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.
   

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

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:

4. In SPM, 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
5. 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.

The Effective Voltage for a Sinusoidal Alternating Current

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:

4. 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. 
5. The r.m.s current is the effective value of the alternating current.
6. The r.m.s. current can be calculated by using the following equation:

 

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 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.
   
   
5. Initially the armature is vertical. No cutting of magnetic flux occurs and hence induced current does not exist. 
6. When the armature rotates, the change in flux increases and the induced current correspondingly increases in magnitude. 
7. 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.
8. When the armature continues to rotate, the change in flux decreases. 
9. 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°. 
10. After rotating 360°, the armature returns to its original position.
11. The direction of the induced current can be determined from Fleming's Right-Hand Rule. 
12. 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.

 

There are 2 principal laws of electromagnetic induction:

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

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

 

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.

 

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.

 

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.

Lenz's Law

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

 

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.

 

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.

3. The direction of the force can be determined by Fleming's left hand rule.
4. 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.
5. Examples of electric equipment whose operation is based on this turning effect are
   1. the direct current motor
   2. the moving coil meter.

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.

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.

 

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.
   
   
3. The catapult field is a non-uniform field where the field at one side is stronger than the other side.
4. As a result, a force is produced to move the current carrying conductor from the stronger field to the weaker field.
5. The force produced by a catapult field is called the catapult force.
6. The direction of the force can be determined by Fleming's left hand rule as shown in Figure below.
   
7. 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.
8. 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.

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)

 

Door 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.

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.

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.

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.