Electricity Flashcards
Which way do electrons flow in a circuit
Positive end to negative end
State what is Ohm’s Law
Current flowing through a metallic conductor is directly proportional to the potential difference across it at a constant temperature
Define potential difference (the volt)
Potential difference is amount of energy transferred when one coulomb of charge is moved between the two points
Define power and state it’s units
Rate of doing work / rate at which energy is transferred
Joules per second = watts
Draw a current voltage graph for a fixed resistor, a diode and a bulb
Differences between a variable resistance and a potential divider
Why does resistance increase for a filament bulb
Filament is made of metal which is made up of metal ions bonded together in a lattice
Electrons have to pass through gap in lattice
As current increases, temperature of filament increases, more number of collapses per second
Causing lattice ions to vibrate more, making it harder for electrons to pass through collision
Why does resistance decrease for a diode
Diode is a semi conductor
Electrons in a semi conductor are weakly bound and require some energy to become free from semi conductor
At a threshold voltage, electrons gain enough energy to become free, so current increases
As vintage increases current increases
Define resistance
Ratio of voltage applied to a current through a material
Name one practical use for a semi-conductor diode
Ensuring current only flows through one way
Rules for series circuit
Rules for parallel circuit
What is Kirchoffs first law
At any junction in a circuit sum of current flowing into the junction of equal to the sum flowing away from it
What is kirchoffs second law
In any complete loop of a circuit sum of p.d’s equals the source p.d
Calculations for resistors in series
Calculation for resistors in parallel
State the rules applied here
What happens to p.d across resistors when resistors in series
P.d will be equally divided across them as largest resistance will get largest p.d
What is meant by NTC (negative temperature coefficient)
State difference of PTC
- what should you assume a thermistor is
As temperature increases, resistance of thermistor decrease
- unless told otherwise, assume it is a NTC
Rule for LDR, light dependent resistor
As light intensity increases, resistance of LDR decreases
Even though 10 ohm resistor and 50 ohm resistor have different voltage and resistance, they must have same current as they are in series so simply do 12/60
Total current needs to be found, not current for single resistor only
Units for charge
Coulombs
Explain what is meant by a potential divider
Two or more resistors placed in series with a power supply
To provide a variable voltage output
Skip part a)
State and explain what happens to ammeter reading when temperature of thermistor increases
If temperature increases, resistance of thermistor decreases, so overall resistance in circuit decreases
From Ohms law, if total resistance decreases, total current increases so ammeter reading increases
What can a rheostat be used for
State what happens if rheostat slides to the right
This circuit represents a rheostat as a potential divider. State what the red arrow represents
Position of the slider
If each strip is 2.6mm wide and 1.1mm thick, what is the area of one strip
Simply times them together
Resistance is 120,000
Find voltage at each resistance
What is a superconductor
Superconductors are materials that conduct electricity with zero resistance and below a critical temperature
Graph for a superconductor
State use of a superconductor
In electrical heats to support high processing speeds
- because below critical temperature, resistance is zero. No energy is wasted as heat
What does emf mean
Amount of energy per unit charge produced inside a cell
What is T.P.D, terminal potential difference
Voltage measured across terminals of a cell
How does power link to the internal resistance equation
Explain what is meant by an emf of 0.7 volts
0.7 Joules of work done by cell per 1 coulomb
B
As potential difference defined as amount of energy per unit charge
Explain why when a battery is supplying a current to a circuit, voltage measured between its terminals is not equal to emf
Resistance in battery so work is done in battery.
Less energy available for external circuits, as there is lost voltage between terminals
If there is a voltmeter connected across battery, state reading when switch is open and reading when switch is closed
Open : emf
Closed : potential difference across circuit, not taking lost volts in internal resistance into account
Why does ammeter reading increase when a motor does more work
More energy needed to provided
We know V = e - Ir, where e is the emf, I is the current and r is the internal resistance
Re arrange for y=mx + c
V = -Ir + e, hence gradient is -r, so A is correct answer
Since diodes are ideal, current only flows through one way and doesn’t go through both junctions
V=IR
12/60 = 0.2, C
C
What happens to a resistance of a filament lamp when current increases, explain
Resistance increases
Electron collisions increase lattice vibrations
More frequent collisions at larger target for electrons
Is a filament lamp an ohmic conductor
An ohmic conductor had current and voltage as proportional and resistance as constant
Filament lamp isn’t as they are not proportional and resistance isn’t constant
Resistance of PQ combination increases and recieves a greater share of voltage, voltage across R decreases
What is meant by potential difference
A combination of resistors in series connected across a voltage source to meet a required potential difference
Define Volt
1 joule per coulomb
Total current = 2.25mA
Use R1 resistance, V = 4.95
Benefit of using high value resistance in a potential divider circuit
Reduced current
Potential difference across a length of cable of 1km is 75 V. Find total power output lost by cable
State signifance of critical temperature of material
Below critical temperature, resistance is zero
What is meant by a superconducting material
No resistance at or below a critical temperature
State an application of a superconductor and explain why is it useful for this application
Particle accelerators, computers
No energy dissipation, high processing speeds
Define emf
Work done per unit charge where charge moved whole way round circuit
For part b) total reasistance decreases
If volume is constant then length 1 x area 1 = length 2 x area 2
If length 2 is 2 times bigger than length 1, then area 2 must be a half of area 1
Sub length 2 and area 2 into resistivity equation to get 4R
How does power link to work done
Power = work done / time
What is meant by internal resistance
Work done by battery per unit charge
Or
Loss of voltage per unit current
X and Y are now joined together in a wire, calculate new reading on ammeter
12 divided by 160 = 75mA
High resistance voltmeter connected between points X and Y
What is the reading on the voltmeter
CHATGPT actually did it with A and B but could also be done with C and D
Current = 2 Amps
Each cell provides same amount of electrical energy before going flat. State which two cells in the circuit would you expect to go flat first
Simplest way to explain is, Electrons in circuit will always have to go through C and D but only need to go through one of A or B
Explain why current of lamp decreases between 0.05 and 0.5 seconds
Resistance increases
As temperature increases as number of collisions increases
State and explain why a filament lamp is most likely to fail as it is switched on
Critical temperature
b) main property of superconductor
Temperature which resistance is zero and it becomes a superconductor
b) a material which has zero resistance
Why it benefits a battery to have a low internal resistance
Less energy wasted
Lets say a voltmeter is added to a circuit and provides a reading. Resistance of resistors stay same as voltmeter connected between resistors. They battery is changed for one with same emf and but a significant resistance. Why does voltmeter reading stay the same
lost volts and reduced current
voltmeter has high/infinite resistance so does not affect circuit
resistance not changing so ratio remains the same
explain why I-V graph for a filament lamp has both of them increasing but not proportionally as it is a curve
An increase in current/voltage leads to an increase in temperature (more heat generated)
This causes an increase in the movement of the
lattice/ions/atoms
And therefore an increase in the rate of collisions with electrons
So the resistance increases as shown by V/I changing/V not proportional to I (on the graph)
When asked for resistance in parallel or series circuit, how do you find it
Total V / Total I
As 3.5 ohms is close to 3.7, this is suitable for making the heating element
Photo does not contain solution for 6.2)
6.2) no potential difference in E, so no currentin E, total current does not change so other lamps are not affected
A
C - because total resistance will increase and due to equation I=V/Rtotal, current decreases so flow of charge decreases
A
C
C
A
B
B
D - as 14 + 9 = 23
As it is a parallel circuit
As temperature of copper wire increases, its resistance…
Increases
As copper is a metal and has free electrons. As temperature increases metal ions vibrate more due to increase in kinetic energy. There’s an increase in frequency of collisions disturbing flow of electrons. Hence resistance increases as it becomes harder for current to flow
Look at branch ammeter is connected in
D
