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Physics (AS) > Chapters 4.1, 4.2, 4.3 - Electricity > Flashcards

Chapters 4.1, 4.2, 4.3 - Electricity Flashcards

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1
Q

Electric Current

A

The rate of flow of charge

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2
Q

Coulomb

A

Unit of charge - 1 Coulomb of charge will pass a point in 1 second by a current of 1 Ampere

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3
Q

Charge on an electron and proton

A

Electron: -e
Proton: +e

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4
Q

What is the net charge on an object always a multiple of and what is this called

A

Net charge is always a multiple of e. This means it is quantised

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5
Q

Physically, what is current in a metal

A

Movement of electrons

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6
Q

Physically, what is current in an electrolyte

A

Movement of ions

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7
Q

Kirchoff’s first law and associated conservation

A

Sum of the currents entering a junction is equal to the sum of the currents leaving a junction (conservation of charge)

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8
Q

Charge carrier

A

What carries the charge in a current (electron/ion)

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9
Q

Mean drift velocity

A

The average velocity of all electrons in a section of a circuit

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10
Q

Current equation in terms of electron drift velocity

A 
I=Anev
where
I is current
A = cross sectional area
n = number of charge carriers per unit volume
e = charge on a charge carrier
v = mean drift velocity
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11
Q

Distinction between conductors, semiconductors and insulators in terms of n

A

Conductors - high n
Insulator - very low n (perfect insulator would have n=0)
Semiconductor - low n

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12
Q

Resistance

A

Energy per coulomb needed to pass a current of 1A through

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13
Q

Ohms law

A

V=IR

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14
Q

Resistivity of a material

A

The resistance of a 1m length of the material with a cross sectional area of 1m^2

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15
Q

Equation for resistance in terms of resistivity

A 
R = ρL/A
where
R is resistance
ρ is resistivity
L is length
A is cross sectional area
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16
Q

Typical order of magnitude of resistivity

A

Very small e.g. 10^-8

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17
Q

Ohmic conductor

A

A conductor which obeys ohms law.

The potential difference across a conductor is directly proportional to the current in the component as long as its temperature remains constant

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18
Q

Experiment to determine resistivity of a wire

A

Determine cross sectional area of wire standard way. Set up a circuit with ammeter in series and voltmeter around the wire. Have one end of the wire connected via crocodile clip that can be moved up and down the wire and have the wire set up next to a ruler. Attach the crocodile clip at various places along and record the current, potential difference and length at each point. Calculate resistance as V/I. Plot R against L. Gradient will be ρ/A

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19
Q

How to improve accuracy of experiment to determine resisrtivity

A

Keep the wire at a constant temperature by only using a small current

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20
Q

Energy transfer in a circuit

A

Work done = potential difference * charge

W = VQ

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21
Q

How to calculate the kinetic energy of an electron that is accelerated over a potential difference

A

W=VQ
W=Ve
(1/2)mv^2=Ve
So the kinetic energy of an electron is equal to the potential difference * its charge

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22
Q

How to investigate I-V characteristics of a component

A

Set up a circuit with an ammeter in series and a voltmeter around the component. Also have a variable resistor in series. Vary the resistance to change the potential difference over the componenet

23
Q

I-V characteristic for a normal resistor

A

Straight line through origin

24
Q

What is the gradient of an I-V graph

A

1/Resistance

25
Q

I-V characteristic for a filament lamp

A

Curved through origin in shape of a sigmoid function. Increasing the voltage increases the temperature and therefore the resistance, hence the gradient gets shallower

26
Q

How temperature affects resistivity of a metal and why

A

High temperature increases resistivity. This is because the positive ions in the metal vibrate more and therefore collide more with electrons, which causes the electrons to lose energy

27
Q

Explain resistivity of a semiconductor

A

Lower than that of a metal due to fewer charge carriers. When energy is supplied, electrons are released since they are given just enough energy, meaning that there are more charge carriers and therefore lower resistivity.

28
Q

Resistance-Temperature characteristic of a thermistor (NTC)

A

y=1/x

Temperature increase results in resistance decrease, so it is the inverse of a normal resistor

29
Q

I-V characteristic of a thermistor (NTC)

A

Looks like a x^3 graph

30
Q

I-V Characteristics of a LDR

A

Essentially the same as a thermistor in all ways except the energy is provided by light instead of thermal energy

31
Q

What does a diode do

A

Only allows current to flow in one direction and only allows the current to flow in that direction if it has a P.D higher than the threshold voltage (minimum voltage to start conducting)

32
Q

I-V characteristic of a diode

A

Very shallow line in negative P.D. In positive P.D. it is 0 up until the threshold voltage where it starts increasing rapidly

33
Q

Forward bias

A

The direction that current is allowed to flow in a diode

34
Q

Equation for power

A 
P=IV
where
P is power
I is current
V is voltage
35
Q

Equation for total work done by a circuit

A 
W=Pt
or
W=VIt
or
(many others from rearranging V=IR)
36
Q

kWh

A

The work done by something with a power of 1 kilowatt in 1 hour = 3.6 million joules

37
Q

One unit of energy

A

One kWh

38
Q

Electromotive force

A

The amount of work a power source does on each coulomb of charge

39
Q

Transformation of energy over a potential difference

A

Electric to another form

40
Q

Transformation of energy by the electromotive force

A

Another form to electric

i.e chemical > electrical

41
Q

Internal resistance

A

Resistance that is inherent to the power source

42
Q

Why is terminal p.d. different to the e.m.f.

A

Some of the potential difference is lost overcoming the internal resistance

43
Q

Lost volts

A

The energy wasted per coulomb overcoming the internal resistance (e.m.f. - terminal p.d.)

44
Q

What is the equation ε = V + v referring to

A

ε is the e.m.f.
V is the terminal p.d.
v is the lost volts

45
Q

What is r and R

A

r is the internal resistance

R is the load resistance (external resistance)

46
Q

effective e.m.f. of cells in series

A

ε + ε + ε…

47
Q

effective e.m.f. of identical cells in parallel

A

ε (no effect)

48
Q

Experiment to determine internal resistance

A

Set up a circuit with a variable resistor, ammeter and voltmeter around the resistor. Vary the resistance and record the p.d. and current and plot them on a graph (with p.d as the vertical axis).
ε = V +Ir
V = ε - Ir
therefore y intercept is ε and gradient is -r

49
Q

Kirchoff’s second law and associated conservation

A

Sum of p.d. in a closed loop (conservation of energy) equals sum of e.m.f in a closed circuit .

50
Q

hopefully u can analyse circuits

A

i hope so

51
Q

Explain potential divider

A

Two resistors in parallel divide the potential among them, proportional to their resistance. Decreasing the resistance of one will decrease the p.d. across it and therefore increase the p.d. across the other resistor (by kirchoff’s 2nd)

52
Q

How a potentiometer works

A

It is just a resistor in series but with a wire running parallel to it. Whats the catch you say? Well the catch is that one end of the wire is just touching the resistor at some point along it and this distance can be change by a slider, essentially simulating a potential divider

53
Q

How to investigate a potential divider

A

Vary the resistance of one of the resistors and record the p.d. across the other resistor as well as the current. Resistance could be varied by the resistor being a thermistor, LDR e.t.c

Physics (AS) (14 decks)

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