Section 1: Magnets and Magnetic Fields Section 2: Magnetism from Electric Currents Section 3: Electric Currents from Magnetism

Transcription

1 Section 1: Magnets and Magnetic Fields Section 2: Magnetism from Electric Currents Section 3: Electric Currents from Magnetism

2 Key Terms Magnetic Poles Magnetic Fields

3 Magnets The name magnet comes from the region of Magnesia which now modern day Greece The first magnetic rocks called Lodestones were found in this region almost 3000 yrs ago. A lodestone is composed of an iron-based material called magnetite

4 Some materials can be made into permanent magnets Some materials like lodestones are always magnetic. These materials are called permanent magnets. However, some materials like iron can become permanent magnets

5 By rubbing a permanent magnet against a piece of iron, the iron will become permanent magnetic. A slower process in producing a permanent magnet from a piece of iron would be placing the iron near a strong magnet. Creating a permanent magnet from a piece of iron does not mean the iron will always be magnetic

6 The magnetism of the iron can be weakened or completely removed. This can be accomplished by heating or hammering the piece of iron. Magnetically Soft a material that is easily magnetized but also loses its magnetism easily. (Iron) Magnetically Hard a material that is hard to magnetize and does not lose their magnetism easily. ( Cobalt and Nickel)

7 Magnets exert magnetic forces on each other Like poles repel, and opposite poles attract Magnets have a pair of poles, the north pole and south pole. It is impossible to isolate a magnetic pole. If you cut a magnet in half it will still have a north and south pole. NO MATTER HOW SMALL

8 Magnetic Fields If two like poles are brought close together and one magnet is free to move, the closer you bring the poles the free moving magnet will start to move away. Magnets are sources of magnetic fields

9 Magnetic force is a field force. When magnets repel or attract each other, it is due to the interaction of their magnetic fields. The strength of a magnetic field depends on what the magnet is made of and the degree to which it has been magnetized Magnetic field lines are used to show magnetic fields

10 The magnetic field gets weaker with distance. The further apart magnetic field lines are represents a weak field. The closer together, the stronger the field Magnetic fields are strongest at the poles Compasses can track magnetic fields. A compass is a magnet suspended on top of a pivot so that the magnet can rotate freely

11 A compass aligns with Earth s magnetic field. The first compasses were made from lodestones. Earth s magnetic field is like that of a bar magnet

12 Earth s magnetic poles are not the same as its geographic poles The magnetic pole in Antarctica is actually a magnetic N pole, and the magnetic pole in northern Canada is actually a magnetic S pole.

13 Key Terms Solenoid Electromagnet Galvanometer Electric Motor

14 Magnetism from Electric Currents In 1820 a Danish science teacher named Hans Christian Oersted first experimented with the effects of an electric current on the needle of a compass Electric currents produce magnetic fields

15 Use the right-hand rule to finde the direction of the magnetic field produced by a current If you imagine holding the wire in your right hand with your thumb pointing in the direction of the positive current, the direction your fingers would curl is in the direction of the magnetic field. The magnetic field of a coil of wire resembles that of a bar magnet

16 By wrapping a wire in to a coil you can increase the magnetic field without increasing a fire hazard. A coil of wire with an electric current is called a solenoid A solenoid will have a north and a south pole just like a magnet

17 The strength of the magnetic field in a solenoid depends on the number of coils or the amount of current in the wire. By increasing the number of coils or the amount of current you can increase the strength of the magnet. Another way to increase the magnetic field of a solenoid is to place a magnetic material in the center of the coil

18 By placing a magnetic material in the center of a solenoid, an electromagnet is created. Magnetism can be caused by moving charges Negatively charged electrons moving around the nuclei of all atoms make magnetic fields. Nuclei also have magnetism because of proton movement.

19 Each electron with in an atom has a property called electron spin, which also produce magnetic fields When a potentially magnetic substance is not magnetized, its domains are randomly oriented When the domains become more uniform the substance becomes magnetized

20 Electromagnetic Devices Galvanometer detect current Galvanometers are devices used to measure current in ammeters and voltage in voltmeters. A galvanometer consists of a coil of insulated wire wrapped around an iron core that can rotate between the poles of a permanent magnet

21 Electric motors convert electrical energy to mechanical energy A device called a commutator is used to make the current change direction every time the flat coil makes a half revolution. Devices called brushes connect the wires to the commutator.

22 Stereo speakers use magnetic force to produce sound In a speaker, when the direction of the current in the coil of wire changes, the paper cone attached to the coil moves, producing sound waves.

23 Key Terms Electromagnetic Induction Generator Alternating Current Transformer

24 In 1831, Michael Faraday discovered that a current can be produced by pushing a magnet through a coil or wire Electromagnetic induction is the process of creating a current in a circuit by changing a magnetic field

25 Electromagnetic induction and Faraday s Law Faraday s Law states that an electric current can be produced in a circuit by a changing magnetic field. It may seem that electromagnetic induction is created from nothing, but it does not violate the law of conservation of energy.

26 An outside source is needed to move a loop through the magnetic field. Moving electric charges experience a magnetic force when in a magnetic field The force is at its maximum value when the charge moves perpendicular to the magnetic field.

27 When the wire in a circuit moves perpendicular to a magnetic field, the current induced in the wire is at a maximum When the wire moves parallel to a magnetic field, there is zero current induced in the wire Generators convert mechanical energy to electrical energy

28 Alternating currents are electric currents that change direction at regular intervals (AC) In an alternating current generator, the mechanical energy of the loop s rotation is converted to electrical energy when a current is induced in the wire. Table 1 pg 579

29 Generators produce the electrical energy you use in your home The mechanical energy used in a commercial power plant comes from a variety of sources. Dams Coal Nuclear Fission Hot Water Solar Power Wind

30 Electricity and magnetism are two aspects of a single electromagnetic force Light is a from of electromagnetic energy. Visible light travels as electromagnetic waves or EM waves, as to other forms of radiation (X rays and radio signals) called EMF electromagnetic frequency waves

31 EM waves are made up of oscillating electric and magnetic fields that are perpendicular to each other EM waves are transverse waves

32 Transformers Transformers are devices that increase or decrease the voltage of alternating current Two wires are coiled around opposite sides of a closed iron loop. One wire is attached to a source of alternating current, such as a power outlet. The other wire is attached to an appliance.

33 Transformers can increase or decrease voltage The voltage induced in the secondary coil of a transformer depends on the number of loops, or turns in the coil When the primary and secondary circuits in a transformer each have one turn, the voltage across each is about equal.

34 When an additional secondary circuit is added, the voltage across each is again about equal When the two secondary circuits are combined, the secondary circuit has about twice the voltage of the primary circuit. Actual transformers may have thousands of turns.

35 When the secondary coil produces a higher voltage than the primary coil this is a step-up transformer. If the voltage on the primary coil is greater than the voltage on the secondary coil this is a step-down transformer. (Transformers of power lines)

36 A step-up transformer is used at or near the power plant to increase the voltage of the current to about 120,000 V. A step-down transformer is use near homes to reduce the voltage to around 120 V.