# Physic Unit 3 Chapter 2

• Created by: A.B.
• Created on: 17-04-13 17:27
• Physics Unit 3 Chapter 3
• Electromagnets
• The force between two magnets:  like poles repel; unlike poles attract.
• A magnetic field line is the line along which a plotting compass points.
• An electromagnet consists of a coil of insulated wire wrapped round an iron core.
• The magnetic field of the wire magnetises the iron bar strongly and when the current is switched off the iron loses most of its magnetism.
• Iron easily loses it magnetism whereas steel is unsuitable because it keeps it magnetism when the current is switched off.
• Electromagnets are used in scrapyard cranes, circuit breakers, electric bells and relays.
• Untitled
• The Motor Effect
• In the motor effect, the force:
• Is increased if the current or the strength of the magnetic field is increased.
• It is right angles to the direction of the magnetic field and to the wire.
• Is reversed if the direction of the current or the magnetic field is reversed.
• An electric motor has a coil which turns when a current is passed through it.
• Fleming's LH Rule
• First finger - field. Second finger - Current. ThuMb - Movement.
• Electromagnetic Induction
• Electromagnetic induction is the process of creating a potential difference using a magnetic field.
• When a conductor cuts the lines of a magnetic field, a potential difference is induced across the ends of the conductor.
• When an electromagnet is used, it needs to be switched on or off to induce a pd.
• Transformers
• A transformer only works on ac because a changing magnetic field is necessary to induce ac in the secondary coil.
• A transformer has an iron switch mode transformer which has a ferrite core.
• A switch mode transformer is lighter and smaller than an ordinary transformer. It operates at high frequency.
• A step up transformer must have more turns (coils) around the secondary coil than the primary coil.
• Transformers in Action
• Transformers are used to step potential differences up or down.
• The transformer equation is primary pd/secondry pd.= number of primary turns/number of secondry turns.
• For a step down transformer, the number of secondary turns is less than the number of primary turns and vice versa.
• For a 100& efficient transformer, power supplied to the transformer = power delivered by the transformer which is primary current x voltage = secondary current x voltage.
• The higher the grid pd, the greater the efficiency of transferring electrical power through the grid.
• Pd = number of turns.
• A Physics Case Study
• We use physics in hospital whenever: -blood pressure (or temperature) is measured - an ECG recording is made - an endoscope is used - a scanner is used.
• We measure blood pressure, ECG potential differences and exposure to ionising radiation in hospitals.
• A CT scanner uses X-rays, which are ionising radiation and therefore can damage living tissue.
• We use physics in hospital whenever: -blood pressure (or temperature) is measured - an ECG recording is made - an endoscope is used - a scanner is used.