1. Electricity and Magnetism
SONNY P. DE LEON MT 1

2. Electric Current and Magnetism
He's not as famous as Darwin or Newton, but if you've ever used a modern gadget, chances are you have this 19th-century Danish physicist to thank.

3. Oersted’s Experiment successfully connected electricity and magnetism
discovered that in addition to the electric field, a charge also has a magnetic field around it when it moves.
when current is present, the needle deflects perpendicular to the wire
when the current is reversed, the needle deflects in the opposite direction

4. An apparatus can be built that shows the magnetic field around a straight wire.
The compass needles all form a circle when the current is switched on in the wire.

5. Sources of Magnetic Field (B)
Magnetic fields exist around current-carrying wires, permanent magnets, and moving charged particles.

6. Oersted’s Principle
“A charge moving through a straight conductor produces a circular magnetic field around the conductor”

7. The Right Hand Rule
is used to determine the direction of the magnetic field:
If you hold a straight conductor in your right hand with your right thumb pointing in the direction of the conventional current, your curled fingers will point in the direction of the magnetic field lines.
Note: If you use the electron flow model instead, you must use your left hand

8. Representing Currents and Magnetic Fields
a cross-section of the wire is often shown
the current can either go into the page (x) or out of the page (.)
this model is based on an arrow
the magnetic fields get farther apart as you move away from the wire to indicate that it is getting weaker

9. Implications led to new technologies like motors and generators
demonstrated using electricity to produce magnetism
can produce magnetic fields with properties that can be controlled:
can turn the magnetism on and off
can change the direction of the magnetic field
can change the strength of the magnetic field

10. (1) Current Carrying Wire loop
Right-hand Rule
It is used to determine the direction of magnetic field.
(1) Current Carrying Wire loop
The magnitude or strength of magnetic field B at a given point is
B = μ0 I / 2πr

11. B = μ0 I / 2 π r (current carrying wire)
Where μ0 is the permeability of free space, I is the current, and r is the distance from the center of the wire to the point of interest.
The SI unit for magnetic field is tesla ( T), named after Nicola Tesla, who invented the modern radio among other things.
The permeability of free space μ0 is 4 π x tesla

12. Magnetic Field Through a Wire loop
A current-carrying wire is more useful as magnet when it is wound in loop or many turns.
The magnitude of magnetic field at the center of a wire loop is B = μ0 N I / 2r

13. Right -hand rule (Coil)
Grasp the coil so that your curled fingers is the direction of the current and the thumb is the direction of the magnetic field.
A current carrying wire is more useful as a magnet when it is wound in a loop or many turns. The result is that the magnetic field at the center of the loop adds up, intensifying the field there.

14. Solenoid
When many turns are made such that the length of the coil is greater than the radius of each turn, the coil is solenoid.
The magnitude of magnetic field at the center of the solenoid at a given point is, B = μ0 N I / L, where L stands for the length of the solenoid.

15. Right - hand rule
Grasp the solenoid so that your curled fingers is the direction of the current and the thumb is the direction of the magnetic field.
Solenoid are described in terms of the ratio n, the number of turns per unit length.
The magnetic field at the center of the solenoid can be computed as B = μ0 N I / L

16. Solenoid
The magnetism of a solenoid may be turned on or off and the magnetic field may be increased through the following ways:
Increasing the current
Increasing the number of turns or windings
Making the winding close and tight
Inserting soft iron core inside the solenoid.

17. Electric Current and Magnetism
Two wires carrying electric current exert force on each other, just like two magnets.
The forces can be attractive or repulsive depending on the direction of current in both wires.

18. The direction of the force can be deduced from the right-hand rule.
If you bend the fingers of your right hand as shown, your thumb, index, and middle
finger indicate the directions of the force, current and magnetic field.

19. Electric Current and Magnetism
The magnetic field around a single wire is too small to be of much use.
There are two techniques to make strong magnetic fields from current flowing in wires:
Many wires are bundled together, allowing the same current to create many times the magnetic field of a single wire.
Bundled wires are made into coils which concentrate the magnetic field in their center.

20. When wires are bundled, the total magnetic field is the sum of the fields created by the current in each individual wire.
By wrapping the same wire around into a coil, current can be “reused” as many times as there are turns in the coil

21. Electric Current and Magnetism
The most common form of electromagnetic device is a coil with many turns called a solenoid.
A coil takes advantage of these two techniques (bundling wires and making bundled wires into coils) for increasing field strength.

22. Coils are used in electromagnets, speakers, electric motors, electric guitars, and almost every kind of electric appliance that has moving parts.

23. The true nature of magnetism
The magnetic field of a coil is identical to the field of a disk-shaped permanent magnet.

24. Electric Current and Magnetism
The electrons moving around the nucleus carry electric charge.
Moving charge makes electric current so the electrons around the nucleus create currents within an atom.
These currents create the magnetic fields that determine the magnetic properties of atoms.

25. Magnetic force on a moving charge
The magnetic force on a wire is really due to force acting on moving charges in the wire.
A charge moving in a magnetic field feels a force perpendicular to both the magnetic field and to the direction of motion of the charge.

26. Magnetic force on a moving charge
A magnetic field that has a strength of 1 tesla (1 T) creates a force of 1 newton (1 N) on a charge of 1 coulomb (1 C) moving at 1 meter per second.
This relationship is how the unit of magnetic field is defined.

27. Magnetic force on a moving charge
A charge moving perpendicular to a magnetic field moves in a circular orbit.
A charge moving at an angle to a magnetic field moves in a spiral.

28. Magnetic field near a wire
The field of a straight wire is proportional to the current in the wire and inversely proportional to the radius from the wire.
Current (amps)
B = 2x10-7 I r
Magnetic field
(T)
Radius (m)

29. Magnetic fields in a coil
The magnetic field at the center of a coil comes from the whole circumference of the coil.
No. of turns of
wire
Magnetic
field
(T)
B = 2p x10-7 NI r
Current
(amps)
Radius
of coil (m)

30. Calculate magnetic field
A current of 2 amps flows in a coil made from 400 turns of very thin wire.
The radius of the coil is 1 cm.
Calculate the strength of magnetic field (in tesla) at the center of the coil.
1) You are asked for the magnetic field in tesla.
2) You are given the current, radius, and number of turns.
3) Use the formula for the field of a coil: B = 2π x 10-7 NI ÷ R
4) Solve:
B =(2π x 10-7)(400)(2A) ÷(.01m)= 0.05 T

31. Electromagnets and the Electric Motor
Key Question:
How does a motor work?

32. Electromagnets and the Electric Motor
Electromagnets are magnets that are created when electric current flows in a coil of wire.
A simple electromagnet is a coil of wire wrapped around a rod of iron or steel.
Because iron is magnetic, it concentrates and amplifies the magnetic field created by the current in the coil.

33. Electromagnets and the Electric Motor
The right-hand rule:
When your fingers curl in the direction of current, your thumb points toward the magnet’s north pole.

34. The Principle of the Electric Motor
An electric motor uses electromagnets to convert electrical energy into mechanical energy.
The disk is called the rotor because it can rotate.
The disk will keep spinning as long as the external magnet is reversed every time the next magnet in the disk passes by.
One or more stationary magnets reverse their poles to push and pull on a rotating assembly of magnets.

35. The Principle of the Electric Motor
To keep the disk spinning, the external magnet must be reversed as soon as magnet (B) passes by.
Once the magnet has been reversed, magnet (B) will now be repelled and magnet (C) will be attracted.
As a result of the push-pull, the disk continues to rotate counterclockwise.

36. Commutation
The process of reversing the current in the electromagnet is called commutation and the switch that makes it happen is called a commutator.

37. Electric Motors Electric motors are very common.
All types of electric motors have three key components:
A rotating element (rotor) with magnets.
A stationary magnet that surrounds the rotor.
A commutator that switches the electromagnets from north to south at the right place to keep the rotor spinning.

38. Electric Motors
If you take apart an electric motor that runs on batteries, the same three mechanisms are there; the difference is in the arrangement of the electromagnets and permanent magnets.

39. Electric motors
The rotating part of the motor, including the electromagnets, is called the armature.
This diagram shows a small battery-powered electric motor and what it looks like inside with one end of the motor case removed.

40. Electric motors
The permanent magnets are on the outside, and they stay fixed in place.
The wires from each of the three coils are attached to three metal plates (commutator) at the end of the armature.
commutator

41. Electric Motors
As the rotor spins, the three plates come into contact with the positive and negative brushes.
Electric current flows through the brushes into the coils.
As the motor turns, the plates rotate past the brushes, switching the electromagnets from north to south by reversing the positive and negative connections to the coils.
The turning electromagnets are attracted and repelled by the permanent magnets and the motor turn

42. Induction and the Electric Generator
Key Question:
How does a generator produce electricity?

43. Induction and the Electric Generator
If you move a magnet near a coil of wire, a current will be produced.
This process is called electromagnetic induction, because a moving magnet induces electric current to flow.
Moving electric charge creates magnetism and conversely, changing magnetic fields also can cause electric charge to move.

44. Induction
Current is only produced if the magnet is moving because a changing magnetic field is what creates current.
If the magnetic field does not change, such as when the magnet is stationary, the current is zero.

45. Induction
If the magnetic field is increasing, the induced current is in one direction.
If the field is decreasing, the induced current is in the opposite direction.

46. Magnetic flux
A moving magnet induces current in a coil only if the magnetic field of the magnet passes through the coil.

47. Faraday's Law
Faraday’s law says the current in a coil is proportional to the rate at which the magnetic field passing through the coil (the flux) changes.
Consider a coil of wire rotating between two magnets

48. 23.3 Faraday's Law
When the coil is in position (A), the magnetic flux points from left to right.
As thecoil rotates (B), the number of field lines that go through the coil decreases.
As a result, the flux starts to decrease and current flows in a negative direction.
At position (C), the largest negative current flows because the rate of change in
flux is greatest.
The graph of flux versus time has the steepest slope at position (C), and that is why the current is largest.
At position (C), no magnetic field lines are passing through the coil at all and therefore the flux through it is zero.
As the coil continues to rotate (D), flux is still decreasing by getting more negative.
Current flows in the same direction, but decreases proportionally to the decreasing rate of change (the slope of flux versus time levels out).
At position (E), the flux through the coil reaches its most negative value.
The slope of the flux versus time graph is zero and the current is zero.
As the coil rotates through (F), the flux starts increasing and current flows in the opposite direction.

49. Generators
A generator is a device that uses induction to convert mechanical energy into electrical energy.
Because the magnet near the coil alternates from north to south as the disk spins,
the direction of the current reverses every time a magnet passes the coil.
This creates an alternating current.

50. Transformers
Transformers are extremely useful because they efficiently change voltage and current, while providing the same total power.
The transformer uses electromagnetic induction, similar to a generator.

51. Transformers
A relationship between voltages and turns for a transformer results because the two coils have a different number of turns.

52. The induced emf in the primary and secondary coil is given by
Ve P = -Np Δ(BA) / Δt ( primary coil)
Ve S = -Ns Δ(BA) / Δt ( secondary coil)
Transformer efficiency is the ratio of the power output to the power input.
E = power output / power input
= Ve S is / Ve P i p

53. Step-up transformer – transformer that produces a larger output voltage.
Step-down transformer - gives a lower output voltage.

54. Solve:
A friend being back from Europe a device that she claims to be the world’s greatest coffee maker.Unfortunately, it was designed to to operate from 240 V line to obtain the 960 W of power that it needs.
a. What can she do to operate it at 120-V?
b. What current will the coffee maker draw from 120-V line?

55. 2. An ac generator that delivers 20 A at 6000 V is connected to a step-up transformer . What is the output current at V if the trnsformer efficiency is 100%? 3. A step-up transformer has 400 secondary turns and only 100 primary turns. A 120V alternating voltage is connected to primary coil.What is the output voltage?

56. Application: Trains that Float by Magnetic Levitation