PHY_10_59_V2_P_Artificial magnets. Solenoid

Lesson Objectives

By the end of the lesson learners will be able:

10.4.1.4 – to make an artificial magnet empirically and explain the scopes of its application;
10.4.1.5 – to explain the factors having an effect on a magnetic field of a solenoid;

Keywords

Hard and Soft Magnetic Materials

Permanent magnets are made of magnetically HARD materials such as steel, alcomax, magnadur. These materials retain their magnetisation once magnetised. Hard to magnetise.

Magnetically SOFT materials, such as iron, mumetal lose their magnetisation easily. They suitable for temporary magnets such as electromagnets. Easy to magnetise.

Making and Destroying Magnets

stroke, coil and solenoid methods
Use direct current.

Heating
Drop it down several times
Use alternating current

Stroke Methods

Direct Current Magnetization

Coil and Solenoid Magnetization

Magnetic Keepers

AC Demagnetization

Induced Magnetism

Magnetism can be induced in a magnetic material if it is placed within a magnetic field.

If the material is magnetically hard it will retain its magnetism once removed from the field.

Certain rocks in the Earth’s crust such as lodestone have been magnetised in this way by the Earth’s magnetic field.

Induced Magnetism

9-May-20

7

Solenoid

A solenoid is a coil of wire carrying an electric current.

The magnetic field is similar in shape to that around a bar magnet.

The strength of the field increases with:
the electric current
the number of turns in the coil

The Right-Hand Grip Rule (for Poles)

Grip the coil with the RIGHT hand.
The fingers are placed in the direction that the eclectic current flows around the coil.
The thumb points towards the north pole end of the coil.

Solenoid Polarity from Side view

Complete the diagrams below:

Electromagnets

An electromagnet consists of a current carrying coil wrapped around an iron core.

You need the following materials:
Large iron nail (approximately 3 inches in length)
Thin coated copper wire.
Dry cell batteries.
Electric tape.
Iron fillings, paper clips and other magnetic items

AfL

1. The magnetic field surrounding a wire is strongest:
A. close to the wire
B. far from the wire
C. the field does not vary with distance

2. Electromagnet polarity can be reversed by:
A. increasing current
B. decreasing current
C. reversing current direction

3. An electromagnet’s strength can be adjusted by:
A. adjusting the number of coils
B. adjusting the amount of current
C. both A and B
D. neither A nor B

KEY

1. A
2. C
3. B

Activity

Explain the applications of electromagnets in
Scrap yard crane
The electric door bell
The relay switch
Car ignition key

Uses of Electromagnets

1. Scrap yard Crane

The iron core of the electromagnet is a SOFT magnetic material.

When current flows the iron becomes strongly magnetised and so picks up the scrap iron and steel.

When the current is turned off the iron loses its magnetisation and so releases the scrap.

When the push switch is closed current flows around the circuit turning on the electromagnet.

The soft iron armature is pulled towards the electromagnet and the hammer hits the gong.

This causes the contact switch to open cutting off the electric current.

The spring now pulls the armature back again closing the contact switch.

Current now flows again and the hammer hits the gong again.

Label the diagram of the electric bell be:

Contact switch

A relay switch is a way of using a low voltage circuit to switch remotely a high voltage (and possibly dangerous) circuit.

When switch A is closed, the small current provided by the cell causes the electromagnet to become magnetised..

The iron armature is then attracted to the electromagnet causing the springy contact switch B to close in the high voltage circuit.

Current normally flows between terminals A and B through the contact and the electromagnet.

When the current in a circuit increases, the strength of the electromagnet will also increase. This will pull the soft iron armature towards the electromagnet.

As a result, spring 1 pulls apart the contact and disconnecting the circuit immediately, and stopping current flow.

The reset button can be pushed to bring the contact back to its original position to reconnect the circuit

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