid) will lead to a decrease in pressure.

Chapter 8 - Temperature

1.

*State the thermometric property that defines the temperature scale in a liquid-in-glass thermometer.

  • Volume of a fixed mass of liquid.

2.

*Describe how you would calibrate an unmarked mercury-in-glass thermometer.

  • Place the bulb of the unmarked thermometer in pure melting ice at a pressure of 1 atmosphere.
  • When the mercury level is steady, mark this position as 00C.
  • Next, place the bulb of the unmarked thermometer in steam above boiling water at pressure of 1 atmosphere.
  • When the mercury level is steady, mark this position as 1000C.
  • Divide the interval between 00C and 1000C into 100 divisions.
  • Each division represents 10C.

3.

*Explain what it meant by responsiveness of the thermometer.

  • A responsive thermometer reacts quickly to changes in temperature.

4.

 *Explain what is meant by sensitivity of the thermometer.

  • A sensitive thermometer gives a more noticeable expansion (large increase in the length of the mercury thread) for small changes in temperature.

5.

*State the set up of a thermocouple and describe how works.

  • A thermocouple consists of two types of wires made of different metals.
  • The ends of the wires are joined to form two junctions.
  • When the two junctions are at different temperature (one hot and one cold), a small voltage (e.m.f) is produced.
  • The greater the difference, the greater the voltage produced across the two junctions. That is, the voltage produced is proportional to the temperature difference.
  • By knowing one reference temperature and measuring the voltage, the unknown temperature can be found.

Chapter 9 - Kinetic Model of Matter

1.

Describe how you would demonstrate Brownian motion of smoke particles in the air. State and explain the observations.

  • Direct light into the side of an enclosed smoke cell/chamber.
  • View the smoke particles from the top, under a microscope.
  • Bright specks of smokes will be seen as light reflects from the smoke particles
  • The bright specks are observed to be in continuous random motion which proves that the smoke particles are in random motion.
  • The smoke particles are in random motion because they continually bombarded unevenly on different sides by the air molecules.

 

2.

*Using the kinetic model of gases, explain how gases exerts a pressure on the walls of its container.

  • When a gas particle collides onto the wall of the container, a force is exerted on it.
  • Numerous such collisions by the many molecules results in an average force exerted on the wall.
  • This force acting per unit area give rise to pressure exerted by the gas molecules on the walls of the container.

 3.

*Using the kinetic model of gases, explain why the pressure exerted by a fixed mass of gas increases when its volume is reduced at constant temperature.

  • When the volume is reduced, the number of particles per unit volume increases.
  • Therefore, the gas particles collides more frequency with the walls, resulting in greater force exerted on the container wall.
  • Since pressure P = F/A, a greater force F results in greater pressure.

4.

*Using the kinetic model of gases, explain why the pressure exerted by a fixed mass of gas increases when its temperature is raised. Assume that the volume and mass of the gas remains constant.

  • When the temperature of the gas is raised, the particles have higher KE and moves faster.
  • They collide with the walls of the container more vigorously and at higher frequency, resulting in greater force exerted on the container wall.
  • Since pressure P = F/A, a greater force F results in greater pressure.

5.

*A gas syringe is being heated and the piston begins to be move outwards and eventually stops. Explain.

  • Upon heating, the gas pressure increases to more than that of the atmospheric pressure.
  • As a result, a resultant force acts outwards which pushes the piston outwards.
  • As the piston moves outwards, the volume of the cylinder increases, causing the gas pressure to decrease.
  • When the gas pressure drops back to a value equal to that of atmospheric pressure, there will be no more resultant outward force and the piston will stop being pushed out.

Chapter 10 - Transfer of Thermal Energy

1.

Describe, in molecular terms, how energy transfer occurs in solids.

  • When a substance (object) is heated at one end, the particles at the region gains thermal energy and vibrate faster.
  • These molecules collide with the less energetic neighbouring particles and transfer KE to them.
  • In this way, thermal energy is transferred along the entire substance (object) by molecular collisions from the hot end of the substance (object) to the cooler end.

2.

Why is metal a better conductor of heat than non-metals?

  • In metals, apart from transferring heat energy by the relatively slow process of molecular collision, another faster process of free electron diffusion takes place.
  • Metals contain free electrons that are mobile.
  • When heated, these free electrons gains KE and will diffuse to the cooler end at high speeds.
  • In the process, they will collide with atoms at the cooler end and transfer kinetic energy to them.

3. 

Explain why materials such as fiberglass, wool, polystyrene, fur are commonly used as heat insulators.

  • Fibreglass, wool, polystyrene and furs are poor conductors of heat because they are non-metals.
  • In addition, they contain trapped, still air, which is a poor thermal conductor as well.

4.

Describe, in terms of density changes, heat convection in fluids.

  • When a fluid is heated at the base, it will expand, become less dense and rise.
  • The cooler fluid at the top being denser, will sink.
  • This continuous, cyclic movement of the fluid known as convection current will eventually heat up the entire whole fluid evenly.

5. 

Explain why the heating element of a kettle should be positioned at the base.

  • Heated liquid rises and cooler liquid will sink.
  • Placing the heating element at the base allows convection current to set up, eventually heating up the entire liquid.

6.

In some water heating system, the heating element is placed in the middle instead of the base. State one advantage of this arrangement.

  • Less energy will be used as only the upper half of the water in the container will be heated.

7. 

Two cars, one white, one black are parked in the sun. After some time, one is much hotter to touch than the other. Which car is it? Explain your answer.

  • The black car feels hotter.
  • This is because black surfaces are better absorbers of infrared radiation than white surfaces.

8.

Two kettle, one silver, one white, contains water at 90 oC. After some time, one is cooler than the other. Which kettle is it? Explain your answer.

  • The black kettle feels cooler.
  • This is because black surfaces are better emitters of infrared radiation than silvered surfaces.

9.

Explain why heat loss from a cooking pan is reduced by fitting it with a lid.

  • The lid prevents hot air from rising, hence reducing heat loss due to convection.
  • It also reduces heat lost by evaporation (if a liquid is present in the pot).

10. 

*Explain why the filament reaches a constant temperature, even though heat is produced continually as current flows through the bulb.

  • As electricity passes through the filament, thermal energy is produced and the temperature of the filament increases.
  • At the same time, thermal energy is lost to the surrounding air by radiation as the temperature of the filament is higher than the surrounding.
  • The filament reaches a final constant temperature when the rate of increase of thermal energy by the filament is equal to the rate of thermal energy loss by the filament to the surrounding.
  • It is said that the filament have reached thermal equilibrium.

Chapter 11 - Thermal Properties of Matter

1.

When thermal energy is supplied to melt a solid, the temperature remained constant during the melting point. Explain what happened to the heat energy supplied.

  • During melting, thermal energy absorbed is used to break the intermolecular bonds between the particles, resulting in increase in potential energy of the substance.

  • Thermal energy is not used to cause an increase in kinetic energy of the particles, hence temperature remains constant.

2.

*Using the kinetic model of gases, explain why evaporation of a liquid causes a fall in temperature.

  • In a sample of water, the molecules move at different speeds, and therefore different kinetic energies.

  • At the water surface, molecules with sufficient energy to overcome the attractive forces of other molecules escape into the atmosphere.

  • When energetic molecules escapes from the surface of a liquid, its leaves behind the less energetic molecules.

  • As a result, the average kinetic energy of the remaining molecules decreases and therefore the temperature of liquid falls.

3.

Which will cause a more severe injury? Scalding by boiling water at 100 degree Celsius or steam at 100 degree Celsius?

  • Scalding by steam at 100 degree Celsius.

  • Although both are of the same temperature, the latent heat of vaporization of steam is greater than specific heat capacity of water.

  • A larger amount of latent heat of vaporization is released as steam condenses on the skin, as compare to thermal energy released as water contacts the skin. 

Chapter 12 - Light

1.

State the properties of the image formed by a plane mirror

  • Upright
  • Image size equals object size
  • Virtual
  • Image distance equals object distance
  • Laterally inverted.

2.

State one advantage of using a plane mirror in optical testing.

  • When a plane mirror is used, the image of the optical test card will be at a distance equal to the object distance. In this way, it can appear further than it actually is.
  • The larger distance between the viewer and the test card can be obtained. This distance can even be longer than the room itself.

3.   

Explain why refraction takes place when light travel from air to glass.

  • Light travel at different speeds in different optical media.
  • Its speed in air is 3.0 x 10^8 m/s while its speed in glass is lower at 2.0 x 10^8 m/s.
  • At the boundary of the two media, the sudden change in speed causes the path of light to bend.

4.   

Explain why TIR can only take place when light is traveling from denser to less dense medium

  • Only when light travels from denser to less dense medium, then will it bend away from the normal.
  • This is essential for the refracted angle in the less dense medium to be 90 degrees for a given incident angle in the denser medium. This incident angle is called the critical angle.
  • When the incident angle is more than the critical angle, TIR will then take place.

5.   

*Why do diamonds have such brilliant sparkle?

  • Having high refractive indexes, diamonds have small critical angles.
  • A large proportion of light at the diamond-air boundary will have incident angle greater than that of the critical angle, hence undergoing TIR.
  • The diamond is also cut in such a way that when light enters, it will undergo TIR a few times before most of them emerge from the top surface.

6.   

State some uses of optical fibres.

  • Telecommunication – transmission of signal.
  • Endoscope – to see organs in the body by directly light through a thin plastic tube into the body.
  • Binoculars – prisms allow for TIR and reduce length of the instrument.
  • Periscope – prisms are positioned to reflect light and give clear images.

7.   

State the advantages of using optical fibres over copper wires in telecommunications.

  • Optical fibres can carry much more information than copper wires.
  • Optical fibres experience less signal loss
  • Optical fibres are lighter and hence easier to install.
  • Optical fibres are cheaper to manufacturer. (many physics questions will state to avoid this economical reason)

Chapter 13 - Waves

1.   

A transverse wave can be demonstrated using a rope. Describe how the hand must move to make a transverse wave of frequency 5 Hz.

  • For a transverse wave to move from left to right, the hand must move in a to and fro motion vertically.
  • There should be 5 oscillations of the hand in 1 second.

2.   

A wave travels in a medium with a certain speed. What must be done to double the wavelength of the wave in the same medium?

  • Since the wave is in the same medium, the speed does not change.
  • From speed of wave = frequency x wavelength, the source of the wave should be at half the frequency in order to achieve a doubled wavelength.

3.   

Describe what happens to water waves as it travel from a deep to shallow region.

  • When water wave travels from a deep to shallow region, its speed is slowed down while its frequency remains constant.
  • Since speed of wave = frequency x wavelength, the wavelength will decrease too.

Chapter 14 - Electromagnetic Waves

1.    State the properties which are common to all forms of electromagnetic radiation.

  • All are transverse waves.
  • All travel with the same speed, 3 x 10^8 m/s in vacuum.
  • They all obey the wave equation:
  • They can be reflected or refracted
  • They transports energy.

2.    How do the frequency, wavelength and speed of electromagnetic waves change when it enters from air to glass?

  • When an EM wave enters glass, its speed is slowed down while its frequency remains constant.
  • Since, the wavelength will decrease too.

3.    State one difference between waves of red light and waves of blue light.

  • Red light has a longer wavelength (or lower frequency) than blue light.

4.    State 3 ways in which an electromagnetic wave differs from the sound wave.

 Sound waves

  • Longitudinal waves
  • Travel at an approximate speed of 330m/s through air
  • Needs a medium to travel, cannot travel through vacuum

Electromagnetic waves

  • Transverse waves
  • Travel at an approximate speed of 3 x 10^8 m/s through air
  • Does not need any medium to travel. Can travel through a vacuum

Chapter 15 - Sound

1.    Describe how a vibrating object produces sound waves. (2 marks)

  • Sound is produced by vibrating sources placed in a medium.
  • When the object (e.g. ruler or tuning fork) vibrates, a series of compressions (high air pressure) and rarefactions (low air pressure) are produced by shifting of air layers.
  • In doing so, a longitudinal wave is produced.

2.    Describe how a vibrating object produces sound waves. (4 marks)

  • Sound is produced by vibrating sources placed in a medium.
  • When the vibrating object moves outwards, it pushes the air layers close together and produces a region of higher pressure known as compression.
  • When the object moves inwards, it pulls the air layers apart and produces a region of lower pressure known as rarefaction.
  • The continuously vibrating object thus produces a series of compression and rarefactions traveling away from the vibrating source as a longitudinal wave.
  • Energy is transferred away from the source via collisions of adjacent particles in the medium.

3.    Describe how the air pressure at a particular point changes when a sound wave passes that point.

  • Air pressure changes from high (compression) to low (rarefaction) alternately.

4.    Describe an experiment which shows that a medium is needed to transmit sound waves.

picture credits to  http://www.alanpedia.com/physics_sound/sound_clip_image004.gif

picture credits to 

http://www.alanpedia.com/physics_sound/sound_clip_image004.gif

 

  • Before air is drawn out from the bell jar, the sound of the ringing bell can be heard.
  • Evacuate air out of the bell jar slowly using the vacuum pump. The sound of the ringing bell decreases until eventually, no sound is heard, although the hammer can still be seen vibrating.
  • This experiment shows that a medium is needed to transmit sound waves.

5.    Suggest briefly how measurements involving echoes can be used to find the depth of water in a sea.

  • SONAR (SOund Navigation And Ranging) is used to send out sound waves until they are reflected by the bottom of the lake.
  • The reflected sound is detected and the time taken, t, is recorded.
  • If the speed of the sound in water v is known, then the depth of the lake, d, can be calculated by the formula:

6.    Describe how you would carry out an experiment to measure the speed of sound in air. Your answer should make clear what measurements you would take and how those measurements would be used to produce the results.

  • Position observers A and B at a large known distance, d.
Picture Credits to http://www.excelatphysics.com/uploads/3/1/7/5/31758667/386706_orig.jpg

Picture Credits to

http://www.excelatphysics.com/uploads/3/1/7/5/31758667/386706_orig.jpg

 

  • The distance d should be large so that it reduces the percentage error due to human reaction.
  • Observer A fires the pistol.
  • Observer B uses his stopwatch to measure the time interval t1 between on seeing the flash of the pistol and hearing the sound.
  • To reduce the measurement error due to the effect of the wind, repeat the experiment with A and B switched place, measuring the the time t2.. Obtain the average of the two times, <t> = (t1+t2)/2
  • The speed of sound, v can be calculated by:

                                                      Speed = Distance/Time = d/<t>

Chapter 16 - Static Electricity

1.    How does an ebonite rod become negatively charged when it is rubbed with a duster? What is the charge of the duster and how does it acquire that charge?

  • The ebonite rod gains electron from the duster and becomes negatively charged as it now has an excess of electrons.
  • The duster lose electrons to the ebonite and becomes positively charged as it now has more protons than electrons.

2.    Two plastic rods repel each other after they have been rubbed with a cloth. Suggest why the plastic rods repel each other.

  • Rubbing the rods with a cloth cause them to acquire the same charges.
  • Since like charges repel each other, the two rods will repel each other.

3.    A positively charged rod is brought near a piece of paper. Explain why the paper is attracted to the rod and sticks to it.

  • When the positively charged rod is brought near the neutral paper, it induces a negative charge on the side of the paper nearer to the rod.
  • Since unlike charges attract each other, the paper will be attracted to the rod.
  • When the paper touches the rod, it does not get discharge as it is an insulator.
  • The paper remains attracted to the rod.

4.    A negatively charge metal ball suspended on an insulating thread is brought near a neutral metal sphere held on an insulated stand. State and explain what will happen to the metal ball.

Negatively Charged ball near neutral conducting sphere
  • The negatively charged metal ball will repel the electrons in the sphere to the far right side, hence inducing a positive charge in the near left side.
  • Since unlike charges attract, the negatively charge ball will now be attracted to the positively charged side of the sphere.

State and explain what will happen to the ball if the metal ball is allowed to touch the sphere.

Charged ball touches conductor
  • Since both are conductors, the excess negative charges (electrons ) in the ball will flow to the sphere, and be redistributed among the ball and sphere.
  • Both the ball and sphere will now be negatively charge.
  • Since unlike charges repel, the ball will repel from the sphere.

5.    A positively charged rod is brought near a metal sphere held on an insulated stand. State and explain the movement of electrons in the sphere that occurs as the rod is brought near.

Charging by Induction
  • The electrons, being negatively charged, are attracted to the positively charged rod.
  • This leaves the left side of the sphere negatively charged and right side positively charged.

The metal sphere is earthed and the metal ball removed thereafter. State and explain what happens.

Earthing of a conducting ball
  • Electrons will flow from the ground, through the earth wire to the right side of the sphere, discharging the right side of the sphere.
  • When the positively charged rod is removed, the negative charges will be redistributed. A negatively charged sphere is obtained.

      *Name this process.

  • Charging by induction.

6.    Describe how the spray painting of objects makes use of electrostatic principle. State the advantages of using this method of spray painting.

  • As the paint leaves the nozzle, the particles of paint become charged by friction.

OR

The nozzle is connected to a positive (negative) terminal so that paint that passes through it will become positively (negatively) charged.

  • The paint particles being likely charged, will repelled one another, spreading out when being sprayed on an object.
  • The charged paint particles will be attracted to the earthed object by induction.
  • This method of spray painting ensures uniform coating of paint and reduces wastage of paint.

 

 

Chapter 17 - Current Electricity

 

1.    Describe an experiment to determine the resistance of a resistor.

Determining resistance of unknown resistor
  • Set up the apparatus as show in the diagram above.
  • Adjust the rheostat to the maximum so that the smallest possible current flows in the circuit.
  • Record the ammeter reading (I) and the voltmeter reading (V).
  • Adjust the rheostat to allow a larger current to flow in the circuit. Record the values of I and V.
  • Repeat the procedure to obtain five sets of I and V readings.
  • Plot a graph of V against I. Draw a best fit line.
  • The gradient of the graph gives the resistance of the resistor.

2.    Define Ohm’s law.

  • Ohm’s law states that for metallic conductor, the current passing through it is directly proportional to the potential difference provided physical conditions such as the temperature remains constant

3.    Explain why the component with a voltage-current graph as shown below obeys Ohm’s law (is an ohmic conductor).

graph of ohmic conductors
  • For an Ohmic conductor, the current passing through it is directly proportional to the potential difference provided that the temperature remains constant.
  • The graph shown is a straight line through the origin which indicates that V is directly proportional to I.

4.    Draw the voltage versus current graph of a filament lamp and explain why Ohm’s law does not apply to the filament lamp. Give a reason for the relationship.

I-V Graph of non-ohmic conductors

 

  • The graph is a curve which indicates that V is not directly proportional to I. Therefore, it does not obey Ohm’s Law.
  • When the current flows through the filament, its temperature increases. This causes its resistance to increase.

Chapter 18 - D.C. Circuits

1.

Explain why electric lights in a building are connected in parallel rather than in series.

  • When the bulbs are connected in parallel, the current through each bulb will be higher and hence each bulb will glow more brightly.

  • Parallel arrangement also allows the bulbs to work independently. That is, when one bulb is fused or blown, the other bulbs can still work.

2. 

Resistor A and B are in parallel. Would the addition of an extra resistor parallel to A and B increase or decrease the current flowing through the battery? Explain your answer.

resistors in parallel
  • Adding a resistor in parallel decreases the effective resistance of a circuit.

  • Using I=V/R, the current through the battery will increases.

Chapter 19 - Practical Electricity

1.   

Explain why the metal case of electrical equipment which is operated from the mains supply should be earthed.

  • If the live wire touches the metal casing accidentally, current will flow through the casing and the user touching the casing will get an electrical shock.
  • The earth wire has a resistance much lower than the human body.This will allow the current to flow through it to the ground instead of through the user’s body.
  • This protects the user from getting electrocuted.

 

2.   

Explain why thin wires should not be used in electrical systems, even through this would be cheap.

  • Since resistance is inversely proportional to cross-sectional area of wire, thin wires have high resistance.
  • Since the heating effect of the wire is given by P=I2R, a thin wire with large R will result in excessive heating. This results in loss of energy and in more serious cases, overheating which may lead to a fire hazard.

 

3.   

Explain why switches should be placed in the live wire, not the neutral wire, in household electrical systems.

  • Switches must be fitted onto the live wire so that opening it will disconnect (isolate) the appliance from the high voltage live wire. (If the switch is placed in the neutral wire, the electrical appliance is still connected to the high voltage live wire even when the switch is opened.)
  • This may cause the user to get an electric shock if he touches the live wire accidentally.

 

4.   

Explain why a fuse should have a rating a little higher than the current normally expected in the circuit.

  • This is to ensure that the appliance will operate when the normal operating current flows in the circuit.
  • The fuse will only melt when the current is higher than the rated value of the fuse.

 

5.   

What is the function of a fuse?

·         A fuse will prevent excessive current from flowing in a circuit. This protects the electrical appliance from being damaged.

 

6.   

How does a fuse work?

  • The wire in a fuse will melt and break the circuit if the current exceeds the rating of the fuse.
  • This ensures that the circuit is broken.

 

7.   

Why should a fuse be placed in the live wire of a mains circuit?

  • The fuse is connected to the live wire so that the appliance will not become charged (have a potential difference of 230 V) after the fuse has melted due to excessive current.
  • Fuses must be fitted onto the live wire so that when it blows, it will disconnect (isolate) the appliance from the high voltage live wire. (If the fuse is placed in the neutral wire, the electrical appliance is still connected to the high voltage live wire even when the switch is opened.)
  • This may cause the user to get an electric shock if he touches the live wire accidentally.

Chapter 20 - Magnetism

1.   

State the properties of a magnet.

  • A magnet can attract another magnetic material such as iron, steel, cobalt and nickel.
  • A magnet has two poles, the North pole and South pole.
  • Like poles repel and unlike poles attract.
  • A freely suspended magnet always points in a North-South direction.

 2.   

With the aid of a diagram, describe how a magnetic material such as a steel bar can be magnetised using the electrical method.

magnetisation of steel. magnetization of steel using direct current (d.c.)
  • A steel bar to be magnetised is placed in a solenoid (wire coil) as shown.
  • The solenoid is connected to a direct current (d.c.) supply and switched on. The current flowing through the solenoid produces a magnetic field that will magnetise the steel bar.
  • The resistance of the rheostat can be reduced to increase the current flowing in the circuit hence increasing the strength of the magnet.

3.   

With the aid of a diagram, describe how a magnet can be demagnetised using the electrical method.

Demagnetisation using alternating current (a.c.)
  • A magnet to be demagnetised is placed in a solenoid.

  • An alternating current is allowed to flow through the solenoid.
  • The magnet is withdrawn in an east-west direction, far away from the solenoid while the current is still flowing.

 4.   

With the aid of a diagram, describe how the magnetic field lines can be plotted with a compass.

plotting magnetic field lines
  • Place a bar magnet on a sheet of paper. Mark the outline of the magnet.
  • Put a small compass near to the North-pole of the magnet.
  • Using a pencil, mark dots 1 and 2, the position of the South and North pole of the compass respectively.
  • Move the compass so that the S-pole of the compass is on dot 2 and mark dot 3 against the N-pole of the compass.
  • Repeat the above method to plot other lines of force on either side of the magnet to obtain a magnetic field pattern around the magnet.

5.   

State the differences between the magnetic properties of steel and iron.

  • Iron can be magnetised and demagnetised easily while steel is harder to magnetised and demagnetised.

6.   

You are given three steel bars. Only two of the bars are magnets. Explain how you would identify the magnets without using any other equipment.

  • Using two of the three bars, check for repulsion between the ends of the two bars. The bars whose ends repel are magnets.

7.   

You are given three apparently identical metal bars, one steel, one soft iron, and one copper. How would you identify them without damaging them in any way?

  • Place all 3 bars in a solenoid with direct current flowing through it to magnetise it.
  • Bring an iron paper clip close to one end of each bar.
  • The bar that does not attract the paper clip at all is the copper.
  • Switch off the current.
  • The bar that still attracts the paper clip is the steel (as it retained its magnetism).