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BMAT Physics > magnetism > Flashcards

magnetism Flashcards

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1
Q

permanent magnet

A

-have two poles, a north pole (N) and south pole (S

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2
Q

how do you demonstrate that something is a permanent magnet?

A

-demonstrate that it can repel another permanent magnet (attraction would only show that it is a magnetic material, not whether it is permanently magnetised

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3
Q

north and south seeking

A
  • ‘north’ pole on a bar magnet is attracted to the north geographic pole of the Earth
  • ‘south’ pole on a bar magnet is attracted toward the geographic south pole of the Earth
  • If a bar magnet is pivoted at its centre so that it is free to rotate, it will align itself in a north-south direction
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4
Q

magnetic compasses

A
  • consists of a bar magnet pivoted about its centre inside a case
  • can be used for navigation, to identify the poles of a magnet and to trace the pattern of magnetic field
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5
Q

magnetic field

A
  • a magnetic field is a region of space in which magnetic forces act on magnets or magnetic material
  • always starts on north poles and ends on south poles
  • the closer the lines are, the stronger the magnetic forces
  • only moving particles generate a magnetic field
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6
Q

soft and hard magnetic materials

A
  • soft magnetic materials – easy to magnetise but also easily lose their magnetisation
  • e.g. iron
  • hard magnetic materials – difficult to magnetise but once they are magnetised, they are difficult to demagnetise
  • e.g. steel
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7
Q

demagnetisation

A
  • heating the magnet

- hitting the magnet against something

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8
Q

induced magnetism

A

-when a pole of a bar magnet is held close to an unmagnetised magnetic material (e.g. iron, steel), there is a force of attraction between the magnet and the material
(never a force of repulsion)
-if a north pole approaches the iron then the induced pole is a south pole, and vice versa
-used to make permanent magnets

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9
Q

electric currents

A
  • electric currents create magnetic fields in the surrounding space
  • if an a.c. current is used, the magnetic field also continually changes its direction
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10
Q

factors affecting the magnetic field created by an electric current

A
  • reversing the direction of the current reverses the direction of the magnetic field
  • increasing the current increases the strength of the magnetic field
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11
Q

magnetic field patterns around current-carrying wires

A
  • consists of concentric circles, that become farther apart at greater distance from the wire
  • use the right-hand grip rule to work out direction (point thumb in the direction of the current and the rest of your fingers curl around showing the direction of magnetic field lines)
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12
Q

coils/solenoids

A
  • one end of the solenoid (where field lines emerge) is the north pole and the other end (where the field lines enter the solenoid) is the south pole
  • the field at the sides of the solenoid is weak and in the opposite direction to the field inside the solenoid
  • the field inside the solenoid is constant in strength and direction in the centre and through most of the coil, decreasing at the ends
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13
Q

identifying poles

A
  • note whether the current circulates clockwise or anticlockwise when you look at the end of the coil
  • clockwise = south pole
  • anti-clockwise = north pole.
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14
Q

factors affecting magnetic field strength around a wire

A
  • increasing current increases magnetic field strength
  • farther from the wire the field is weaker
  • adding a core material e.g. soft iron can increase the field strength
  • increasing number of wire loops increases magnetic field strength
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15
Q

electromagnet (EM) vs permanent magnet (PM)

A
  • EM can be switched on/off, PM are continuous
  • EM strength can be varied by changing the current in the coil, PM strength is constant
  • EM polarity can be reversed by reversing current direction, PM polarity is constant
  • EM core is made from soft magnetic material, PM made from hard magnetic material
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16
Q

the motor effect

A
  • when a current carrying wire passes through a magnetic field such that its direction crosses the field lines, there is a force on the wire
  • reversing the direction of the current/magnetic field reverses the direction of the motor effect force
  • reversing the directions of both current and magnetic field results in no change in direction of the motor effect force
  • the force can be calculated using W=mg
17
Q

Fleming’s LH rule

A

F (thumb): direction of motor effect force
B (index finger): direction of magnetic field (N -> S)
I (middle finger): direction of current (+ -> -)

18
Q

factors affecting the magnitude of the force on a wire in a magnetic field

A
  • the greater the current the greater the force
  • the greater the magnetic field strength the greater the force
  • the greater the length of the wire the greater the force
  • force is greatest when the current and magnetic field are at 90° and are zero at 0°
19
Q

formula for motor effect force

A

𝐹=𝐵𝐼𝐿

  • F = the motor effect force in newtons (N).
  • B = the magnetic field strength in tesla (T).
  • I = the current in the wire in amps (A).
  • L = the length of wire at 90° to the magnetic field in metres (m)
20
Q

electromagnetic induction

A
  • when a voltage is induced in a conductor because:
    1. it cuts across the lines of a magnetic field
    2. the magnetic field passing through it changes
  • always results in an induced voltage but will only produce a current if there is a closed circuit
21
Q

the magnitude of an induced voltage is increased by:

A
  • increasing the rate at which a wire cuts magnetic field lines
    • increasing the rate at which the magnetic field changes
    • using a stronger magnet
    • the ac frequency is increased
    • the ac amplitude is increased
    • increasing number of turns in coil
22
Q

transformers

A
  • converts ac electricity to a higher/lower voltage
  • a step-up transformer increases the voltage, decreases the current, no. of turns on secondary coil > no. of turns on primary coil
  • a step-down transformer decreases the voltage, increases the current, no. of turns on primary coil > no. of turns on secondary coil
23
Q

transformer formula

A

𝑽𝒑/𝑽𝒔= 𝒏𝒑/𝒏𝒔 )
V=voltage
n=number of turns
p=primary (input coil) , s=secondary (output coil)

VpIp = VsIs (I=current)

24
Q

why do transformers not have 100% efficiency?

A

some input power is transferred to heat as a result of:

  • the resistance in the wires on the coils
  • heating effects in the core as it magnetises and demagnetises
  • currents induced in the core by the changing magnetic field
25
Q

generator

A

causes a coil to rotate in a magnetic field, generating an electric current by induction

26
Q

electric field

A
  • a region where charges experience a force
  • the direction of the arrow shows the way a +ve charge will be pushed
  • the closer together the arrows are, the stronger the field and the greater the force experienced by charges in that field

BMAT Physics (7 decks)

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