Unit 3

Chapter 2 applications on magnetic fields

Dr. Mohamed Adel

Chapter 2 : Applications On magnetic fields.

• In this chapter we will study some applications on

magnetic fields, which are :

• 1- moving coil galvanometer.

• 2- Ammeter.

• 3-Voltameter.

But before we study these applications we should first study the concept of :

Magnetic Torque

Magnetic Torque

Force and torque (couple) acting on a rectangular coil carrying current

placed in a magnetic field:

If we have a rectangular coil ( abcd ) whose plane is parallel to the lines of the magnetic

flux

a- The sides ( ad ) and ( cb ) are parallel to the magnetic flux lines

(The force acting on each wire is zero)

b- The sides ( ab ) and ( cd ) are perpendicular to the magnetic flux lines

(The 2 wires are affected by 2 forces equal in magnitude and opposite in direction and are

parallel)

Magnetic Torque

- Each force equal ( F = B I Lcd = B I Lab ) and separated by distance ( Lad or Lbc )

- So the coil will be affected by a torque which will cause the soil to rotate around its axis

- The torque = force × the perpendicular distance between the 2 equal forces

τ = B I Lcd × Lbc as A (area of the rectangle) = Lcd × Lbc

- If the rectangular coil consists of number of turns:

(N: number of turns)

Illustrating video

Measuring instruments depends on the torque

1- The sensitive moving coil galvanometer:

It is a meter used to detect, measure and determine the direction of very weak electric current.

Its uses:

a- Detect very small currents in a circuit

b- Measure their intensities and determine their polarities

Its idea:

- Its operation depends on the torque that is generated in a current – carrying coil moving in a

magnetic field

Its structure:

1- Rectangular thin wire coil is wrapped around aluminum frame mounted on a soft iron

core

2- The frame is pivoted on jeweled bearings.

3- The coil rotates between the poles of U – shaped Magnet ( horse shoe )

4- Its rotational motion is restrained by a pair of spiral Control springs.

5- A pointer is fixed on the coil on a regular scale

How does it works? How does it work?

Iron core

1- When a current flows in the coil from the right side upwards and emerges from the other side

2- Then the magnetic flux force generates a torque which makes the coil rotate clockwise

3- The pointer deflects until it stops at a certain reading when the torque is balanced with the

spring torque which is counterclockwise (in the opposite direction of the torque produced from

the magnetic force)

4- We can read the current value

Illustrating video

2- The direct current (DC) ammeter:

The ammeter : it is a sensitive moving coil galvanometer whose pointer deflects in one

direction connected with a small resistance in parallel connect in circuit in series and it is used

to measure currents with high intensities ( in amperes )

Its uses:

- Measure the electric current intensity of a direct current

Its connection in circuit:

- It is connected in series as the current intensity is the same in case of series connection

The galvanometer is an ammeter of limited range due to its coil sensitivity

Its idea:

- Connecting a small resistance (shunt resistance Rg) parallel to the galvanometer

Its structure:

1- A sensitive moving coil galvanometer connected to a small resistance (shunt resistance Rg)

in parallel

RG

Is

How does it works?

How does it work?

1- Connect the ammeter in series in the circuit (so that the current passes through the ammeter

is the same current passes through the circuit)

2- The current divided where most of the current (Is) passes through the shunt resistance ( Rs) (

low resistance ) while only small current (Ig) passes in the galvanometer coil

( Rg)

3- The direct current (DC) voltmeter:

The voltmeter: it is a sensitive moving coil galvanometer whose pointer deflects in one

direction connected with large multiplier resistor in series connect in circuit in parallel

Its uses:

- Measure the potential difference (electromotive force) across 2 points in an electric circuit

Its connection in circuit:

- It is connected in parallel as the potential difference is the same in case of parallel connection

Its idea:

- Connecting a large multiplier resistance (Rm) in series to the galvanometer

Its structure:

1- A sensitive moving coil galvanometer connected to a large multiplier resistance in series

A large multiplier resistance connected in series with the voltmeter (GR)

- So it draws a negligible current to give a right reading of the voltage

m

How does it works?

How does it work?

1- Connect the voltmeter in parallel in the circuit (so that the voltage in the voltmeter is the

same voltage in the circuit)

2- The resistance of galvanometer is (Rg) and the multiplier resistance (Rm) connected together

in series and a current (Ig) passes through them

3- and a total current (I) passes through the circuit.

Illustrating video