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Physics > Magnets > Flashcards

Magnets Flashcards

(50 cards)

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1
Q

Motor effect

A

Where a force is exerted on a wire carrying a current in a magnetic field at right angles. This causes the wire to move

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2
Q

Calculating the motor effect force

A 
F= B I l
Force = magnetic flux density (measured in tesla T) x current x length (in metres)
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3
Q

Magnetic flux density def

A

The strength of a magnetic field (how many field lines there are in a region)

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4
Q

Natural magnetic materials

A

Iron (steel), cobalt and nickel

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5
Q

Magnet def

A

Attract magnetic materials

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6
Q

Magnetic materials def

A

Materials that are attracted to magnets

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7
Q

Induced magnets def

A

Permanent magnets have their own magnetic fields. Temporary magnets made from magnetic materials that have been put into a magnetic field e.g. paper clips on a bar magnet. When the magnetic field is taken away, the induced magnets lose their magnetism and stop producing a magnetic field

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8
Q

A moving charge (current) creates…

A

A magnetic field (an area surrounding a magnet that can exert a force on magnetic materials)

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9
Q

What is the field like surrounding a wire

A

Concentric circled perpendicular to the wire in the centre

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10
Q

How to see field lines

A

Iron filings: shape of field, compasses: direction

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11
Q

How to increase the magnetic force on a wire

A

Increase current, use a stronger magnet, increase the length of the conductor inside the magnetic field

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12
Q

Changing the direction of the current changes the…

A

Direction of the magnetic field

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13
Q

Right hand rule

A

Thumb shows direction of current in wire. Curved fingers show direction of magnetic field around the wire

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14
Q

Current going towards you out the paper

A

Magnetic field going anti-clockwise

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15
Q

Current going away from you into the paper

A

Magnetic field going clockwise

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16
Q

Solenoid

A

A series of coils of wire next to each other

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17
Q

In the middle of a solenoid…

A

The magnetic field is uniform (in rows) and strong (many lines close together) because they all go in the same direction

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18
Q

How does using a solenoid increase the strength of a magnetic field

A

The field lines around each loop line up with each other. They all point in the same direction and very close to each other

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19
Q

How to increase the field strength of a solenoid

A

Add more coils. Add an iron core (block of iron in the centre). When a current flows, it becomes an induced magnet - an electromagnet as the magnetic field can be turned on and off with an electric current

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20
Q

How does the motor effect work? When does a full force happen?

A

The magnetic field of wire and the magnetic field of the magnets interact and causes them to exert a force on each other, causing the wire to move. A full force happens when the wire is at 90° to the magnetic field. If they are parallel there will be no force at all

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21
Q

How to show the direction of the force in the motor effect

Study These Flashcards
A

Apply a current to a set of rails inside a horseshoe magnet. A bar is placed on the rails which completes the circuit and generates a force to roll it along when a current is applied

22
Q

Flemings left hand rule

Study These Flashcards
A

At right angles 90° to each other
thuMb: movement/motion, direction of the motor effect force on the conductor carrying the current, F
Forefinger: field (N to S), lined with magnetic field lines, B
seCond finger: current (pos to neg), I

23
Q

How to reverse the direction of the force on the motor effect (Flemings left-hand rule)

Study These Flashcards
A

Reverse the direction of the current and the magnetic flux density

24
Q

Cycle of power in the national grid

Study These Flashcards
A

Power station > step up transformer > pylon > step down transformer > heavy industry / light industry / homes

25
Purpose of transformers
Reduce danger, increases efficiency (high current means more collisions means more heat lost means more energy wasted means less efficiency)
26
For the same power, decreasing the current...
Increases the voltage
27
Equation for conservation of power
Vp x Ip = Vs x Is p is primary/incoming s is secondary/out going
28
How do dc motors work
Forces act on the two side arms of a coil of a wire that is carrying a current. The coil is on a spindle so it rotates. It is connected to a split-ring commutator which swaps the contacts every half turn to keep it rotating in the same direction (disconnects the current but the coil momentum keeps it rotating)
29
How to reverse the direction of the dc motor
Reverse the current by swapping the polarity of the dc supply. Or reverse the field by swapping the magnetic poles over
30
How do loud speakers and headphones work
Because of the motor effect. They use electromagnets. An ac is sent through the coil attached to the base of a paper cone. The coil surrounds one pole of a permanent magnet and is surrounded by the other pole. The current crests a force on the coil, causing the cone to move. When the current reverses, the force acts in the opposite direction so the cone does too. Variations in the current make the cone vibrate making the air around it vibrate causing variations in pressure to cause a sound wave. The frequency of the wave is the same as the ac so you can alter the sound wave by controlling the ac
31
The generator effect
The induction of a potential difference (and current if it is a complete circuit) in a wire which is moving relative to a magnetic field or experiencing a change in magnetic field
32
Process of the generator effect
Move a magnet in a coil of wire or move a conductor (wire) in a magnetic field - cutting field lines. Shifting the magnet side to side create a little current if it is in a complete circuit. If the magnet is moved in the opposite direction then the potential difference will be reversed. If the magnet keeps moving forwards and backwards then the potential difference produced will cause an alternating current
33
How to create a generator effect
By turning a magnet end to end in a coil or turning a coil inside a magnetic field. As the magnet turns, the magnetic field in the coil changes. This induces a potential difference which can make a current flow. After half a turn, the direction of the magnetic field reverses and the current flows in the opposite direction. If the magnet turns in the same direction, there will be an alternating current produced
34
How does induced current oppose the charge that made it
A change in magnetic field induces a current. This creates another field around the wire which acts against the current
35
How to change the size of the induced potential difference
Change the rate that the magnetic fuel is changing - increasing speed of movement (cutting more magnetic field lines in a given time), increasing the strength of the magnetic field (more field lines to be cut)
36
Alternators
Generate alternating current. Rotate a coil in a magnetic field. As it spins, a current is induced which changes direction every half turn. They have slip rings and brushed so the contacts stay the same
37
Dynamos
Generate direct current. Rotate a coil in a magnetic field. As it spins, a current is induced which changes direction every half turn. Have split ring commutators which swap the connection every half turn to keep the current flowing in the same direction
38
Oscilloscope
Show how potential difference generated in the coil changes overtime
39
Microphones. How they work
Sound waves hit a flexible diaphragm attached to a coil of wire wrapped around a magnet chasing the coil to move in a magnetic field generating a current. Louder sounds make the diaphragm move further so the microphone can convert the pressure variations into variations in a current
40
The ratio of cooks determines...
If the voltage is increases or decreased in the transformer
41
Turns ratio equation for transformers
Vp Np — = — Vs Ns N: turns Works out either the missing voltage or missing number of turns
42
How to increase current in transformers
Increase speed, increase number of cooks (turns), stranger magnetic field
43
What type of current do transformers only work in
AC
44
What do dynamos do
Turn AC in coils to DC in circuit using a commutator
45
What do commutators do
Turn DC supplies into AC in coils so it always rotates
46
What type of current do electric motors have
DC from battery
47
What do transformers do
Change the size of the potential difference in AC (change the size of an alternating voltage)
48
How do transformers work
They have a primary and secondary coil joined by an iron core. When an alternating voltage is applied across the primary coil, the iron core magnetises and demagnetises quickly. This changing magnetic field induces an alternating voltage in the secondary coil. In a complete circuit, this induces a current. The alternating current in the primary circuit can therefore induce a current in the secondary circuit
49
Potential difference in step up
Vs > Vp
50
Potential difference in step down
Vs < Vp

Physics (12 decks)

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