## December 18, 2011

### 6.7 Uniform Magnetic field

Know how to use two permanent magnets to produce a uniform magnetic field pattern

If the lines of a magnetic field are close together the magnetic field is strong, likewise further apart magnetic field lines show a weak field. If the magnetic field lines are parallel to each other we have a field of constant strength – a uniform field

Making a Uniform magnetic field
An easy way to produce a roughly uniform magnetic field is to hold two opposite poles close to each other so that they attract [see image below].

### 6.6 Magnetic field patterns

Sketch and recognise the magnetic field pattern for a permanent bar magnet and that between two bar magnets.

Things to note:
• The arrows always point from North to South
• The lines are closest at the poles.

### 6.5 Magnetic materials

Understand that magnetism is induced in some materials when they are placed in a magnetic field.

Practical
1. Stroke a magnet along a steel bar and an iron bar
2. Try picking up some bar clips
3. Bang both bars on the desk
4. Now try picking up the paperclips again

Through this experiment we have magnetised the steel and iron bars so that we can pick up lightweight magnetic objects.

Tap the magnetised bars onto a table gently and try to pick up the paperclip again. What you should notice is that the steel retains some of its magnetism and can pick up the paperclip but the iron bar loses its magnetism completely. This is because steel is magnetically hard.

### 6.4 Magnetic Field Line

Understand the term ‘Magnetic field line’

Using iron filings we can see the magnetic field of the magnet. These “line” we see traveling from the North pole to the South pole of the magnet are called the Magnetic field lines.

The video below shows how the lines look in 3 dimensions.

### 6.3 Magnetic materials

Recall the properties magnetically hard and soft materials

Magnetically hard materials can be permanently magnetised by a strong magnetic field. Steel and other special alloys of iron are magnetically hard.

Magnetically soft materials can be magnetised very easily, but the magnetism induced in them is only temporary and easy to remove. Iron is magnetically soft material.

### 6.2 Magnets

Recall that magnets repel and attract other magnets, and attract magnetic substances

Magnets and Magnets:
Magnets can either attract or repel other magnets: you should already know that opposite poles attract i.e. South and North attract, but South and South repel

Magnetic substances:
There 5 magnetic materials: 3 elements and 2 compounds
• Iron (Fe : Element)
• Cobalt (Co : Element)
• Nickel (Ni : Element)
• Steel (an alloy or iron)
• Magnetite/Lodestone (Fe3O4 : an oxide of iron)

### Topic 6

Magnetism and Electromagnetism
Objectives:
6b) Magnetism
6.2 recall that magnets repel and attract other magnets, and attract magnetic substances
6.3 recall the properties of magnetically hard and soft material
6.4 understand the term ‘magnetic field line’
6.5 understand that magnetism is induced in some materials when they are placed in a magnetic field
6.6 sketch and recognise the magnetic field pattern for a permanent bar magnet and that between two bar magnets
6.7 know how to use two permanent magnets to produce a uniform magnetic field pattern

6c) Electromagnetism
6.8 recall that an electric current in a conductor produces a magnetic field round it
6.9 describe the construction of electromagnets
6.10 sketch and recognise magnetic field patterns for a straight wire, a flat circular coil and a solenoid when each is carrying a current
6.11 appreciate that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field
6.12 recall that a force is exerted on a current-carrying wire in a magnetic field, and how this effect is applied in simple d.c. electric motors and loudspeakers
6.13 use the left hand rule to predict the direction of the resulting force when a wire carries a current perpendicular to a magnetic field
6.14 recall that the force on a current-carrying conductor in a magnetic field increases with the strength of the field and with the current.

6d) Electromagnetic induction
6.15 recall that a voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it; also recall the factors which affect the size of the induced voltage
6.16 describe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field; also describe the factors which affect the size of the induced voltage
6.17 recall the structure of a transformer, and understand that a transformer changes the size of an alternating voltage by having different numbers of turns on the input and output sides
6.18 explain the use of step-up and step-down transformers in the large-scale generation and transmission of electrical energy
6.19 recall and use the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer:
$\frac{input\, (primary)\, voltage}{output\, (secondary)\, voltage}=\frac{primary\: turns}{secondary\: turns}$
6.20 recall and use the relationship (for 100% efficiency):