Section 6: Magnetism and electromagnetism
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a) Units
6.1: Use the following units: ampere (A), watt (W), volt (V)
Ampere: Used to measure electrical current
Watt: Used to measure power
Volt: Used to measure energy transferred
Ampere: Used to measure electrical current
Watt: Used to measure power
Volt: Used to measure energy transferred
b) Magnetism
6.2: Understand that magnets repel and attract other magnets and attract magnetic substances
Magnets are able to attract substances made of magnetic materials like iron, nickel, steel and cobalt. Magnets cannot attract non-magnetic materials such as plastic and wood. The strongest part of a magnetic is called a pole. Most magnets have two poles known as the north pole and the south pole. Similar poles repel - if you try to push two north poles together, they will swing away from each other. However, opposite poles attract - if you try to push a north and a south pole together, they will willingly stick.
Magnets are able to attract substances made of magnetic materials like iron, nickel, steel and cobalt. Magnets cannot attract non-magnetic materials such as plastic and wood. The strongest part of a magnetic is called a pole. Most magnets have two poles known as the north pole and the south pole. Similar poles repel - if you try to push two north poles together, they will swing away from each other. However, opposite poles attract - if you try to push a north and a south pole together, they will willingly stick.
6.3: Describe the properties of magnetically hard and soft materials
Permanent magnets are made from magnetically hard materials. A magnetically hard material retains its magnetism once it has been magnetized. Magnetically soft materials such as iron lose their magnetism easily and can be used as temporary magnets.
Permanent magnets are made from magnetically hard materials. A magnetically hard material retains its magnetism once it has been magnetized. Magnetically soft materials such as iron lose their magnetism easily and can be used as temporary magnets.
6.4: Understand the term 'magnetic field line'
There is a volume of space surrounding every magnet where magnetism can be detected. This volume of space is called a magnetic field and normally cannot be seen, however, the magnetic field line can be plotted to help visualize the main features of the magnetic field. These lines show the shape of the magnetic field, the direction of the magnetic field, and the strength of the magnetic field (when the field lines are closer together, the field is stronger).
There is a volume of space surrounding every magnet where magnetism can be detected. This volume of space is called a magnetic field and normally cannot be seen, however, the magnetic field line can be plotted to help visualize the main features of the magnetic field. These lines show the shape of the magnetic field, the direction of the magnetic field, and the strength of the magnetic field (when the field lines are closer together, the field is stronger).
6.5: Understand that magnetism is induced in some materials when they are placed in a magnetic field
Magnetism can be induced in a material by leaving it in a magnetic field. This may be done deliberately to make a magnet. For example, iron filings are temporarily magnetized when they are placed near a magnet. However, this can also happen to steel food cans - if they are left stationary in one position, the Earth's magnetic field will gradually induce magnetism in them.
Magnetism can be induced in a material by leaving it in a magnetic field. This may be done deliberately to make a magnet. For example, iron filings are temporarily magnetized when they are placed near a magnet. However, this can also happen to steel food cans - if they are left stationary in one position, the Earth's magnetic field will gradually induce magnetism in them.
6.6: Describe experiments to investigate the magnetic field pattern for a permanent bar magnet and that between two bar magnets
Iron filings
Plotting compass
Iron filings
- Place a bar magnet under a piece of paper
- Shake iron filings onto the piece of paper
- The shape of the magnetic field can be seen around the bar magnet because of the iron filings attracted to it
Plotting compass
- Place a bar magnet on a piece of paper
- Surround the bar magnet with plotting compasses, arranged in a semicircle; i.e. starting at north and curving around to south
- The needle of each plotting compass lines up with the field line, pointing from north to south
6.7: Describe how to use two permanent magnets to produce a uniform magnetic field pattern
If two magnets are placed near each other, their magnetic fields can affect each other. Various field patterns are possible. When north and south poles are placed near each other there is an almost uniform field between the poles.
If two magnets are placed near each other, their magnetic fields can affect each other. Various field patterns are possible. When north and south poles are placed near each other there is an almost uniform field between the poles.
c) Electromagnetism
6.8: Understand that an electric current in a conductor produces a magnetic field around it
When a current flows in a wire a magnetic field is produced around the wire. The field is continuous and extends over the whole length of the wire. The field lines represent the direction of the force a north pole would feel and the spacing of the field lines shows that the strength of the field decreases with distance from the wire. Increasing the current will increase the strength of the magnetic field produced.
When a current flows in a wire a magnetic field is produced around the wire. The field is continuous and extends over the whole length of the wire. The field lines represent the direction of the force a north pole would feel and the spacing of the field lines shows that the strength of the field decreases with distance from the wire. Increasing the current will increase the strength of the magnetic field produced.
6.9: Describe the construction of electromagnets
A piece of wire is wrapped around a soft magnetic material. When there is a current in the wire, a magnetic field is induced in the metal. This field can be made stronger by increasing the current through the coil or increasing the number of turns in the coil.
A piece of wire is wrapped around a soft magnetic material. When there is a current in the wire, a magnetic field is induced in the metal. This field can be made stronger by increasing the current through the coil or increasing the number of turns in the coil.
6.10: Sketch and recognize magnetic field patterns for a straight wire, a flat circular coil and a solenoid when each is carrying a current
6.11: Understand 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
If something with a charge is moving across a magnetic field, it will experience a force from the field, unless its motion is parallel to the field, in which case it will not experience a force.
If something with a charge is moving across a magnetic field, it will experience a force from the field, unless its motion is parallel to the field, in which case it will not experience a force.
6.12: Understand 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
When a current is passed through a wire placed in a magnetic field a force is produced which acts on the wire. The motor effect can be used in loudspeakers: the signal current produced by an amplifier is alternating and by passing it through a coil in a magnetic field the current results in alternating forces on the coil. The coil is attached to a paper cone and this transfers the vibrations to the air. The motor rule is also used in simple d.c. motors. In its simplest form a DC motor consists of a single turn coil of wire that is free to rotate in a magnetic field about an axle. Carbon brushes make contact with the ends of the coil that are connected to a commutator so that a current can be passed through the coil.
When a current is passed through a wire placed in a magnetic field a force is produced which acts on the wire. The motor effect can be used in loudspeakers: the signal current produced by an amplifier is alternating and by passing it through a coil in a magnetic field the current results in alternating forces on the coil. The coil is attached to a paper cone and this transfers the vibrations to the air. The motor rule is also used in simple d.c. motors. In its simplest form a DC motor consists of a single turn coil of wire that is free to rotate in a magnetic field about an axle. Carbon brushes make contact with the ends of the coil that are connected to a commutator so that a current can be passed through the coil.
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
You need to be able to use Fleming's Left Hand Rule to work out the direction of the force that acts on the wire. This is also called the Motor Rule. The force acting upon the wire will make the wire move. The thumb of your left hand is used to determine the direction of movement caused by the force on the wire. The index finger of your left hand is used to determine the direction of the magnetic field. The middle finger of your left hand is used to determine the direction of the current.
You need to be able to use Fleming's Left Hand Rule to work out the direction of the force that acts on the wire. This is also called the Motor Rule. The force acting upon the wire will make the wire move. The thumb of your left hand is used to determine the direction of movement caused by the force on the wire. The index finger of your left hand is used to determine the direction of the magnetic field. The middle finger of your left hand is used to determine the direction of the current.
6.14: Describe how the force on a current-carrying conductor in a magnetic field increases with the strength of the field and with the current
The force exerted on a wire in a magnetic field will increase if the magnetic field gets stronger and if the current is increased.
The force exerted on a wire in a magnetic field will increase if the magnetic field gets stronger and if the current is increased.
d) Electromagnetic induction
6.15: Understand 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 and describe the factors which affect the size of the induced voltage
When a conductor is in a changing magnetic field a voltage will be induced in the conductor. The size of the induced voltage in a coil can be increased by increasing the rate of change of the strength of the magnetic field, by having more turns on the coil and by having a coil of greater area.
When a conductor is in a changing magnetic field a voltage will be induced in the conductor. The size of the induced voltage in a coil can be increased by increasing the rate of change of the strength of the magnetic field, by having more turns on the coil and by having a coil of greater area.
6.16: Describe the generation of electricity by the rotation of a magnetic within a coil of wire and of a coil of wire within a magnetic field and describe the factors which affect the size of the induced voltage
Bicycle dynamos consist of a strong, small bar magnet which is spun by the wheel of a bicycle. It spins between the faces of a U-shaped iron core on which a coil of wire is wound. The changing magnetic field induces an alternating voltage in the coil.
Generators have spinning coils of wire in the magnetic field of strong permanent magnets (or, in large generators, a magnetic field produced by electromagnets). Since the coil in which the voltage is being induced is spinning, the electrical connections are made by brushes which slide over slip rings.
The size of the induced voltage in a coil can be increased by increasing the rate of change of the strength of the magnetic field, by having more turns on the coil and by having a coil of greater area.
Bicycle dynamos consist of a strong, small bar magnet which is spun by the wheel of a bicycle. It spins between the faces of a U-shaped iron core on which a coil of wire is wound. The changing magnetic field induces an alternating voltage in the coil.
Generators have spinning coils of wire in the magnetic field of strong permanent magnets (or, in large generators, a magnetic field produced by electromagnets). Since the coil in which the voltage is being induced is spinning, the electrical connections are made by brushes which slide over slip rings.
The size of the induced voltage in a coil can be increased by increasing the rate of change of the strength of the magnetic field, by having more turns on the coil and by having a coil of greater area.
6.17: Describe 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
The function of a transformer is to change the size of an alternating voltage. This is done by having two separate coils with different numbers of turns. Transformers consist of a core made from thin sheets of a magnetically soft material clamped together. Two separate coils of wire, insulated from one another, are tightly wound onto the core. Transformers are designed to perform the job of changing voltage with very little power loss; they are approximately 100% efficient.
The function of a transformer is to change the size of an alternating voltage. This is done by having two separate coils with different numbers of turns. Transformers consist of a core made from thin sheets of a magnetically soft material clamped together. Two separate coils of wire, insulated from one another, are tightly wound onto the core. Transformers are designed to perform the job of changing voltage with very little power loss; they are approximately 100% efficient.
6.18: Explain the use of step-up and step-down transformers in the large-scale generation and transmission of electrical energy
Transformers are used in the transmission of electrical energy over large distances. Transmission lines have low but not zero resistance, and this means that power losses between the power station and the consumers would be unacceptably large.
If a transformer is used to step up the generated alternating voltage 50 times, the current is stepped down 50 times. This will reduce the power loss by a significant amount. The voltage is then stepped down using transformers close to consumers, again with very little power loss because of the near 100% efficiency of the transformer.
Transformers are used in the transmission of electrical energy over large distances. Transmission lines have low but not zero resistance, and this means that power losses between the power station and the consumers would be unacceptably large.
If a transformer is used to step up the generated alternating voltage 50 times, the current is stepped down 50 times. This will reduce the power loss by a significant amount. The voltage is then stepped down using transformers close to consumers, again with very little power loss because of the near 100% efficiency of the transformer.
6.19: Know and use the relationship between input (primary) and output (secondary) voltage and the turns ratio for a transformer
6.20: Know and use the relationship for 100% efficiency
input power = output power
VPIP = VSIS
input power = output power
VPIP = VSIS