7.1.4 The Effect of Electric Field

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Effect of Electric Field on a Ping Pong Ball Coated with Conducting Material

  1. A ping ball coated with conducting material is hung by a nylon thread.
  2. When the ping pong ball is placed in between 2 plates connected to a Extra High Tension (E.H.T.) power supply, opposite charges are induced on the surface of the ball. The ball will still remain stationary. This is because the force exert on the ball by the positive plate is equal to the force exerted on it by the negative plate.
  3. If the ping pong ball is displaced to the right to touch the positive plate, it will then be charged with positive charge. Since like charges repel, the ball will be pushed towards the negative plate.
  4. When the ping pong ball touches the negative plate, it will be charged with negative charge. Again, like charge repel, the ball will be pushed towards the positive plate. This process repeats again and again, causes the ping pong ball oscillates to and fro continuously between the two plates.

Candle Flame in an Electric Field

  1. Normally, with absent of wind, the flame of a candle is symmetry.
  2. The heat of the candle flame removes electrons from the air molecules around it, and therefore ionised the molecule. As a result, the flame is surrounded by a large number of positive and negative ions.
  3. If the candle is placed in between 2 plates connected to a Extra High Tension (E.H.T.) power supply, the positive ions will be attracted to the negative plate while the negative ions will be attracted to the positive plate.
  4. The spreading of the flame is not symmetry. This is because the positive ions are much bigger than the negative ions; it will collide with the other air molecule and bring more air molecule towards the negative plate.

 

7.1.3 Electric Field

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  1. An electric field is a region in which an electric charged particle experiences an electric force.
  2. Electric field is represented by a number of lines with arrows, called electric lines of force or electric field lines.
  3. The direction of the field at a point is defined by the direction of the electric force exerted on a positive test charge placed at that point.
  4. The strength of the electric field is indicated by how close the field lines are to each other. The closer the field lines, the stronger the electric field in that region.
    Example
  5. The lines of force are directed outwards for a positive charge and inwards for a negative charge.
  6. The electric line of force will never cross each other.
  7. The figure shows a few examples of the field pattern that you need to know in the SPM syllabus.

 

 

7.1.2 Current

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  1. An electric current I is a measure of the rate of flow of electric charge Q through a given cross-section of a conductor.
  2. In other words, current is the measure of how fast the charge flow through a cross section of a conductor.

Equation


or


Direction of Current

  1. Conventionally, the direction of the electric current is taken to be the flow of positive charge.
  2. The electron flow is in the opposite direction to that of the conventional current.
  3. In a circuit, current flow from the positive terminal to the negative terminal.
  4. In a circuit, electrons flow from the negative terminal to the positive terminal.

Unit of Current

  1. The SI unit for current is the ampere (A).
  2. The current at a point is 1 ampere if 1 Coulomb of electric charge flows through that point in 1 second. Therefore, 1 A = 1C/s.

Example 1:
If 30 C of electric charge flows past a point in a wire in 2 minutes, what is the current in the wire?

Answer:
Charge flow, Q = 30C
Time taken, t = 2 minutes = 120s

Current,


Example 2:
Current of 0.5A flowed through a bulb. How many electrons had flowed through the bulb in 5 minute? (The charge of 1 electron is equal to -1.6×10-19 C)

Answer:
Current, I = 0.5A
Time taken, t = 5 minutes = 300s

Charge of 1 electron, e = -1.6×10-19 C
Number of electrons, n = ?

 

7.1.1 Electric Charge

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  1. There are two kind of electric charge, namely the positive charge and the negative charge.
  2. Like charge repel each other.
  3. Unlike charge attract each other.
  4. A neutral body can be attracted by another body which has either positive or negative charge.
  5. The SI unit of electric charge is Coulomb (C).
    Example
    Charge of 1 electron = -1.6 x 10-19C
    Charge of 1 proton = +1.6 x 10-19C

Charge and Relative Charge


Sum of Charge

Sum of charge
= number of charge particles × charge of 1 particle
Q=ne

Example:
Find the charge of 2.5 x 1019 electrons.
(Charge of 1 electron is   -1.6 x 10-19C)

Answer:
Number of electrons, n = 2.5 x 1019
Charge of 1 electron, e = -1.6 x 10-19C

 

6.2.3.3 Refraction in Daily Life

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  1. The effect of refraction causes seaside near to a cape is stony while sea near to a bay is sandy.
  2. At the middle of the sea, the wavefront is a linear line.
  3. This linear form is due to the nearly equal similar depth of water.
  4. When waves move close to the coast line, the wavefronts start to curve and follow the topography of the coast line, due to the effect of refraction.
  5. At the bay, the energy of the have spread to a wider area, and cause the amplitude to reduce.
  6. At the cape, the energy of the wave is converged to a smaller area, therefore the amplitude of the wave increases.

 

6.4.3 Sources, Properties and Applications of Electromagnetic Waves

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(This file is shared by Philip Ronan under the Creative Commons Attribution-Share Alike 3.0 Unported license.)
  1. The full range of electromagnetic waves, arrange orderly in their wavelengths and frequencies is called the electromagnetic spectrum.
  2. Figure above shows the electromagnetic waves in electromagnetic spectrum.
  3. In SPM, you need to know
    1. The order of the electromagnetic waves in electromagnetic spectrum.
    2. The sources of the electromagnetic waves
    3. The applications of the electromagnetic waves

Sources of Electromagnetic Waves

Electromagnetic Waves Source
Radio Wave Electrical oscillating circuit (consists of a capacitor and an conductor connected in series)
Microwave Oscillating electrical charge in a microwave transmitter
Infrared Hot bodies, the sun and fires
Visible Light The sun, hot objects, fires, light bulbs, fluorescent tubes
Ultraviolet Very hot objects, the sun, mercury vapour lamps
X-ray X-ray tubes where high energy electrons bombarding a metal plate.
Gamma Ray Radioactive substances

 

6.4.2 Properties and Applications of Electromagnetic Waves

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Radio Wave

  1. Telecommunications
  2. Broadcasting: Radio and television transmission
  3. Astronomy study

Microwave

  1. Satellite transmissions
  2. Radar systems to detect objects(size, form and position)
  3. Cooking

Infrared

  1. Night vision
  2. Thermal imaging and physiotherapy
  3. Remote controls for TV/VCR
  4. Heating in physiotherapy
  5. Thermometer
  6. Cooking

Visible Light

  1. Sight
  2. Photosynthesis in plants
  3. Photography

Ultraviolet

  1. Identification of counterfeit notes
  2. Production of fluorescent effects
  3. Production of vitamin D in the skin
  4. Sterilisation to destroy germs
  5. Pest control

X-ray

  1. Disinfecting drinking water
  2. Radiotherapy
  3. Radiography (X-ray photograph)
  4. Detection of cracks in building structures

Gamma Ray

  1. Crystallography
  2. Cancer treatment
  3. Sterilisation of equipment
  4. Pest control in agriculture

 

 

6.4.1 Electromagnetic Waves

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  1. Electromagnetic waves come from a whole variety of sources. They differ greatly in their wavelengths and in their effects, but they have certain fundamental properties in common.
  2. The common properties of electromagnetic waves are as below:
    1. They are transverse waves:
      1. Electromagnetic waves are transverse wave.
      2. The moving wave effect is produced by oscillating electric and magnetic fields oscillate at right angles to the direction of travel and at right angles to each other
    2. They can travel in Vacuum:
      Electromagnetic wave can travel in vacuum

    3. They travel with the same speed in vacuum:
      1. In vacuum, all the electromagnetic waves travel at the same speed commonly known as the speed of light (3 × 108m/s).
      2. In other medium, different electromagnetic wave may travel at different speed.
    4. They can be polarised:
      1. A transverse wave can be polarized.
      2. Plane polarized light will be produced when light travels through a polarizing material like polaroid.
      3. Polaroid is a type of material that only allows light waves of one plane to pass through.
    5. They are neutral:
      Electromagnetic waves are electrically neutral.

 

6.3.3 Applications of Sound Wave

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Sonar


Q & A

Explain how sonar is used to measure the depth of a sea. for visualizing subcutaneous body structures including tendons, muscles, joints, vessels and internal organs for possible pathology or lesions.
  1. In the sonar system, the broadcasting equipment emits ultrasonic pulses.
  2. The pulses are reflected back from the base of the sea or object in the sea and create an echo.
  3. The echo is traced by a hydrophone.
  4. The time interval t between transmitting the pulses and receiving them again is recorded by electronic equipment.
  5. If the ultrasonic velocity is v, then depth of the sea d is given by d = ½ vt

Disintegration of Kidney Stone

High intensity ultrasound wave is used to break kidney stone in a patient's body.

Ultrasound Scanning

  1. Ultrasound is used for the scanning of foetus in thge womb.
  2. Ultrasound is used in ultrasonography for visualizing body structures including tendons, muscles, joints, vessels and internal organs.


Cleaning

Jewelers use ultrasound to clean rings and watches.

 

6.3.2 Loudness and Pitch

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  1. Loudness of a sound depends on the amplitude of the sound wave.
  2. The greater the amplitude of a sound wave, the louder the sound is.
  3. The pitch of a sound is high or low of the sound.
  4. The pitch of sound is determined by its frequency. The higher the frequency, the higher the pitch.