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BMAT > Phy- Magnetism > Flashcards

Phy- Magnetism Flashcards

1
Q

Examples of magnetic materials

A
  • nickel
  • iron
  • cobalt
  • steel
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2
Q

How do you test for a permanent magnet?

A

repels another perm magnet

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3
Q

Which pole does the needle in a magnetic compass point to?

A
  • lines up w/ magnetic field of magnet
  • point away from N, towards S
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4
Q

What is a magnetic field?

A

region of space in which magnetic forces act on magnets/ magnetic materials

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5
Q

How do you use iron filings to see the mag field pattern?

A
  • place white card on magnet
  • sprinkle iron filings evenly across
  • tap card
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6
Q

Egs. + uses of soft & hard magnetic materials

A

soft- iron, used in electromagnets
hard- steel, perm magnets

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7
Q

What is an electromagnet?

A
  • soft iron core
  • coil of many turns
  • current creates field –> magnetises core
  • field of electromagnet = field from current + field from mag. core
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8
Q

How can permanent magnets potentially be demagnetised?

A

heating/ hitting

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9
Q

How are perm magnets made from hard mag materials?

A
  • placed in strong field eg. solenoid
  • induced poles at ends
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10
Q

2 factors affecting field created by current

A
  1. direction of current –> direction of field
  2. size of current –> strength of field
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11
Q

How is a mag field created around a wire?

A
  • beam of charged particles (e-/ ions) moving thru vacuum
  • moving electric charges in current –> field
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12
Q

3 factors affecting strength of field around wire

A
  1. current
  2. distance from wire
  3. medium around wire (↑ B w/ magnetic media eg. iron)
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13
Q

What is a ferromagnetic material?

A

each atom acts like a bar magnet w/ N & S ends
- eg. iron

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14
Q

3 ways of increasing strength of field in a solenoid

A
  1. ↑ turns in same length
  2. soft iron core
  3. ↑ current in coil
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15
Q

How and why does the current in an electromagnet needs to be limited?

A

why-
strong field –> strong heating effect (could melt insulation around wire)
how-
superconducting coils (w/ 0 R)
(has to be cooled to ex. low temp w/ liquid helium)

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16
Q

Uses of electromagnets.

A
  1. loud speaker
  2. lifting heavy objects (eg. cars)
17
Q

How does a loudspeaker work?

A
  • current thru voice coil –> motor effect pushes it back & forth
  • cone creates compressions & rarefractions in air
  • sound
18
Q

How is an electromagnet used in lifting heavy objects?

A
  • coils w/ many turns of insulated copper wire
  • around a soft iron core
  • high current
  • ➕ switched on & off –> lift & release
19
Q

What is the motor effect?

A
  • current carrying wire passes thru m field
  • direction crosses field lines
  • force on wire (max when perpendicular)
20
Q

Factors affecting F on wire in field

A
  1. magnitude of current
  2. strength of field
  3. length of wire
  4. angle betw. field & current
21
Q

Formula for motor effect

A

F = B I L
- B = field strength (Tesla (Nm-1A-1))
- L = length of wire (m)

22
Q

What does a split-ring commutator do in a dc motor?

A
  • rotates w/ coil
  • connect to dc power supply
  • reverses direction every half turn
  • changes polarity every half turn
23
Q

What are the brushes in a dc motor made of?

A

graphite
- ↓ friction
- good electrical contact

24
Q

What is the split-ring commutator made of?

A

copper/ brass

25
Q

Factors increasing turning effect in coil of dc motor

A
  1. ↑ current
  2. ↑ area of coil
  3. ↑ field
  4. ↑ no. of turns
  5. add soft iron core
  6. coil in plane of field (perpendicular)
26
Q

How is voltage induced in a conductor?

A

movement
1. conductor cuts lines of field
2. field passing thru conductor changes

27
Q

When is a current induced in a conductor

A

complete circuit

28
Q

Factors increasing induced voltage

A
  1. magnet moved faster (↑ rate of cutting lines)
  2. stronger magnet (↑ density of lines)
  3. ↑ freq. of ac
  4. ↑ ac amplitude
  5. ↑ no. of turns in coil
  6. ↑ area of turns
29
Q

What does the freq. of output ac voltage suggest about the coil’s rotation f?

A

=

30
Q

What do transformers use electromagnetic induction for?

A

changing voltage of ac supply

31
Q

How does a transformer work?

A
  • current thru primary coil –> creates field in soft iron core
  • electromagnetic induction generates v. in 2° coil
  • when field changes
  • ONLY INDUCED V W/ AC! NEEDS TO BE Δ
32
Q

Equation for power in an ideal transformer

A

Vp Ip = Vs Is

33
Q

Why aren’t real transformers 100% efficient?

A
  1. R in wires on coils
  2. heating effect in core as mag. & demagnetise
  3. current induced in core ∵ Δ field
34
Q

Formula for power wasted in transmission lines

A

P = I²R

35
Q

Typical voltage output for power station generators

A

～11kV / 33kV

36
Q

Typical voltage output for transmission

A

275kV / 400kV

37
Q

Typical voltage output for consumers

A
  • domestic = 230V
  • railways = 25kV

BMAT (1 decks)

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