1

ELECTRODYNAMICS

2

Force on a current-carrying wire in a magnetic field. When a wire carrying a current is placed inside a magnetic

field, it experiences a force that causes the wire to move. The force is the result of the interaction between the

magnetic field of the magnets and the magnetic field of the wire.

3

The force is at a maximum when the wire moves at 90o to the magnetic field and the force is zero when the wire moves parallel to the magnetic field.

The direction of the force can be found by using right hand screw rule

Conventional current is used,

that is, current that flows from

the positive towards the

negative terminal.

4

The Direct Current (DC) Motor Electrical energy

(current) in the motor is converted into mechanical energy (the movement of the motor).

Using the Left Hand Motor Rule, the left side is forced up and the right side is forced down.

5

A simple DC motor consists of a rectangular coil of wire mounted on an axle that can rotate between the two poles of a magnet.

Each end of the coil is connected to half of a split-ring commutator that consists of two copper segments that rotate with the coil.

Two carbon blocks, the brushes, are pressed lightly against the split rings.

The brushes are connected to the power supply.

The split-ring commutator ensures that the coil turns in one direction only.

6

Step 1 shows the coil in the horizontal position. ab experiences an upward force and cd a downward force.

Step 2. The torque causes the coil to rotate into a vertical position. Now the openings between the half-rings of the split-ring commutator are opposite the brushes and the commutator loses contact with the brushes. The current stops flowing through the coil. However, the momentum of the coil carries it past the vertical position.

7

Step 3: The commutator makes contact with the brushes again, but the current in the coil is reversed (in the direction abcd). This allows the torque to continue acting in the same direction. The side ab now experiences a downward force, and side cd an upward force.

Step 4: The coil continues to rotate until it reaches a vertical position again and the current is broken.

The rotating shaft is usually connected to other rotating parts in the system, by means of gears or pulleys.

8

Increasing the current in the coil. Increasing the number of turns on the coil. Increasing the strength of the magnetic

field.

The turning effect on the coil can be increased by:

9

The changing force experienced by the coil as it rotates through 360o, results in the simple motor not running smoothly.

In practice, motors turn very smoothly and at high speeds.

In these motors the coil consists of a soft iron core, surrounded by many loops.

Such a coil is called an armature. Most armatures have many coils which are placed

at different angles. Each coil in the armature has its own commutator.

Motors in everyday life

10

Some motors, such as those in electric drills, can run on AC, because they contain electromagnets, rather than permanent magnets.

As the current flows through the coil, the magnetic field changes direction to match it.

This enables the motor to keep turning in the same direction.

Electric motors have almost limitless useful applications. These vary from the tiny motors found in moving electric toys and disc players, to the driving force behind water pumps – all the way to the giant motors that drive the winches on construction cranes.

11

The design of an electric motor depends on the task it must perform – sometimes it must turn fast as in the dentist drill, or slower as in a clock and sometimes in steps as in the motor in a printer which feeds the paper line by line.

12

ELECTROMAGNETIC INDUCTION When a magnet moves near a

conductor or when a wire moves in the magnetic field of a magnet, the change in the magnetic field induces an emf and a current flows in the conductor.

This phenomenon is called electromagnetic induction.

The induced current will be maximised when the motion of the conductor is perpendicular to the direction of the magnetic field, and minimised when it travels parallel to the field.

13

Moving a conductor in a magnetic field

14

Faraday’s Law The size of the induced current is directly

proportional to the rate of change of magnetic flux linkage.

What this means is that the induced current is most effectively produced when the number of magnetic field lines being ‘cut’ by the conductor is greatest.

The size of the induced emf (and hence the induced current) can be increased by

- Moving the conductor faster- Using stronger magnets - Increasing the length of the conductor (more

turns on the coil)

15

THE AC GENERATOR N & S are the field

magnets that provide the magnetic flux.

abcd indicates one turn of a rectangular coil of insulated copper wire and represents the armature.

S1 & S2 are a pair of slip-rings consisting of copper around an insulated cylinder. Each end of the coil remains connected to its own slip ring.

16

B1 & B2 are carbon or copper brushes for ‘collecting’ the induced current.

A is an insulated shaft that enables the coil and slip-rings to rotate as a single unit.

The handle stresses the fact that kinetic energy must be supplied to the armature and the lamp,L, represents the electrical device in which the generated current is used.

Generators use mechanical energy to produce electrical energy.

17

Working of an AC generator When the coil is in the

vertical position, no magnetic field lines are cut by ab and dc; no emf is induced and no current flows.

While the coil rotates it cuts the field lines and causes a change in magnetic flux, an emf is induced which causes an electric current flow in the circuit.

18

As the coil stats moving to the vertical position again, the induced emf decreases so the current also decreases, till it is zero.

When the coil is rotated further, there is a change in magnetic flux again, but now the induced current flows in the opposite direction.

19

When the coil of an AC generator rotates in a magnetic field, a constantly varying emf is induced across the coil’s ends.

One complete change in the direction of the current during one revolution of the coil is called a cycle of alternating current.

The induced emf varies according to a sine wave and the induced current is slightly less than the induced emf.

20

Uses of the AC generator Alternators: in cars, the movement of the car’s engine

produces electricity via the alternatort to recharge the car’s battery.

Back-up power: generators are used to produce electricity where there is a power outage. Places that are left vulnerable or cannot function in the event of a power cut, such as hospitals, fresh produce distributors and high security areas, use generators.

Power stations: huge dynamos are turned using steam from heated water, to produce electricity.

21

The DC Generator (dynamo) The slip-rings of the AC generator are replaced

by a split-ring commutator to convert the AC generator to a DC generator.

The DC generator produces current that flows in one direction only.

The carbon brushes are arranged in such a way that contact is broken between the coil and the brushes for a brief instant when the coil is vertical.

When contact is re-established, the brushes come into contact with a different part of the coil.

So the current continues to flow through the brushes with no switch in its direction.

22

Uses of AC Generators The main generators in nearly all electric power plants

are AC generators. This is because a simple

electromagnetic device called a transformer makes it

easy to increase or decrease the voltage of alternating

current. Almost all household appliances utilize AC.

Uses of DC Generators Factories that do electroplating and those that produce

aluminium, chlorine, and some other industrial

materials need large amounts of direct current and use

DC generators. So do locomotives and ships driven by

diesel-electric motors. Because commutators are

complex and costly, many DC generators are being

replaced by AC generators combined with electronic

rectifiers. 23

24

Alternating Current Alternating current (AC) is an electric current

which reverses its direction of flow fifty times every second – it has a frequency of 50 Hz.

Why do we use AC and not DC? Electricity needs to be distributed through the

country at high voltages to reduce energy losses in the power cables.

To increase the voltage at the power stations and reduce it again before it reaches your home, transformers must be used.

Transformers can only work on AC, since a changing magnetic field is required to induce a current.

25

In addition, generating AC at the power stations is easier – no complex modifications to the AC generator is necessary.

Most appliances can operate on AC, including all those with a heating element (light bulbs, kettles, toasters, stoves), but some require the AC to be converted to DC first as they are direction sensitive (laptops, cell phone chargers)

Characteristics of AC One of the characteristics of alternating current is that it

causes self-inductance in the wires that carry it. It means that when the current changes direction, the

magnetic field associated with it is in such a way as to oppose that change (Lenz’s Law).

26

Because of self-inductance, some of the electrical energy of the current is wasted – appliances will get a lower maximum voltage than the peak value.

This lower maximum voltage is known as the root-mean-square value (RMS value)

2maxV

VRMS

2maxI

I RMS

rms rms rmsP I V

27

In SA our mains supply is 220V (rms) AC (50 Hz).

What is the peak or maximum voltage?

max 2

2 220

311.13

rmsV V

V

V

66666666666666