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Higher physics > Electricity > Flashcards

Electricity Flashcards

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1
Q

Describe direct current

A

Current is one direction only and may vary value

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2
Q

Give an example of a DC supply

A

Battery

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3
Q

Describe alternating current

A

Current repeatedly changes in direction and varies with value in time.

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4
Q

Give an example of an AC supply

A

Mains supply

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5
Q

The root mean square of ac voltage is the equivalent…

A

d.c. voltage that would produce the same power in a circuit as the a.c. voltage

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6
Q

The rms value is always less than…

A

the peak voltage of an a.c. supply

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7
Q

what is the rms formula

A

Vpeak = √2 Vrms
both measured in Volts (V)
√2 = 1.41…

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8
Q

That mains supply in the UK has an rms value of

A

230V

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9
Q

How can you find the frequency of an AC signal

A

The frequency of a signal can be found from the period (time for one complete wave) using T = 1/f

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10
Q

How can the period of an AC signal be found

A

The period can be found from the waveform on an osciliscope and looking at the time base setting.

Period (T) = number of divisions for one wave * time base

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11
Q

What is the peak voltage of the mains supply in the UK

A

325V

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12
Q

How can the peak voltage of an AC signal be found

A

Can be found from the amplitude of the waveform multiplied by the Y gain setting

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13
Q

How is rms current calculated

A

in the same way as RMS voltage

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14
Q

What is the rms current formula

A

I(peak) = √2 I(rms)

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15
Q

How is average power (power rating) calculated

A

using the rms values of current and voltage (P = I(rms)V(rms))

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16
Q

How is peak power calculated

A

using the peak values of current and voltage
Ppeak = IpVp

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17
Q

State the work done formula (electricity)

A

Ew = QV
Where:
Q = charge in coulobs (C)
V = Voltage in volts (V)
Ew= Work done in Joules (J)

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18
Q

State the definition of the potential difference of a power supply

A

the potential difference of a power supply is the energy supplied per coloumb of charge

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19
Q

Describe the function of an ammeter

A

Measires current through a component and is connected in series

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20
Q

Describe the function of a voltmeter

A

Measures potential difference across a component and is connected in parallel.

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21
Q

Formula for combining resistance in series

A

Rt = R1+R2+R3…

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22
Q

Formula for combining resistance in parallel

A

1/Rt = 1/R1 + 1/R2 + 1/R3…

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23
Q

State ohms law

A

V=IR
Where:
V=Voltage in volts (V)
I=Current in amperes (A)
R=Resistance in ohms

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24
Q

What is power

A

Power is the energy transfered per second

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25
State the 4 power relationships
P=E/t, P=IV. P=V^2/R, P=I^2R
26
State the two relationships used in voltage dividers
V1 = (R1/R1 + R2)Vs, V1/V2 = R1/R2
27
Define EMF
EMF(Electromotive force) is the total electrical potential energy given to each coloumb of charge through the supply. It is fixed for a particular supply whatever the current
28
Define Lost volts
Lost volts is the energy per coloumb (Voltage) which is "lost" to heat due to current passing through the internal resistance.
29
Define TPD (terminal potential difference)
the tpd (terminal potential difference) is the energy per coloumb (Voltage) availablle to the circuit (left over voltage after losing the "lost volts) This is what is measured across the circuit with a voltmeter. It decreases as current increases.
30
When is EMF equal to tpd
When I = 0A
31
The resistance of a cell is called
the internal resistance of the cell.
32
A cell can be thought of as
a source of electromotive force (EMF) with a resistor
33
State the internal resistance relationship
E = IR +Ir Where: E =EMF in volts V V (IR) = tpd in volts V I = current in amperes A R = "load" resistance (resistance of circuit) in ohms r = internatl resistance in ohms
34
How do you find the internal resistance from the graph of tpd against current.
-r = m
35
How do you find emf from a graph of tpd against current
Y-int = E
36
on a graph of tpd against current the short circuit current is...
the x intercept
37
Describe a capacitor
A capacitor has two parallel plates with an insulating gap between them. The plates can hold charge
38
Define capacitance
Capacitance is the ratio of charge, Q, to potential difference, V (ie the gradient of the QV graph)
39
The charge on the plates of a capacitor is _______ to the potential difference across the plates
directly proportional
40
State the capacitance formula
C = Q/V Where: C= capacitance measured in Farads (F) V = Potential difference measured in volts V Q = Charge measured in coulombs (C)
41
1 Farad is equal to...
1 Coulomb per volt
42
How can you find the energy stored in a capacitor
Area under the QV graph.
43
State 3 uses of capacitors
Flash gun for a camera Timing devices such as clock pulse generator Radio tuning circuit Blocking DC signals Back up "battery" to stop data loss when batteries are being changed in media devices.
44
Describe conductors
Conduct an electric current
45
Give examples of conductors
all metals and some semi metals like graphite and arsenic
46
Describe insulators
Do not conduct an electric current
47
Give examples of insulators
Plastic, air, rubber
48
Describe semi conductors
Materials that behave like insulators but can be made to conduct by heating, applying a voltage or adding an impurity.
49
Give examples of semi conductors
Silicon and Germanium
50
What is the valence band
The highest energy band that has electrons in it at absolute zero.
51
What is the conduction band
The Lowest unoccupied band at absolute zero
52
What must happen for conduction to occur
Electorns have to move from atom to atom under the influence of an electric field.
53
An electron can only move to another atom if...
there is a vacant energy level for it as two electrons cannot occupy identical energy levels.
54
For a material to conduct electricity it must have a...
partially filled band so there are spaces for electrons to move into.
55
In insulators the valence band is...
full so there are no vacant energy levels and electrons cannot flow through the material
56
Why can insulators not conduct
To conduct, electrons would need to move into the empty energy band above (conduction band) however the energy gap is too large for electrons to do this.
57
Why can conductors conduct
One or more bands are partially filled. Some metals have free electrons and partially filled valence bands , therefore they are highly conductive. Some metals have overlapping valence and conduction bands. Each band is partially filled and therefore electrons are free to move between them.
58
Describe how semi conductors can change conductivity
The gap between the valence band and conduction band is small. At room temp some electrons have enough energy to cross the band gap. An increase in temperature increases the conductivity of a semi conductor since more electrons have the energy to cross the band gap into the conduction band.
59
Holes in the valence band act as
Positive charge carriers
60
In an intrinsic semiconductor there will be an equal number of
Electrons and holes involved in conduction.
61
What is doping
Doping is when an impurity is added to the semiconductor to increase conductivity
62
What is the difference between p type and n type semiconductors
n type semi conductors have an extra electron not involved in bonding allowing current to flow as it is free moving. P type semi conductors have a "missing electron" leaving a positive hole which electrons can move into allowing current to flow.
63
What is a PN junction
A semiconductor which is grown to be half p type and half n type and functions as a diode.
64

Higher physics (7 decks)

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