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10.1.4Magnetism and the Motor Effect

The Earth’s Magnetic Field

5 min read 6 parts Lesson 5 of 11 1 quiz

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Section 1 of 6The Earth as a magnet

Shape of the field
  • Shape of the field The Earth has a magnetic field like that of a giant bar magnet, with a field that extends far out into space.
  • Outside the planet the field lines come out near the geographic south pole and go back in near the geographic north pole.
Origin
  • Origin The field is produced by the movement of molten iron and nickel in the liquid outer core, not by a permanent magnet in the Earth.
Magnetic poles
  • Magnetic poles The magnetic poles are near, but not exactly at, the geographic poles.
    • They also move slowly over time.

Section 2 of 6How a compass works

1A small magnet
  • A small magnet A compass contains a small bar magnet, the needle, which is free to turn.
2Field acts on it
  • Field acts on it The Earth's magnetic field exerts forces on the poles of the needle.
3Aligns
  • Aligns The needle turns until it lines up with the field.
4Points north
  • Points north The end that points to geographic north is the north-seeking pole, attracted to a magnetic south pole in the northern hemisphere.

Section 3 of 6Magnetic poles and fields

RuleDetail
Like polesRepel each other
Unlike polesAttract each other
Field line directionFrom the north pole to the south pole of a magnet
Field strengthStrongest where the field lines are closest together
Compass near a magnetPoints along the field line, away from a north pole and towards a south pole

Section 4 of 6Investigating a field

1
Place a bar magnet in the middle of a sheet of paper.
2
Put a plotting compass near one pole and mark the positions of both ends of the needle with dots.
3
Move the compass so that its first end sits at the previous second dot, and mark again.
4
Keep going until you reach the other pole, then join the dots to form a smooth field line.
5
Repeat from other starting points to see the whole shape. Far from the magnet, the compass swings to point north because of the Earth's field.

Section 5 of 6Key terms

TermMeaning
Magnetic fieldThe region around a magnet where another magnet or magnetic material feels a force.
CompassA small magnet on a pivot that shows the direction of a magnetic field.
Field lineA line showing the direction of the force on a north pole placed in the field.
Outer coreThe liquid layer of molten iron and nickel in which the Earth's field is generated.
Magnetic poleA place on a magnet where the magnetic force is strongest.

Section 6 of 6Exam technique

1
Saying the Earth's north pole is a magnetic north pole. The geographic north pole is near a magnetic south pole of the Earth's field.
2
Thinking that the field comes from a solid magnet at the centre. It comes from moving liquid metal.
3
Drawing field lines which cross each other or have no arrows.
4
Saying that a compass points to a place, rather than lining up with the field.
Transformer Golden Equations Physics Standard

Transformer Turns Ratio & Efficiency

Ratio: Vp / Vs = Np / Ns = Is / Ip

  • Step-up reduces grid thermal loss (P = I2R).
  • Laminated core prevents eddy currents.