Download electricity and magnetism - lesson2

Electricity and Magnetism
Science Posse
University of Wyoming
What is ELECTROMAGNETISM?
 The fundamental relationship between electric
fields and magnetic fields
Electromagnetism
 A moving electric field creates a magnetic field
that rotates around it
 The Right Hand Rule helps describe this
The Right Hand Rule
 First define positive electric current as flowing
from the positive (+) end of a battery, through
an electric circuit, and back into the negative (-)
end.
 Next define a magnetic field as always pointing
away from a North pole and towards a South
pole.
The Right Hand Rule
 Extend your thumb. This is the direction of the
current.
 If your thumb is the direction of the electric
current, your fingers are curling along with a
rotating magnetic field!
The Right Hand Rule
A transformer is a device for increasing or
decreasing voltage.
Structure of Transformer
Circuit Symbol for Transformer
How Transformer works
Laminated soft iron
core
Input voltage
Output voltage
(a.c.)
(a.c.)
Primary coil
Secondary coil
All transformers have three parts:
1. Primary coil – the incoming voltage Vp
(voltage across primary coil) is connected
across this coil.
2. Secondary coil – this provides the output
voltage Vs (voltage across the secondary coil)
to the external circuit.
3. Laminated iron core – this links the two coils
magnetically.
Notice that there is no electrical connection between the two coils,
which are constructed using insulated wire.
Two Types of Transformer
A step-up transformer increases the
voltage - there are more turns on the
secondary than on the primary.
A step-down transformer decreases the
voltage - there are fewer turns on the
secondary than on the primary.
To step up the voltage by a factor of 10,
there must be 10 times as many turns on the
secondary coil as on the primary. The turns
ratio tells us the factor by which the voltage
Formula for Transformer
voltage across the primary coil
number of turns on primary

voltage across the secondary coil number of turns on secondary
Vp
Vs

Np
Ns
Where Vp = primary voltage
Vs = secondary voltage
Np= Number of turns in primary coil
Ns = Number of turns in a secondary coil.
The diagram shows a transformer. Calculate the
voltage across the secondary coil of this
transformer.
Step-up transformer!
Solution
VP N P

VS N S
Substituting
12 180

VS 540
Crossmultiplying
180.VS  12 x 540
12 x 540
 VS 
180
 VS  36 V
Worked example No. 2
A transformer which has 1380 turns in its primary coil is to be used to
convert the mains voltage of 230 V to operate a 6 V bulb. How many
turns should the secondary coil of this transformer have?
VP = 230 V
VS = 6 V
NP = 1380
NS = ?
Obviously, a Step-down transformer!!
Solution
VP N P

VS
NS
Substituting
230 1380

6
NS
Crossmultiplying
2300.N S  6 x 13800
6 x 1380
 NS 
230
 N S  36 turns
The Electromagnet
 By the Right Hand Rule, a coil of wire with
current flowing in it will create a magnetic field
 The strength of the magnetic field depends on
 The amount of current in a wire – More current
means stronger magnetic field
 The number of turns in the coil – More turns
means stronger magnetic field
 The material in the coil – Magnetic materials like
iron and steel make the magnetic field stronger
The Electromagnet
 A simple electromagnet can be made by coiling some wire
around a steel nail, and connecting a battery to it.
 As current rotates around the nail, a magnetic field is created
with the North pole at the bottom and the South pole at the
top
The Electromagnet
 Electromagnets are found in many everyday
objects such as
 Motors and generators
 Doorbells
 Speakers and headphones
 Hard drives
 VHS and Audio cassette tapes
 Telephones
The Electric Motor
 An electric motor is a device that converts
electromagnetic energy into mechanical energy
 It works by using special electromagnets in the
case of the motor that create North and South
poles that are constantly spinning in a circle.
 These spinning poles pull along magnets that are
on the rotor (the part that spins), causing it to
rotate
The Electric Generator
 An electric generator works in the opposite of
a motor; it converts mechanical energy into
electromagnetic energy
 An outside force moves magnets in a circle past
several coils of wire.
 By the Right Hand Rule, electric currents are
developed in the coils of wire.