Slide 1

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 magneticforce ion a current carrying wire inside a magnetic field.

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

•Use F = Bvq to calculate the size and direction of the magnetic force on amoving 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 awire across a magnetic field.

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

Magnetic fields

Magnetic field lines behave in a similar way toelectric field lines: Like poles attract andopposite poles repel; the closer the magneticfield lines are the stronger the magnetic field.

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

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

Understand that an electric current creates a magnetic field around itself

Current Carrying wire:

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

Describe the magnetic field created by a current carrying wire

Motor force on a current carrying wire in amagnetic field



Lowconcentration

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

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

Right hand slap rule

•Place you right thumb along the direction of the current in the wire, rotateyour hand so that your fingers point in the direction of the magnetic fieldlines. The palm of the right hand gives the direction of the force acting onthe current carrying wire.

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direction ofmagnetic field (B)

direction offorce (F)

(slap)

direction ofcurrent (I)

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

Motor force

Forcemeasured in N

Magnetic fieldstrengthmeasured in T

Current flowing inwire measured in A

Length of wire withinthe magnetic field andperpendicular to thefield lines.

Measured in m

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

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Example Exercise

1What is the forceacting on a wire carrying0.25 A that crosses a 0.20m uniform magnetic fieldwith a strength of 23 mT?

•Magnetic field linesdrawn from N to S.

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

•Using the RH slap rulefingers are alignedwith the magneticfield lines and thethumb with thedirection of current.

•The palm of the handslaps outwards this isthe direction of theforce.

Working out the direction and size of theMotor force

Force acting on a charged Particle in a magneticfield

Forcemeasured in N

Magnetic fieldstrengthmeasured in T

Charge of particlemoving throughthe magnetic field

Velocity of thecharged particle inthe magnetic field.

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

•Magnetic field linesare going into thepage.

•The positive chargeis traveling to theright.

•Using the RH slaprule fingers arealigned with themagnetic field linesand the thumbwith the directionof positive charge.

•The palm of thehand slaps upwardsthis is the directionof the force.

Example Exercise

1What is the forceacting on a proton, 1.60 x10-19 C as it moves at 8.00kms-1 across a uniformmagnetic field of strength27.3 μT?

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

•As the charged particle movesthrough the magnetic field aforce is producedperpendicular to the velocity.

•This force causes the particleto accelerate, changedirection.

•This force causes the particleto constantly changes directionand forces it is a circular path.

The path of a charged particle movingthrough a magnetic field

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

Voltage induced across a wire moving througha magnetic field

Voltagemeasured in v

Magnetic fieldstrengthmeasured in T

Length of the wiremoving throughthe magnetic field

Velocity of thecharged particle inthe magnetic field.

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

•By aligning our thumb with thevelocity and our fingers withmagnetic field lines coming out ofthe page we can work out thatpositive charge is pushed towardsthe right.

•As more and more positivecharge accumulates at the rightend of the wire it becomesincreasingly harder for thepositive charge to move there.Eventually the charge slopsflowing and the right becomespositively charged. A potentialdifference, voltage is induced.

-ve

+ve

Example Exercise

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

Voltage induced across a wire moving througha magnetic field

1.5 A

1A 20cm copper rod issits on conducting rails, in a 1.5mT magnetic field. The rails are15cm apart an connected to apower supply supplying a currentof 1.5 A. In what direction does aforce act on the rod? What is themagnitude of this force?

The current flows anti-clockwise.The current flows down throughthe rod. Using the RH slap rule aforce pushes the rod to the left.

The current only flows throughthe 0.15m of the rod connectedto the rails.

Example Exercise

2A 30cm copperrod is rolled on conductingrails, through a 3.5 mTmagnetic field at 3.0ms-1.The rails are 20cm apart. Inwhat direction does thecurrent flow? What is themagnitude of the voltageinduced?

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

The current is pushed upways so it flows clockwise.

Example Exercise

1.An aeroplane with a wingspan of 26 meters flies through a 30 μTmagnetic 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 thevelocity is halved the induced voltage is halved.

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