O Level – Physics

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Electromagnetic Waves

Some useful applications of electromagnetic waves include:

(a) Gamma-rays in radiation therapy (or cancer treatment)
(b) X-rays in medical (X-ray images) and everyday applications (X-ray scanners)
(c) Ultraviolet radiation in sunbeds and sterilisation of medical equipment
(d) Visible light in optical fibres for medical uses and telecommunications
(e) Infrared radiation in remote controllers and intruder alarms
(f) Microwaves in microwave ovens and satellite television
(g) Radio waves in radio and television communications

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Electromagnetic Waves – Key Concepts

1. Some useful applications of electromagnetic waves include:

(a) Gramma-rays in radiation therapy (or cancer treatment)
(b) X-rays in medical (X-ray images) and everyday applications (X-ray scanners)
(c) Ultraviolet radiation in sunbeds and sterilisation of medical equipment
(d) Visible light in optical fibres for medical uses and telecommunications
(e) Infrared radiation in remote controllers and intruder alarms
(f) Microwaves in microwave ovens and satellite television
(g) Radio waves in radio and television communications

2. Some effects of absorption of electromagnetic waves by humans include:

(a) Infrared heating
(b) Damage to different tissues and organs of the human body due to ionising radiation

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Sound – Key Concepts

1. Sound is produced by vibrating sources placed in a medium.

2. A sound wave is an example of a longitudinal wave and comprises of a series of compressions and rarefactions in the medium.

3. There are three types of graphs to describe sound waves i.e. pressure-distance graph, displacement-distance graph and displacement-time graph.

4. Ultrasound are sound waves that have frequencies above 20 kHz.

5. Some uses of ultrasound include quality control and prenatal scanning

6. Pitch is a subjective quantity that is related to the objective quantity of frequency.

7. Loudness is a subjective quantity that is related to the objective quantity of amplitude.

8. Pitch is a subjective quantity that is related to the objective quantity of frequency.

9. Loudness is a subjective quantity that is related to the objective quantity of amplitude.

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Electrostatic – Key Concepts I

1. There are two types of charges—positive and negative.

2. Like charges repel, unlike charges attract

3. Charge is measured in coulombs (C).

4. Electrostatic charging by friction involves a transfer of electrons.

5. Electrical insulators (e.g. rubber) do not conduct electricity because their electric charges (electrons) are not free to move about.

6. Electrical conductors (e.g. copper) are able to conduct electricity because their electric charges (electrons) are free to move within them.

7. Charged insulators can be neutralised by heating and humid conditions.

8. Conductors can be charged by induction.

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Electrostatic – Hazards and Applications of Electrostatics

1. An electric field is a region where an electric charge experiences an electric force.

2. The direction of the field is the direction of the force on a small positive charge.

3. Some hazards of electrostatic charging are lightning and electrostatic discharge.

4. Some uses of electrostatic charging are the laser printer, photocopier and electrostatic precipitators.

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Electrostatic – Key Concepts II

Some hazards of electrostatics

1. Lightning

Thunderclouds are charged by friction between the water molecules in the thunderclouds and air molecules.

It then ionises the air and the ionised air provides a conducting path for electric charge to be discharged to the nearest or sharpest object on the ground.

2. Electrostatic discharge

Excessive charges may build up due to friction. Sudden discharge may cause sparks and ignite flammable materials nearby

Electronic equipment, such as computer boards and hard drive, can be easily damaged.

Such equipments are usually packed in antistatic packaging materials.

See movie : Static @ Petrol.

Practical applications of electrostatics

1. Laser printer

2. Electrostatic precipitator

3. Spray painting

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

The paint particles contain like charges thus they spread out when they are sprayed on a car body.

The charged paint particles will be attracted to the metallic car body.

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Misconception Analysis – Measurement

Stop and Think, are the following questions True or False

1. A Physical quantity must have both magnitude and unit.

2. Base quantities and base units are the same.

3. Derived quantities are not physical quantities.

4. The SI units for length, mass and time are the metre, gram and second respectively .

5. Prefixes are used to express big numbers only

Ans TFFFF

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Misconception Analysis – Kinematics

Stop and Think, are the following questions True or False

1. Speed is defined as the rate of change of distance

2. The gradient of a displacement-time graph at any point gives the average velocity

3. The area under the velocity-time graph is the displacement

4. When an object is moving, it must have acceleration.

5. The SI unit for acceleration is ms^-2

Ans TFTFT

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Misconception Analysis – Dynamics

Stop and Think, are the following questions True or False

1. A resultant force is needed to start a motion.

2. A resultant force is needed to keep an object moving with constant velocity.

3. When there is no resultant fore acting on an object, the object may be at rest.

4. A resultant force will cause the object to move faster.

5. According to Newton, forces always act in pairs

Ans – TFTFT

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Misconception Analysis – Mass Weight and Density

Stop and Think, are the following questions True or False

1. Mass is always constant.

2. Weight increases as mass increases.

3. Weight cannot be measured by beam balance.

4. Gravitational field strength is numerically equal to the acceleration due to gravity.

5. An object with bigger mass has higher inertial than an object with smaller mass.

Ans : TTFTT

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Measurement – Key Points

1. Micrometer screw gauge has better precision than the vernier calipers. Its maximum reading is smaller than the vernier calipers, thus, if is only suitable for measuring small objects like diameter of wire.

For more key points, please contact Mr Ong @9863 9633

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Key Points – Dynamics

1. In solving kinematics questions. We can define motion to the right is positive or negative.
All vectors quantities example displacement, velocity and acceleration in that direction will be positive.

2. A ball thrown upwards will experience a constant acceleration of g during the upward and downward motion with air resistance neglected.

For more key points and exam based questions with full worked solution, please contact Mr Ong @9863 9633

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Key Points – Forces

F = ma
F = resultant force acting on the body
m = mass of the body
a = acceleration of the body

1. Zero resultant force does not imply that the body is stationary.

2. If resultant force acting on the body is zero, it only implies that the acceleration of the body is zero.

3. Zero acceleration only implies that velocity is constant.

For more key points and exam based questions with full worked solution, please contact Mr Ong @9863 9633

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Key Points – Mass, Weight and Density

1. Inertia depends only on the mass of the object. Even if temperature or density of the object changes, as long as the mass remains constant, its inertia remains constant.

2. When ice melts, its density increases. Mass remains constant since the number of molecules remains the same.

Density = Mass/Volume

Therefore volume decreases

For more key points and exam based questions with full worked solution, please contact Mr Ong @9863 9633

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Key Points – Turning Effect of Moment

1. The moment of a force about a point is the product of the force F and the perpendicular distance d from the pivot to the line of action of the force

Moment = F x d

2. The SI unit ofr moment of a force is Newton meter – Nm

3. moment of a force is a vector. The direction of the moment is either clockwise or anticlockwise about the pivot.

4. There are two conditions for an object in equilibrium.

a) Net force is zero ie F(net) = 0
b) Net moment due to the external force is zero

5. The principle of moments states that when an object is in equilibrium, the sum of the clockwise moments is equals the sum of the anticlockwise moments.

6. The centre of gravity of an object is the point through which its entire weight appears to act.

7. CG of regular-shape objects can be determined by geometrical symmetry.

8. CG of irregular-shape objects can be determined by the plumbline experiments.

9. The stability of an object increases when the base area increases or the CG is lowered.

For more key points and exam based questions with full worked solution, please contact Mr Ong @9863 9633

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