Magnetism and electromagnetism - Shawon Notes

Physics > Section 6: Magnetism and Electromagnetism

a) Units

6.1 use the following units: ampere (A), volt (V), watt (W).

Unit of current: ampere (A)
Unit of potential difference: volt (V)
Unit of power: watt (W)

b) Magnetism

6.2 understand that magnets repel and attract other magnets and attract magnetic substances

Magnets are able to attract objects made from magnetic materials such as iron, steel, etc. Other objects like plastic, rubber are non-magnetic substance. They can attract magnet. Magnets have two poles: North Pole and South Pole. North Pole and South Pole attract each other. Similar poles like North Pole and North Pole or South Pole and South Pole repel each other.

6.3 describe the properties of magnetically hard and soft materials

Magnetically hard materials:

  • Needs time to become magnetized
  • Once magnetized, the magnetism remains permanently
  • Magnets with magnetically hard materials are known as permanent magnets
Eg: Steel

Magnetically soft materials:

  • Easily gets magnetized
  • Loses its magnetism easily
  • Magnets with magnetically soft materials are known as temporary magnets
Eg: Iron

6.4 understand the term ‘magnetic field line’

Magnetic field line is imaginary line which represents where the magnetism is acting. Magnetic field line starts from north pole to south pole.

6.5 understand that magnetism is induced in some materials when they are placed in a magnetic field

If you keep a material between the magnetic field, eventually after a period of time, that material will be magnetized.

6.6 describe experiments to investigate the magnetic field pattern for a permanent bar magnet and that between two bar magnets

Using a compass moving around the magnet you can detect the magnetic field pattern. As you move so the needle will move. The compass can show direction of magnetic field, which from North to South.

6.7 describe how to use two permanent magnets to produce a uniform magnetic field pattern.

If you put two bar magnets together with their north and south touching, then they will form the same magnetic field as if there were one bar magnet.

c) Electromagnetism

6.8 understand that an electric current in a conductor produces a magnetic field round it

When a current flows through a wire a magnetic field is created around the wire. This phenomenon is called electromagnetism. The field around the wire is quite weak and circular in shape. The direction of the magnetic field depends up the direction of the current and can be found using the right-hand grip rule.

6.9 describe the construction of electromagnets

If a temporary magnet is wrapped with a wire into a coil and pass current to it, the magnet will become magnetized. This way electromagnets can be constructed.

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

A field around a straight wire is simply a series of circles around the wire.

A field around a solidness is similar to that of a bar magnet.

A field around a flat coil is basically like a single wire, but there are two.

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

A charged particle moving through a magnetic field experiences a force, as long its motion is not parallel to the field. If we pass a current through a piece of wire held at right angles to the magnetic field of a magnet, the wire will move.

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

As current passes around the loop of wire, one side of it will experience a force pushing it upwards. The other side will feel a force pushing it downwards, so the loop will rotate. This is used to produce movements in machines, cars etc.

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

The left hand rules shows the direction of force, magnetic field and current when a wire carries a current perpendicularly to a magnetic field.
The pointing finger points the magnetic field from North to South
The middle finger points the direction of current.
The thumb shows the resulting force.

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.

Ways to increase the force produced in motors:

  • Increase the number of turns or loops of wire
  • Increase the strength of magnetic field
  • Increase the current flowing through the loop of wire

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

If we move a wire across a magnetic field at right angles, a voltage is induced in the wire. This phenomenon is called electromagnetic induction.
The size of the induced voltage can be increased by:

  1. moving the wire more quickly
  2. using a stronger magnet
  3. wrapping the wire into a coil so that more pieces of wire move through the magnetic field.

6.16 describe the generation of electricity by the rotation of a magnet 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

We can generate a voltage and current by pushing a magnet into a coil. The size of induced voltage can be increased by:

  1. moving the magnet more quickly
  2. using a stronger magnet
  3. using a coil with a larger cross-sectional 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

A transformer is a device that helps to reduce or increase voltage in a wire or electric line. This is made of two soft iron core linking to the coils at the two end of the transformer. The first coil is called the primary coil and the second one is called the secondary coil. When alternating current is passed through a coil, the magnetic field around it is continuously changing. The changing magnetic field will pass through the secondary coil and induce voltage in it, and that’s how current is passed. If the secondary coil has more turns, the voltage will increase and if the secondary coil has less turns, the voltage will decrease.

6.18 explain the use of step-up and step-down transformers in the large- scale generation and transmission of electrical energy

A transformer that is used to increase voltage is called step-up transformer. One that is used to decrease voltages is called step-down transformers. After generating electricity, electric currents are passed to a step-up transformer which increase the voltage and decrease the current. This is because higher currents need wide and expensive wire to pass through. Or else, energy is lost in form of heat. Using transformers mean we can have a solution to this problem. Before the electricity reaches home, those are passed through step-down transformers to decrease the voltage and increase the current at the same time.

6.19 know and use the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer:

input(primary)voltage / output(secondary)voltage = primary turns / secondary turns
Vp/Vs=np/ns

6.20 know and use the relationship: for 100% efficiency

Input power = output power
VP IP = VS IS

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