P9 Flashcards

(31 cards)

1
Q

pd

A

energy transferred per coulomb of charge
i.e when electrical energy is transferred to other forms e.g thermal energy
work is done by charge carriers so essentially they LOSE energy as they pass through components

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2
Q

emf

A

total energy gained by charge carriers per coulomb of charge
work is done ON the charge carriers e.g gaining energy when passing through battery
more emf means more energy per coulomb transferred from chemical to electrical energy

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3
Q

emf equation

A

E = W/Q
Q = charge
W = energy transferred by charge
usually total pd = total emf
W = VQ or W = EQ

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4
Q

How do electron guns work

A

Small metal filament heated by electric current
electrons gain ke, some gain enough to escape from metal surface - thermionic emission
placed in vacuum and high pd between filament and anode then filament is cathode, so electrons accelerate to anode gaining ke
if anode has small hole then electrons can pass through creating a beam with specific ke

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5
Q

equation linking current energy to kinetic energy

A

eV = 1/2mv^2
work done on electron/ charge carriers = gain in kinetic energy

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6
Q

what is resistance

A

all components resist the flow of charge carriers through them
energy is required to push electrons through a component, and resistance shows how much energy is required to do so (probably energy per coulomb of charge)

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7
Q

Ohms law

A

for a metal conductor at constant temperature the current in wire is directly proportional to pd across ends

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8
Q

Why does resistance vary with temperature

A

More heat means positive ions in wire have more internal energy and vibrate higher amplitude around mean positions.
Collisions between charge carriers and positive ions increase so charge carriers do more work - transfer and ‘lose’ more energy as they travel through the wire, hence more resistance

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9
Q

to investigate IV characteristics circuit set up

A

connect battery ammeter component and variable resistor in series, and voltmeter parallel to component

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10
Q

Fixed resistors (including e.g wires) IV graph

A

Direct proportion line - ohmic conductor as obeys law
behaves same way regardless of polarity
wires can be seen as resistors with very low resistance

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11
Q

Filament lamp IV graph

A

Increases with decreasing rate of increase (opposite for 3rd quadrant)
non-ohmic, unaffected by polarity
because it heats up

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12
Q

Diode

A

only allows current in one direction

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13
Q

LED

A

emit light when they conduct
of a single specific wavelength i.e colour
different threshold pd related to colour
take very little energy to run so often use as indicators of current direction - light up shows that part of circuit is live

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14
Q

LED IV graph

A

flat in 2nd quadrant then exponential rise after about x = 0.7 (for silicon, thresholds vary slightly)
resistance very high (aka infinite) when voltage is wrong direction
non-ohmic, non-constant resistance

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15
Q

resistivity

A

electrical property of a material

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16
Q

4 factors that affect resistance in wire

A

temperature
material
length
CSA

17
Q

what is resistivity

A

electrical property of a material - which affects resistance

18
Q

equations for resistance and resistivtt

A

R = pL/A
or
p = RA/L
(p is resistivity)

19
Q

Experiment to find resistivity

A

Find I, V and then R of a wire with different lengths then draw a graph where the gradient will be p/A (because p = gradient x A)
so times gradient by CSA to get resistivity

20
Q

thermistor function

A

semiconductor component with a negative temperature coefficient i.e resistance drops as temperature increases
as more electrons are freed so number density of charge carriers increases - less R more I

21
Q

uses of thermistors

A

thermometers and thermostats
monitor and measure temp - device components/ engines from overheating, electrical homeware devices

22
Q

how to detect thermistor resistance setup

A

put thermistor in parallel with ohmmeter then measure resistance at different temperatures using a water bath
usually resistance change will be biggest over a small range of temperatures

23
Q

thermistor IV graph

A

positive correlation, unaffected by polarity, increases with an increasing rate of increase
similar to lamp but curved other way

24
Q

LDR function

A

higher light intensity means more electrons freed so higher number density - less R more I

25
light intensity vs resistance of LDR
as light intensity increases resistance decreases with a decreasing rate of decrease
26
what happens when current passes through a component
electrical energy is transferred from power source to that component - e,g electrical to heat and light in a lamp, or sound and heat in a loudspeaker
27
How to derive P = VI
P = W/t and V = W/Q so W = VQ P = VQ/t as Q/t = I then P = VI subsequently W = VIt
28
what is a kWh
the energy transferred by a device with power 1kW running for 1 hour 1 kWh = 3.6MJ
29
Equation for kwh and SI units
E = power in kW x time in hours J = W x s
30
How are electricity bulls calculated
operating companies set a base price per kWh and charge this and possibly tax prices usually between 6-15p per kWh
31
resistivity values for insulators and conductors (ohm meters)
conductors - x10^-8 insulators - x10^-16 semi conductors somewhere in between