Chapters 4.1, 4.2, 4.3 - Electricity Flashcards by Michael Matthews

Electric Current

The rate of flow of charge

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Coulomb

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

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Charge on an electron and proton

Electron: -e
Proton: +e

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

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

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Physically, what is current in a metal

Movement of electrons

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Physically, what is current in an electrolyte

Movement of ions

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Kirchoff’s first law and associated conservation

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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Charge carrier

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

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Mean drift velocity

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

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Current equation in terms of electron drift velocity

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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Distinction between conductors, semiconductors and insulators in terms of n

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

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Resistance

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

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Ohms law

V=IR

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Resistivity of a material

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

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Equation for resistance in terms of resistivity

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

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Typical order of magnitude of resistivity

Very small e.g. 10^-8

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Ohmic conductor

A conductor which obeys Ohms law (when at a constant temperature)

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Experiment to determine resistivity of a wire

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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How to improve accuracy of experiment to determine resisrtivity

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

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Energy transfer in a circuit

Work done = potential difference * charge

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

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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How to investigate I-V characteristics of a component

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

I-V characteristic for a normal resistor

Straight line through origin

What is the gradient of an I-V graph

1/Resistance

I-V characteristic for a filament lamp

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

How temperature affects resistivity of a metal and why

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

Explain resistivity of a semiconductor

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.

Resistance-Temperature characteristic of a thermistor (NTC)

y=1/x | Temperature increase results in resistance decrease, so it is the inverse of a normal resistor

I-V characteristic of a thermistor (NTC)

Looks like a x^3 graph

Characteristics of a LDR

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

What does a diode do

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

I-V characteristic of a diode

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

Forward bias

The direction that current is allowed to flow in a diode

Equation for power

``` P=IV where P is power I is current V is voltage ```

Equation for total work done by a circuit

``` W=Pt or W=VIt or (many others from rearranging V=IR) ```

kWh

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

One unit of energy

One kWh

Electromotive force

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

Transformation of energy over a potential difference

Electric to another form

Transformation of energy by the electromotive force

Another form to electric

Internal resistance

Resistance that is inherent to the power source

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

Some of the potential difference is lost overcoming the internal resistance

Lost volts

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

What is the equation ε = V + v referring to

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

What is r and R

r is the internal resistance | R is the load resistance (external resistance)

effective e.m.f. of cells in series

ε + ε + ε...

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

ε (no effect)

Experiment to determine internal resistance

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

Kirchoff's second law and associated conservation

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

hopefully u can analyse circuits

i hope so

Explain potential divider

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)

How a potentiometer works

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

How to investigate a potential divider

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