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Physics 6B Magnetic Forces and Fields Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB
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Physics 6B

Feb 10, 2016

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Physics 6B. Magnetic Forces and Fields. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB. What creates a magnetic field?. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB. What creates a magnetic field? Answer: MOVING CHARGES. - PowerPoint PPT Presentation
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Page 1: Physics 6B

Physics 6B

Magnetic Forces and Fields

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Page 2: Physics 6B

What creates a magnetic field?

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Page 3: Physics 6B

What creates a magnetic field?

Answer: MOVING CHARGES

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Page 4: Physics 6B

What creates a magnetic field?

Answer: MOVING CHARGES

What is affected by a magnetic field?

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Page 5: Physics 6B

What creates a magnetic field?

Answer: MOVING CHARGES

What is affected by a magnetic field?

Answer: MOVING CHARGES

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Page 6: Physics 6B

What creates a magnetic field?

Answer: MOVING CHARGES

What is affected by a magnetic field?

Answer: MOVING CHARGESWe have a formula for magnetic force on a moving charge:

)sin(BvqBvqFmag

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Page 7: Physics 6B

What creates a magnetic field?

Answer: MOVING CHARGES

What is affected by a magnetic field?

Answer: MOVING CHARGESWe have a formula for magnetic force on a moving charge:

)sin(BvqBvqFmag

This is a vector cross-product. We need a right-hand-rule to find the direction of this force.

How to find the direction:0)Use your RIGHT HAND1) Fingers start in the direction of the charge’s velocity.2) Curl fingers toward the direction of the magnetic field.3) Thumb points in the direction of the magnetic force on the charge.4) If the charge is negative, flip your hand over.

Important notes: If velocity is aligned with the magnetic field, force is zero. Magnetic force is always perpendicular to both the velocity and the B-field.

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Page 8: Physics 6B

X X X X X X

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X X X X X X

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Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

B

A uniform magnetic field is directed into the page.Find the direction of the magnetic force on charge A, a positive charge.

A

Av

Page 9: Physics 6B

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

X X X X X X

X X X X X X

X X X X X X

X X X X X X

B

A uniform magnetic field is directed into the page.Find the direction of the magnetic force on charge A, a positive charge.

A

Av

-Fingers start along A’s velocity (up in the picture)-Fingers bend toward B-field (into the page)-Thumb sticks out to the left.magF

Page 10: Physics 6B

X X X X X X

X X X X X X

X X X X X X

X X X X X X

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

B

A uniform magnetic field is directed into the page.Find the direction of the magnetic force on charge B, a positive charge.

Bv

B

Page 11: Physics 6B

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

X X X X X X

X X X X X X

X X X X X X

X X X X X X

B

A uniform magnetic field is directed into the page.Find the direction of the magnetic force on charge B, a positive charge.

Bv

B

-Fingers start along B’s velocity (up and to the left in the picture)-Fingers bend toward B-field (into the page)-Thumb sticks out down and to the left.

magF

Page 12: Physics 6B

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

X X X X X X

X X X X X X

X X X X X X

X X X X X X

B

A uniform magnetic field is directed into the page.Find the direction of the magnetic force on charge C, a negative charge.

C Cv

Page 13: Physics 6B

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

X X X X X X

X X X X X X

X X X X X X

X X X X X X

B

A uniform magnetic field is directed into the page.Find the direction of the magnetic force on charge C, a negative charge.

C Cv

magF

-Fingers start along C’s velocity (to the right in the picture)-Fingers bend toward B-field (into the page)-Thumb sticks out upward.-Force is downward. (negative charges are pushed in the opposite direction)

Page 14: Physics 6B

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

X X X X X X

X X X X X X

X X X X X X

X X X X X X

B

A uniform magnetic field is directed into the page.Now that we can find the force on a charge, we should be able to predict its trajectory.Notice that the force is always perpendicular to the velocity. This will yield a circular path. In other words, the magnetic force is a centripetal force.

A

Av

magF

C Cv

magF

Notice that in this diagram, A is a positive charge, and moves counter-clockwise.

Since B is a negative charge, it moves the opposite direction - clockwise.

So you can tell the sign of the charge by which direction it rotates in a magnetic field.

What path would a neutral charge follow?

Page 15: Physics 6B

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

X X X X X X

X X X X X X

X X X X X X

X X X X X X

B

A uniform magnetic field is directed into the page.Find the direction of the magnetic force on the wire, with current flowing as shown.

magF

IPretend that positive charges are flowing in the direction of the current.-Fingers start along A’s current (up in the picture)-Fingers bend toward B-field (into the page)-Thumb sticks out to the left.

There is a formula for the force on a wire in a B-field:

)sin(BLIFmag

Length of wire

Page 16: Physics 6B

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Mass SpectrometerIons are fired into a region of constant magnetic field.Magnetic force pushes it into a circular path and the impact location will determine the mass/charge ratio of the ion.

Velocity selector

Page 17: Physics 6B

Application: Torque on a current loop.

The square loop of wire has current I running through it as shown. If a uniform magnetic field passes through the loop it will rotate due to the magnetic forces on the 4 sides of the loop. Note that the net force will be zero, but the loop will spin in an attempt to align itself with the magnetic field (so that the maximum number of field lines pass through the loop).

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Page 18: Physics 6B

How can you create a magnetic field?

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Page 19: Physics 6B

How can you create a magnetic field?

Answer: move some charges (e.g. make current flow in a wire)

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Page 20: Physics 6B

How can you create a magnetic field?

Answer: move some charges (e.g. make current flow in a wire)

Magnetic Field Near a Long Straight Wire:

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

R2IB 0

AmT7

0 104

R = distance from wireThis formula gives the magnitude of the magnetic field near a wire. The B-field takes the shape of concentric rings centered on the wire.

A right-hand-rule will give the orientation:-Put your thumb along the wire in the direction of the current.-Curl your fingers around the wire (as if you are grasping the wire)-Your fingers are the field lines.

Page 21: Physics 6B

2 current-carrying wires will put magnetic forces on each other.

Wire #1 and #2 below both have current flowing to the right.Find the direction of the magnetic force on each wire.

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Wire #1 I1

Wire #2 I2

Page 22: Physics 6B

2 current-carrying wires will put magnetic forces on each other.

Wire #1 and #2 below both have current flowing to the right.Find the direction of the magnetic force on each wire.

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Wire #1 I1

Wire #2 I2

1B

The magnetic field created by wire #1 is shown. The field points into the page in the vicinity of wire #2, creating magnetic force upward on wire #2.

Page 23: Physics 6B

2 current-carrying wires will put magnetic forces on each other.

Wire #1 and #2 below both have current flowing to the right.Find the direction of the magnetic force on each wire.

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Wire #1 I1

Wire #2 I2

2B

Now we see the field created by wire #2. The field points out of the page in the vicinity of wire #1, creating magnetic force downward on wire #1.

Page 24: Physics 6B

2 current-carrying wires will put magnetic forces on each other.

Wire #1 and #2 below both have current flowing to the right.Find the direction of the magnetic force on each wire.

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Wire #1 I1

Wire #2 I2

2B

Basic result:Parallel curents - wires will attract each otherAnti-parallel currents - wires will repel.

Page 25: Physics 6B

The field near a straight wire is curved into loops.

How can you create a straight magnetic field?

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Page 26: Physics 6B

The field near a straight wire is curved into loops.

How can you create a straight magnetic field?

Answer: Bend the wire into a loop (or lots of loops – make a coil of wire).

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Page 27: Physics 6B

The field near a straight wire is curved into loops.

How can you create a straight magnetic field?

Answer: Bend the wire into a loop (or lots of loops – make a coil of wire).

Prepared by Vince ZacconeFor Campus Learning Assistance Services at UCSB

Magnetic Field in a Solenoid (aka coil)InB 0 n is the number of loops per meter

Notice the direction of the field – it looks very much like a bar magnet. The B-field lines go in through the south pole and come out through the north pole. Inside the coil the field is nearly uniform and points along the axis.

To find the direction you can use a right-hand-rule:

-Curl your fingers around the coil in the direction of the current.-Stick out your thumb: this is the direction of the B-field inside.

The strength of the field can be increased by inserting an iron core.