P2 - Magnetic fields

Question 1

What is a magnetic field?
How can these be represented?
How do flux lines work?

Answer 1

A region where a force is exerted on a magnetic or magnetically susceptible material.
Can be represented by field lines (flux lines)
The lines go from north to south and the closer together they are the stronger they are.

Question 2

What happens when a current flows through a wire?
How can we work out the direction of the magnetic field, current of a wire?
What are the two components of the right hand fist rule?

Answer 2

When current flows through a wire a magnetic field is induced around the wire.
Use of Flemings right hand rule
Thumb points towards the current
Fingers wrap around and show the direction of the field lines.

Question 3

what is a solenoid? what do the lines look like?

Answer 3

A solenoid is a coil of wire around one plane. straight lines through middle from north to south, then loop around outside and back through.

Question 4

What happens when you put a force on a current carrying wire ( a current carrying wire between a external magnetic field)?
What does the size of this force depend on?
What does the direction of the force depend on?

Answer 4

The field around the wire and the field around the magnets are added together causing a resultant field.

The size of the force depends on the component of the magnetic field that is perpendicular to the current.

The direction is always dependant on both the direction of the current and the direction of the magnetic field

Question 5

What happens if the current is parallel to the field lines

Answer 5

The size of the force is 0 as there is no component perpendicular to the magnetic field

Question 6

How do you find the direction of the current or external magnetic field or the direction of the force on the wire?
What is the acronym for flemings left hand rule?

Answer 6

Use of fleming’s left hand rule
First Finger For Field
seCond finger for Current
thuMb for Motion

Question 7

What direction is the current going if the diagram shows a circle with a solid dot?
What about a circle with cross?

Answer 7

Circle shows the current flowing out the page at you
Cross shows the current flowing into the page
Like an arrow

Question 8

What happens to a current conducting wire through a magnetic field when the current is alternating?

Answer 8

vibrates as the direction of motion is constantly being flipped from up to down.

Question 9

What is magnetic flux density? what is it measured in?
In equations what is magnetic flux density shown as?
what is L?

Answer 9

The force on one metre of wire carrying a current of one amp at right angles to the magnetic field.
T, teslas
B = magnetic flux density
L = length of wire

Question 10

What happens when a charged particle moves through a magnetic field?

Answer 10

A force act so the charge particle as long as it is moving.

It is always perpendicular to the current so it turns into circular motion

Question 11

How does a particle accelerator work? (cyclotrons)

Answer 11

in a uniform magnetic field, two D,s with different pd so one side the current is negative and the other is positive, allow for the charged particles to move in circler motion around gaining speed.

Question 12

what is magnetic flux?

Answer 12

the density of flux times the area is the total magnetic flux.
weird looking symbol o with line through it

Question 13

what is electromagnetic induction?

Answer 13

If there is relative motion between a conducting rod and a magnetic field the electrons in the rod will experience a force which causes them to accumulate at one end of the rod creating an e.m.f

Question 14

when is an e.m.f induced?

Answer 14

when the conductor cuts the magnetic flux

Question 15

what is flux linkage?
what does N stand for?
What does A stand for?

Answer 15

Flux linkage is the product of the magnetic flux and the number of turns on the coil cutting the flux
N = number of turns on the coil cutting the flux lines
Area of the coil

Question 16

what do you do if the flux linkage is at an angle to the lines?

Answer 16

Use trig, to find the vector component

Question 17

What is faradays law?

Answer 17

Induced e.m.f is directly proportional to the rate of change of flux linkage.

Question 18

What is lenz’s law?

Answer 18

The induced e.m.f is always in such a direction as to oppose the change that caused it.

Question 19

What is an alternating current?

Answer 19

An alternating current is one that changes direction with time. This means the voltage across a resistance goes up and down in a regular pattern, from positive to negative.

Question 20

What is an oscilloscope?

What does sinusoidal mean?

Answer 20

oscilloscope are just like volt meters as they display the voltage of an alternating current

sinusoidal is when the curve is like a sine curve, a constant wave

Question 21

What is the 230v value for mains electricity in the uk?

Answer 21

the rms

Question 22

What’s a transformer?

Answer 22

Transformers are devices that make sue of electromagnetic induction to change the size of the voltage for an alternating current

Question 23

What is a magnetically soft material?

Answer 23

A magnetically soft material is one whose magnetisation disappears after the current is removed

Question 24

Describe how transformers work

Answer 24

An alternating current flowing in the primary coil causes the core to magnetise, demagnetise, and remagnetise continuously in opposite directions. This produces a rapidly changing magnetic flux across the core. The rapidly changing magnetic flux in the iron core passes through the secondary coil, where it induces an alternating voltage of the same frequency but different voltage assuming the number of turns is different

Question 25

What’s the difference between a step up and a step down transformer?

Answer 25

a step up transformer increases the voltage by having more turns on the secondary coil than primary
A step down transformer reduces voltage by having fewer turns on the secondary coil

Question 26

How is efficiency lost in transformers?

how is it minimalised?

Answer 26

eddy currents, causing the core to heat up and energy be lost
heat is generated by resistance in the coils
energy is needed to magnetise and demagnetise the core.
use of low resistivity wires,
laminating the core with thin layers of insulator

Question 27

What are eddy currents?

Answer 27

eddy currents are looping currents induced by the changing magnetic flux in the core, they create a magnetic field that acts against the field that induced them reducing field strength

Question 28

How is electricity sent around the national grid? why?

Answer 28

minimal possible current to minimalize energy lost as heat