Electricity & Magnetism Revision Slide 1 Electricity & Magnetism Revision Revision Magnets. 1. Draw a diagram to show the lines of force around a simple bar magnet: 2. What two main things do the lines of force tell you about the strength and the direction of the magnetic field in the different areas around the magnet? i)…………………………………………………………………………….. ii)…………………………………………………………………………….. 3. Name the two types of magnetic pole. i)………………………………. ii)…..……………………… 4. Complete the sentences. i) Like poles …………………. ii) Unlike poles ……………… E J Vine.

N S Slide 2 If you sprinkle iron filings around a bar magnet you can see a pattern form. What is this pattern? What do the arrows on the lines show? What does it tell you about the strength of the magnetic field where the lines are: Close together? Far apart? Mark on the diagram the points where the field is the weakest. What causes the steel bar to have a magnetic field? E J Vine.

S N S ? ? N S ? S N What are the magnetic field patterns between the above arrangements of magnets? (Fill in the areas where you see the question marks) ? N N E J Vine.

Magnetism is caused by the movement of positive or negative charges. An Atom Atoms have electrically charged particles inside. The positive particles called PROTONS ATTRACT the negative ones called ELECTRONS This force of attraction keeps the electron in orbits around the centre of the atom called the NUCLEUS. + Magnetism is caused by the movement of positive or negative charges. Moving Negative electrons in atoms give substances magnetic properties. Look at the picture which shows the inside of an atom. Name the main particles which are found in atoms and write down what charge each carries. Particle name ……………………… Charge it carries…………….. Electrons are negatively charged. PROTONS are positively charged. What charge do the neutrons have?............... The cluster of particles in the middle of the atom are held together with very strong forces, this cluster is called the NUCLEUS of the atom. The opposite charges attract each other and this is what keeps the electrons orbiting the nucleus of the atom. What stops the electrons just flying off and escaping from the atom?.................................................................................................................................................... Neutrons have no electrical charge E J Vine.

Neutrons have no electrical charge An Atom Atoms have electrically charged particles inside. The positive particles called PROTONS ATTRACT the negative ones called ELECTRONS This force of attraction keeps the electron in orbits around the centre of the atom called the NUCLEUS. + Orbiting electrons produce tiny magnetic fields. Any positive or negative particle when moving produces a tiny magnetic field. Which metal produces the strongest magnetic field when magnetised? (hint. It is used to make magnets) ……………………………………………………………. Neutrons have no electrical charge E J Vine.

Atomic magnets North South Iron is the element which produces the strongest magnetic fields If you could see these tiny atomic magnets you would see that they form magnetic groups called domains. The domains can be lined up inside an iron or steel bar. All you need is a magnetic field to line them up. Electrons circling in atoms make the atoms behave like tiny magnets: This effect is very strong in iron E J Vine.

No magnetic force field An iron or steel bar which has not been magnetised. S N S N No magnetic force field The atomic magnets point at each other N to S and S to N. When they are like this then the magnetic fields effectively cancel and the bar has no external magnetic field. Atomic Magnets all jumbled pointing at each other: Their fields cancel each other and this iron or steel bar would not be magnetic E J Vine.

Unmagentised steel or iron bar The field from the red permanent magnets will gradually line up the magnetic domains. E J Vine.

The magnetic force field from the permanent magnets line up the atomic magnets and will make the steel or iron bar magnetic. N S The field from the red permanent magnets gradually line up the magnetic domains. E J Vine.

The magnetic force field from the permanent magnets lines up the atomic magnets and will make the steel or iron bar magnetic. N S The field from the red permanent magnets gradually line up the magnetic domains. E J Vine.

The magnetic force field from the permanent magnets lines up the atomic magnets and will make the steel or iron bar magnetic. N S The field from the red permanent magnets gradually line up the magnetic domains. E J Vine.

The magnetic force field from the permanent magnets lines up the atomic magnets and will make the steel or iron bar magnetic. N S The field from the red permanent magnets gradually line up the magnetic domains. E J Vine.

The magnetic force field from the permanent magnets lines up the atomic magnets and will make the steel or iron bar magnetic. N S S N The blue bar now produces its own magnetic field. E J Vine.

S The blue magnet now has its own magnetic field N The field from the red permanent magnets has lined up the domains and the blue bar has now become magnetic. E J Vine.

S N N S Complete the following sentence: TEMPORARY MAGNETS ARE MADE OF IRON : When the domains are lined up the bar becomes magnetic. The domains in pure iron are easily lined up, iron is easy to magnetise. However, even the warmth of the room gives the domains enough energy to jumble up again and the bar easily looses its magnetic field. N S S N TEMPORARY MAGNETS ARE MADE OF IRON : When the domains are lined up the bar becomes magnetic. The domains in pure iron are easily lined up, iron is easy to magnetise. However, even the warmth of the room gives the domains enough energy to jumble up again and the bar easily looses its magnetic field. The bar becomes magnetic and it produces its own magnetic field. Complete the following sentence: Temporary magnets are made of pure………………………. Temporary magnets are easy to ………………………………….. And easy to de…………………………………… List TWO WAYS that a temporary magnet may be demagnetised. …………………………………………………………………………………………………………………….. E J Vine.

Heat from the room is sufficient to give the domains energy, this destroys the orderly lines, the magnetic field will disappear. S N Heat from the room is sufficient to give the domains energy, this destroys the orderly lines, just like a line of pupils will fall part when left!!!!!!! heat E J Vine.

heat The magnetic force field gradually disappears. N The field gradually……………………………………… heat E J Vine.

No field left!!!! E J Vine.

Permanent MAGNETS ARE MADE OF Steel plus impurity atoms like Al aluminium, Ni nickel, Co cobalt They are called AlNiCo Magnets N S S N The impurity atoms make it difficult to magnetise Al Ni Co magnets but they also lock the domains in position once they are lined up. This makes them into good material for Permanent magnets. What is it about permanent magnets that makes them different from temporary magnets:…………………………………………………………………… Why is it difficult to demagnetise permanent magnets? ………………………………….. And difficult to de…………………………………… Explain TWO WAYS that a permanent magnet may be magnetised. …………………………………………………………………………………………………………………….. Al Ni The impurity atoms make it difficult to magnetise Al Ni Co magnets but they also lock the domains in position once they are lined up. Co E J Vine.

Permanent MAGNETS ARE MADE OF Steel plus impurity atoms like Al aluminium, Ni nickel, Co cobalt They are called AlNiCo Magnets N S S N The impurity atoms make it difficult to magnetise Al Ni Co magnets but they also lock the domains in position once they are lined up. This makes them into good material for Permanent magnets. What is it about permanent magnets that makes them different from temporary magnets:…………………………………………………………………… Why is it difficult to demagnetise permanent magnets? ………………………………….. And difficult to de…………………………………… Explain TWO WAYS that a permanent magnet may be magnetised. …………………………………………………………………………………………………………………….. Al Ni heat Co E J Vine.

Big Numbers An electric current in a wire is actually a flow of electrons 1 Amp flowing through a bulb means that 6,250,000,000,000,000,000 electrons Pass through it every second!!! 1Amp = 6 ¼ million, million, million electrons per second E J Vine.

The electrons are produced by a hot wire in a electron gun. In a hot cathode ray television (CRT) the picture is formed on a screen by Three beams of electrons striking the screen inside a tube which has all the air removed from it. (a vacuum tube ) The electrons are produced by a hot wire in a electron gun. The moving negative electrons produce a magnetic Field in the shape of a cylinder. Screen Hot filament The Magnetic field pattern forms cylinders around the beam of electrons Beam of electrons E J Vine.

When electrons move they create a magnetic field around the electron beam: you can see the field here as concentric cylinders around the beam of electrons Hot filament Beam of electrons E J Vine.

Screen 2 Hot filament Beam of electrons E J Vine.

To show how an electron beam can be moved using a Screen 2 To show how an electron beam can be moved using a Magnetic field from a magnet. Screen 1 Hot filament No air inside the tube Beam of electrons Teltron e/m deflection tube demo E J Vine.

N Teltron e/m deflection tube demo Look at the next few slides; screen Hot filament Look at the next few slides; What happens to the path of the electron beam as the magnet is brought closer? N Teltron e/m deflection tube demo E J Vine.

Teltron e/m deflection tube demo screen Force Hot filament N Teltron e/m deflection tube demo E J Vine.

screen Force Hot filament N E J Vine.

Electron flow Electrons flowing in a wire Produce a magnetic field just like those in the vacuum tube Wire Electron flow Magnetic field lines E J Vine.

- ve + Magnetic coils If you bend a wire round to make a coil, the circular magnetic fields produce a new pattern which is rather like that of a bar magnet!! An electric current on circuit diagrams shows A direction which is OPPOSITE to the electron flow!!!! + If you bend a wire round to make a coil, the circular magnetic fields produce a new pattern which is rather like that of a bar magnet!! You can see this more easily when you look from above Current Electrons Actually go the other Way! - ve Electron flow E J Vine.

With many coils you can create a strong magnetic field. If you compare this with the slide of the bar magnet you can see that the field pattern is very similar. With many coils you can create a strong magnetic field. What is the other advantage of using a coil of wire and an electric current to create magnetic fields? E J Vine.

If you compare this with the slide of the bar magnet you can see that the field pattern is even more similar when more coils are used. With many coils you can create a strong magnetic field. What is the other advantage of using a coil of wire and an electric current to create magnetic fields? E J Vine. 4 coils of wire

Once the electricity had been switched on then the magnetic field from the coils can be used to line up the domains in an iron bar and make it very strongly magnetic. This is called an electromagnet. What do you think would happen once the electricity was switched off? Iron bar E J Vine. 4 coils of wire

What has happened to the magnetic domains (the atomic magnets)? What do you think has happened to the magnetic field? E J Vine.