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Before faraday’s discovery we knew…. * The source of magnetism is moving charge: Tiny and weak magnetic fields: spinning orbiting electrons create atomic.

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Presentation on theme: "Before faraday’s discovery we knew…. * The source of magnetism is moving charge: Tiny and weak magnetic fields: spinning orbiting electrons create atomic."— Presentation transcript:

1 before faraday’s discovery we knew…. * The source of magnetism is moving charge: Tiny and weak magnetic fields: spinning orbiting electrons create atomic magnets which cancel another completely in all materials except iron, nickel,& cobalt Large but weak magnetic fields: earth (toy magnet is 1000x stronger!) Small and strong magnetic fields any wire carrying a large current electromagnet (current + magnet)

2 Question 1 Which magnet is probably stronger, a. Refrigerator magnet b. Magnet in a pigeon head c. Earth d. Human blood?

3 Which is the correct unit for magnetic field, a.Tesla b.Gauss One Tesla (T) = 10,000 Gauss Source Magnetic Field (Gauss) Earth 0.5 Appliance 10 Bar magnet 100 Human limit 2000 Large electro- magnet 50,000

4 These are mostly a. transformers b. inductors c. electromagnets d. MRI machines

5 Question 4 Which would strengthen an electromagnet ? a. stronger core magnet b. more current c. less resistive wire d. more wire turns e. all of the above

6 Magnetic field lines of a coiled wire (electromagnet) B points A. left B. right

7 Recall magnetic fields around wires Straight wires: curved wires (solenoid) moving charges & iron magnets act like two magnets… why? Since moving charges create their own magnetic fields (i.e. force field) The force between moving charges and magnets is perpendicular to both-weird!

8 Charges repel another in straight lines, gravity pulls straight down, but magnets push charges sideways? Force = charge * velocity * magnetic field The most force comes from a large charge moving very fast past a strong magnet. Examples: deflection of cosmic rays at earth’s poles deflection of electrons inside a TV tube Deflection of a current wire in a motor

9 Magnets push charges sideways!

10 Question If a positive charge is moving horizontally from left to right And a magnet points vertically towards the ceiling Which “sideways” is the charge deflected? a.In a circle b.Towards the ground c.Towards you d.Right to left

11 Force is A. left B. right C. Up D. down

12 Magnetic Force on Moving Charges (enriched) Units: F = Force in Newtons (N) q = charge in Coulombs (C) B = magnetic field in Teslas (T) I = current in Amperes (A) l= length in meters (m) Definition of Magnetic Field (B): 1 Tesla = 1 N / Am = 1 N / (Cm/s) 1 Tesla = 10,000 Gauss

13 Motors The motor effect: the magnetic force pushes charges perpendicular to the wire so the entire wire moves instead of charges down the wire- S N I Which way is the wire pushed? 1. Up 2. down 3. left 4.right

14 Motors The motor effect: the magnetic force pushes On the charges perpendicular to the wire so the entire wire moves instead of just the charges down the wire--- S N I F

15 Motor simulation Why is the motor shown a DC motor? 1.There is a battery used for voltage 2.the magnets don’t move 3.The contact points switch the current so the loop turns in the same direction 4.All of the above

16 Motor/generator parts Label each: Stator (magnet) Brushings (contacts) Current (wire leads)

17 AC vs. DC motors AC motors: uses ac current so the current changes direction and the force changes direction to keep the motor turning- no special contacts Ex: plug in tools/ motion appliance DC motors: current doesn’t reverse so do it by reversing the battery connections every ½ cycle Ex: battery powered tools

18 Question Which is a dc motor? a.Washing motion b.Flashlight c.Electric toothbrush d.Plugged inVacuum cleaner

19 Faraday’s Discovery The generator (induction) effect: steam from a power plant pushes a wire past a magnetic field. This generates an EMF- electromotive force which pushes charges along the wire thereby generating current S N I move

20 Induction question IF moving charges (current)in a wire produces magnetism around it What is created by moving a magnet past a bare wire? A. changing magnetic field B. Voltage C. An force on the charges in the wire D. All of above E. Only A and B

21 “induce” Def: to create or generate ex. Induce child birth do not induce vomiting! Physics: create a voltage (“emf”) in a conductor by placing it in a changing magnetic field

22 Magnetic Induction The Missing Link: -Oersted discovered Electric Currents produce Magnetism. Will Magnetism produce Currents? YES since Faraday! But… only if the magnet field is changing!

23 How can you change the magnetic field near a wire? A. move a magnet across the wire B. Move the wire across a magnet C. Suddenly turn on or off the magnet if it’s an electromagnet D. Suddenly change the number of magnet lines intersecting the wire by rotating the magnet E. All of the above

24 Faraday’s Law Faraday’s Law of Magnetic Induction: emf = Electro-Motive Force (Volts) N = # of loops in circuit A = Area of loop (m 2 ) B = magnetic field (Teslas) t = time (seconds) Emf (voltage)= -  NBA) /  t

25 Why do moving magnets create current? 1.Just accept it! Electromagnetism along w/gravity, weak, strong forces exist our universe 2.It’s the same thing! Moving a magnet past a wire is moving a wire past a magnet. But moving a wire which has charges in it means your moving a charge,ie. current

26 Really why? (magnetism= electrostatics + special relativity) 3. Einstein’s special relativity! When you move a wire it actually shrinks! This means the charges in the wire are now closer, ie. Denser charges means stronger electrostatic field, which pushes the charges, i.e current!

27 Why 2 wires with current in same direction attract e e e e >>>> e e e e >>>> Electrons eeeee moving in 2 wires Past protons >>>> which don’t move if you’re an electron in the top wire you see protons in the bottom wire moving backwards. Due to special relativity they are closer together, ie. More of them! a net charge exists causing attraction! e e e e >>>>>> e e e e >>>> This true electrostatic attraction is called “magnetism”

28 Which is not an example of induction a. Walking past a magnet at a store with a stolen good which has a metal tag inside of it b. swiping a credit card c. using your flash (jump) drive d. copying files on your hard drive

29 Tricky induction: instead of moving a magnet you change a magnet by a. turning it on/off b. changing its current c. rotating the wire d. using an ac current all of these affects the number of magnet lines “caught by your wire”

30 Rotating a wire in a generator Which picture shows the wire catching no magnet lines? Which shows the wire loop catching the max # of B lines?

31 Induction by pulsing a current Left wire connect to a battery Right side is just a wire If current stays on or off, no new magnet lines going into right side so no new current But if suddenly change current, new magnet lines appear or disappear, and current is induced in the right side!

32 Ac electromagnet demos Pulse using alternating current! creating alternating magnetic fields Which induce alternating currents in any nearby metal These alternating currents create their own alternating secondary magnetic fields!

33 Ac generator is like having someone wave a magnet

34 Transformers Must use ac currents in primary! Power in = power out Ip*Vp = Is*Vs Vp/ Vs = # primary turns/# secondary turns Emf depents on # of coils

35 Transformer questions If the primary coil has 200 volts and 2x as many coils, then the voltage of the secondary circuit=____ To keep the same power, then the secondary coil has _____ as much current.

36 Lenz’s Law (energy conservation) The new magnetic fields always oppose the original magnetic source Why? Otherwise we would electrical energy to get even more mechanical (motion) energy. the new currents always weaken the original magnetic field That’s how they detect you stealing!

37 Summary 1. Moving charge (current) produces magnetic field - direct current produces a constant B -alternating creates an alternating B field, which then induces a counter current 2. Voltage or emf is generated in objects sitting in a changing magnetic field a. magnet or wire is moving b. dc electromagnet is pulsed c. use an ac electromagnet

38 Right-Hand Rules Hints (enriched) * I is in direction of positive charges as move from + battery to – battery ends (for electrons flip or use left hand) * Force is zero if the magnetic field is exactly parallel to the flow of charge * B magnetic field flows from N to S pole

39 Example Problem (enriched) current loop in a magnetic field Helga’s Super Vacuum F = ? (magnitude & direction!) B = 200 Teslas I = 20 Amps l= 5.8 m of copper wire in motorbrush = into page (tail feathers of arrow) = out of page (tip of arrow)

40 Helga’s current loop (enriched) (vacuum cleaner) = into page (tail feathers of arrow) = out of page (tip of arrow) Is this drawing correct? 1.Yes 2. No

41 Right-Hand Rules (enriched) Current-Carrying Wire: Thumb: points in direction of I, Fingers: curl in direction of B in concentric circles around wire. Loop: Fingers: curl in direction of current Thumb: points in direction of B. Force on Moving Charge: Thumb : direction of v Fingers : direction of B Palm : pushes in direction of F (or use middle finger!) Note all 3 Quantities are at 90 degrees !

42 Are ac circuits safe? Edison said no, Tesla yes Who won? What’s in your house?


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