Electromagnetism
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Electromagnetism. Understand that an electric current creates a magnetic field around itself Describe the magnetic field created by a current carrying wire Use the Right Hand Slap Rule to predict the direction of the magnetic force ion a current carrying wire inside a magnetic field.

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Electromagnetism

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Electromagnetism

Electromagnetism

  • Understand that an electric current creates a magnetic field around itself

  • Describe the magnetic field created by a current carrying wire

  • Use the Right Hand Slap Rule to predict the direction of the magnetic force ion a current carrying wire inside a magnetic field.

  • Use F = BIL and F = BILsin(θ) to calculate the size of the magnetic force on a current carrying wire in a magnetic field.

  • Use F = Bvq to calculate the size and direction of the magnetic force on a moving charge inside a magnetic field.

  • Describe the circular motion of a charged particle inside a magnetic field

  • Understand electromagnetic induction in terms of the relative motion of a wire across a magnetic field.

  • Use V = BvL to calculate the voltage induced across a wire moving through a magnetic field.


Magnetic fields

Magnetic fields

Understand that an electric current creates a magnetic field around itself

Magnetic fields are created when magnetic field lines travel from the North pole to the South pole of a magnet. The strength of magnetic field (B), is measured in Tesla (T).

Magnetic field lines behave in a similar way to electric field lines: Like poles attract and opposite poles repel; the closer the magnetic field lines are the stronger the magnetic field.

A uniform magnetic field can be produced between the North and South poles of magnets.


Current carrying wire

Current Carrying wire:

Describe the magnetic field created by a current carrying wire

  • A current carrying wire creates a magnetic field. We can use the right hand grip rule, with our thumb in the direction of the current, and our fingers in the direction of the magnetic field, to discover the field direction.


Motor force on a current carrying wire in a magnetic field

+

Motor force on a current carrying wire in a magnetic field

  • When a current carrying wire is placed in a uniform magnetic field the combination of the two magnetic fields produce a force on the wire. This is called a motor force and the direction of the force can be deduced by using the right hand slap rule.

=

Low concentration

  • The wire is pushed into the space with a low concentration of field lines.


Right hand slap rule

Right hand slap rule

Use the Right Hand Slap Rule to predict the direction of the magnetic force ion a current carrying wire inside a magnetic field.

  • Place you right thumb along the direction of the current in the wire, rotate your hand so that your fingers point in the direction of the magnetic field lines. The palm of the right hand gives the direction of the force acting on the current carrying wire.

direction of current (I)

direction of force (F)

(slap)

direction of magnetic field (B)


Motor force

Motor force

Use F = BIL and F = BILsin(θ) to calculate the size of the magnetic force on a current carrying wire in a magnetic field.

Magnetic field strength measured in T

Force measured in N

Length of wire within the magnetic field and perpendicular to the field lines.

Measured in m

Current flowing in wire measured in A


Working out the direction and size of the motor force

Working out the direction and size of the Motor force

S

Example Exercise

  • Magnetic field lines drawn from N to S.

  • Current travels from +ve to –ve in wire.

  • Using the RH slap rule fingers are aligned with the magnetic field lines and the thumb with the direction of current.

  • The palm of the hand slaps outwards this is the direction of the force.

1What is the force acting on a wire carrying 0.25 A that crosses a 0.20 m uniform magnetic field with a strength of 23 mT?

B

+

-

I

F

N


Force acting on a charged particle in a magnetic field

Force acting on a charged Particle in a magnetic field

Use F = Bvq to calculate the size and direction of the magnetic force on a moving charge inside a magnetic field.

Velocity of the charged particle in the magnetic field.

Force measured in N

Magnetic field strength measured in T

Charge of particle moving through the magnetic field


Working out the direction and size of the force on the charged particle

Working out the direction and size of the force on the charged particle

  • Magnetic field lines are going into the page.

  • The positive charge is traveling to the right.

  • Using the RH slap rule fingers are aligned with the magnetic field lines and the thumb with the direction of positive charge.

  • The palm of the hand slaps upwards this is the direction of the force.

Example Exercise

F

B

1What is the force acting on a proton, 1.60 x 10-19 C as it moves at 8.00 kms-1 across a uniform magnetic field of strength 27.3 μT?

v


Electromagnetism

The path of a charged particle moving through a magnetic field

Describe the circular motion of a charged particle inside a magnetic field

  • As the charged particle moves through the magnetic field a force is produced perpendicular to the velocity.

  • This force causes the particle to accelerate, change direction.

  • This force causes the particle to constantly changes direction and forces it is a circular path.


Voltage induced across a wire moving through a magnetic field

Voltage induced across a wire moving through a magnetic field

Use V = BvL to calculate the voltage induced across a wire moving through a magnetic field.

Velocity of the charged particle in the magnetic field.

Voltage measured in v

Length of the wire moving through the magnetic field

Magnetic field strength measured in T


Voltage induced across a wire moving through a magnetic field1

Voltage induced across a wire moving through a magnetic field

  • By aligning our thumb with the velocity and our fingers with magnetic field lines coming out of the page we can work out that positive charge is pushed towards the right.

  • As more and more positive charge accumulates at the right end of the wire it becomes increasingly harder for the positive charge to move there. Eventually the charge slops flowing and the right becomes positively charged. A potential difference, voltage is induced.

-ve

+ve

Example Exercise

1What is the induced voltage produced by a 12 m wire moving at 200 ms-1 through a 25 μT magnetic field?


Electromagnetism

Example Exercise

1A 20cm copper rod is sits on conducting rails, in a 1.5 mT magnetic field. The rails are 15cm apart an connected to a power supply supplying a current of 1.5 A. In what direction does a force act on the rod? What is the magnitude of this force?

1.5 A

The current flows anti-clockwise. The current flows down through the rod. Using the RH slap rule a force pushes the rod to the left.

The current only flows through the 0.15m of the rod connected to the rails.


Electromagnetism

Example Exercise

2A 30cm copper rod is rolled on conducting rails, through a 3.5 mT magnetic field at 3.0ms-1. The rails are 20cm apart. In what direction does the current flow? What is the magnitude of the voltage induced?

The current is pushed up ways so it flows clockwise.

Although a voltage is induced across the whole length of the rod, only the difference between the voltage where the rod connects the rails is measured. ∴ This is 0.20m.

V


Electromagnetism

Understand electromagnetic induction in terms of the relative motion of a wire across a magnetic field.

An aeroplane with a wingspan of 26 meters flies through a 30 μT magnetic field at 250 ms-1. What is the voltage induced across its wings?

How will the voltage change if the plane halves its velocity?

Given that V=BvL we can see that

As the voltage is directly proportional to the velocity, if the velocity is halved the induced voltage is halved.


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