Electricity

Question 1

what is charge (Q)?

Answer 1

it is a fundamental property of an object measured in Coulombs (C)
the charge of an electron is 1.60*10^-19C

Question 2

what is current (I)?

Answer 2

it is the rate of flow of charge
measured in Amperes (A) using an ammeter (connected in series)

Question 3

what is the formula linking current, charge and time?

Answer 3

I = ΔQ/Δt

where:
I = current
Q = charge
t = time (s)

Question 4

what is potential difference?

Answer 4

the work done per unit charge
the higher the potential difference across a component, the higher the current through it
measured in Volts (V) using a voltmeter (connected in parallel)
1 V = 1 J C-1

Question 5

what is the formula linking potential difference, work done and charge?

Answer 5

V = W/Q

where:
V = potential difference
W = work done

Question 6

what is resistance?

Answer 6

the ratio of potential difference across a component to the current flowing through it
a measure of how much a component opposes the current flowing through it
measured in Ohms (Ω)

Question 7

what is the formula linking resistance, potential difference and current?

Answer 7

R = V/I

where:
R = resistance

Question 8

how can resistance be explained?

Answer 8

it is caused by the collisions between delocalised electrons carrying charge and metal ions in the wire/conductor

Question 9

what is power (and an equation)?

Answer 9

it is the rate of transfer of energy or the rate at which work is done
measured in Watts (W)
P = ΔE/Δt = ΔW/Δt

Question 10

what are the 3 equations that can be used to calculate electric power?

Answer 10

P = IV
P = (I^2)R
P = (V^2)/R

found by subsituting in variations of R = V/I

Question 11

how do you calculate total energy transferred to a component in a given time?
(hint: equation features current + potential difference)

Answer 11

E = I t V
(can get different variants by subbing in V = IR etc)

where:
E = energy transferred
I = current
t = time
V = potential difference

Question 12

what is Ohm’s law?

Answer 12

the current through a conductor is directly proportional to the potential difference across it provided the physical conditions (eg. temperature) do not change

Question 13

what is an ohmic conductor?

Answer 13

any conductor that obeys Ohm’s law and has a fixed resistance

Question 14

how can you measure the I-V characteristics of different components?

Answer 14

connect the component to a circuit that uses a rheostat as a potential divider (better than a variable resistor as this allows the full range of pds to be tested 0V-EMF of power supply)

Question 15

why use a ballast resistor when measuring the I-V characteristics of a diode?

Answer 15

used to limit the current through the diode to prevent a high current that can burn out the diode

Question 16

what is the I-V characteristic graph of an ohmic conductor?

Answer 16

a straight line through the origin (since current is directly proportional to pd)
a steeper gradient means lower pd
current is on the y-axis, pd on x-axis

Question 17

what is the I-V characteristic graph of a filament lamp?

Answer 17

• an s-shaped graph where gradient decreases as pd increases (shows resistance of filament lamp increases)
• gradient never = 0

Question 18

why does a filament lamp produce its I-V characteristic graph?

Answer 18

• as current flows through the filament, its temperature increases
• this causes atoms and ions to vibrate more in the wire and they collide more often with electrons flowing through the wire
• more collisions means more resistance in the wire
• this means there is less current per volt increase in pd so gradient of the graph decreases.

Question 19

what is the I-V characteristic graph produced by a diode?

Answer 19

it is like a deathslide - it has basically 0A of current until the pd is above a certain threshold voltage to let current flow through it (around 0.6V)
after the threshold pd current increases exponentially pretty much

Question 20

why does a diode produce its I-V characteristic graph?

Answer 20

it only lets current flow in one direction (the forward bias)
it has very high resistance in reverse bias (the opposite direction)

Question 21

what is a variable resistor?

Answer 21

a special resistor where the value of resistance can be changed

Question 22

what is a negative temperature coefficient (NTC) thermistor- how does it work + what graph does it produce?

Answer 22

• resistance decreases as temperature increases
• no of charge carriers (conduction electrons) increases with temp -> more current per volt of pd across thermistor -> smaller resistance

Question 23

what are thermistors used for and why?

Answer 23

• temperature sensing circuits
• change in resistance per kelvin is much greater than for a metal so its really effective

Question 24

what is a light dependent resistor (LDR)?

Answer 24

a special resistor where as intensity of light incident on it increases, its resistance decreases

Question 25

what is resistivity?

Answer 25

- measured in ohm metres - a measure of how difficult it is for a current to flow through a material - defined as the resistance of 1m of the material with a cross sectional area of 1 sq m

Question 26

what is the resistivity equation?

Answer 26

ρ = (RA)/L where: ρ = resistivity (Ωm) R = resistance A = cross sectional area L = length

Question 26

why does resistivity of a material change depending on temperature?

Answer 26

particles vibrate differently

Question 27

how does resistance change with temperature in a metal?

Answer 27

- it increases with temperature - positive temperature coefficient - higher temp -> atoms + metal ions vibrate more -> higher chance of collisions with charge carriers -> harder to pass through the metal -> less current per volt of pd across the material -> higher resistance - change in resistance with temperature is material dependent - may not always be a linear equation - pure metal - theoretically 0 kelvin = 0Ω resistance - impure metal - gets close to 0Ω but never 0

Question 28

what is a superconductor?

Answer 28

a material which has zero resistivity at and below a critical temperature which depends on the particular material

Question 29

what kind of temperatures are the critical temperatures of superconductors?

Answer 29

they are really low - the highest known one is -135 degrees C (cuprate)

Question 30

what happens when current flows through a superconductor below the critical temperature?

Answer 30

- it has zero resistance - since V=IR, there will be no pd as well - current flowing through the wire will not have any heating effect - no energy is lost

Question 31

what are some uses of superconductors?

Answer 31

- producing strong magnetic fields - MRI scanners, particle accelerators - power transmission cables

Question 32

why would superconductors be really good for power transmission cables?

Answer 32

- there would be much more efficient electrical energy transfer since none of it would be lost to heating - superconducting wires would also increase efficiency of electric motors and generators

Question 33

what is the law of conservation of charge?

Answer 33

charge cannot be created not destroyed, so the amount of charge in a closed system remains the same

Question 34

what is Kirchoff's first law? (the current one)

Answer 34

the sum of currents entering a junction is equal to the sum of currents leaving a junction ΣI(in) = ΣI(out)

Question 35

what is potential difference between 2 points on a circuit?

Answer 35

the energy transferred per coulomb of charge from one point to another

Question 36

what is a potential gain vs a potential drop?

Answer 36

- potential drop = pd between 2 points on the circuit if charge carriers lose energy - potential gain = pd between 2 points on the circuit if charge carriers gain energy

Question 37

what is Kirchoff's second law? (the pd one)

Answer 37

the sum of emfs/potential gains is equal to the sum of pds/potential drops around any closed loop - works due to the law of conservation of energy Σpd = Σemf

Question 38

how do you find pd using pens of potential?

Answer 38

1. find negative terminal of the cell - assign any wire in contact with it a pd of 0V 2. label wires in contact with the positive terminal of the cell the same pd as the cell 3. divide potentials between different components 4. use different colours for each different potential in the wires

Question 39

how does potential difference get split up in series vs in parallel?

Answer 39

- series: total pd across all components = sum of pd of each component - parallel: pd across all components is the same (or each branch)