Physics - Electromagnetism

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Magnetic Field Line The magnetic filed of a magnet is represented by the magnetic field line. The magnetic field line flowing out from the North pole and flowing into the South pole. 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. Compass in a magnetic Field The pattern and the direction of a magnetic field can be determined by a compass. The pointer of the compass is always in the direction of the magnetic field. 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. 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. This will discuss in electromagnetism. <iframe allowfullscreen="" frameborder="0" height="344" src="http://www.youtube.com/embed/HQdLFEiVeCA?fs=1" width="459"></iframe> <span class="fullpost"> </span> Alternating Current Direct 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:

f = \frac{1}{T}

The effective voltage for a sinusoidal alternating current The maximum potential difference supplied by an a.c source is known as the peak voltage V_P. 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:

V_{rms}  = \frac{{V_p }}{{\sqrt 2 }}

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. Magnetic Field Pattern

[Figure (a)]

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.

[Figure (a)]

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

[Figure (c)]

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.   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 (a)]

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.

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.

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.

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.

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.

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.

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