unit 3 chapter 2 applications on magnetic fieldsfptcu.com/gep files/physics 2/lecture 7...
TRANSCRIPT
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