Circuits revision Heres the circuit for the flashing neon bulb. - - PowerPoint PPT Presentation

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Circuits revision

• Here’s the circuit for the

flashing neon bulb.

• What is the period of the

flash in seconds ?

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Magnetic Fields revision

Please try problem 15 in Ch 24 on page 825. “What is the magnetic field at the center of the loop….”

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Magnetic Fields and Forces

• Magnetism
• Magnetic field shapes and direction
• Fields near electric currents
• Magnetic forces
• Moving charges and magnetism
• Magnetic machines
• Magnetic materials

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Magnetism

• Fundamental force of nature
• Related to electricity, but not the same

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Experimental Observations

• Magnetism does not move an electroscope, it

does not act on stationary charges

• Long range force (action over a distance)
• There are 2 poles, north and south, and they

come in pairs

• Like poles repel, unlike poles attract
• Poles attract magnetic materials

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Magnetic Field lines

• Magnetic Fields around

a bar magnet

• Similar to an electric

dipole

• Start at north pole,

terminate at south pole

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Like and unlike poles

Magnetic field lines between poles

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Electric Currents and Magnetic Fields

Oersted found that a current can move a magnetic compass

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Direction of Magnetic field

We use the right handed rule to find which way a magnetic compass would point

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Magnetic field near a loop

• Bend the wire into a loop.
• Dots - field is coming out of the page.
• Crosses - field is going in to the page

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Field near a solenoid

• Many loops will concentrate the field inside

the coil

• Called a solenoid – contains a uniform

magnetic field

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Magnetic field due to a current

Experimentally, the field strength, B, is proportional to current, I, and inversely proportional to distance, r. Units of Tesla, where μ0 is the permeability constant – 1.257x10-6 TmA-1

r I B   2 

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Tesla is a large unit

• Magnets in the lab – 0.1 to 1 T
• Kitchen magnets – 5x10-3 T
• Earths magnetic field – 5x10-5 T
• Superconducting magnets – in accelerators

and maglev trains – 10 T

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Magnetic Field at the center of a current loop

Inside a loop radius R:

R I B 2  

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Magnetic Field at the center of a current loop with N turns

If the loop has N turns, but its not yet a solenoid we have:

R NI B 2  

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Magnetic field inside a solenoid

The uniform field in a solenoid is For a solenoid with N turns, Length L and current I. Note: independent of the coil radius. Field is uniform.

L N I B  

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Magnetic Forces

• The magnetic fields

around two wires will attract or repel, just like bar magnets.

• A magnetic field exerts a

force on a current, or moving charge

• Currents in the same

direction attract

• Opposite currents repel

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Direction of Magnetic Force

• The force on a

wire with a current is perpendicular to both the magnetic field the direction of the current.

• We use another

right hand rule

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Magnitude of the Magnetic Force

The force between a magnetic field and a current along a wire length L perpendicular to the field is:

ILB F 

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Magnitude of the Magnetic Force

The force between a magnetic field and a current along a wire length L at an angle, α to the field is: If the current and B field are parallel – there is no force.

 sin ILB F 

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Force on a moving charge

• A current, I, is a

moving charge.

• The charge q moves

along the wire length L in time Δt

• The velocity will be

L/Δt

• We find that qv=IL

qv IL L qv t q I t L v      

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Magnitude of the Magnetic Force

The force between a magnetic field and a charge, q, moving with a velocity, v perpendicular to the field is:

qvB F 

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Magnitude of the Magnetic Force

The force between a magnetic field and a charge, q, moving at velocity, v, at an angle, α to the field is: If the moving charge and B field are parallel – there is no force.

 sin qvB F 

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Direction of Magnetic Force

• The force on a moving

charge is perpendicular to both the magnetic field the direction of the charge.

• Note the thumb is now

the direction of the +ve charge, instead of the current I.

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Path of charges in a magnetic field

• The force on a

charged particle in a magnetic field is perpendicular to its direction of motion.

• We always get

circular or spiral paths of charged paths in a magnetic field

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Path of charges in a magnetic field

• Centripetal force of

an object in a circle

m RqB v qvB r mv F    

2

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Path of charges in a magnetic field

• If we accelerated the ions

in an electric field V, the charge to mass ratio can be measured, 2 2 2

2 2 1 R B V m q mv qV E    

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Mass spectrometer

• First

measurement of e/m for the electron

• Used to

distinguish different types of atoms and isotopes

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Aurora Borealis

• Solar wind from the sun

(protons & electrons) gets deflected by Earth’s magnetic field.

• Portion of velocity

perpendicular to the field lines, curves the ionizing particles into spirals

• Ionize O2 and N2 in the

ionosphere

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Magnetic forces between currents

• Consider two wires

carrying currents I1 and I2.

• The field at the top wire

is

d I LI F L I B F d I B     2 2

2 1 12 1 2 12 2 2

  

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Magnetic forces between currents

From the field from the single wire, we can deduce the force between 2 wires carrying currents I1 and I2 is

d I LI F

wires parallel

  2

2 1





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Torques and Magnetic Moments

• Torque was defined in chapter 7
• Quantity to measure the force applied near a

pivot

• Useful for calculating rotational motion

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Torque

Torque, τ, measures the effectiveness of a force at causing an

• bject to rotate about

a pivot

  sin rF 

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Torque on a current loop in a B field

• Current loop in a

uniform field

• The forces on the top

and bottom wires will rotate the loop

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Torque on a current loop in a B field

• The total torque, τ, will

be the sum of the torques on the top and bottom wires.

• Loop height L, wire

length W

   sin sin 2 2 BIWL L F  

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Torque on a current loop in a B field

• In general, the torque on

a loop area A will be: The loop is forced to align with the magnetic field

  sin IAB 

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Using torque - MRIs

• Magnetic Resonance

Imaging (MRI) uses the protons magnetic moment in hydrogen atoms in high 1T fields.

• The rate of the emitted

radio waves from the excited states are detected

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Using Torque – Electric motor

Using commutators, the loop can be made to spin, to produce rotational movement

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Permanent Magnets - Ferromagnetism

• Ferromagnetism is a property of certain

elements – the ability to maintain a permanent magnetic field

• Depends on the crystalline structure of the

metal

• Found in alloys of iron, cobalt, nickel,

gadolinium, dysprosium, europium

• Half full electron shells, the magnetic dipole of

the electrons can align

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Periodic Table

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Crystalline structure aligned

• The magnetic

dipoles are grouped in micron size crystals, domains

• The dipoles can be

aligned by applying a magnetic field

• Can be destroyed

by heating (Curie point) or dropping

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Electromagnets

• An iron core near a

solenoid will align the domains inside the iron

• This increases the

magnetic field (factor of 100)

• Used to amplify the

magnetic field

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Summary

• Magnetism
• Magnetic field shapes and direction
• Fields near electric currents
• Magnetic forces
• Moving charges and magnetism
• Magnetic machines
• Magnetic materials

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Homework problems

Chapter 24 Problems 20, 21, 31, 41, 48, 53, 56, 57