Chapter 22,23 - electric and magnetic fields

Question 1

define magnetic flux density

Answer 1

Magnetic flux density (B) is defined as the force acting per unit current per unit length on a wire placed at right angles to the magnetic field.

Question 2

define one tesla

Answer 2

the magnetic flux density when a wire, carrying a current of 1A is placed perpendicular to the magnetic field, experiences a force of 1N per metre.

Question 3

what does linking the equation of circular motion to force acting on a charged particle show?

Answer 3

• faster moving particles travel in bigger circles as v is proportional to r
• bigger particles have bigger radius’s
• smaller magnetic fields relate to smaller circles
• particles with greater charge move in smaller circles

Question 4

What is the idea behind a velocity selector

Answer 4

• two parallel horizontal plates connected to a power supply
• they produce a uniform electric field between the plates
• a uniform magnetic field is produced perpendicular to the plates
• The charged particles travelling at different speeds to be sorted will enter a narrow slit
• the electric and magnetic fields deflect them by producing an optimum velocity that the particles need to be at to not be deflected by the fields, if this velocity is exhibited, the force from the fields will cancel out and the particle will travel in a straight line.

Question 5

Define magnetic flux

Answer 5

product of the magnetic flux density passing perpendicular to the cross sectional area
so φ=BAcosθ

Question 6

si unit for magnetic flux

Answer 6

Weber (Wb)

Question 7

define magnetic flux linkage

Answer 7

product of the magnetic flux and the number of turns in the coil

Question 8

when is an e.m.f. induced

Answer 8

whenever there is a change in the magnetic flux linking the circuit, so by changing B, A or θ

Question 9

Faradays law and what lenz law does to it

Answer 9

magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage.
Lenz law adds a - sign as a constant of proportionality

Question 10

What is Lenz’s law

Answer 10

the direction of the induced e.m.f or current is opposes the change producing it

Question 11

why does Lenz’s law add a minus sign

Answer 11

reminder that energy cannot be created from nothing

Question 12

step up transformer vs step down

Answer 12

step up has more turns on the secondary coil whereas step down has more on the primary

Question 13

how to increase efficiency of a transformer

Answer 13

• insulating core reduced eddy currents into core
• low resistance wires reduces heating effect
• core made of soft iron so can be magnetized and demagnetized easily, allows little loss of magnetic forces

Question 14

approximate radius for a nucleus

Answer 14

10^-14

Question 15

Describe the observations of Rutherford’s alpha particle scattering expt.

Answer 15

• most when all the way through showing the atom is mostly empty space
• Some were deflected at angles showing most of the mass is collected in a small dense nucleus
• very little were deflected straight back, showing the nucleus is positively charged

Question 16

What is the charge on a nucleus

Answer 16

Ze,

where Z is the atomic number, bottom number e.g. proton number