P7 - Magnetism and electromagnetism
P7 - Magnetism and electromagnetism
All magnets have two poles, a north, and south pole. All magnets produce a magnetic field. It is a region of magnetic force that interacts with other magnets or magnetic materials. Magnetic forces are a non-contact force.
Magnetics fields can be represented by magnetic field lines. The lines will always go from north to south and they show which way a force would act on a north pole if it was put at the point in the field. The closer the lines are together the stronger the magnetic field. The further the distance from the magnet the weaker the force of attraction is. The magnetic field is strongest at the poles of the magnet. If the magnets are placed near each other they exert a force on one another, either repulsion or attraction.
Compasses show the direction of magnetic fields. Inside the compass, there is a small bar magnet (the needle) and the north pole of this magnet is attracted to the south pole of any magnet it is near. So the compass needle points in the direction of the magnetic fields it is in.
You can use a plotting compass to plot the magnetic field around a magnet using the direction of the needle when moving it about the magnet.
The motor effect
A current in a magnetic field experiences a force. When a current carrying wire is put between magnetic poles, the magnetic field around the wire interacts with the magnetic field it has been placed in. This force experienced is called the motor effect and can cause the wire to move.
To experience the full force of the motor effect the wire has to be at 90 degrees to the magnetic field. If the wire is parallel to the magnetic field, it won't experience any of the force at all. At angles between the force will be small
The size of the force can be found by using 3 different variables. The magnetic flux density (represents the strength of the magnetic field). The size of the current flowing through the conductor and finally the length of the conductor that is in the magnetic field. This is liked together in the equation F=BIL.
The direction the force is acting on can be found using f lemmings left hand rule. Using a left hand the first finger represents the direction of the field. The second finger in the direction of the current and the thumb will point in the direction of the force.
The Generator effect
Cutting through field lines generates a potential difference. The generator effect: The induction of a potential difference (and current if there's a complete circuit) in the wire which is experiencing a change in magnetic field. The size of the induced potential difference can be changed. This can be done by increasing the speed of the movement or by increasing the strength of the magnetic field.
The generator effect can be demonstrated by moving a magnet through as solenoid. Or moving a conductor in a magnetic field as this would cut through the filed lines.
Rotation can also demonstrate the motor effect. A magnet can be spun from end to end in a coil of wire, this can either be done to produce a.c current or d.c current. An alternating current is created as after every half turn the polarity switches and reverses the induced flow of current.
Induced current opposes the change that made it. When a current is induced a new magnetic field is created around the wire with the current flowing in it. This acts against the induced current and opposes the change that made it.
A magnet can be permanent or induced. Permanent magnets produce their one magnetic field. Induced magnets are magnetic materials that turn into a magnet when they're put into a magnetic field. The force between a permanent and an induced magnet is always attractive. When you take away the magnetic field, the induced magnet quickly loses its magnetism.
A moving charge creates a magnetic field. When current flows through a wire, a magnetic field is created around the wire. The field is made up of concentrated circles around the wire. A compass can be used to plot these lines around the wire. Changing the direction of the current changes the direction of the magnetic field.
The right-hand thumb rule can determine the direction of the current or the magnetic field. The thumb is the direction of the current and the fingers curl inwards to represent the direction of the magnetic field.
A solenoid is a coil of wire. You can increase the strength of the magnetic by wrapping the wire into coils (a solenoid). This happens because you get many more field lines that line up an are all in the same direction creating field lines that are close together and therefore are stronger.
The magnetic field inside a solenoid is very strong and uniform and is even the entire way through the solenoid. Outside the solenoid, the magnetic field is the same as the magnetic field for a bar magnet. The magnetic field strength can be increased further by adding an iron core (the iron core becomes an induced magnet) or increasing the coils in the solenoid. If the current stops flowing then the magnetic field will disappear. A solenoid with an iron core is called an electromagnet and can be turned on and off.
Electromagnets have many uses. Electromagnets can be switched on and off and can vary in power depending on the current and the number of coils in the solenoid. Such as in speakers. Electromagnets can be used with other circuits to act as switches, this could be an example of an electric starter motor in a car as this separates the high voltage circuits and low voltage circuit increasing the safety of the motor and switch mechanism. Another example is an electric doorbell
Alternators and Dynamos
Alternators generate alternating current. Generators rotate a coil in a magnetic field. Their construction is similar to that of a motor. As the coil spins, a current is induced in the coil. This current changes direction every half turn. Instead of a split-ring commutator, alternators have slip ring commutators and brushes so the contacts don't swap every half turn.
Dynamos generate D.C current. Dynamos work in the same way as alternators, except they have a split-ring commutator. This swaps the connection every half turn to keep the current flowing in the same direction.
An oscilloscope can be sued to generate see a generated potential difference. A.c current goes above and below the normal, D.C current stays above the normal line.
Transformers change the potential difference but only for alternating current.Transformers have two coils of wire. A primary coil and a secondary coil. They are joined with an iron core. When an alternating potential difference is applied across the primary coil, the iron core magnetises and demagnetises quickly. This changing magnetic field induces an alternating potential difference on the second coil. The ratio between the number of turns is the same as the ratio between the potential difference.
Transformer equation 1 --> vp/vs = np/ns.
Transformer equation 2 --> VsIs = VpIp
Loudspeakers and microphones
Loudspeakers work because of the motor effect. An alternating current is sent through a coil of wire attached to the base of a cone. The coil surrounds one pole of a permanent magnet and is surrounded by the other pole. The current causes a force to be exerted on the magnet. This causes the cone to move. When the current reverses the force acts in the opposite direction, which causes the cone to move in the opposite direction. The variations in current cause the cone to vibrate which makes the air around the cone vibrate, creating variations in air pressure that cause sound waves. The frequency of the wave is the same as the frequency of the a.c current.
Microphones generate current from sound waves. Microphones are loudspeakers in reverse. Sound waves hit the diaphragm and cause the magnets to move through the coil of wire. This induces a potential difference and current. The movement of the coil or magnet depends on the frequency of the waves hitting the diaphragm.
A current-carrying coil of wire rotates in a magnetic field. Forces act on either side of a coil of wire in opposite directions causing the wire to gain rotational spin. These forces are due to the magnetic field that the wire experiences when a current flows through it. The coil is on an axle that allows rotational spin. A split ring commutator flips the polarity each half turn to maintain the same directional spin of the motor. The motor can be reversed by switching the polarity of the poles or the current flowing from the power source.