electromagnetism bruh

Question 1

What is a magnetic field (flux)?

Answer 1

It is a region where a force will be felt on a magnet.

Question 2

What methods can be used to visualise the field lines around a single bar magnet?

Answer 2

• Sprinkling iron filings near it.

- Plotting the direction in which a small compass points when placed around the magnet.

Question 3

What is the difference between flux and flux density?

Answer 3

Flux: The number of lines coming from a pole. [webers]

Flux density: The concentration of lines (i.e lines per square metre) [webers/m^2 or Tesla]

Question 4

What are Ferromagnetic materials?

Answer 4

• They are ones that become strongly magnetic when placed in a magnetic field.
• Field lines become diverted towards the ferromagnetic substance thereby concentrating the field lines and making the flux density larger.

Question 5

List the Ferromagnetic metals.

Answer 5

Iron, steel, nickel and cobalt.

Question 6

What is the Angle of Dip?

Answer 6

It is the angle that the Earth’s field lines make with the horizontal at any point on the Earth.

-In Perth the angle of dip is about 68 degrees.

Question 7

When does the strength of the field lines around a circle reduce?

Answer 7

The field circles around the wire, reduces with distance away from the wire.

Question 8

What does a coil (or solenoid) consists of ?

Answer 8

• It consists of lots of loops side by side.

- The fields from each loop add to give an overall flux pattern that is similar to that of a bar magnet.

Question 9

What is a Motor Effect?

Answer 9

It is the motor effect is the term used when a current-carrying wire in the presence of a magnetic field experiences a force.

Question 10

What is the result of a uniform (parallel) field from a magnet combined with the circular field around a current-carrying wire?

Answer 10

This produces a distorted resultant field.

• The resultant field above the wire is now much stronger.
• The resultant field below the wire is weaker ( or 0 in one position).

-This causes a resultant force to act on the wire from the strong to weak field region.

Question 11

What does the size of the magnetic force on a current-carrying wire depend on?

Answer 11

1. The Strength of the magnetic field (B)
2. The size of the current. (I)
3. The length of the wire in the field. (L)
4. The angle of the wire to the field.

Question 12

What is a moving charged particle classified as?

Answer 12

A current.

Question 13

What will the direction the force of a positive charge moving in a magnetic field be?

Answer 13

It will be in the same direction as that on a wire carrying a conventional current.

Question 14

What will the direction the force of a negative charge moving in a magnetic field be?

Answer 14

It will be in the opposite direction as that on a wire carrying a conventional current.

Question 15

What is an Electric Motor?

Answer 15

An electric motor is an electrical machine that converts electrical energy into mechanical energy.

-The reverse of this would be the conversion of mechanical energy into electrical energy and is done by an electric generator.

Question 16

Why does a current-carrying coil rotate when it is placed in a magnetic field?

Answer 16

• This occurs because the current direction on one side of the coil is opposite to the current direction on the other side.
• One force (F) is upwards on the right and one force (F) downwards on the left. This produces a two rotating torques on the coil (a ‘couple’).

Question 17

How to make a motor continue to turn in the same direction?

Answer 17

• A method is needed to reverse the current to the coil when it becomes vertical.
• Nikolai Tesla devised a way of doing this by connecting two split rings to make a commutator.

Question 18

What is the function of a commutator ?

Answer 18

A commutator allows current to flow into the coil through two carbon brushes that make a sliding connection with it.

Question 19

The total flux in a magnetic field equal to=

Answer 19

The total number of lines of force present.

Question 20

What did Faraday’s experiment showed?

Answer 20

Faraday’s experiments showed that when a wire was moved through a magnetic flux, a voltage or Electro Motive Force (emf) was produced between the ends of the wire.

Question 21

What does the size of the induced voltage depend on?

Answer 21

1. The strength of the field.
2. The speed of motion of the wire.
3. The length of the wire in the field.

Question 22

What are the two generalised rules made by Faraday?

Answer 22

1. A voltage is induced in any conductor when the magnetic flux around it changes.
2. The magnitude of the induced voltage depends on the rate at which the flux is changing.

Question 23

What causes a potential difference (Emf) to be set up across the ends of the wire?

Answer 23

The wire is “cutting through” magnetic flux, so the flux change occurring around the wire is what causes a potential difference (Emf) to be set up across the ends of the wire.

Question 24

What is Lenz’s law?

Answer 24

a law stating that the direction of an induced current is always such as to oppose the change in the circuit or the magnetic field that produces it.

Question 25

What does a coil act like?

Answer 25

A coil really acts like lots of small wires connected in series, so the induced voltages in each wire will be added together to produce an overall large voltage.

Question 26

What is an Eddy Current?

Answer 26

Currents can be induced in any metal object that is in a position where there is a flux change occurring near it.

Question 27

What is a Generator?

Answer 27

A generator is a device that converts mechanical energy to electrical energy for use in an external circuit.

• Alternating or direct current can be generated by rotating a coil in a magnetic field.

Question 28

How does a Generator work ?

Answer 28

The generator gives alternating current out to some external device connected to the slip-rings via the brushes, so current will go into ring 1 and out of ring 2.

However, when side AB of the coil is going down of the right the current will reverse to flow from B to A and out of ring 1.

Question 29

How to produce a Direct Current?

Answer 29

To produce direct current split rings are attached to the coil so that, as the current begins to reverse, the connection is switched to the other brush. In this way one brush is always at a positive potential.

Question 30

What happens when a magnet is pushed towards a coil?

Answer 30

There will be a flux change occurring in the coil and so a voltage will be induced in the coil.

-If the coil connects to a complete circuit then a current will flow, otherwise a voltage will be present but no current.

Question 31

Why does repulsion have to occur during the induction in coils?

Answer 31

By repelling, work is done by the magnet which equals the energy produced in the coil.

Question 32

What happens when the magnet is withdrawn from the coil?

Answer 32

The flux in the coil will decrease.

Question 33

What is a Transformer?

Answer 33

In a transformer a current change in one coil can induce a voltage in a second coil close to it.

Question 34

What happens during an Alternating Current?

Answer 34

• With alternating current, the voltage is constantly switching from positive to negative 50 times per second.
• This means that if an AC voltage is connected to the Primary coil of the transformer then the flux linked to the Secondary will constantly be changing and so an AC output from coil 2 will be produced which is out of phase with the input at coil 1.

Question 35

What is a Step-up transformer?

Answer 35

It is when a the number of coils on the output side (secondary) is greater than those on the input side (primary) and the voltage is stepped up.

Question 36

What is the difference between Faraday’s first transformer and the modern transformers?

Answer 36

Faraday’s - The primary and secondary were wound onto a circular (or rectangular) iron core so the flux linkage between the two coils was 100%.

Modern - Rectangular, laminated iron-alloy core with low magnetic energy loss. The laminations reduce energy lose through eddy currents as these cannot flow from one lamination to another.

Question 37

What are other forms of energy loss in transformers?

Answer 37

• Hysteresis loss: the energy needed to magnetise and then demagnetise the core.
• Joule Heating: heat energy produced when a current runs through a wire.

Question 38

What does any energy loss within a transformer mean?

Answer 38

It means that the power output would be smaller than the power input.

Question 39

What are ways to minimise these energy losses in efficient transformers?

Answer 39

• A rectangular, laminated core design with efficient flux linkage from Primary to Secondary.
• Cores made from special alloys with low remanence (low retention of magnetic field) and low coercivity (a small amount of energy is required to magnetise the core).
• Thick, low resistance coil winding wires (usually copper).

Question 40

How are energy lost through cables ?

Answer 40

In the form of Joule heating.

Question 41

How to minimise energy loss in AC Power Transmission (cables that run for hundreds of kilometres)?

Answer 41

Power is transmitted at a high voltage and low current.