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Physics The Good One > 07 electric and magnetic fields > Flashcards

07 electric and magnetic fields Flashcards

1
Q

Magnetic Fields

A

A region surrounding a magnet or current carrying wire which acts upon any other magnet or current carrying wire placed within the field

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2
Q

Electric Field

A

A region of space in which a (small, positively) charged particle feels a force

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3
Q

Electric Field Strength

A

Force per unit charge, E = F/Q

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4
Q

What do field lines show

A

Strength and direction. Closer together lines show a stronger field

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5
Q

Uniform fields

A

Same strength at any point in the field. Evenly spaced lines, arrows from + to -

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6
Q

Radial fields

A

Strength decreases as distance from the point charge increases. All equal angles, arrows from + to -

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7
Q

Field line drawing rules

A

Lines can never cross

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8
Q

Electric neutral/ null point

A

Where there are no field lines: the forces from the field are balanced.

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9
Q

Electric field strength of a uniform field

A

E = v/d

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10
Q

Charging

A

Electrons are transferred from one material to another

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11
Q

Inducing a charge in an object without touching the object

A

A negatively charged strip is brought close to 2 touching metal spheres. Electrons from sphere A are repelled to sphere B. The spheres are separated with the strip nearby. The strip is removed and the charge spreads out so it is distributed evenly. B is now negatively charged and A is positively charged.

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12
Q

Coulomb meter

A

Measures charge

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13
Q

Force between two charges

A

Directly proportional to each of the charges Q1 and Q2. Inversely proportional to the square of their separation.

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14
Q

Coulomb’s law

A

F = kQ1Q2 / r^2. F can be repulsive or attractive.

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15
Q

Electric potential

A

The potential energy that each coulomb of positive charge would have if placed at that point in the field.

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16
Q

Potential difference

A

The energy transferred when one coulomb of charge passes from one point to another point. W = VQ

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17
Q

Equipotentials

A

Positions within a field with zero potential difference between them. They are perpendicular to field lines.

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18
Q

Equipotentials between parallel plates

A

Even spaced perpendicular to field lines

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19
Q

Equipotentials for a point charge

A

Concentric circles, closer together near the point charge

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20
Q

Charge moving along equipotential

A

No work is done because the potential energy does not change.

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21
Q

Capacitor

A

An electrical component that stores and releases charge (and therefore energy)

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22
Q

Capacitor uses

A

Defibrillators & Camera Flashes

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23
Q

Charging up a capacitor

A

When the switch is closed, electrons flow from the negative terminal of the battery onto the first plate of the capacitor, this becomes negatively charged. Electrons are repelled from the second plate around the circuit. There is a force of attraction between the plates. This charging process continues until the pd. across the capacitor is equal to the pd. of the supply.

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24
Q

Discharging a capacitor

A

Disconnect the battery. Initially there is a large current due to the large potential difference across the plates. The current drops as pd drops. Current flows opposite way round the circuit when discharging. The charge drops quickly at first then more slowly.

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25
Capacitance
The ability to store charge on the plates of a capacitor. The charge stored per unit of pd. across it. Q = VC
26
Capacitance Unit
Farads
27
Energy stored in a charged capacitor
1/2 QV (proof is VQ graph, area under graph = area of triangle). (Also can sub in Q = CV, V = C/Q to have a different form)
28
Capacitors in parallel
C = C1 + C2 ….
29
Capacitors in parallel proof
Q = Q1 + Q1 ... Q = CV. CV = C1V2 + C2V2 ... V is constant in parallel. C = C1 + C2...
30
Capacitors in series
1/C = 1/C1 + 1/C2 ….
31
Capacitors in series proof
V = V1 + V2. V = Q/C. Q/C = Q1/C1 + Q2/C2. Q is constant in series. 1/C = 1/C1 + 1/C2
32
The greater the capacitance
The greater the charge stored.
33
As the pd. falls during the discharge of a capacitor the time…
For the pd to drop takes longer and longer.
34
Time constant
The time it takes for the charge to drop to 37% (1/e) of the original value (=RC)
35
RC / time constant unit proof
RxC = V/I*Q/V = Q/I = Q * t/Q = t
36
Capacitance discharging graph (Qt, It, Vt)
Exponential decay
37
Capacitance charging graph (Qt, It, Vt)
Current asymptotically reaches 0 (gradient decreases as it goes up)
38
Exponential
Same fractional change in y for each interval change in x
39
To plot a linear graph from discharge equations
Take logs from both sides and follow log rules
40
Magnetic field lines
Lines of flux (always from N pole to S pole)
41
Magnetic Flux Density, B
The flux per unit area (an indication of the strength of the magnetic field).
42
Magnetic Flux Density Unit
Tesla, T
43
Magnetic Neutral point
The point between two north poles where the magnetic field cancels and the resultant field strength is zero.
44
Flux Φ
The flux passing through an area A perpendicular to the magnetic field B is defined as Φ=BA
45
Flux Linkage
The product of magnetic flux and the number of turns on the coil NΦ = BAN
46
Flux/Flux Linkage Unit
Weber, W (sometimes Flux linkage is Weber Turns)
47
Right hand rule
Always a magnetic field around a current carrying wire
48
Fleming's left hand rule fingers
Thumb - Force. Index (First Finger) - Field. Middle (Second Finger) - Current
49
Fleming's Left hand rule
The conductive wire cuts lines of flux. A force acts perpendicular to both current and magnetic field.
50
If the current is parallel to the lines of flux
No force acts
51
The force due to Fleming's left hand rule is greatest when
The wire and field are perpendicular, otherwise only a component of the field strength is acting.
52
The force due to Fleming's left hand rule depends on
Size of current, magnetic field strength, length of wire, angle of wire to magnetic field.
53
Force on current carrying wire equation
F = BILsinθ
54
Force on single moving charged particle in a magnetic field
F = BQvsinθ. (from F = BIL, L = vt and I = Q/t)
55
Shape of the path of charged particle in a magnetic field
Circular
56
Why is the path of a charged particle in a magnetic field circular?
By Fleming's left hand rule the force on a moving charge is always perpendicular to its direction of travel, this is the condition for circular motion.
57
EMF Induction
Movement of a conductor in a B-Field. Cuts lines of flux/change in flux linkage. An emf is induced. (- If part of a complete circuit an induced current flows)
58
Faraday’s Law
The induced emf is directly proportional to the rate of change in flux linkage. Ɛ = d(NΦ)/dt
59
Lenz’s Law
The induced emf is always in such a direction to oppose the change that caused it. Ɛ = - d(NΦ)/dt
60
Why is Lenz’s Law good
It agrees with the principle of conservation of energy.
61
Factors affecting the emf induced in a coil
Angle between the coil and the field. Number of turns on the coil. Area of coil. Magnetic Field Strength. Angular Speed of Coil. Magnitude/frequency current in the wire. Add an iron core.
62
Factor: angle between coil and field reasoning (emf induction)
Smaller angle = less lines of flux cut = lower emf.
63
Factor: Number of turns on the coil reasoning (emf induction)
More turns = more points which cut each flux line = higher emf.
64
Factor: Area of the coil reasoning (emf induction)
Larger area = more lines of flux pass through = higher emf.
65
Factor: Magnetic Field Strength reasoning (emf induction)
Higher flux density = more lines of flux per unit area = coil cuts more lines of flux = higher emf.
66
Factor: Angular Speed of Coil reasoning (emf induction)
Increased angular speed = increased number of lines of flux cut in a certain time = higher emf.
67
Using Lenz’s law to predict direction of induced emf
Fleming's left hand rule... force is opposite direction to motion, B field as told, then current shows direction of induced emf.
68
EMF induction using two coils
One coil has an AC current passed through producing a changing magnetic field. Second stationary coil then cuts lines of flux. Induces an emf in the second coil.
69
Eddy Currents
If the conductor is large enough small rings of induced currents form. These will grow depending on the size of the conductor. These lose energy as heat. If a conductor is moving through a B-field and the induced eddy currents are big enough it will stop the motion as all the KE is transferred to heat.
70
Conductor with cracks moving through B-field
The motion will not be stopped/slowed as much because the cracks stop large eddy currents from forming.
71
Relation between electric field and electric potential
Electric potential is a property of electric fields.
72
Where is there always a magnetic field?
Around a current carrying wire.
73
x
Into the page
74
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Out of the page
75
Solenoid
A long coil with many turns.
76
Magnetic field of a solenoid
Lines through the center looping around to the opposite end of the solenoid.
77
Flux density in the center of a solenoid
Constant.
78
Why is the magnetic field of a solenoid greatly increased when a magnetic material is placed inside it
The atomic magnets of the core line up along the lines of flux inside the solenoid so the core is magnetized.
79
What factors affect the force of a charged particle in a B field?
The magnetic flux density, charge on the particle, the velocity of the particle.
80
Time period
Is the time for one complete cycle.
81
Frequency
The number of cycles in one second.
82
Peak values
Are the largest voltages or currents an AC supply produces.
83
Root-mean-square value
When you square all the values, take the mean then square root the values.
84
Other method of finding the rms value
Divide by √2.
85
What type of current does a transformer change?
AC.
86
How does a transformer work?
An alternating current flows in the primary coil. This produces an alternating magnetic field in the soft iron core. This means the flux linkage of the second coil is constantly changing so an alternating voltage is induced across it.
87
Transformer equation
Vs/Vp = Ns/Np
88
Step-up transformer
Increases the ac voltage (more turns on secondary coil).
89
Step-down transformer
Decreases the ac voltage (less turns on secondary coil).
90
Where is energy lost in transformers?
Eddy currents are induced in the soft iron core.
91
How is energy loss reduced in transformers?
The core is made of laminated iron sheets.
92
Is the time taken for a capacitor to lose half its energy greater or less than the time taken to lose half its charge?
W = QV/2. Q and V both increase over time. W will decrease faster so it takes LESS time to half in value.
93
Advantage of data loggers which isn’t about reaction time
Take multiple readings at the same time. More readings can be taken in a shorter time / higher sample rate.
94
How does pd vary with charge for a capacitor
pd is directly proportional to charge.
95
Exponential decay curve
x decreases by proportional fractions in equal time intervals. Decreases very rapidly then more slowly.
96
When asked to work out the resistance of a capacitor circuit
RC = time constant. Time constant is the time it takes for I/V/C to drop to 37% of the original value.
97
Frequency in capacitor circuits
f = 1/t, t = RC.
98
Arrows on a uniform electric field
Positive to negative.
99
Electric field strength
Force per unit charge acting on a small positive charge.
100
Faraday's law of EM induction
The induced e m.f. is proportional to the rate of change of magnetic flux linkage.
101
What to do to a DC current carrying wire to induce an emf in a coil
Make it AC, move either coil, turn the power on/off.
102
What does an iron core do in emf induction
It becomes magnetized and increases the magnetic field.
103
Why is there a minus sign
Lenz’s law/conservation of energy. Induced emf opposes the change that caused it.
104
Capacitance
The ability to store charge.
105
AC root mean square
Square root of the arithmetic mean of the squares.
106
What energy transfers occur in the motor effect?
Electrical to kinetic.
107
Electric potential, V =
V = kQ/r.
108
Difference between EM induction and motor effect
EM is when a conductor moves in a B field. Motor effect is when a current carrying wire in a B field feels a force.
109
Motor effect
If two magnetic fields interact, the force is felt perpendicular to both fields.
110
Question
Answer
111
Size of force in motor effect is affected by
Increasing the magnetic field strength, increasing the length of the wire in the field, increasing the current in the wire.
112
How to show an electric field in the lab
Set up two parallel plates with a pd. across them. Put a charged oil drop in the region between the two plates to show the force acting on the droplet.
113
What happens to a capacitor circuit when the switch changes to the power supply
Capacitor charges up so that the pd across the capacitor equals the pd of the supply. It has opposite charge on both plates.
114
As a capacitor charges…
Current drops to zero.
115
What happens to a capacitor circuit when the switch changes to the resistor circuit
Discharges over a period of time.
116
Why may a diode be used when inducing an emf
So the current is not discharged by alternating emf from AC supply.
117
Discharging a capacitor
Electrons/charge are transferred from the negatively charged plate to the positively charged plate through the resistor. Hence the charge on the capacitor decreases until the charge on the capacitor equals 0.

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