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IG Physics > P9. Magnets & Currents > Flashcards

P9. Magnets & Currents Flashcards

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1
Q

Magnet

A

A magnet is an object or material that can attract ferromagnetic materials (iron, nickel, cobalt); it has a magnetic field around it, and a North Pole and a South Pole

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2
Q

Induced magnetism

A

When a material (eg. iron, steel) becomes a magnet itself because it touches or is brought near to a permanent magnet

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3
Q

Magnetic material

A

A material which can be magnetised and is attracted to magnets

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4
Q

What causes magnetic forces?

A

Magnetic forces are due to interactions between magnetic fields

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5
Q

Methods of magnetisation

A
  1. Stroking with a magnet (stroke several times with the same pole of a permanent magnet)
  2. Use d.c. in a coil and switch on circuit
  3. Hammering in a magnetic field
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6
Q

Methods of demagnetisation

A
  1. Hammering/dropping
  2. Heating (with a bunsen burner)
  3. Use a.c. in a coil and withdraw rod from solenoid
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7
Q

Temporary magnets & use

A

Soft iron, nickel and cobalt (soft magnetic materials) are easily magnetised but do not retain their magnetism for long
Use: in electromagnets

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8
Q

Permanent magnets & use

A

Steel (hard magnetic material) will retain its magnetism for a longer time (although it is difficult to magnetise)
Use: in compasses, fridge doors

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9
Q

Right-hand grip rule

A
  1. Imagine gripping the coil with right hand so that fingers point in conventional current direction, and thumb points towards N pole of the coil
  2. Thumb points in the conventional current direction, fingers point in the direction of the magnetic field lines
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10
Q

Ways to increase the strength of the magnetic field of a solenoid/current-carrying coil

A
  1. Increase the current
  2. Increase the number of turns on the coil
  3. Use a soft iron core
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11
Q

Direction of a magnetic field line at a point

A

The direction of a magnetic field line at a point is the direction of the force on the N pole of a magnet at that point

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12
Q

Uses of electromagnets - Circuit-breaker

A
  1. When a large current enters the circuit, the magnetic field around the solenoid gets stronger, attracting the iron catch.
  2. This causes the moving contact to break the circuit; current can no longer enter the house.
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13
Q

Uses of electromagnets - Electric bell

A
  1. When the switch is pressed, current flows through the coil and the electromagnet attracts the iron armature, and the hammer hits the gong.
  2. When this happens, the circuit breaks (is open) and the iron armature is released to its original position.
  3. The circuit is closed once again and the process repeats.
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14
Q

What is the effect on the magnetic field if the current is reversed?

A

The direction of the magnetic field is reversed

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15
Q

What happens to a current-carrying coil when it is placed in a magnetic field?

A

The current-carrying coil experiences a turning effect

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16
Q

Ways to increase the turning effect on a coil in a d.c. motor

A
  1. Increase the number of turns on the coil
  2. Increase the current
  3. Increase the strength of the magnetic field / Use a stronger magnet
17
Q

What are the functions of a split-ring commutator and carbon brushes (d.c. motor)?

A
  1. Split-ring commutator: to reverse the direction of current flow in the coil every half-cycle, to keep the coil rotating in the same direction
  2. Carbon brushes: to enable current from the external circuit to flow into the coil
18
Q

What is a centre-zero galvanometer?

A

The needle of the galvanometer points to the left or right of the zero, depending on the current direction

19
Q

Ways to increase the magnitude of an induced e.m.f. (and current)

A
  1. Move the wire or magnet faster / Cut the flux lines faster
  2. Use a stronger magnet
  3. Increase the number of turns on the solenoid / Increase the length of wire in the magnetic field
20
Q

Faraday’s law

A

The magnitude of the induced e.m.f. in a conductor is directly proportional to the rate of change of magnetic flux in the circuit (to the rate at which magnetic field lines are cut by the conductor)

21
Q

Lenz’s law

A

An (induced e.m.f and hence) induced current always flows in a direction such that it opposes the motion or change which produced it

22
Q

Differences between Fleming’s left-hand rule and right-hand rule

A

Left-hand rule
1. When a current causes motion (induced force)
2. Motor, presence of battery
Right-hand rule
1. When motion causes a current (induced current)
2. Generator, no battery

23
Q

Difference between d.c. and a.c.

A

d. c. flows in only one direction

a. c. flows alternately backwards and forwards (constantly changes direction)

24
Q

Explain a rotating-coil (a.c.) generator and the use of slip rings

A
  1. When a force is applied to turn the coil between the magnets, the magnetic field passing through the rectangular coil changes
  2. This induces a current to flow in the coil
  3. The induced current flows to an external circuit through one brush to another
  4. The SLIP RINGS lead the current in and out of the coil to the external circuit
    * purpose: provide continuous connection while coil is rotating
  5. The direction of the induced current changes every half-rotation. Hence, the current is alternating (a.c.)
25
Q

Principle of operation of a transformer

A

A transformer is used to increase or decrease the voltage in a circuit. It works by mutual induction.

  1. When a.c. flows in the primary coil, it causes a changing magnetic field
  2. The secondary coil cuts the changing field
  3. This changing field induces an e.m.f. in the secondary coil - mutual induction
26
Q

Step-up transformer

A

A step-up transformer has more turns on the secondary coil than on the primary coil, so it increases the secondary voltage

27
Q

Step-down transformer

A

A step-down transformer has fewer turns on the secondary coil than on the primary coil, so it decreases the secondary voltage

28
Q

Why is high-voltage transmission of electricity used?

A

Power losses are encountered due to long-distance transmissions. So to reduce power loss, high voltage is used. Since P = IV and power needs to be the same, the current will be very low. Power loss is given by P = I^2R. Therefore, reducing the current means lower power losses in cables.

29
Q

What is a 100% efficient transformer known as?

A

An ideal transformer

30
Q

Experiment to determine the pattern and direction of the magnetic field around a bar magnet (using a compass/suspended small magnet)

A
  1. Use compass/suspended small magnet
  2. Observe needle/magnet on one field line
  3. Observe needle/magnet on another field line
  4. Mark on card; needle/magnet shows direction of field
31
Q

Experiment to determine the pattern and direction of the magnetic field around a bar magnet (using iron filings & compass/suspended small magnet)

A
  1. Sprinkle iron filings
  2. Tap card
  3. Direction/alignment of iron filings shows field
  4. Use compass/suspended small magnet to show field direction

IG Physics (11 decks)

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