5. Magnetic Fields

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5. Electric currents also produce magnetic fields and these fields are used in different devices in the home

5.1 - Describe the behaviour of the magnetic poles of bar magnets when they are brought close together

5.2 - Define the direction of the magnetic field at a point as the direction of force on a very small north magnetic pole when placed at that point

5.3 - Describe the magnetic field around pairs of magnetic poles

A field is a region of influence such that an object placed inside it may potentially experience a force

A magnetic field is a region in which objects that possess magnetic properties will experience a force

Magnetic fields are vector quantities with both magnitude and direction

The strength is determined by the size of the magnetic force acting on a current-carrying wire

The direction of a magnetic field at a given point is the direction of the force acting on a north magnetic pole placed at that point

Magnetic fields may be represented using field lines - the density of the field lines represents the strength of the magnetic field, while the arrows indicate the direction

Field lines running in the same direction will add, while those in opposing directions will cancel. Field lines never cross each other

The magnetic field leaves the north pole, and enters the south pole

Like poles repel and unlike poles attract

The magnetic field around a permanent bar magnet is made up of a magnetic field leaving the north pole and looping around to enter the south pole

When two or more bar magnets are placed next to each other, the magnetic field will leave one magnet to enter the other magnet of a north pole is facing a south pole

If two like poles are facing each other, the magnetic fields may cancel, as they are in opposite directions

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The Earth has its own magnetic field created by the circulation of ions within the mantle

This is the basis by which a compass functions

The magnetic field of the earth behaves in a way similar to that of a bar magnet

However the earth's geographic north pole acts like a magnetic south pole, and the earth's geographic south pole acts like a magnetic north pole

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5.4 - Describe the production of a magnetic field by an electric current in a straight current-carrying conductor and describe how the right hand grip rule can determine the direction of current and field lines

Oersted accidentally discovered that when electricity flowed in a conducting wire, the needle of a compass placed in its vicinity aligned itself perpendicular to the wire

A straight current-carrying wire produces a magnetic field around it - the nature of the field is that they are concentric circles around the wire

Magnetic fields are vector quantities - the magnetic field produced by a straight current carrying wire can be described using magnitude and direction

The magnitude is directly proportional to the size of the current passing through the wire, and is inversely proportional to the distance from the wire

The direction can be determined using the right-hand grip rule

The thumb points in the direction of the conventional current, while the fingers curl in the direction of the magnetic field

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5.5 - Compare the nature and generation of magnetic fields by solenoids and a bar magnet

A solenoid is a tightly wound cylindrical coil of wire

As for a straight wire, when current is fed into a solenoid, it produces a magnetic field

The pattern of this magnetic field is identical to that around a bar magnet, however since the solenoid is made up of a tightly wound coil, the solenoid has an overall polarity which is dependent on the direction of current flow

This can also be determined through the right hand grip rule

For a solenoid, when the fingers of the right hand curl in the direction of flow of the conventional current, the thumb points in the direction of the north pole

Unlike a bar magnet, magnetic field lines are able to pass through the hollow solenoid

An electromagnet also uses electricity to produce a magnetic field

However unlike a solenoid, it has a soft iron core inside the hollow coil - when a solenoid is wound on a soft iron core, it becomes an electromagnet

This does not influence the pattern of the magnetic field, but but helps to concentrate and intensify the magnetic field produced by the coil

The magnetic field produced by an electromagnet is much stronger than that produced by an air-core solenoid

The field strength is determined by the current passing through it and the number of turns of coil

Advantages of using electromagnets include:

Electromagnets can provide much stronger magnetic fields than permanent magnets

Electromagnets can be switched on or off my switching the current passing through the coil on or off, allowing for greater control

Electromagnets can have their magnetic field strength adjusted

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