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Physics year 12 > Electricity Chapters 8/9/10 > Flashcards

Electricity Chapters 8/9/10 Flashcards

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1
Q

Charge

A

Q = IT

Charge = current x time

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2
Q

Number of electrons from charge

A

Total charge/ 1.6x10^-19

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3
Q

Mean drift velocity equation

A

I = nAev

Current = charge carrier density x area of wire x charge on an electron x mean drift velocity

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4
Q

Work done/ energy transferred (4)

A

W = VQ
Work done = voltage x charge

W = EQ
Work done = EMF x charge

W = VIt
Work done = voltage x current x time

W= PT

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5
Q

Resistance from resistivity

A

R= pL/A

Resistance = reisitivity x length/ area

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6
Q

Resistors in series

Resistors in parallel

A

R = R1 + R2

1/R = 1/R1 + 1/R2 etc

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7
Q

Power (3)

A 
P = IV
Power = current x voltage 
P = I^2 R 
Power = current squared x resistance 
P= V^2/R
Power = voltage squared / resistance
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8
Q

Internal resistance

Actual and factorised

A 
EMF = V + Ir
EMF = I(R+r)
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9
Q

Voltage (2)

A

V = IR
(VIR tree)

V= W/Q
Work done/ charge

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10
Q

Charge equation

A

Q= IT

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11
Q

eV equation for an electron gun

A

eV = 1/2 x mass x velocity^2

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12
Q

Potential dividers

A

Do as a fraction of total p.d.

Where a greater resistance gives a greater P.D.

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13
Q

Kirchhoff’s first law

And what it shows

A

Sum of current in = sum of current out

Shows conservation of charge

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14
Q

Kirchhoff’s second law

What does this show

A

Sum of EMF in a in a loop = sum of voltages in the same loop

Conservation of energy

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15
Q

Define potential difference

And define the volt

A

Potential difference is an energy transfer per unit charge

V = w/q

And the volt is equal to one joule per coulomb

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16
Q

State ohms law

A

For a metallic conductor at a constant temperature, current is directly proportional to potential difference

Resistance is constant

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17
Q

Define resistivity

A

Resistivity at a given temperature is the equal to the product of resistance and area per unit length

P = RA/L

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18
Q

Define the kilowatt hour

A

The kilowatt hour is the energy transferred when 1000w is run for an hour

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19
Q

Define current

A

Current is the rate of flow of charge, or the charge per unit time and is defined by

I = Q/t

1 coulomb per second

20
Q

How to investigate internal resistance

A

Set up a cell in series with a variable resistor an ammeter and a voltmeter in parallel over the cell

Take reading of current and voltage for varying resistances

Plot an V-I characteristic graph

The emf is the y intercept
The gradient is -r

21
Q

What is the charge on an electron

A

1.6 x 10^-19

22
Q

What is the coulomb

A

The unit of charge equal to the charge flowing past a point in one second with a current of one amp

23
Q

Calculating net charge

A

Positive charges - negative charges

Should be a multiple of e as charge is quantised

24
Q

What was Milikin’s experiment

A

Tiny Droplets of negatively charged oil were sprayed between two oppositely charged plates

When these droplets were stationary he knew
Weight = electrostatic charge

From here the charge could be calculated

25
Q

What is the difference between charge in wires/metals and charge in electrolytes

A

In metals electrical charge is the movement of electrons

In electrolytes electrical charge is the movement of ions

26
Q

2 ways to increase electrical current

A

Faster moving electrons

Or more electrons

27
Q

Difference between conventional current and electron flow

A

Conventional current flows +ve to -ve

Electrons flow -ve to +ve

28
Q

What is number density and what are typical values for
Conductors
Semiconductors
Insulators

A

The number of charge carriers per m^-3

Conductors = around 10^28
Semiconductors = around 10^17
Insulators = very few
29
Q

Difference in circuit symbols of thermistor and variable resistor

A

Thermistor had a bent line

Variable resistor has an arrow

30
Q

Difference between p.d. And emf

A

Potential difference is a measure of work done by charge carriers

EMF is a measure of work done to charge carriers

31
Q

What should an ideal voltmeter and ammeter have as resistances

A

A voltmeter would ideally have infinite resistance

An ammeter would ideally have no resistance

32
Q

Describe the electron gun experiment

A

1) a metal filament (which acts as a cathode -ve) is heated using a p.d.
2) some electrons gain enough energy to escape from thermionic emission

3) they accelerate towards the anode
4) it is assumed all the work done on the electrons is converted to K.E.
5) W = VQ and Q for an electron is 1.6x10^19 (e)
6) thus, eV = 1/2 x mass x velocity ^2

33
Q

Define resistance and define the ohm

A

Resistance is the force opposing electrical current
Defined as the ratio between potential difference and current where
1 ohm = 1 VA^-1

34
Q

What happens to most components’ resistances when temperature increases and why

A

Resistance increases because the positive ions of the metal vibrate about their fixed postings more causing more collisions with the electrons thus increasing resistance as the charge carriers have to do more work

35
Q

I-V characteristic of a resistor

A

Straight line where a
low gradient = high resistance
High gradient = low resistance

36
Q

I-V characteristic for a filament lamp

A

Curved where it’s like a semi -n^2 on positive v and a semi n^2 on negative v

37
Q

What is the unique property of a diode and how is this possible

A

They only allow current in one direction by having a very large resistance in one direction and in the other (above a threshold value) the resistance is very low.
This is because above a certain P.D. the number of charge carriers increases

38
Q

I-V characteristic of a diode

A

Flat line then after Y axis curving upwards

39
Q

Resistance-temperature

characteristic of an NCT thermistor and why

A 
Decreasing curve (concave)
This is because in greater temperatures there is enough energy to release a greater number of charge carriers from the structure of the thermistor, thus allowing more charge to flow and decreasing resistance
40
Q

Resistance - light intensity curve for an LDR

A

Resistance decreases with light intensity because the energy releases more charge carriers

Concave curve down

41
Q

Methods of determining resistivity

A

1) measure the diameter of a wire using micrometers at different points and take an average
2) calculate cross-sectional area using A= pi x (d/2)^2

3) record the length of the wire
4) connect in series to a cell, with an ammeter in series and a voltmeter in parallel
5) calculate resistance using R = V/I
6) calculate resistivity using p = RA/L

OR

do 1,2,3,4
repeat steps 4,5 for different lengths
plot a graph of resistance against length
gradient is p/a

42
Q

I-V characteristic of a thermistor

A

Concave curve up in positive V

Convex curve down in negative v

43
Q

Why is there internal resistance and what is the only point when terminal pd = emf

A

When there is a current in a power source work is done so volts are “lost”

When current = 0

44
Q

Producing a variable PDout using different components

A

Thermistors and LDRs in series
For example if the temperature increases then the resistance of a thermistor will decrease this gives a lower p.d. Over the thermistor or a greater P.D over the other resistor which could be monitored

45
Q

What is the purpose of a potentiometer

A

It varies resistance and thus varies potential difference

46
Q

define mean drift velocity

A

the average change in displacement of an electron per unit time through a wire, in the opposite direction to conventional current. The electrons constantly collide with the lattice of metal ions

47
Q

what happens to the resistance of a parallel system when the resistance of one of its components decreases

A

the overall resistance decreases but not linearly

Physics year 12 (12 decks)

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