O Level – Physics

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Key Points – Current Electricity Part 2

7. Resistance R of an electrical component is the ratio of the potential difference V across it to the electric current I going through it.

R = V/I

8. SI unit of resistance is ohm

9. Ohm’s law states that the current I passing through a conductor is directly proportional to the potential difference V across it when the physical conditions are unchanged.

I is proportional to V

10. Conductors that obeys Ohm’s Law are said to be ohmic. Their resistance remains constant under constant physical conditions an their I-V graphs are linear.

11. Non-ohmic conductors do not obey Ohm’s Law. Their resistance varies.

12. Resistivity p is a measure of how much a material opposes the flow of electric current.

13. The resistance R of a wire is directly proportional to its length, inversely proportional to its cross-sectional area A, and directly proportional to the reisitivity p of the material.

R = (pl)/A

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Key Points – DC Circuits

1. Total emf Et of n identical cells arranged in series
Et = E1 + E2 + E3 …… + En

2. Total emf Et of n identical cells arranged in parallel
Et = E1 = E2 = E3 ……+ En

3. At any point in the circuit, total incoming current is equal total outgoing current

4. The resistance of thermistors decreases as temperature increases.

5. The resistance of light-dependent resistors (LDRs) decreases as light intensity increases.

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Key Points – Practical Electricity Part 1

1. Electricity is essential in modern homes to power electrical appliances, which covert electrical energy into light, kinetic, sound and heat energy.

2. Heating elements in household appliances such as electric kettle and heater convert electrical energy to thermal energy. The heating elements are usually made of nichrome because of its high resistance and high melting point.

3. The amount of electrical energy E converted can be calculated by the following equations

E = VIt
E = (I^2)Rt
E = (V^2/R)t

V = Voltage (V)
I = Current (A)
t = Time (s)
R = Resistance (Ohm)

4. Power, P can be calculated by the following equations
P = IV
P = (I^2)R
P = (V^2)/R

5. Electrical energy consumption is measured in kilowatt hour (kWh).
One kilowatt hour is the amount of electrical energy used by 1 kW appliance in 1 hr.

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Key Points – Practical Electricity Part 2

6. Fossil fuels and nuclear energy are non-renewable energy sources. Solar energy, wind energy and hydroelectric generation are renewable energy sources.

7. Generally, non-renewable energy source produce electricity cheaper than
renewable energy.

8. A short circuit causes excessive current to pass through the circuit and produce a large amount of heat within a Short period of time.

9. Electricity can be dangerous due to
– damaged insulation which can cause electric shocks
– overheating of cables which can cause fire.
– damp conditions which can cause electric shocks.

10. Live wire carry current into homes while neutral wires carry current out of homes. Earth wires are connected to the ground for safety purposes.

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Key Points – Practical Electricity Part 3

11. The earth wire provides a low resistance path for electric currents to flow from the metal parts of a faulty electrical appliance to the ground. Appliances with three-pin plugs are earthed.

12. The fuse is a safety device that is connected to the live wire. It contains a metal wire that melts to to disconnect a circuit when excessive current flows through it. The current rating of a fuse is the minimum amount of current that will cause it to melt.

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Key Points – Practical Electricity Part 4

13. A circuit breaker is a safety device that automatically switches off a circuit when excessive current flows through it. This protect electrical appliances from damage arising from circuit faults.

14. Double insulation is a safety feature that uses layers of insulation that keeps electricity within the circuit and protects the appliances’s internal components.

15. Double insulated electrical appliances use two-pin plugs because that there is no need for the earth wire, and they normally do not have a metal casting.

16. Switches, fuses and circuit breakers are connected to the live wires. When a circuit is disconnected by these safety devices, current is no longer supplied to the faculty circuit or electrical appliance.

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Key Points – Magnetism Part 1

1. A magnetic field exists around a magnet.

2. A magnetic field is a region in which another magnet or magnetic material experiences a magnetic force.

3. This magnetic force is either repulsion or attraction.

4. A magnet always has a north pole and south pole. The magnetic field of a magnet is the strongest at the poles.

5. Magnetic materials are attracted to both ends of a magnet. Magnets do not affect non-magnetic materials.

6. Like poles repel while unlike poles attract.

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Key Points – Magnetism Part 2

7. Only repulsion can differentiate magnets from other materials.

8. Magnetisation is the process of aligning the magnetic domains in a magnetic material in order to convert it into a magnet.

9. Demagnetisation is the process of removing the magnetic properties of a magnet.

10. Soft magnetic materials are easily magnetised and lose their magnetic properties easily.

11. Hard magnetic materials are difficult to be magnetised and retain their magnetic properties much longer.

12. When a magnetic material is bought near a magnet, the magnetic material becomes an induced magnet. The magnetic material can also be magnetised by repeated stroking with a magnet or by putting it in a solenoid with a direct current.

13. A magnet can be demagnetised by heating, subjecting it to heavy ;physical impact or by slowly withdrawing the magnet from a solenoid with an alternating current.

14. The direction of the magnet field is indicated by the direction in which the compass needle points.

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Key Points – Electromagnetism Part 1

1. A wore carrying an electric current induces a concentric circular magnetic field around it.

2. According to the right hand grip rule, if the thumb points in the direction of the current flow, the fingers will indicate the direction of the magnetic field lines around a current-carrying conductor.

3. Fleming’s Left-hand rule gives the direction of the magnetic force (represented by the thumb) which is 90 deg to the magnetic field (represented by the forefinger)and the conventional current current (represented by the second finger).

4. The direction of motion of negatively charged particles is taken to the opposite of positively charged particles.

5. A current-carrying conductor (or moving charged particle) will experience a magnetic force in a magnetic field unless the direction of current (or the motion of charged particle) and the magnetic field are parallel.

6. Stationary charged particles do not experience magnetic force.

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Key Points – Electromagnetism Part 2

7. Magnetic field lines are drawn to represent magnetic fields. Lines that are closer together indicate a greater magnetic field strength where a charged particle experiences a greater magnetic force.

8. An induced magnetic field is strongest near the conductor. It weakens as the distance from the conductor increases. Magnetic field strength also increases as the current in the conductor increases.

9. A charged particle moving in a magnetic field perpendicular to its direction of motion experiences a magnetic force that causes it to
deflect in a circular path.

10. A pair of parallel wires attracts each other if the currents in both wires are in the same direction. A pair of wires repels each other if the currents they carry are in opposite directions.

11. A rectangular coil carrying an electric current in a magnetic field experiences a turning effect due to the magnetic forces acting on each side of the coil. This turning effect is used in the action of a simple d.c. motor.

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Key Points – Electromagnetic Induction

1. Faraday’s law of electromagnetic induction states that the electromotive force induced in a conductor is proportional to the rate of change of magnetic flux linkage through the conductor.

2. The magnitude of the induced e.m.f. is increased by
(a)increasing the strength of the magnetic field by using stronger magnets;
(b)increasing the number of turns in the solenoid;
(c)increasing the speed of motion in the magnetic field;
(d)winding the coil on a soft iron core to strengthen the magnetic field through the coil.

3. Lenz’s law states that the direction of the induced e.m.f., and hence the induced current in a closed circuit, always opposes the motion or change that produces it.

4. Fleming’s right-hand rule gives the direction of the induced e.m.f. or current (represented by the second finger) which is 90° to the direction of motion (represented by the thumb) and the magnetic field (represented by the forefinger).

5. The simple alternating current (a.c.) generator uses a rotating coil in a magnetic field to convert kinetic energy to electricity

6. A simple transformer has two circuits consisting of a primary coil and a secondary coil, each at opposite ends of a laminated soft iron core.
(a) Step-up transformers increase voltages and decrease current. There are fewer turns on the primary coil Np than on the secondary coil Ns.
(b) Step-down transformers decrease voltages and increase current. There are more turns on the primary coil than on the secondary coil.

7.The turns ratio of a transformer is given below.

Ns/Np = Vs/Vp = Ip/Is

where
Ns = number of turns in secondary coil
Np = number of turns in primary coil Vs = secondary output voltage
Vs = secondary input voltage
Vp = primary input voltage
Is = current in the secondary coil
Ip = current in the primary coil

8. The cathode-ray oscilloscope (C.R.O.) is a device used to measure d.c. and a.c. voltages, study waveforms and measure time and frequency.

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Kinematics

Scalars and vectors

Scalar quantities are physical quantities that have magnitude only.

Vector quantities are physical quantities that have both magnitude and direction.

Scalar
Distance
Speed
Mass
Energy
Time

Vector
Displacement
Velocity
Acceleration
Force

Distance
• The total length covered by a moving object regardless of the direction of motion
• A scalar quantity (i.e. has magnitude only)
• SI unit: metre (m)

Displacement
• The distance measured in a straight line in a specified direction
• A vector quantity (i.e. has both magnitude and direction)
• SI unit: metre (m)

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Kinematics

Basic Kinematic Quantities

1. Distance is the total length covered between two points. Distance is a scalar quantity.

2. Displacement is the measurement of distance from one reference point to another in a certain direction. Displacement is a vector quantity.

3. The SI unit of distance and displacement is metre (m).

4. Speed is the rate of change of distance. Speed is a scalar quantity.

5. Velocity is the rate of change of displacement. Velocity is a vector quantity. Speed is the magnitude of velocity.

6. The SI unit of speed and velocity is m s–1 or m/s.

7. For constant (or uniform) speed, its value is given by:

Speed v = (distance travelled s)/(time taken t)

8. For non-constant speed, the average speed is given by:

Average speed = (total distance travelled)/(total time taken)

9. If the speed of an object is zero, the object is not moving (or at rest).

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Kinematics

Acceleration of Free Fall

1. Free fall is defined as the motion of an object under the influence of gravity only. In other words, the only force acting on the object must be its own weight.

Examples:
A coin dropped in a vacuum column
A hammer thrown upwards by an astronaut on the surface of the Moon

2. Any object that moves in air experiences a force called air resistance that tends to slow its motion down. Therefore, objects that experience air resistance are not moving in free fall. However, the effect of air resistance is often assumed as negligible to simplify calculations.

Examples:
A ball thrown upwards in air (with air resistance ignored)
A rock dropped from a tower (with air resistance ignored)

Exam Tip
Always assume air resistance is negligible while solving questions on falling objects, unless explicitly stated otherwise. The effect of air resistance is a common reason why the calculated value differs from actual value obtained in experiments.

3. The acceleration of an object falling freely, without air resistance, is known as acceleration of free fall, g. It is determined to be approximately 10 m s-2. This value varies slightly at different places on the Earth’s surface because the Earth’s gravitational pull varies.

4. In common experiments, the effect of air resistance is usually not negligible. It can be seen
in the following ways:
(a) It always opposes the motion of objects.
(b) It increases with the speed of the object.
(c) It increases with the cross-sectional area of the object.
(d) It increases with the density of air.

5. Air resistance increases as speed increases. Eventually, the acceleration of the object becomes zero, and it reaches its maximum speed (or magnitude of velocity). This maximum constant velocity is called terminal velocity. The direction of terminal velocity is understood to be downwards for a falling object.

6. The value of terminal velocity of a falling object depends on the cross-sectional area and weight of the object. An opened parachute greatly increases air resistance. This reduces the terminal velocity of a parachutist who has jumped out of a plane so that he can land on the ground at a safe speed.

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Mass,Weight and Density

KEY TERMS AND CONCEPTS

1. The mass of an object is a measure of the amount of matter in the object. The SI unit of mass is kilogram (kg).

2. Inertia is the resistance of an object to a change in its state of motion or rest. The larger the mass, the greater is its inertia.

3. The attractive force between any two masses is known as gravitational force.

4. The gravitational field is a region in which a mass experiences
a gravitational force.

5. Gravitational field strength g is the gravitational force acting per unit mass. The unit for g is m s-2 or N kg-1. The gravitational field strength is also known as the acceleration due to gravity.
F = mg

6. Gravitational field strength g is usually taken to be a constant at 10 N kg-‘ on the surface of the Earth. It varies slightly according to the location.

7. Weight is the gravitational force acting on an object. It is equal to the product of mass and gravitational field strength. The SI unit for weight is the newton (N).
W = mg

8. Density p of an object is its mass per unit volume.

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