Electromagnetism

WHAT IS ELECTROMAGNETISM? 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. Since the magnet is produced electric current, it is called the electromagnet. 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. In short, when current flow through a conductor, magnetic field will be generated. When the current ceases, the magnetic field disappear.

MAGNETIC FIELD PATTERN

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

Sometime, the magnetic field pattern may be given in plan view, as shown in figure (b). 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.

Direction of the Magnetic Field The direction of the magnetic field formed by a current carrying straight wire can be determined by the Right Hand Grip Rule or the 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 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.

Strength of the Magnetic Field The strength of the magnetic field form by a current carrying conductor depends on the magnitude of the current. A stronger current will produce a stronger magnetic field around the wire as shown in Figure (e) below.

The strength of the field decreases out as you move further out. This is illustrated in figure (f) below. Thus, you must be very careful when you are asked the draw the magnetic field in your exam. The distance of the field lines must increase as it is further out form the wire.

COIL Field Pattern Figure (a) below shows the field pattern produced by a current flowing in a circular coil. In SPM, you need to know the field pattern, the direction of the field and the factors affect the strength of the field. 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 (b) shows the plan view of the field pattern.

Factors affecting the strength There are 2 ways to increase the strength of the magnetic field: 1. increase the current and 2. increase the number of turns of the coil.

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 The figure (a) illustrated the field pattern produced by a solenoid when current pass through it. The field lines in the solenoid are close to each other, showing that the magnetic field is stronger inside the solenoid. 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. 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)] Determining the Pole of the Magnetic Field The pole of the magnetic field of a solenoid can be determined by the Right Hand Grip Rule. 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. Since the magnetic field line is always coming out from the North Pole, therefore the thumb points towards the North Pole.

[Figure (b)] There is another method can be used to determine the pole of the magnetic field forms by the solenoid. Try to visualise that you are viewing the solenoid from the 2 ends as illustrated in figure (c) below. 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.

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.

Uses of Electromagnet - Electric Bell

When the switch is on, the circuit is completed and current flows. 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. 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. When the circuit is connected, the electromagnet regain its magnetism and pull

the armature and hence the hammer to strike the gong again. This cycle repeats and the bell rings continuously.

Uses of Electromagnet - Electromagnetic Relay

A relay is an electrical switch that opens and closes under the control of another electrical circuit. The switch is operated by an electromagnet to open or close one or many sets of contacts. 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. The electromagnet is magnetised and attracts one end of the iron armature. The armature is then closes the contacts (2nd switch) and allows current flows in the second circuit. 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.

Uses of Electromagnet - Circuit Breaker

The figure shows the structure of a circuit breaker. A circuit breaker is an automatic switch that cut off current in a circuit when the current become too large. 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. As a result, the spring pulls apart the contact and disconnects the circuit immediately, and the current stop to flow. 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.

Uses of Electromagnet - Telephone Earpiece

An electromagnet is used in the earpiece of a telephone. The figure shows the simple structure of a telephone earpiece. When you speak to a friend through the telephone, your sound will be converted into electric current by the mouthpiece of the telephone. The current produced is a varying current and the frequency of the current will be the same as the frequency of your sound. The current will be sent to the earpiece of the telephone of your friend. When the current passes through the solenoid, the iron core is magnetised. The strength of the magnetic field changes according to the varying current. When the current is high, the magnetic field will become stronger and when the current is low, the magnetic field become weaker. 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. The frequency of the sound produced in the telephone earpiece will be the same as your sound.

Force on a Current Carrying Conductor in a Magnetic Field When a current-carrying conductor is placed in a magnetic field, the interaction between the two magnetic fields will produce a force on the conductor, which called a catapult force. The direction of the force can be determined by Fleming's left hand rule as shown in Figure below.

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. The strength of the force can be increased by: 1. Increase the current 2. Using a stronger magnet

The Catapult Field The interaction of the two magnetic fields produces a resultant field known as catapult field as shown in the figure below.

Force between 2 Current-Carrying Conductors When 2 current carrying conductors are placed close to each other, a force will be generated between them. 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. This force is due to the interaction between the magnetic field of the 2 conductor. The figure below shows the catapult field produced by 2 current carrying conductors when their current is in the direction or opposite direction.

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.

Turning Effect of a Current Carrying Coil in a Magnetic Field

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. The diagram below shows the catapult field produced.

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

Direct Current Motor

. Electric motor converts electrical energy to kinetic energy. Diagram above shows the structure of a simple direct current motor (DC motor). It consist a rectangular coil of wire placed between 2 permanent magnets. The coil are soldered to a copper split ring known as commutator. 2 carbon brushes are held against the commutator. The function of the brush is to conduct electricity from the external circuit to the coil and allow the commutator to rotate continuously. 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.

Electromagnetic Induction When a magnet is moved into and out of the solenoid, magnetic flux is being cut by the coil. The cutting of magnetic flux by the wire coil induces an e.m.f in the wire. When the solenoid is connected to a closed circuit, the induced current will flow through the circuit. 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.

Law of Electromagnetic Induction Faraday's Law The magnitude of the induced e.m.f is determined from Faraday's Law.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.

Lenz's Law 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. Lenz's Law states that the induced current always flows in the direction that opposes the change in magnetic flux. 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. 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.

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

Direct Current Generator

A simple d.c generator essentially the converse of a d.c. motor with its battery removed. Initially the armature is vertical. No cutting of magnetic flux occurs and hence induced current does not exist. When the armature rotates, the change in flux increases and the induced current correspondingly increases in magnitude. 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. When the armature continues to rotate, the change in flux decreases. 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°. After rotating 360°, the armature returns to its original position. The direction of the induced current can be determined from Fleming's RightHand Rule. 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.

Alternating Current Generator

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. 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. When the armature rotates, the change in magnetic flux increases and the induced current increases until its maximum value at the horizontal position. The direction of the induced current can be determined from Fleming's Right Hand rule. As the armature continues on its rotation, the change in magnetic flux decreases until at the vertical position, no induced current exists. 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. 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. 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

Alternating Current Direct current (d.c) is usually supplied by acid-based batteries or dry cells. A common example of acid-based (electrolyte) batteries is the car battery. Direct current is uniform current flowing in one fixed direction in a circuit. Alternating Current Alternating current (a.c) is generated from alternating current generators such as hydroelectric power generators. The electricity supplied to households is

alternating current. Household electricity (alternating current) changes direction 50 times every second. Its magnitude also changes with time. Period And Frequency

The time taken for one complete cycle is known as the period, T. The frequency f is defined as the number of complete cycles in 1 second. The relationship between the frequency and the period is:

The effective voltage for a sinusoidal alternating current

The maximum potential difference supplied by an a.c source is known as the peak voltage VP. 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. 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:

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. The r.m.s current is the effective value of the alternating current.

What is transformer? 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

[ 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. 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. Types of Current in A Transformer 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. 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. The alternating in the secondary circuit can be converted into direct current by using a pair of diode. Symbol of A Transformer

The figure on the left shows the symbol of a transformer. The 2 lines in between the coil denote the core.

Types of Transformer Step-up transformer 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. The number of windings in the secondary winding is greater than the number of

windings in the primary coil. The current in the primary coil is greater than the current in the secondary coil. Step-down transformer 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. The number of windings in the primary winding is greater than the number of windings in the secondary coil. 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 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