Flashcards in electromagnetism bruh Deck (41):
What is a magnetic field (flux)?
It is a region where a force will be felt on a magnet.
What methods can be used to visualise the field lines around a single bar magnet?
- Sprinkling iron filings near it.
- Plotting the direction in which a small compass points when placed around the magnet.
What is the difference between flux and flux density?
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]
What are Ferromagnetic materials?
-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.
List the Ferromagnetic metals.
Iron, steel, nickel and cobalt.
What is the Angle of Dip?
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.
When does the strength of the field lines around a circle reduce?
The field circles around the wire, reduces with distance away from the wire.
What does a coil (or solenoid) consists of ?
-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.
What is a Motor Effect?
It is the motor effect is the term used when a current-carrying wire in the presence of a magnetic field experiences a force.
What is the result of a uniform (parallel) field from a magnet combined with the circular field around a current-carrying wire?
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.
What does the size of the magnetic force on a current-carrying wire depend on?
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.
What is a moving charged particle classified as?
What will the direction the force of a positive charge moving in a magnetic field be?
It will be in the same direction as that on a wire carrying a conventional current.
What will the direction the force of a negative charge moving in a magnetic field be?
It will be in the opposite direction as that on a wire carrying a conventional current.
What is an Electric Motor?
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.
Why does a current-carrying coil rotate when it is placed in a magnetic field?
-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').
How to make a motor continue to turn in the same direction?
-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.
What is the function of a commutator ?
A commutator allows current to flow into the coil through two carbon brushes that make a sliding connection with it.
The total flux in a magnetic field equal to=
The total number of lines of force present.
What did Faraday's experiment showed?
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.
What does the size of the induced voltage depend on?
1. The strength of the field.
2. The speed of motion of the wire.
3. The length of the wire in the field.
What are the two generalised rules made by Faraday?
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.
What causes a potential difference (Emf) to be set up across the ends of the wire?
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.
What is Lenz's law?
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.
What does a coil act like?
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.
What is an Eddy Current?
Currents can be induced in any metal object that is in a position where there is a flux change occurring near it.
What is a Generator?
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.
How does a Generator work ?
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.
How to produce a Direct Current?
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.
What happens when a magnet is pushed towards a coil?
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.
Why does repulsion have to occur during the induction in coils?
By repelling, work is done by the magnet which equals the energy produced in the coil.
What happens when the magnet is withdrawn from the coil?
The flux in the coil will decrease.
What is a Transformer?
In a transformer a current change in one coil can induce a voltage in a second coil close to it.
What happens during an Alternating Current?
-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.
What is a Step-up transformer?
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.
What is the difference between Faraday's first transformer and the modern transformers?
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.
What are other forms of energy loss in transformers?
- Hysteresis loss: the energy needed to magnetise and then demagnetise the core.
- Joule Heating: heat energy produced when a current runs through a wire.
What does any energy loss within a transformer mean?
It means that the power output would be smaller than the power input.
What are ways to minimise these energy losses in efficient transformers?
- 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).
How are energy lost through cables ?
In the form of Joule heating.