Plotting magnetic fields

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課程筆記

Magnetic Fields and Compasses

Iron filings show the shape of a magnetic field but not its direction.
A plotting compass is a small bar magnet that is free to rotate.
The north pole of the compass needle points towards magnetic south.
To plot a field: place a bar magnet on paper, move a plotting compass around, mark the needle direction at each position, then join the arrows to show field lines.
Compasses are also used for navigation.

Earth's Magnetic Field

Earth's outer core is made of molten iron and nickel, producing a magnetic field.
The field shape is similar to that of a bar magnet.
The geographic North Pole is near (but not exactly at) Earth's magnetic south pole.
A compass needle points towards magnetic south, which is near the geographic North Pole.

Key Facts and Questions

True or false: a compass points towards Earth's magnetic north. False (it points to magnetic south).
True or false: a compass can be used for navigation. True.
True or false: the geographic North Pole is near Earth's magnetic south pole. True.
A compass is made of a small bar magnet (magnetised material).
The Earth's outer core is made of iron and nickel; the third magnetic metal is cobalt.
A compass shows direction of the field, unlike iron filings which only show shape.
Electronic devices produce magnetic fields that can deflect the compass needle, giving false readings.
The magnetic field is strongest at the poles (field lines are closest together there).
Without Earth's magnetic field, compasses would not work for navigation because there would be no consistent direction to point.

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練習題

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1.What does a plotting compass consist of?
Easy
- AA small bar magnet free to rotate
- BA needle made of plastic
- CA piece of iron filing
- DA liquid crystal display
2.Which pole of a plotting compass points towards magnetic south?
Easy
- ANorth pole
- BSouth pole
- CBoth poles
- DNeither pole
3.What is the Earth's outer core made of?
Easy
- AMolten iron and nickel
- BSolid rock
- CLiquid water
- DMolten aluminium
4.Which of the following is NOT a magnetic metal?
Easy
- ACopper
- BIron
- CNickel
- DCobalt
5.What is an advantage of using a plotting compass over iron filings to investigate a magnetic field?
Medium
- AIt shows the direction of the field
- BIt is cheaper
- CIt shows the shape of the field more clearly
- DIt works without a magnet
6.Why should a compass not be used near an electronic device?
Medium
- AThe device's magnetic field will affect the compass reading
- BThe compass will damage the device
- CThe compass will run out of battery
- DThe device's electric field will repel the compass
7.Where is Earth's magnetic field strongest?
Medium
- AAt the poles
- BAt the equator
- CIn the middle of the oceans
- DIn the atmosphere
8.If Earth had no magnetic field, could compasses be used for navigation?
Hard
- ANo, because compasses rely on Earth's magnetic field
- BYes, because compasses use the stars
- CYes, because compasses use the Sun
- DNo, because compasses would point to geographic north

更多題目

9.What shape is Earth's magnetic field similar to?
Easy
- AThat of a bar magnet
- BThat of a horseshoe magnet
- CThat of a ring magnet
- DThat of a solenoid
10.The geographic North pole of Earth is near which magnetic pole?
Medium
- AMagnetic south pole
- BMagnetic north pole
- CEquator
- DNeither
11.What happens to a plotting compass when placed near a bar magnet?
Medium
- AIts north pole points towards the magnet's south pole
- BIts north pole points towards the magnet's north pole
- CIt spins continuously
- DIt stops working
12.Why can't iron filings show the direction of a magnetic field?
Hard
- AThey are not magnetic
- BThey align along field lines but have no polarity to indicate direction
- CThey are too small
- DThey repel each other
13.A student places a plotting compass near the north pole of a bar magnet. Which direction will the north pole of the compass needle point?
Medium
- ATowards the bar magnet's north pole
- BTowards the bar magnet's south pole
- CAway from the bar magnet
- DPerpendicular to the bar magnet
14.The diagram shows the magnetic field lines around a bar magnet. At which point is the magnetic field strongest?
Medium
- APoint A, near the north pole
- BPoint B, near the south pole
- CPoint C, midway along the magnet
- DPoint D, far from the magnet
15.A student uses a plotting compass to map the magnetic field around a bar magnet. She places the compass at several positions and marks the direction of the needle. Which of the following best describes the pattern of the arrows she draws?
Hard
- AArrows point from the north pole to the south pole outside the magnet
- BArrows point from the south pole to the north pole outside the magnet
- CArrows point randomly around the magnet
- DArrows point away from both poles
16.Which of the following materials is magnetic?
Easy
- ACopper
- BAluminium
- CIron
- DPlastic
17.The Earth's magnetic field is similar in shape to the field of a:
Medium
- AHorseshoe magnet
- BBar magnet
- CElectromagnet
- DCircular magnet
18.A student places a plotting compass near a wire carrying a direct current. The compass needle deflects. What does this demonstrate?
Hard
- AThe wire is made of iron
- BAn electric current produces a magnetic field
- CThe compass is faulty
- DThe wire is magnetised
19.A plotting compass is placed at point X near a bar magnet. The compass needle points towards the south pole of the magnet. What does this indicate about the magnetic field direction at X?
Medium
- AThe field points towards the south pole
- BThe field points away from the south pole
- CThe field is zero at X
- DThe field points towards the north pole
20.A student sets up an experiment to compare the strength of the magnetic field at different distances from a bar magnet. She uses a plotting compass and measures the angle of deflection from north. Which of the following should she keep constant?
Hard
- AThe distance from the magnet
- BThe orientation of the magnet
- CThe type of compass
- DThe size of the paper
21.The Earth's outer core is made of molten iron and nickel. Why does this produce a magnetic field?
Medium
- ABecause iron and nickel are magnetic materials
- BBecause the core is hot
- CBecause the core is solid
- DBecause the core rotates
22.In an experiment, a student places a bar magnet under a sheet of paper and sprinkles iron filings on top. The filings form a pattern. Which of the following statements about this method is correct?
Hard
- AThe pattern shows the direction of the magnetic field
- BThe pattern shows the shape but not the direction of the magnetic field
- CThe pattern shows both shape and direction
- DThe pattern shows the strength but not the shape
23.Which of the following is the correct method to map the magnetic field around a bar magnet using a plotting compass?
Easy
- APlace the compass at one point, mark the direction of the needle, then move the compass to the next point and repeat.
- BPlace iron filings on paper and then place the compass on top of the filings.
- CHold the compass stationary and move the magnet around it.
- DPlace the compass at the north pole of the magnet and then move it in a straight line away from the magnet.
24.A student uses a plotting compass to map the magnetic field around a bar magnet. She places the compass at position A and the north pole of the compass needle points toward the south pole of the magnet. She then moves the compass to position B and the north pole of the needle points toward the north pole of the magnet. What can be concluded about positions A and B?
Medium
- APosition A is closer to the magnet's south pole than position B.
- BPosition B is closer to the magnet's south pole than position A.
- CThe compass needle is defective at position B.
- DThe magnet has two north poles.
25.A student places a bar magnet under a sheet of paper and sprinkles iron filings on top. The filings form a pattern showing the magnetic field lines. She then places a plotting compass at a point where the filings are dense. The compass needle aligns at an angle to the visible field line. What is the most likely explanation?
Hard
- AThe compass shows the direction of the field, which may differ from the line of filings if the filings are not perfectly aligned.
- BThe compass is affected by the Earth's magnetic field, which distorts the local field.
- CThe iron filings have become magnetised and are creating their own field.
- DThe compass needle is pointing to the geographic North Pole.
26.A plotting compass is placed near a wire carrying a large direct current. The compass needle deflects. What does this demonstrate?
Medium
- AA current-carrying wire produces a magnetic field.
- BThe wire is made of a magnetic material.
- CThe compass is faulty.
- DThe Earth's magnetic field has been cancelled.
27.A student sets up a circuit with a battery, a switch, and a long straight wire. She places a plotting compass near the wire. When the switch is closed, the compass needle deflects. She reverses the connections to the battery so the current flows in the opposite direction. What will happen to the compass needle?
Medium
- AIt will deflect in the opposite direction.
- BIt will deflect in the same direction.
- CIt will not deflect.
- DIt will point to magnetic north.
28.A student wants to compare the strength of the magnetic field at different distances from a bar magnet. She uses a plotting compass and measures the angle of deflection from north. Which of the following procedures will give the most reliable results?
Hard
- APlace the compass at various distances along a line from the magnet, and measure the angle of the needle each time.
- BPlace the compass at one distance, measure the angle, then move the compass to another distance and measure again, repeating several times.
- CUse the same compass and repeat the measurement at each distance three times and calculate the average angle.
- DUse different compasses at each distance to avoid systematic error.
29.A student places a plotting compass near the north pole of a bar magnet. The north pole of the compass needle points away from the magnet. She then moves the compass to the south pole of the magnet. Which direction will the north pole of the compass needle point?
Hard
- AAway from the magnet
- BToward the magnet
- CToward geographic north
- DIt will not point in a consistent direction
30.A student places a bar magnet on a table and uses a plotting compass to map its magnetic field. She notices that the compass needle does not point exactly along the expected field line direction. Which of the following is the most likely reason?
Hard
- AThe Earth's magnetic field is also affecting the compass.
- BThe compass needle is not magnetised.
- CThe bar magnet is not strong enough.
- DThe paper is too thick.
31.What is a plotting compass made of?
Easy
- AA small bar magnet that is free to rotate
- BA piece of iron that is fixed
- CA coil of wire with a current
- DA plastic needle that points north
32.Which of the following statements is true?
Easy
- AA compass points towards Earth's magnetic north.
- BA compass points towards Earth's magnetic south.
- CA compass points towards Earth's geographic north.
- DA compass points towards Earth's geographic south.
33.A student uses a plotting compass to map the magnetic field around a bar magnet. She places the compass at several positions and marks the direction of the needle. Which of the following best describes what the final pattern of arrows shows?
Medium
- AThe direction of the magnetic field at each point
- BThe strength of the magnetic field at each point
- CThe shape of the magnet
- DThe temperature of the magnet
34.A student places a bar magnet under a sheet of paper and sprinkles iron filings on top. The filings form a pattern. She then places a plotting compass at point X and the compass needle points towards the south pole of the magnet. Which of the following is a correct conclusion?
Hard
- AThe magnetic field at X points towards the south pole
- BThe magnetic field at X points away from the south pole
- CThe iron filings show the direction of the field at X
- DThe compass needle points towards geographic north
35.A student sets up a circuit with a battery, a switch, and a long straight wire. She places a plotting compass near the wire. When the switch is closed, the compass needle deflects. What does this demonstrate?
Medium
- AA current-carrying wire produces a magnetic field
- BA magnetic field produces an electric current
- CThe compass is faulty
- DThe wire becomes magnetic
36.A student wants to compare the strength of the magnetic field at different distances from a bar magnet. She uses a plotting compass and measures the angle of deflection of the needle from its usual north-south alignment. Which of the following is a valid conclusion?
Medium
- AThe greater the deflection, the stronger the magnetic field
- BThe smaller the deflection, the stronger the magnetic field
- CDeflection is unrelated to field strength
- DThe compass cannot measure field strength
37.A student places a plotting compass near the north pole of a bar magnet. The north pole of the compass needle points away from the magnet. She then moves the compass to a point where the needle points towards the magnet. Which of the following could be the new position?
Hard
- ANear the south pole of the magnet
- BNear the north pole of the magnet
- CFar away from the magnet along the axis
- DAt the midpoint of the magnet
38.A compass needle points towards Earth's magnetic south pole. Which geographic pole is near Earth's magnetic south pole?
Easy
- AGeographic North Pole
- BGeographic South Pole
- CEquator
- DBoth poles
39.The diagram shows magnetic field lines around a bar magnet. At which point is the magnetic field strongest?
Medium
- AAt the poles
- BAt the centre
- CMidway between the poles
- DFar away from the magnet

複習筆記

## Plotting Magnetic Fields - **Iron filings** show the shape of a magnetic field but **not its direction**. - To show **direction**, use a **plotting compass** (a small bar magnet free to rotate). - The compass's north pole points towards **magnetic south**. - **Method**: Place a bar magnet on paper, put a plotting compass near a pole, mark the compass direction, move it to other positions, and join the arrows to show the field lines. - **Earth's magnetic field** is similar to a bar magnet's field. The outer core (molten iron and nickel) generates it. - **Geographic North Pole** is near Earth's **magnetic south pole** (but not exactly). A compass points to magnetic south. - **Key fact**: Compasses are used for navigation. ## Key Points - Compass = small bar magnet. - Earth's outer core = molten iron + nickel (magnetic metals). The third magnetic metal is **cobalt**. - Compass shows direction; iron filings only show shape. - Electronic devices produce magnetic fields that can disturb a compass. - Magnetic field is strongest where field lines are closest (e.g., at the poles).

Plotting compass: A small bar magnet that is free to rotate, used to show the direction of a magnetic field.
Magnetic field: The region around a magnet where magnetic forces are exerted.
Magnetic south: The pole of a magnet that attracts the north pole of a compass; Earth's magnetic south is near the geographic North Pole.
Geographic North Pole: The northernmost point on Earth, near but not exactly at Earth's magnetic south pole.
Outer core: The layer of Earth made of molten iron and nickel that generates Earth's magnetic field.

字卡

What is a plotting compass?

A plotting compass is a small bar magnet that is free to rotate.

What does the north pole of a plotting compass point towards?

The north pole of a plotting compass points towards magnetic south.

Why can't iron filings show the direction of a magnetic field?

Iron filings show the shape of the magnetic field but not the direction.

What is used to show the direction of a magnetic field?

A plotting compass is used to show the direction of a magnetic field.

Describe how to plot the magnetic field of a bar magnet using a plotting compass.

Place a bar magnet on paper, place a plotting compass near one pole, mark the direction of the compass, move the compass to other positions, mark each direction, then join the arrows to show the field shape.

What is the Earth's outer core made of?

The Earth's outer core is made of molten iron and nickel.

What shape is Earth's magnetic field?

Earth's magnetic field has the same shape as that of a bar magnet.

Where is Earth's magnetic south pole located relative to the geographic North pole?

The geographic North pole is near (but not precisely at) Earth's magnetic south pole.

True or false: A compass points towards Earth's magnetic north.

False. A compass points towards Earth's magnetic south.

True or false: A compass can be used for navigation.

True.

True or false: The geographic North pole of Earth is near its magnetic south pole.

True.

What is a compass made of?

A compass is made of a small bar magnet.

Name the two magnetic metals found in the Earth's outer core.

Iron and nickel.

Name the third magnetic metal (besides iron and nickel).

Cobalt.

What is a benefit of using a compass over iron filings to visualize a magnetic field?

A compass shows the direction of the field, while iron filings do not.

Why should a compass not be used near an electronic device?

Electronic devices produce a magnetic field that can affect the compass reading.

Where is Earth's magnetic field strongest?

At the poles, as indicated by the closer spacing of field lines in diagrams.

Could we use compasses to navigate if Earth had no magnetic field?

No, because a compass relies on Earth's magnetic field to point north/south.

What does a compass always point to on Earth?

Earth's magnetic south.

What is the geographic North pole near?

It is near Earth's magnetic south pole.

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