Magnetism

Question 1

Introduction to Magnetism (Revision)

1. A magnet can attract certain type of metal.
2. The metals that can be attracted by a magnet are called the “magnetic materials” of “ferromagnetic materials”. Examples of magnetic materials are iron, steel, nickel and cobalt.
3. A magnet has 2 poles-the North Pole and the South Pole.
4. There is a magnetic field surrounding the magnet. A magnetic field is a region in the surrounding of a magnet which a magnetic material experiences a detectable force.

Question 2

Magnetic Field Line

(The magnetic field is represented by the magnetic field lines)

1. The magnetic filed of a magnet is represented by the magnetic field lines. The magnetic field lines flow out from the North pole and flow into the South pole.
2. The distance between the field lines represent the strength of the field, the closer the field line, the stronger the field. In the diagram, the magnetic field A is stronger than magnetic field B because the line in magnetic field A is closer.

Question 3

Compass in a Magnetic Field

Figure(a):

(The pointer of a compass point towards the North pole of a magnet)

Figure(b):

(The direction of the pointer of a magnet is always in the same direction of the magnetic field)

1. The pattern and the direction of a magnetic field can be determined by a compass.
2. First of all, we need to know that, in VET, normally we use a circle with an arrow to represent compass. The arrow represents the pointer of a compass and it always points towards the North pole of a magnet.
3. Second, we also need to know that the pointer of a compass is always in the direction of the magnetic field.
4. In figure (b) above, we can see that when a few compasses are put near to a bar magnet, the pointer of the compasses are all in the direction of the magnetic field.
5. If a compass is placed near to a current carrying wire, the pointer of the compass will point along the direction of the magnetic field generated by the current (as shown in the figure below). This will be discussed in electromagnetism.

Question 4

Electromagnetism and Electromagnets

1. When current passes through a conductor, magnetic field will be generated around the conductor and the conductor become a magnet. This phenomenon is called electromagnetism.
2. Since the magnet is produced by electric current, hence it is called the electromagnet.
3. An electromagnet is a type of magnet in which the magnetic field is produced by a flow of electric current. The magnetic field disappears when the current ceases.
4. The magnetism of an electromagnet is switched on or off using electric current.
5. In short, when current flow through a conductor, magnetic field will be generated. When the current ceases, the magnetic field disappear.

An electromagnet is a type of magnet in which the magnetic field is produced by a flow of electric current.

Question 5

Magnetic Field Pattern

Figure (a)

1. The magnetic field generated by a straight wire are concentric circles around the wire as shown in figure (a) above.
2. Take notes that when the direction of the current is reversed, the direction of the magnetic field line is also reversed.
3. The direction of the magnetic field line can be determined by the Maxwell’s Screw Rule or the Right Hand Grip Rule.

Figure (b)

The plan view of the magnetic field generated by a straight wire.

1. Sometime, the magnetic field pattern may be given in plan view, as shown in figure (b).
2. In plan view, a dot in the wire shows the current coming out from the plane whereas a cross in the wire shows the current moving into the plane.

Figure (c)

A dot indicates the current move out from a plane whereas a cross indicates the current move into the plane

Question 6

Direction of the Magnetic Field
The direction of the magnetic field formed by a current carrying straight wire can be determined by the

1. Right Hand Grip Rule or the
2. Maxwell Screw Rule.

Right Hand Grip Rule
Grip the wire with the right hand, with the thumb pointing along the direction of the current. The other fingers give the direction of the magnetic field around the wire. This is illustrated in the figure below.

Figure (d)

The Maxwell’s Screw Rules
The Maxwell Screw Rules sometime is also called the Maxwell’s Corkscrew Rule. Imagine a right handed screw being turn so that it bores its way in the direction of the current in the wire. The direction of rotation gives the direction of the magnetic field.

(Figure (e))
Strength of the Magnetic Field

1. The strength of the magnetic field form by a current carrying conductor depends on the magnitude of the current.
2. A stronger current will produce a stronger magnetic field around the wire as shown in Figure (f) below. Figure (f)
3. The strength of the field decreases out as you move further out. This is illustrated in figure (g) below. Thus, you must be very careful when you are asked to draw the magnetic field in your exam. Figure (g)
4. The distance of the field lines must increase as it is further out form the wire.

Question 7

Field Pattern

1. Figure (a) below shows the field pattern produced by a current flowing in a circular coil.
2. In VET, you need to know the field pattern, the direction of the field and the factors affect the strength of the field.
3. The direction of the field can be determined by the Right Hand Grip Rule. Grip the wire at one side of the coil with your right hand, with thumb pointing along the direction of the current. Your other fingers will be pointing in the direction of the field.

Figure (a)

1. Figure (b) shows the plan view of the field pattern.

Figure (b)

Question 8

Factors affecting the strength
There are 3 ways to increase the strength of the magnetic field:

1. increase the current and
2. increase the number of turns of the coil.
3. use coil with smaller radius

Question 9

Magnetic Effects of a Current-Carrying Conductor - Solenoid

A solenoid is a long coil made up of a numbers of turns of wire.

Magnetic Field Pattern

1. Figure (a) illustrates the field pattern produced by a solenoid when current pass through it.
2. The field lines in the solenoid are close to each other, indicates that the magnetic field is stronger inside the solenoid.
3. We can also see that the field lines are parallel inside the solenoid. This shows that the strength of the magnetic filed is about uniform inside the solenoid.
4. We can also see that the magnetic field of a solenoid resembles that of the long bar magnet, and it behaves as if it has a North Pole at one end and a South Pole at the other.

Figure (a)

Magnetic field pattern of a solenoid

Question 10

Determining the Pole of the Magnetic Field

1. The pole of the magnetic field of a solenoid can be determined by the Right Hand Grip Rule.
2. Imagine your right-hand gripping the coil of the solenoid such that your fingers point the same way as the current. Your thumb then points in the direction of the field.
3. Since the magnetic field lines always come out from the North Pole, hence the thumb points towards the North Pole.

Figure (b)

1. There is another method can be used to determine the poles of the magnetic field forms by a solenoid.
2. Try to visualise that you are viewing the solenoid from the 2 ends as illustrated in figure (c) below.
3. The end will be a North pole if the current is flowing in the aNticlockwise, or a South pole if the current is flowing in the clockwiSe direction.

Figure (c)

Question 11

Strength of the Magnetic Field
The strength of the magnetic field can be increased by

1. increasing the current,
2. increasing the number of turns per unit length of the solenoid,
3. using a soft-iron core within the solenoid.

Question 12

Uses of Electromagnet - Electric Bell

1. When the switch is on, the circuit is completed and current flows.
2. The electromagnet becomes magnetised and hence attracts the soft-iron armature and at the same time pull the hammer to strike the gong. This enables the hammer to strike the gong.
3. As soon as the hammer moves towards the gong, the circuit is broken. The current stops flowing and the electromagnet loses its magnetism. This causes the spring to pull back the armature and reconnect the circuit again.
4. When the circuit is connected, the electromagnet regain its magnetism and pull the armature and hence the hammer to strike the gong again.
5. This cycle repeats and the bell rings continuously.

Question 13

Uses of Electromagnet - Electromagnetic Relay

1. A relay is an electrical switch that opens and closes under the control of another electrical circuit.
2. The switch is operated by an electromagnet to open or close one or many sets of contacts.
3. A relay has at least two circuits. One circuit can be used to control another circuit. The 1st circuit (input circuit) supplies current to the electromagnet.
4. When the switch is close, the electromagnet is magnetised and attracts one end of the iron armature.
5. The armature is then closes the contacts (2nd switch) and allows current flows in the second circuit.
6. When the 1st switch is open again, the current to the electromagnet is cut, the electromagnet loses its magnetism and the 2nd switch is opened. Thus current stop to flow in the 2nd circuit.

Question 14

Uses of Electromagnet - Circuit Breaker

1. Figure above shows the structure of a circuit breaker.
2. A circuit breaker is an automatic switch that cut off current in a circuit when the current become too large.
3. When the current in a circuit increases, the strength of the electromagnet will increase in accordance; this will pull the soft iron armature towards the electromagnet.
4. As a result, the spring pulls apart the contact and disconnects the circuit immediately, and the current stop to flow.
5. We can reconnect the circuit by using the reset button. The reset button can be pushed to bring the contact back to its original position to reconnect the circuit.

Question 15

Uses of Electromagnet - Telephone Earpiece

1. An electromagnet is used in the earpiece of a telephone. The figure shows the simple structure of a telephone earpiece.
2. When you speak to a friend through the telephone, your sound will be converted into electric current by the mouthpiece of the telephone.
3. The current produced is a varying current and the frequency of the current will be the same as the frequency of your sound.
4. The current will be sent to the earpiece of the telephone of your friend.
5. When the current passes through the solenoid, the iron core is magnetised. The strength of the magnetic field changes according to the varying current.
6. When the current is high, the magnetic field will become stronger and when the current is low, the magnetic field become weaker.
7. The soft-iron diaphragm is pulled by the electromagnet and vibrates at the frequency of the varying current. The air around the diaphragm is stretched and compressed and produces sound wave.
8. The frequency of the sound produced in the telephone earpiece will be the same as your sound.

Question 16

Force on a Current Carrying Conductor in a Magnetic Field

1. We have learned that when current flows in a conductor, a magnetic field will be generated.
2. When the current-carrying conductor is placed in a magnetic field, the interaction between the two magnetic fields will produce a resultant field known as the catapult field as shown in the figure below.

1. The catapult field is a non-uniform field where the field at one side is stronger than the other side.
2. As a result, a force is produced to move the current carrying conductor from the stronger field to the weaker field.
3. The force produced by a catapult field is called the catapult force.
4. The direction of the force can be determined by Fleming’s left hand rule as shown in Figure below.

1. The fore finger, middle finger and the thumb are perpendicularly to each other. The forefinger points along the direction of the magnetic field, middle finger points in the current direction and the thumb points along the direction of the force.
2. The strength of the force can be increased by:
   
   1. Increase the current
   2. Using a stronger magnet
   3. using a longer wire
   4. arranging the wire perpendicular to the direction of the magnetic field.

Question 17

Turning Effect of a Current Carrying Coil in a Magnetic Field

1. If a current carrying coil is placed in a magnetic field (as shown in diagram above), a pair of forces will be produced on the coil. This is due to the interaction of the magnetic field of the permanent magnet and the magnetic filed of the current carrying coil.
2. The diagram below shows the catapult field produced.

1. The direction of the force can be determined by Fleming’s left hand rule.
2. Since the current in both sides of the coil flow in opposite direction, the forces produced are also in opposite direction. The 2 forces in opposite direction constitute a couple which produces a turning effect to make the coil rotate.
3. Examples of electric equipment whose operation is based on this turning effect are
   
   1. the direct current motor
   2. the moving coil meter.

Question 18

Application of the Force on a Current Carrying Conductor in a Magnetic Field - Moving Coil Meter

Light Indicator

A light indicator which has lower inertia is used to increase the sensitivity of the meter.

Linear Scale

1. Due to the radial magnetic field and the cylindrical soft-iron core, a linear scale is produced.
2. A linear scale is more accurate and easier to be read.

Mirror

1. A mirror is used to prevent parallax error.
2. When the observer’s eye is exactly above the indicator, the indicator will cover its own image on the mirror.
3. This can used to prevent parallax error.

Curved Permanent Magnet

1. A curved permanent magnet is used to produce a radial field.
2. A radial field is a magnetic field where the field lines are either pointing away or toward the center of the field.
3. A radial can be focused by a cylindrical soft-iron core.

Rectangular Coils

1. When a current flows through the coils, a force will be generated due to the interaction between the magnetic field of the permanent magnet and the coil.
2. The force will turn the coils, which in turn move the indicator.

Cylindrical Soft-Iron Core

1. A cylindrical soft iron core is placed inside the radial field produced by the curved magnet.
2. A soft-iron core can focus the magnetic field of the permanent magnet.

Hair Spring

1. The deflection of the coil and the indicator stops when the force is balanced by the opposing force from the hair spring.
2. The angle of deflection is directly proportional to the magnitude of the current in the coil.

Question 19

Direct Current Motor

1. An electric motor converts electrical energy to kinetic energy.
2. Diagram above shows the structure of a simple direct current motor (DC motor).
3. It consist a rectangular coil of wire placed between 2 permanent magnets.
4. The coil are soldered to a copper split ring known as commutator. 2 carbon brushes are held against the commutator.
5. The function of the brush is to conduct electricity from the external circuit to the coil and allow the commutator to rotate continuously.
6. The function of the commutator is to change the direction of the current in the coil and hence change the direction of the couple (the 2 forces in opposite direction) in every half revolution. This is to make sure that the coil can rotate continuously.
7. The operation principle of a direct current motor are explained in detail in the second Youtube video below.

Question 20

Application of the Force on a Current Carrying Conductor in a Magnetic Field - Loudspeaker

Loud Speaker

1. The loud speaker contains a cylindrical coil which is free to move in a radial magnetic field set up by a strong cylindrical permanent magnet.
2. The magnet has a central South Pole and a surrounding North Pole. The field lines are therefore radial and at right angles to the turns of the-coil.
3. When varying the current flows through the coil, a force of varying magnitudes will act on the coil. This will cause the coil to move to and fro according to the magnitude of the force.
4. The paper cone then vibrates to produce sound waves.

Question 21

Force between 2 Current-Carrying Conductors

1. When 2 current carrying conductors are placed close to each other, a force will be generated between them.
2. If the current in both conductors flow in the same direction, they will attract each other, whereas if the current are in opposite direction, they will repel each other.
3. This force is due to the interaction between the magnetic field of the 2 conductor.
4. The figure below shows the catapult field produced by 2 current carrying conductors when their current is in the same direction or opposite direction.

(Magnetic field generated when 2 current carrying conductors with currents move in the same direction are brought close to each other. The field will cause the 2 conductors attract each other)

(Magnetic field generated when 2 current carrying conductors with currents move in the opposite direction are brought close to each other. The field will cause the 2 conductors repel each other)

• *Summary**:
  1. A force will be produced between 2 current carrying conductors.
  2. If the currents are in the same direction, the 2 wire will attract each other.
  3. If the current are in opposite direction, the 2 wire will repel each other.

Question 22

Electromagnetic Induction

1. When a magnet is moved into and out of the solenoid, magnetic flux is being cut by the coil.
2. The cutting of magnetic flux by the wire coil induces an e.m.f in the wire.
3. When the solenoid is connected to a closed circuit, the induced current will flow through the circuit.
4. The production of electric current by changing magnetic field is called electromagnetic induction.
5. Current/emf is induced only when there is relative motion between the magnetic field and the conductor.
6. The direction of the induced current and the magnitude of the induced e.m.f due to the cutting of the magnetic flux can be determined from Lenz’s Law and Faraday’s Law.

Question 23

Law of Electromagnetic Induction

There are 2 principal laws of electromagnetic induction:

1. The Faraday’s law
2. The Lenz’s law

Question 24

Faraday’s Law

1. The magnitude of the induced e.m.f is determined from Faraday’s Law.
2. Faraday’s Law states that the magnitude of the induced e.m.f is directly proportional to the rate of change of magnetic flux through a coil or alternatively the rate of the magnetic flux being cut.
3. Therefore, the induced emf can be increased by
   
   1. using a stronger magnet
   2. increase the speed of the relative motion
   3. increase the number of turns of the coil

Question 25

**Lenz's Law** [](http://upload.wikimedia.org/wikipedia/commons/c/c7/Emil_Lenz.jpg) 1. When a magnet is moved into and out of a coil, the induced current that flows through the coil can be determined from Lenz's Law. 2. Lenz's Law states that the induced current always flows in the direction that opposes the change in magnetic flux. 3. Lenz's Law obeys the principle of conservation of energy. Work is done to move the magnet against the repulsive force. This work done is converted to electric energy which manifests as an induced current. 4. For a conductor in a closed circuit moving perpendicular to a magnetic field and hence cutting its magnetic flux, the direction of the induced current is determined from Fleming's Right-Hand Rule. 5. Fleming's Right-Hand Rule is used to determine the direction of the induced current that flows from the wire when there is relative motion with respect to the magnetic field

Question 26

Induced EMF and Current in a Straight Wire [](http://3.bp.blogspot.com/-2xT5c8fISUw/UlIyx91veRI/AAAAAAAADLQ/5zXQn2ft_NE/s1600/induction3.png) 1. When a straight conductor (or wire) moves and cut a magnetic field, emf will be induced across the conductor. 2. If the conductor is in a complete circuit, current will flow in the conductor. 3. The direction of the current induced can be determined by using Fleming's Right Hand Rule, as explained in the video below.

Question 27

Induced EMF and Current in a Solenoid [](http://1.bp.blogspot.com/-ekwZRtDB1NI/UlIjyD7XQGI/AAAAAAAADK0/3I09tBEHLlc/s1600/induction2.png) 1. When a bar magnet is inserted into a solenoid, the solenoid will cut the magnetic flux of the bar magnet. This will induce a current and emf in the solenoid. 2. The induced current will produce another magnetic field around it. 3. The pole of the magnetic field and direction of the induced current can be determined by using Lenz's Law as explained in the video below.

Question 28

Direct Current Generator [](http://3.bp.blogspot.com/-aGF-qicPGYI/UlCU-5XSFII/AAAAAAAADGA/QUSd5sfbDbE/s1600/generator-33.png) 1. Figure above shows the illustration of a simple direct current (d.c.) generator. 2. You should notice that the simple d.c generator is almost the same as the [d.c. motor](http://spmphysics.onlinetuition.com.my/2008/06/direct-current-motor.html) except that the battery in the d.c. motor is removed and replaced by a resistor. 3. The direct current generator produces electric current (or voltage) base on the principle of electromagnetic induction. 4. Figure below shows the change of the induced voltage when the coil is at different postion. [](http://1.bp.blogspot.com/-OjVD_CbS6GI/UlCTQ5EyQdI/AAAAAAAADF0/jQ1VP5ky2WQ/s1600/Picture35.png) 1. Initially the armature is vertical. No cutting of magnetic flux occurs and hence induced current does not exist. 2. When the armature rotates, the change in flux increases and the induced current correspondingly increases in magnitude. 3. After rotating by 90°, the armature is in the horizontal position. The change in magnetic flux is maximum and hence the maximum induced e.m.f is produced. Maximum induced current flows through the galvanometer. 4. When the armature continues to rotate, the change in flux decreases. 5. At the 180° position, there is no change in flux hence no induced current exists.The induced current is achieves its maximum value again when the armature is at 270°. 6. After rotating 360°, the armature returns to its original position. 7. The direction of the induced current can be determined from Fleming's Right-Hand Rule. 8. Even though the magnitude of the induced current or d.g.e is dependent on the orientation of the coil, the current in the external circuit always flows in one direction. This uni-directional current is known as direct current.

Question 29

Alternating Current Generator [](http://2.bp.blogspot.com/-K9Zjk-LmRvM/UlCPIeJ1hQI/AAAAAAAADFg/AaDsEPV4wO0/s1600/Picture34.png) 1. Generator can be modified to an a.c generator by replacing its commutators with two (separate) slip rings. The two slip rings rotate in tandem with the armature. 2. Carbon brushes connect the armature to the external circuit. The armature is initially at the vertical position. No magnetic flux is cut and hence no induced current exists. 3. When the armature rotates, the change in magnetic flux increases and the induced current increases until its maximum value at the horizontal position. 4. The direction of the induced current can be determined from Fleming's Right Hand rule. 5. As the armature continues on its rotation, the change in magnetic flux decreases until at the vertical position, no induced current exists. 6. Subsequently upon reaching the horizontal position again, the induced current is maximum, but the direction of the induced current flowing through the external circuit is reversed. 7. The direction of the induced current (which flows through the external circuit) keeps changing depending on the orientation of the armature. This induced current is also known as alternating current. 8. The current is positive (+) in one direction and negative in the other (-). The smooth rings play a critical role in the generation of alternating current.

Question 30

**Direct Current** 1. Direct current is a uniform current flowing in one fixed direction in a circuit 2. Direct current (d.c) is usually supplied by acid-based batteries or dry cells. 3. A common example of acid-based (electrolyte) batteries is the car battery. 4. Figure below shows the graph of current supplied by a dry cell over time. [](http://4.bp.blogspot.com/-cuT3nlcaIzU/UlCf8W_rGhI/AAAAAAAADGQ/_FIOcIVhQuw/s1600/Picture37.png)

Question 31

**Alternating Current** 1. Alternating current is an electric current in which the direction of flow of the electrons reverses periodically 2. Alternating current (a.c) is generated from alternating current generators such as hydroelectric power generators. 3. The electricity supplied to households is alternating current. 4. Household electricity (alternating current) changes direction 50 times every second. Its magnitude also changes with time. **Period And Frequency** [](http://bp3.blogger.com/_PyvjcvYfRLk/SGCSYwrOnkI/AAAAAAAAAG4/atrN0C3dINg/s1600-h/graph+period.png) 1. The time taken for one complete cycle is known as the period, T. 2. The frequency f is defined as the number of complete cycles in 1 second. 3. The relationship between the frequency and the period is:  1. In VET, you need to know the effect of both the direct current and alternating current on 1. a bulb 2. a capacitor 3. a moving coil loudspeaker 2. Table below give the summary of the comparison of the effect of direct current and alternating on a bulb, a capacitor and a moving coil loudspeaker. **Direct Current** **Alternating Current** **Effects on a bulb** The bulb lights up The bulb lights up **Effects on a capacitor** Current is detected at the very beginning and then ceased to become zero afterward. Current is detected **Effects on moving coil loudspeaker** No sound produced Sound produced

Question 32

Root Mean Square Voltage/Current **The Effective Voltage for a Sinusoidal Alternating Current** [](http://bp1.blogger.com/_PyvjcvYfRLk/SGCSZSIKsfI/AAAAAAAAAHA/RVJXBh8_pDk/s1600-h/graph+amplitude.png) 1. The maximum potential difference supplied by an a.c source is known as the peak voltage V_P. 2. The effective potential difference for an a.c is equal to the potential difference of a alternating current if both results in the same heating effect. 3. The effective potential difference for a.c is known as the root mean square voltage (r.m.s) of the a.c. and is given y the following equation:  [](http://3.bp.blogspot.com/-k6Ef7J1VcxY/UlCoHnVGiYI/AAAAAAAADGg/F5_kSmPttdw/s1600/Picture41.png) 1. The root-mean-square (r.m.s) value of an alternating current is the value of the steady direct current which produces the same power in a resistor as the mean power produced by the alternating current. 2. The r.m.s current is the effective value of the alternating current. 3. The r.m.s. current can be calculated by using the following equation: 

Question 33

[](http://4.bp.blogspot.com/-Evtk9j--jEc/UlCpbhQEGmI/AAAAAAAADGs/lXx-o05ey-U/s1600/Picture42.png) Diagram above shows a graph of potential difference, V against time, t of an alternating current. Find the V_r.m.s. of the power supply.

Answer 33


Question 34

[](http://1.bp.blogspot.com/-PpwQ6ychCcA/UlCpjOkHPEI/AAAAAAAADG0/k6dCCfKavy4/s1600/Picture43.png) The diagram above shows the wave form of an a.c. supply. What is the root mean square value of the current?

Answer 34


Question 35

Transformers [](http://upload.wikimedia.org/wikipedia/commons/7/7a/Transformer-hightolow_smaller.jpg) [This image is original released under ***[GNU Free Documentation license](http://en.wikipedia.org/wiki/GNU_Free_Documentation_License "w:GNU\_Free\_Documentation\_License")*** ] What is a transformer? A transformer is a device that is used to raise or lower down the potential difference of an alternating current. Function: The function of a transformer is to increase or decrease the potential difference of an alternating current supply. Structure and Technical Terms 1. A transformer consist of 3 parts, namely 1. The primary circuit 2. The core 3. The secondary Circuit [](http://upload.wikimedia.org/wikipedia/commons/thumb/6/64/Transformer3d_col3.svg/763px-Transformer3d_col3.svg.png) [This image is released under the [GNU Free Documentation License](http://commons.wikimedia.org/wiki/GNU_Free_Documentation_License "GNU Free Documentation License").] **Primary Circuit:** The primary circuit is the circuit that connected to the input energy source. The current, potential difference and coil (winding) in the primary circuit are called the primary current (Ip), primary potential difference (Vp) and primary coil respectively. **Core:** The core is the ferromagnetic metal wound by the primary and secondary coil. The function of the core is to transfer the changing magnetic flux from the primary coil to the secondary coil. **Secondary Circuit:** The secondary circuit is the circuit that connected to the output of the transformer. The current, potential difference and coil (winding) in the secondary circuit are called the secondary current (Is), secondary potential difference (Vs) and secondary coil respectively.

Question 36

**Working Principle of a Transformer** 1. A transformer consists of a primary coil and a secondary coil wound on a soft iron core. 2. When an alternating current flows in the primary coil, a changing magnetic flux is generated around the primary coil. 3. The changing magnetic flux is transferred to the secondary coil through the iron core. 4. The changing magnetic flux is cut by the secondary coil, hence induces an e.m.f. in the secondary coil. 5. The magnitude of the output voltage can be controlled by the ratio of the number of primary coil and secondary coil.

Question 37

**Types of Current in A Transformer** 1. The current in the primary circuit must be alternating current because alternating current can produce changing magnetic flux. A changing magnetic flux is needed to induce e.m.f. in secondary coil. 2. The induced current in secondary is also an alternating current. The frequency of the alternating current in secondary coil is same as the frequency of the primary current. 3. The alternating in the secondary circuit can be converted into direct current by using a pair of diode. #### Symbol of A Transformer [](http://3.bp.blogspot.com/-YgXcGwROWDo/UlCrJswS0vI/AAAAAAAADHE/aGDg7eoNKF4/s1600/Transformer_symbol.png)The figure on the left shows the symbol of a transformer. The 2 lines in between the coil denote the core.

Question 38

**Types of Transformer** There are 2 types of transformer, namely 1. the step up transformer 2. the step down transformer **Step-up Transformer** [](http://3.bp.blogspot.com/-Hv6WfW2JGZo/UlCtUITtN1I/AAAAAAAADHM/iTTp70z9BgM/s1600/Picture47.png) 1. A step-up transformer is one where the e.m.f. in the secondary coil is greater than the e.m.f. in the primary coil. It is used to increases the potential difference. 2. The number of windings in the secondary winding is greater than the number of windings in the primary coil. 3. The current in the primary coil is greater than the current in the secondary coil. [](http://4.bp.blogspot.com/-HjQ8lxerCoY/UlCtZxMwlRI/AAAAAAAADHU/m-l7Morzw54/s1600/Picture46.png) **Step-down transformer** 1. Conversely, a step-down transformer is one where the e.m.f. in the secondary coil is less than the e.m.f. in the primary coil. It is used to reduce the potential difference. 2. The number of windings in the primary winding is greater than the number of windings in the secondary coil. 3. The current in the primary coil is lesser than the current in the secondary coil. Calculation of Potential Difference Change  ``` Vp = input (primary) potential difference Vs = output (secondary) potential difference ``` ``` Np = Number of Windings (primary) Ns = Number of Windings (secondary) ``` ``` Ip = input (primary) current Is = output (secondary) current ``` Calculation of Current Change **Ideal Transformer**  **Non-ideal transformer**  ``` Vp = input (primary) potential difference Vs = output (secondary) potential difference Ip = input (primary) current Is = output (secondary) current ```

Question 39

**Factors that affect the efficiency** 1. The heating effect of current in a coil. Power is lost as heat I₂R whereby I is the current flowing through the coil and R is the resistance of the coil. **Methods to increase the efficiency** Use thick copper wires of low resistance. Use coolant to decrease the temperature of the transformer. 2. Heating effect of induced eddy currents [](http://2.bp.blogspot.com/-UI2UHI9lW34/UlCviLBAUWI/AAAAAAAADHg/Vki4BR7cAc4/s1600/Picture48.png) In the iron core. When the magnetic field in the iron core fluctuates, eddy currents are generated in the iron core. **Methods to increase the efficiency** Use a laminated iron core whereby each layer is insulated with enamel paint to prevent the flow of eddy currents. The high resistance between layers of the iron core decrease the prevalence of eddy currents and heat. 3. Magnetization of the Iron Core. The energy used in the magnetization and de-magnetization of the iron core each time current changes its direction is known as hysterisis. This energy is lost as heat which subsequently heats up the iron core. **Methods to increase the efficiency** Use a soft iron core that is easily magnetized and de-magnetized. 4. Flux leakage. Some of the induced magnetic flux from the primary coil is not transmitted to the secondary coil, therefore the e.m.f in the secondary coil is decreased. The secondary coil(windings) are intertwined tightly with the primary coils. The iron core should form a closed loop. **Methods to increase the efficiency** The secondary coil (windings) is intertwined tightly with the primary coils. The iron core should form a closed loop.

Question 40

**Renewable and Non-renewable Energy** The frequently asked questions related to this topic are: what is renewable energy? the examples of Renewable and non-renewable energy and the advantages of using renewable energy. Renewable energy sources are the energy sources which can be continuously replaced. Examples of renewable energy are: 1. Hydro energy 2. Solar energy 3. Wind energy 4. Geothermal energy 5. Biomass 6. Tidal energy

Question 41

**Advantages of Using Renewable Energy** 1. Clean and do not pollute the environment. 2. Easily obtain. 3. Can be replenished once it has been used. 4. Can prevent energy crisis.

Question 42

**Hydroelectric** [](http://upload.wikimedia.org/wikipedia/commons/d/de/Stwlan.dam.jpg) The advantages of using hydroelectric as a source of energy to generate electricity are: 1. It is a renewable energy. 2. Building a dam does not pollute the environment. 3. In a lot of country, water can be easily obtained and is free. 4. Building a hydroelectric plant does not involve very high technology as nuclear power plant. The disadvantages of using hydroelectric as a source of energy are: 1. Building a dam will cause a large area flooded with water, and hence seriously destroys the ecosystem nearby. 2. The flooded area causes the loss of wild life habitat and agriculture land. 3. Dam failure happens will cause a disaster to the lower reaches area of the river. 4. The cost to build a dam is very high.

Question 43

**Fossil Fuel** The advantages of using fossil fuel such as petroleum, natural gas and coal as a source of energy to generate electricity are: 1. It is relatively easy to be collected as they are present in large amount in one place. 2. It contains high energy capacity. The disadvantages of using fossil fuel as a source of energy are: 1. It releases a lot carbon dioxide when it is burned. Large amount of carbon dioxide presence in the atmosphere will cause the increase of the temperature of the atmosphere, known as global warming. Global warming is the most serious environmental problem that we are facing now. 2. Fossil fuels are non-renewable energy. 3. Pollute the air. Burning of fossil fuel will release particles and some hazardous gases such as sulphur dioxide and nitrogen dioxide which will pollute the air.

Question 44

**Solar power** [](http://upload.wikimedia.org/wikipedia/commons/4/45/Giant_photovoltaic_array.jpg) The advantages of using solar energy as a source of energy to generate electricity are: 1. It is a renewable energy. 2. Sunlight is free and can be obtained easily. 3. It does not pollute the air. The disadvantages of using solar energy as a source of energy are: 1. A large area is needed to install the solar plate. 2. The amount of sunlight at non-tropical area depends on the season. 3. The energy collected need to be store in a cell so that it can be used at the time sunlight is not present. 4. Its capacity is limited by the capacity of the cell. Normally a cell has relatively low capacity.

Question 45

Nuclear Power The advantages of using nuclear energy as a source of energy to generate electricity are: 1. The nuclear fuel such as uranium and plutonium has high energy capacity. Small amount of uranium can produces huge amount of energy. 2. Nuclear energy does not pollute the air. The disadvantages of using nuclear energy as a source of energy are: 1. Hazardous radioactive waste is produced during nuclear reaction. 2. The radioactive waste will cause pollution in a very large area if leakage happens in the nuclear power plan. 3. The cost to maintain a nuclear plant is very high. 4. Building a nuclear plant involving very high technology. Only a few countries own such technology.

Question 46

Biomass [](http://3.bp.blogspot.com/-Pp7uympPwQk/UlI6PLMuE8I/AAAAAAAADLs/TRnNzbS37Po/s1600/biomass.jpg) 1. Biomass is biological material derived from living organisms such as wood and animal waste. 2. As a renewable energy source, biomass can either be used directly via combustion to produce heat, or indirectly after converting it to various forms of biofuel. 3. Heat energy produced from the burning of biomass can be used to generate electricity. Advantages 1. It is renewable energy. 2. Using biomass to generate electricity can reduce the disposal of organic waste. 3. Liquid biofuels will not pollute the air because they are lead-free and sulphur-free. Disadvantages 1. Large storage space is required to store the biomass. 2. Land utilization can be considerable. Can lead to deforestation. 3. Overall process can be expensive

Question 47

**Wind Power** [](http://upload.wikimedia.org/wikipedia/commons/e/e0/Windenergy.jpg) [This image is shared by Wagner Christian. [This file](http://upload.wikimedia.org/wikipedia/commons/e/e0/Windenergy.jpg "Windenergy.jpg") is licensed under the [Creative Commons](http://en.wikipedia.org/wiki/Creative_Commons "w:Creative\_Commons") [Attribution ShareAlike 2.5](http://creativecommons.org/licenses/by-sa/2.5/ "http://creativecommons.org/licenses/by-sa/2.5/") License. ] The advantages of using wind as a source of energy to generate electricity are: 1. It is a renewable energy. 2. It does not pollute the environment. 3. It is suitable for mountain area because the wind speed in mountain area is relatively high. The disadvantages of using wind as a source of energy are: 1. A large area is needed to build the windmills. 2. The windmills will produce high level of noise. 3. The power generated is inconsistent due to the inconsistent of the wind speed and wind direction.

Question 48

Transmission of Electricity **Q:** Why the voltage is stepped up before transmitting electricity over long distance? **A:** Stepping up the voltage can reduces the current in the cable. This reduces the power loss in the cable during transmission. [](http://upload.wikimedia.org/wikipedia/commons/e/ea/Electric_wiring_near_Helsinki.JPG)[This image is shared by its author Aatu Liimatta in [Wikipedia Image](http://en.wikipedia.org/wiki/Image:Electric_wiring_near_Helsinki.JPG). *[This file](http://upload.wikimedia.org/wikipedia/commons/e/ea/Electric_wiring_near_Helsinki.JPG "Electric wiring near Helsinki.JPG") is licensed under the [Creative Commons](http://en.wikipedia.org/wiki/Creative_Commons "w:Creative\_Commons") [Attribution ShareAlike 2.5](http://creativecommons.org/licenses/by-sa/2.5/ "http://creativecommons.org/licenses/by-sa/2.5/") License.*] Problems Involving Electricity Transmission. 1. Power Loss During Transmission. 2. The high voltage transmission cable is very dangerous. 3. The cost of the cables are high. 4. Charge leakage may happen between cables and earth. 5. Pylons may be struck by lightning. 6. Pylons and cables may be struck by light aircraft.

Question 49

**What is a national Grid Network?** A national **Grid Network** is a **network of cable** that **connects all the power stations** in a country to transmit electricity to the consumers throughout the nation. **The advantages of the National Grid Network** 1. Reduces**power lost during transmission.** The potential difference is increased before transmission. This can reduce the current and hence reduces the energy lost during transmission. 2. **Electricity supply is more stable and reliable.**This ensures a continuous supply of electrical energy to the whole country. 3. **Electric current can be distributed to different users according to the voltage requirement.**Transformer is used to step down the voltage to certain level according to the needs of the consumers. 4. **Maintenance and repair work can be done at anytime.** This is because any power stations can be shut down without affecting users in other areas.