1. Magnetic Field Around A Wire

• A wire contains current going into the page as shown. What will the magnetic field look like?

• A wire contains current going right as shown. What will the magnetic field look like?

1. Magnetic Field Around A Wire

• A wire contains current going into the page as shown.

• A wire contains current going right as shown.

2. Magnetic Field Inside a Solenoid• This (“rule 2”) is just an application of Rule 1.• The fingers follow the current (positive to negative)

and the thumb represents the field INSIDE the solenoid.

Recall x = field ‘into page’ and a dot = ‘out of page’

“The Trick”• Start with your hand ‘clenched’.• Run your fingers around in the direction of the current.• Think “ Does it go right or left”• Does it go into or out of the page first?

http://www.waowen.screaming.net/Maghandrules.htm

Solenoid / Permanent Magnet comparison

The magnetic field comes OUT of the North pole and goes to the south pole OUTSIDE.

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/imgmag/barsol.gif

Therefore

• Find the direction of the magnetic field.• Will it attract or repel the permanent magnet?

N S

+ -

Therefore

• Find the direction of the magnetic field.• Will it attract or repel the permanent magnet?

N S

+ -

S N

- +

- +

S N

N S

- +

- +

N S

-

http://www.pschweigerphysics.com/images/rhr2.jpg

Magnetic Force on a CurrentIf a wire carries a current through a magnetic field, it creates a

force on the wire. The force depends on:• The current• The length of the wire• The strength of the magnetic field.

The most common application is a motor. It uses lots of wire loops to create more force. Only one wire is shown here.

http://www.bbc.co.uk/scotland/education/bitesize/standard/img/physics/electricity/movement/motor.gif

Right Hand Slap Rule

• Three dimensional• Fingers – magnetic field• Thumb – current• Slap Force on the

electric current

Force

Current (+-> -)

Current

http://commons.wikimedia.org/wiki/File:Right-hand-rule.jpg

N S

Current into

page

N S

Current into

page

Another One

Another One

Another One

The current is parallel to the magnetic field, so no force results.

Questions

• Pages 163 (all).

The DC Motor

• A DC motor applies the motor effect (strangely) to make wire spin inside a magnetic field.

• A commutator is necessary to keep it spinning in one direction.

• The force on the wire (and therefore the torque) changes as the position changes.

• Usually many loops are used (coils) to increase the force; this is necessary for a real-world motor to work.

• A motor can (usually) be used as a generator if forced around - more on this soon.

• An AC motor is quite different (and not to be covered now!).

The DC Motor

http://www.walter-fendt.de/ph11e/electricmotor.htm

http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=912.0

Problems

• Electric motor page 165

Electromagnetic Induction

• If a wire is moved through a magnetic field, the magnetic field ‘pushes’ the electrons around the circuit. This creates a voltage and (if a circuit is connected) a current.

• The voltage (and current) can be increased by increasing the length of the wire, the speed of the movement or the strength of the magnetic field.

• The wire must ‘cut’ magnetic field lines, or else no voltage will be produced.

Right Hand Rule

• The diagram showed that:• FINGERS = magnetic field• THUMB = movement• Slap = induced voltage, or the direction conventional current (positive to negative) will flow if a circuit is connected.

Electromagnetic Induction Example

Conductor (normally wire)

+-

v

The “positive charges” move up and the negative charges move down. This generates a voltage difference across the wire.

Electromagnetic Induction ExampleCalculate the current in the circuit.

The “positive charges” moved anticlockwise in the animation.

+10Ω

+

++

+

Motor Effect and Electromagnetic Induction

MOTOR EFFECT

•Fingers = Field

•Thumb = current

•Slap = Force

EM INDUCTION

•Fingers = Field

•Thumb = movement / force

•Slap = induced Voltage (EMF)

Cause

Effect

Always Magnetic Field

http://commons.wikimedia.org/wiki/File:Right-hand-rule.jpg

Another One

Another One

The current is parallel to the magnetic field, so no force results.

Generators and Alternators

• A DC motor produces direct current (electricity which flows in one direction only) when it is turned, however the flow is uneven.

• Most generators produce alternating current, which flows backwards and forwards. The frequency is the number of times the current flows each way per second, which is determined by the number of times the generator turns per second. These generators are called alternators.

Japan’s Electricity• Mains electricity is A.C.• Some countries use 50HZ (eg ________).• Other countries use 60Hz (eg _________).• Japan is unusual because it uses two frequencies:

50Hz in Tokyo (and north) and 60Hz in Osaka (and south).

• The reason is historical: generators were purchased from the US (60Hz) and Europe (50HZ) in the nineteenth century before the entire country was connected to the mains electricity (known as the ‘national grid’).

Questions

• Page 169 & 171

Transformers• An electromagnet produces a m_______ f______.• If the voltage is increased, the current increases, which in

turn increases the magnetic field. This has the same effect as moving a magnet into the coil.

• If the voltage – and therefore the current – is changed, the magnetic field changes. This changing magnetic field can induce a voltage – and therefore a current – in another (“secondary”) coil of wire if the magnetic field flows through it as well.

• If the number of turns in the primary and secondary coils are different, the voltages in each coil will be different. Transformers use two coils with different numbers of turns to transform voltage. Transformers only work with _______.

Transformer Diagram

http://commons.wikimedia.org/wiki/File:750px-Transformator.png

Ignore the writing – unless you can read it! Please note that a transformer is made of two coils with different numbers of turns, connected by an iron (why?) core.

Transformer Equation

• Example: A transformer consisting of a primary coil of wire with 100 turns and a secondary coil of 25 turns is connected to the Japanese mains voltage (100V). Calculate the voltage of the secondary coil.Answer: 25 volts.

€

input voltage

output voltage=

turns on input coil

turns on output coil