Physics 220 Class 8

1
Physics 220 Class 8

• Today you will
• learn about how charged particles move in
  electric and magnetic fields.
• learn about several devices which use electric
  and magnetic fields for practical purposes.

2
Reminders

• Chapter Test 2 is due Friday
• Walk-in Lab 2 should be finished Friday and
  turned in Monday
• HW 3 is due Tuesday
• Chapter Test 3 is due Wednesday

3

The Lorentz Force Law
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Electric Field and Electric Potential

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Electric Field and Electric Potential

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1. Field Vectors

• We can also draw field vectors.
• Magnitude length of arrow
• Direction direction of arrow

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2. Field Lines

• When we draw field lines
• Magnitude number of lines/area
• Direction tangent to field lines

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3. Field Contours

• When we draw field contours
• Magnitude number of surfaces/length
• Direction normal to field contours

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Static Electric Fields and Potentials in a
Conductor

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Motion of Charges in a Constant Electric Field

• Constant force just like .
• Acceleration is .

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Motion of Charges in a Constant Electric Field



charge from rest
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Motion of Charges in a Constant Electric Field



charge from rest
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Motion of Charges in a Constant Electric Field



- charge from rest
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Motion of Charges in a Constant Electric Field



- charge from rest
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Motion of Charges in a Constant Electric Field



- charge with initial velocity
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Motion of Charges in a Constant Electric Field



charge with initial velocity
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Forcesin a Constant Electric Field

0 V
200 V
charge at rest q 1.50 mC
2.00cm
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Forcesin a Constant Electric Field

0 V
200 V
charge at rest q 1.50 mC
2.00cm
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Forcesin a Constant Electric Field

0 V
200 V
charge at rest q 1.50 mC
2.00cm
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Forcesin a Constant Electric Field

0 V
200 V
charge at rest q 1.50 mC
2.00cm
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Kinetic and Potential Energyin a Constant
Electric Field

0 V
200 V
charge at rest q 1.50 mC
2.00cm
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Kinetic and Potential Energyin a Constant
Electric Field
For a charge starting at rest and accelerated
through an electric potential ?V
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Motion of Charges in a Constant Magnetic Field

• This is just weird
• The force is perpendicular to the velocity, so it
  can only change the direction of motion without
  speeding up or slowing down the charge.

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Motion of Charges in a Constant Magnetic Field
v Parallel to B

• If the charge moves parallel (or antiparallel) to
  B, there is no force at all.

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Motion of Charges in a Constant Magnetic Field
v Perpendicular to B

• The direction of the force is given by the
  right-hand rule. It is in a plane perpendicular
  to B and is at 90 to v.
• This force keeps the charge moving in a circle.

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Motion of Charges in a Constant Magnetic Field
v Perpendicular to B

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Motion of Charges in a Constant Magnetic Field
v Perpendicular to B

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Motion of Charges in a Constant Magnetic Field
v Perpendicular to B

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What happens when there is a component of v
parallel to B and a component perpendicular to B?

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Motion of Charges in a Constant Magnetic Field
v General

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Two Useful EquationsKnow how to derive these!

Cyclotron Radius
This even works for relativistic particles as
long as we use the relativistic momentum.
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Two Useful EquationsKnow how to derive these!

Cyclotron Frequency
This is good relativistically if we use the
relativistic mass.
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Linear Accelerators

• Series of drift tubes.
• E inside drift tubes is very small.
• Change V at a high frequency so it always pushes
  the particle forward.
• Drift tubes get longer as v increases.

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Cyclotrons

• Dees instead of drift tubes.
• Path in dees naturally gets longer.

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Mass Spectrometers

• Accelerate ions in electric field.
• Bend ions in magnetic field.