ELECTROMAGNETISM Unlike an ordinary magnet, electromagnets can be switched on and off. A simple electromagnet consists of: - a core (usually iron) - several turns of insulated copper wire When current flows through the coil, it produces a magnetic field. This magnetizes the core, creating a magnetic field about a thousand times stronger than the coil by itself. Iron is temporarily magnetized and so the magnetization in the electromagnet is lost once the current is switched off. On the other hand, steel is not a good core, since steel is permanently magnetized. The strength of the magnetic field is increased by: 1. the number of turns 2. An increase in current
A wire on its own does not have a magnetic field, however, if an electric current is passed through a wire, a weak magnetic field is produced! The magnetic field has these features: - The magnetic field lines are circular - The field is strongest close to the wire - The magnetic field increases (stronger) with an increase in current, and decreases (weaker) with a decrease in current. The direction of the magnetic field produced by the current can be found using the RIGHT HAND GRIP RULE!! Imagine gripping the wire with your Right hand, such that, your thumb points in the direction of the current. Your fingers will then point in the same direction as the field lines. Revision: Current Current is flow of electrons Conventionally current flows from the positive to the negative Magnetic fields from coils If a wire is wound up in a coil, its current produces a stronger magnetic field. A long coil is called a SOLENOID.
The magnetic field produced by a current-carrying coil has the following features: 1. The field is similar to that from a bar magnet, and there are magnetic poles at the ends of the coil 2. The strength of the magnetic field increases (is stronger) with an increase in current and decreases (is weaker) with a decrease in current 3. The strength of the magnetic field increases (is stronger) with an increase in the number of turns, and decreases (is weaker) with a decrease in the number of turns. Finding the field direction for solenoids Imagine gripping the coil with your right hand so that your fingers point in the conventional current direction. Your thumb then points towards the North pole of the solenoid. Follow the convention current with the current leaving from the + to the -, your fingers will be pointing at the direction of the current in the solenoid and your thumb will be pointing towards the North. Making steel magnets In the previous chapter, we magnetised an iron bar by stroking, this way possible because iron is a soft magnetic material. However, it is impossible to magnetise a steel bar using the stroking method, for this reason a steel bar is magnetised using a solenoid.
This can be done by placing a steel rod into a solenoid. When a current is passed through the solenoid, the steel rod becomes magnetised. The steel rod helps the solenoid to produce a stronger magnetic field itself. Once, the current is switched off, the steel rod, now a magnet can be removed from the solenoid. Remember: - Steel is permanently magnetized! Demagnetising a steel magnet A steel magnet is difficult to demagnetise!! However, it can be demagnetised by removing the steel magnet slowly out a solenoid carrying an A.C. This is because in an A.C. the current changes direction 50times per second and so the dipoles in the steel magnet change their direction 50times per second!
What happens if a copper wire is placed between magnets? Copper is non-magnetic, so it is not affected by magnets. However, if a current is passes through the wire, then the wire experiences a force on it. The force arises because the current produces its own magnetic field which acts on the poles of the magnet. How can we find the direction in which the wire will move? Fleming s Left Hand M F C If you hold the thumb and first two fingers of your left hand at right angles, and point the fingers as shown, the thumb will give you the direction of the force (the direction in which the wire will move). Things to remember: 1. By convention, Current moves from the Positive to the Negative 2. The magnetic field is from North to South.
The force experience by the wire will increase if: - The current is increased - The magnets used are stronger - The length of the wire is increased Applications: - The moving coil loudspeaker The cylindrical magnet produces a strong radial (spoke-like) magnetic field at right angles to the wire in the coil. The coil is free to move backwards and forwards and is attached to a sniff paper or plastic cone. Electric motors (Turning effect on a coil) The coil lies between the poles of a magnet. The current flows in opposite directions along the two sides of the coil. So, according to Fleming s Left Hand Rule, one side is pushed up and the other side is pushed down thus, there is a turning effect! This is the model of many electric motors!!
A simple DC motor The diagram on the right shows a simple electric motor. This is a motor which runs on D.C. as the current (from the battery) is flowing in one direction. The coil is made of insulated copper wire. The coil is free to rotate between the poles of the magnet. The cummutator (or split-ring) is fixed to the coil and rotates with it, the brushes are two contacts which rub against the cummutator and keep the coil connected to the battery. The brushes is usually made of carbon. When the coil is horizontal, the forces are furthest apart and have their maximum turning effect (leverage) on the coil. The coil would eventually come to rest in the vertical position when there is no change in force. As the coil hits the vertical position, the commutator changes the direction of the current through it. So the force changes direction and push the coil further until it is vertical again and so on in this way the coil keep rotating clockwise, half a turn a time. If the battery or the poles of the magnets are reversed, then the coil will rotate in the opposite direction (anti-clockwise). The turning effect on the coil can be increased by: - increasing the current - using a stronger magnet - increasing the number of turns on the coil - increasing the area in the coil (using a larger coil)
Practical uses: - electric drillers Electromagnetic induction A current produces a magnetic field. However, a magnetic field can be used to produce a current. Induced EMF and current in a moving wire When a wire is moved across a magnetic field, as shown in the diagram, a small EMF (voltage) is generated in the wire. The effect is called ELECTROMAGNETIC INDUCTION. Scientifically speaking, an EMF is induced in the wire. If the wire forms part of a complete circuit, the EMF makes a current flow. This can be detected using a GALVANOMETER (a meter used to detect very small currents)
Its pointer moves to the left or right of the zero, depending on the current direction. The induced EMF can be increased by: - moving the wire faster - using a stronger magnet - increasing the length of wire in the magnetic field. The above can be summarized in FARADAY S LAW OF ELECTROMAGNETIC INDUCTION: The induced EMF in a conductor is proportional to the rate at which magnetic field lines are cut by the conductor. Remember: Field lines are used to represent:- a. the strength of the magnetic field b. the direction of the magnetic field To reverse the direction of the induced EMF and current: - reverse the direction of the wire - reverse the position of the magnets (such that the poles are reversed) If the wire is not moving or parallel to the field lines, there is no induced EMF / current.
INDUCED EMF AND CURRENT IN A COIL If a bar magnet is pushed into a coil, an EMF can be induced in the coil. In this case, the magnetic field is moving rather than the wire. However, the effect is the same; field lines are being cut. The induced EMF (and current) can be increased by: - moving the magnet faster - using a stronger magnet - increasing the number of turns on the coil Applications and interesting facts: From the coil-magnetic experiment, one can draw the following conclusions: - If the magnet is pulled out, the direction of the induced EMF (and current) is reversed. - If the magnet is pulled in, the direction of the induced EMF (and current) is reversed. Thus opposing the magnet s direction in both cases; this is an example of the law of conservation of energy. Since energy is spent when a current flows around a circuit, this energy has to first be spent to induce the current. If the magnet is held still, no field lines are cut, so there is no induced EMF or current.
The pick-ups under the strings of this guitar are tiny coils with magnets inside them. The steel strings become magnetized. When they vibrate, current is induced in the coils, boosted by an amplifier, and used to produce sound. Transformers A transformer consists of (at least) two coils of wire wound onto the same core of soft ferromagnetic material. Transformers are used to change an alternating EMF in one of the coils to a different EMF in the other coil. There is hardly any energy loss between the two circuits if the transformer is well-designed. However, some eddy currents (e.g. wires heat up) in the form of heat loss cannot be totally eliminated, though reduced by using laminated soft iron core. Application: Electricity supply: - Power Station
The transformer consists of a primary coil (input) and secondary coil (output) AC voltages can be increased or decreased using a transformer. When AC flows through the primary coil (input), it sets up an alternating magnetic field in the core and in the secondary coil (output). TURNS RATIO, STEP-UP AND STEP-DOWN TRANSFORMERS: TURNS RATIO: The number of turns in the secondary coil compared to the number of turns in the primary coil. This turn ratio is also the ratio of the EMF in the secondary coil to that in the primary coil. STEP-UP TRANSFORMER The EMF in the secondary coil is greater than the EMF in the primary coil. The EMF in the primary coil is greater than the EMF in the secondary coil.
Equations: (Ideal Transformer) V p.i p = V s.i s Worked out example: Ans: = 120V
Now try these on your own: 1. 2. Find I 2 3. Find V s and I s
4. Find I p, N s and its efficiency