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3.1 MAGNETIC EFFECT OF A CURRENT-CARRYING CONDUCTOR What is an electromagnet? An electromagnet can be made by sending an electric current through a coil of wire wound around an iron core. When a current flows through the coil, it produces a magnetic field . The soft iron core becomes temporarily magnetized when the current is switched on. When the current is switched off, it loses its magnetism. What is a magnetic field pattern? A magnetic field pattern can be represented by field lines that show the shape of the field. Magnetic field lines which are close together represent a strong field. The field direction is defined as the direction indicated by a compass needle placed in the magnetic field. Determine the direction of the magnetic field around a current- carrying wire The Right-Hand Grip Rule Grip the wire using the right hand , with your thumb pointing in the direction of the current . Your other fingers now point round the wire in the direction of the magnetic field. When the direction of the current is reversed, the magnetic field direction also is reversed. 61

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Activity 1 Aim Draw the magnetic field pattern due to a current in a 1.straight wire, 2. coil 3. solenoid Determine the direction of the magnetic field. Procedure 1. Use small plotting compasses and place them on the cardboard to determine the direction of the magnetic field. 2. Observe and sketch the pattern produced. 3. Reverse the current supply connections to see its effect on the compass needle. 4. Remove the compasses and springkle iron filings onto the cardboards and tap the sheets gently with a pencil. Sketch your observation. Straight Wire 62
Coil Solenoid Note: The right hand grip rule for a solenoid: The thumb points towards north pole of the magnetic field while the other fingers indicate the direction of the current in the solenoid. 63

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factors that affect the strength of the electromagnet. Describe application of Electromagnet 64 Electric Bell When the bell push is pressed , a current flows in the coils of the electromagnet, causing it to be magnetized . The magnetized electromagnet attracts the soft-iron armature, causing the hammer to strike the gong . The movement of the armature breaks the contact and causes the electromagnet to lose it magnetism. The light spring pulls the armature back, remaking the contact and completing the circuit again. The cycle is repeated so long as the bell push is pressed and continuous ringing occurs. Magnetic Relay A solenoid switch worked by an electromagnet. It is used in a circuit which controls the operation of another circuit, especially if the current is large in the second circuit. Circuit 1 requires only a small current. When the switch is closed,
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