Electricity and Electromagnetism

What are Magnets ?

Understanding Magnets A magnet is an object made of certain materials which create a magnetic field.  Every magnet has at least one north pole and one south pole.   Like poles (eg. N & N; S & S) repel each other and opposite poles (eg. N & S) attract each other. Activity 1 – Drawing the Magnetic Field

For Every North, There is a South Every magnet has at least one north pole and one south pole.  By convention, we say that the magnetic field lines leave the North end of a magnet and enter the South end of a magnet.  If you take a bar magnet and break it into two pieces, each piece will again have a North pole and a South pole.  If you take one of those pieces and break it into two, each of the smaller pieces will have a North pole and a South pole.  No matter how small the pieces of the magnet become, each piece will have a North pole and a South pole.  S N S N S N

No Monopoles Allowed It has not been shown to be possible to end up with a single North pole or a single South pole, which is a monopole ("mono" means one or single, thus one pole).  Note: Some theorists believe that magnetic monopoles may have been made in the early Universe. So far, none have been detected. S N

Magnetic Fields Magnetic field lines describe the structure of magnetic fields in three dimensions Field lines converge where the magnetic force is strong, and spread out where it is weak. For instance, in a compact bar magnet or "dipole," field lines spread out from one pole and converge towards the other. the magnetic force is strongest near the poles where they come together. A magnet has a ‘magnetic field’ distributed throughout the surrounding space

What Happens when a Magnet is cut in half?

The earth is like a giant magnet The nickel iron core of the earth gives the earth a magnetic field much like a bar magnet.

Drawing The Magnetic Field Equipment: Bar Magnet Iron fillings Sheet of white paper Greylead Questions: Where is the magnetic field the strongest? How do you know? What happens to the magnetic field as you move further away from the magnet?

Types of Magnets

Temporary and Permanent Magnets

What are magnetic domains? Magnetic substances like iron, cobalt, and nickel are composed of small areas where the groups of atoms are aligned like the poles of a magnet. These regions are called domains. All of the domains of a magnetic substance tend to align themselves in the same direction when placed in a magnetic field. These domains are typically composed of billions of atoms.

Temporary Magnets vs Permanent Magnets Permanent magnets have a north and south pole Within a permanent magnet there are a number of smaller ‘mini-magnets’ These mini-magnets are all aligned so the north poles all face the same direction, and all of the south poles face in the same direction N S N N S S N S N S N S N S N S

Temporary Magnets vs Permanent Magnets In materials that aren’t permanent magnets, these mini-magnets still exist, however, they are all out of alignment In temporary magnets, these mini-magnets can be manoeuvred so that they become aligned S N N S S N N S N S N S S N

Temporary Magnets vs Permanent Magnets Temporary magnets can be stroked by a permanent magnet to align the poles Temporary magnets can also be created by using electricity = electromagnetism Activity 2 – Making Temporary Magnets

Temporary Magnets vs Permanent Magnets Number of Strokes Number of Paperclips ?

Electromagnets

Electromagnets The magnetic field produced around a current carrying wire is circular If the wire is looped, several circular magnet fields are produced, therefore producing a stronger magnetic effect When several of these loops are placed together, a solenoid is created, and the magnetic effect is even stronger

Electromagnets An electromagnet is a solenoid with an iron core that further concentrates the field List all of the different uses of electromagnets you know... Practical 1 – Magnetism and Electric Current Solenoid Iron core

Electromagnets in action Speakers in television sets, telephones and sound systems need to use magnetic fields produced by electric currents to convert electrical energy into sound energy

Electromagnets in action Copper wire is wound around the base of the cone. The coil produces its own magnetic field because of the current flowing through it. In a speaker, the electric current is rapidly changing direction. Because of this, the coil moves up and down alternately repelled and attracted to the base of the magnet. Cone Solenoid Base Permanent magnet

Electromagnets in action The cone then vibrates as the coil moves, causing the air nearby to vibrate, producing sound. The higher the pitch of a note, the more rapidly the electric current changes direction. Cone Solenoid Base Permanent magnet

Motors An electric motor is a device that converts electrical energy into kinetic energy An electric motor turns because it contains coils that produce a magnetic field when electric current flows through them

Motors The armature is the turning point of the motor on which coils are wound DC motors Field coils are magnets that do not move The commutator turns as the armature turns The shaft turns as the armature turns The rotor coils are wound on the armature. When electric current flows they produce a magnetic field The brushes connect to the power supply

Motors Practical 2 – Making a simple electric motor

Electric Generators Electric current is generated by running a coil of wire through a magnetic field The size of the current generated can be increased by: Number of turns of the wire in the coil Strength of the magnet Speed of the relative movement between the coil and the magnetic field Practical 3 – Making an electric generator

Transformers Electric power generated at power stations is very large Wires carrying this electricity would get very hot if the size of the current stayed the same from station to home Transformers allow us to increase or decrease the voltage of the electricity Step-up transformers increase the voltage, and therefore decrease the size of the electric current Step-down transformers decrease the voltage, and therefore increase the size of the electric current

Transformers Where might step-up and step-down transformers be used in following diagram? E = 16.5 kV C = D = F = 66 kV B = 220kV-500kV A = 240 V

Transformers In step-down transformers, the primary coil has more turns. In step-up transformers, the secondary coil has more turns. A changing electric current produces a changing magnetic field inside the secondary coil.

Waves There are two main types of waves Longitudinal Transverse Particles vibrate backwards and forwards Transverse Particles vibrate up and down

Waves Reading wavelengths

Waves Light consists of electromagnetic waves Because light is considered to be made up of a series of changing magnetic and electric fields Light waves travel at 300 000 000 metres per second They may travel a long way before enough energy is lost to destroy them

Waves The electromagnetic spectrum

Waves Your task – In groups of 3-4, take the envelope and remove the cut up pieces of information. Try and place the information in the correct order. Once you have completed the task, show the teacher and have them check you are correct. You are then to summarise the information in your group as present it to the class.