[PHY] Chapter 21 - Electromagnetism

DARRELL ER (COPYRIGHTED) © TOPIC 21: ELECTROMAGNETISM DARRELL ER (COPYRIGHTED) ©

CHAPTER ANALYSIS THE ABOUT MASTERY EXAM WEIGHTAGE DARRELL ER (COPYRIGHTED) ©

MAGNETIC EFFECT OF CURRENT RIGHT HAND GRIP RULE SOLENOID KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

MAGNETIC EFFECT OF CURRENT A current carrying conductor generates a magnetic field around itself. To determine the direction of magnetic field, use the ‘right hand grip rule’. To remember, visualise an arrow. If the arrow is flying towards you, it is a ‘dot’. If the arrow is flying away from you, it is a ‘cross’. Hence, ‘dot’ is out of the paper, towards you & ‘cross’ is into the paper away from you. DARRELL ER (COPYRIGHTED) © MAGNETIC EFFECT OF CURRENT 3D view 2D view

MAGNETIC EFFECT OF A SOLENOID Magnetic field is stronger in the region inside the solenoid as the field lines in the region inside the solenoid are parallel. (has same strength along most of the inner part of the solenoid) Magnetic field lines outside the solenoid are similar to those of a bar magnet because the solenoid has poles. To determine the polarity of the solenoid, use the right hand grip rule. DARRELL ER (COPYRIGHTED) © SOLENOID Imagine gripping the wire with your right hand. Your finger should curl to the direction of the current. The direction your thumb is pointing towards is the north pole of the solenoid. * ‘Fingernails’ is direction of current! To increase magnetic field strength of solenoid: - use a larger current - increase number of coils - Place a soft iron core in the solenoid to concentrate the magnetic field

APPLICATION OF ELECTROMAGNETISM CIRCUIT BREAKER, MAGNETIC RELAY, BELL MOTOR EFFECT KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

CIRCUIT BREAKER Device designed to switch off the current in a circuit when the current exceeds a certain value. When current is too high, electromagnet will be strong sufficiently magnetised to attract the contact. This will break the circuit. Spring will then pull the springy metal to the right and separate the contacts to stop the current from flowing. After the fault is repaired; the contacts can be pushed back by pressing the reset button on the outside of the circuit breaker box. DARRELL ER (COPYRIGHTED) © CIRCUIT BREAKER When current is normal: When current exceeds:

ELECTRIC BELL When the switch is pushed, the circuit is closed and the soft iron cores are magnetized. The armature is then attracted to the electromagnet and hammer strikes the bell. The forward movement of the armature breaks contact and cuts off current. The soft iron cores lose their magnetism and the armature is no longer attracted. The steel spring causes armature to return to its original position and remake contact, causing the cycle to repeat again. The bell will continue to ring as long as the switch is closed. DARRELL ER (COPYRIGHTED) © ELECTRIC BELL

MAGNETIC RELAY A magnetic relay uses an electromagnet in one circuit to switch on another circuit. A small current in the first circuit can be used to control a second circuit that requires a large current. How it works 1) The first circuit is a simple electromagnet which requires only a small current. 2) The current flows and magnetise the soft iron core to attract the soft iron armature. 3) The top end of the armature is raised as it swings about the pivot, closing the switch contacts of the second circuit. 4) When the switch for the first circuit is open, the soft iron core is demagnetised and the armature will return to its original position. This device enables circuits to be constructed more efficiently. Furthermore, it can also be attached to thermistors or LDRs which can run on low current. DARRELL ER (COPYRIGHTED) © MAGNETIC RELA Y

MAGNETIC FORCE ON A CURRENT CARRYING CONDUCTOR Current + Magnetic Field  Direction of force (motor effect) DARRELL ER (COPYRIGHTED) © MAGNETIC FORCE ON A CURRENT CARRYING CONDUCTOR 1. A current carrying conductor will have its own magnetic field. 2. When the wire’s magnetic field interacts with the external magnetic field, this will result in a stronger magnetic field on one side. 3. A magnetic force will act on the wire as a result of the uneven magnetic field. FLEMING’S LEFT HAND RULE Tips to remember: Goldilocks & the 3 bears story. Force is father, Magnetic field is mother, Current is child. The mother bear ownself cook but ownself eat all the porridge. So: Father bear is like ‘you good you good.’ (thumbs up) Mother bear is the culprit. (point at her with index finger) Child is is ‘FML’ (middle finger)