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🔱Physics > 3. Electricity > Flashcards

3. Electricity Flashcards

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1
Q

What’s the charge of a single electron?

A

-1.6x10^-19C

As there’s 6.25x10^18 electrons in 1C of charge
As 1C = nx1.6x10^-19 (Q=ne)

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2
Q

Amount of electrons to for 1C

A

6.25x10^18
As Q=ne

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3
Q

How electric current occurs

A

Experiences an electric force, like a battery, accelerating it (which is the voltage)
So the voltage push the electrons around, making a current

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4
Q

How do electrons pass voltage?

A

The elections carry energy, in coulombs
They give the energy within the coulombs to the lamp
Then leaves a potential difference (p.d) across the lamp

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5
Q

What is potential difference?

A

The difference in potential energy before vs after given to the circuit
As potential energy is the energy from each coulomb transferred
It’s the work done per unit of charge
V=E/Q

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6
Q

What is Ohm’s Law?

A

States that R is constant
So Voltage is directly proportional to Current
V=IR

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7
Q

What’s an Electronvolt (eV)?

A

eV is a unit of energy (J)
It’s the energy held by 1 electron
E=VxQ
Q=e, so 1.6x10^19
as thats the charge of 1 electron

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8
Q

What is Electromotive force (emf)?

A

emf is any type of dry cell that provides electrical energy
Measured in Volts
ε=E/Q same as p.d
But counts internal resistance inside of emf

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9
Q

How do batteries work?

A

Batteries don’t have electricity inside, just chemical energy
- stored in lithium
frees electrons in chemical reaction between metal and electrolyte
transfers chemical to electrical energy

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10
Q

What is Work Done, W?

A

Work done is the same as energy transferred
W=VQ
As W=E

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11
Q

What is electrical Power, P?

A

Power (Watts,W) is the rate of which energy (J) is transferred
P=E/t
so E=Pxt
W=Pxt

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12
Q

What is power in terms of voltage and current?

A

As P=E/t
P=VQ/t
P=VIt/t
so P=VI

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13
Q

What other power formulas can be derived?

A

P=I^2R
P=V^2/R

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14
Q

What’s a conventional current?

A

A current that moves from the positive terminal to negative
Were chosen during discovery of electricity
Used in exams like this
In reality, electrons in a d.c current flows from negative to positive terminal

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15
Q

What is a diode?

A

A diode only lets a current flow one way
Called semi-conductor diodes
Extremely high resistance in other direction
Gradient of diode, m=1/R
Has a threshold voltage, where it needs enough current for loads of current to flow through it
E.g a Light Emitting Diode (LED)

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16
Q

What’s the threshold voltage?

A

The voltage where the diode can let electrons go
So where the gradient shoots up

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17
Q

How does the colour of the LED effect the diode?

A

It changes the threshold voltage, due to the frequency of the light
Red has a lower frequency than violet, so less energy is needed
- so there’s a lower threshold voltage needed

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18
Q

What is resistivity?

A

Resistivity (p (rho)) is a measure of how easily a material conducts electricity
so its how easily electricity passes through a material (fixed value)
p=RA/L
The higher the resistivity = harder it is to move through substance

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19
Q

What is conduction of electricity?

A

Allows electricity to flow through it
So a conductor is a substance that allows electricity to flow through it

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20
Q

What is resistivity dependant on?

A

dependant on environmental factors
- e.g tempuarture

21
Q

What is the standard wire gauge (swg) of a wire mean?

A

A measurement of how thick a wire is
The higher the swg, the thinner the wire

22
Q

How do electrons actually move?

A

Electrons actually move slowly
- around 0.02 cm per sec
They all basically touch/ repel each other, and current is very fast as they are all pushed up very fast.

23
Q

What is the drift velocity?

A

The drift velocity is the average velocity of the electrons through a wire
Overall, is a slow movement
v=L/t

24
Q

How do electrons move in a metal conductor?

A

Metals have a regular lattice structure with the atoms sharing electrons.
This leaves spare electrons, that are pushed towards the positive end of the field with an emf

25
Q

What’s the transportation equation?

A

Finds the current (I) exactly, using the amount of electrons flowing (nAL)
It finds the exact charge of the whole wire (nALe) divided by time
As current is the rate of the change in charge
so I=nALe/t
I=nAve / I=nqvA , where q=e

26
Q

What happens to the coulombs if the circuit splits into 2 separate ways?

A

The coulombs double
This is why the V is equal on all the different branches

27
Q

How does V work in parallel circuits?

A

When there’s 2 different ways of the circuit, the coulombs double, leading to voltages being the same as V(total) on each segment, as all voltage is used up (Kirchoff’s 2nd law)

28
Q

How does R work in parallel circuits?

A

As V is constant in a parallel circuit,
I(1)=V/R(1)
I(total)=V(1/R(1)+1/R(2)+1/R(3)…)
As I/V=1/R
1/R(total)=1/R(1)+1/R(2)+1/R(3)…

29
Q

What’s Kirchhoff’s 1st Law?

A

The total current entering a junction = total leaving it
I(1)=I(2)+I(3)

30
Q

What’s Kirchhoff’s 2nd Law?

A

All the voltage is used up in a circuit
So the total emf = sum of drops in a closed circuit

31
Q

How does V work in series circuits?

A

The total p.d across the resistors = the p.d of the emf
V(t)=V(1)+V(2)+V(3)…

32
Q

How does R work in series circuits?

A

The combined R is the sum of the separate resistors in series
R(t)=R(1)+R(2)+R(3)…

33
Q

How does current get used?

A

Due Kirchhoff’s 1st Law, it splits up at junctions, then it travels through all the resistors on each branch.
Most the current will go where there’s less resistance, as V=IR

34
Q

What is a potential divider circuit?

A

A potential divider circuit is just a circuit with more than 1 resistor
So it splits its voltage up, as the resistors will share it
You can put a variable resistor in place to control the V coming out the other one, V(out)

35
Q

How do we find V(out) on a potential divider circuit?

A

The ratio of V = ratio of R
due to V=IR so resistance is proportional to voltage
So…
R(2)/R(t)=V(out)/V(in)
As R(t)=R(1)+R(2) then:
V(out)=V(in) x R(2)/R(1)+R(2)

36
Q

What is a variable resistor (VR)?

A

VR’s can be used to
- control the voltage (potentiometer)
- control the current (rheostat)
Used in potential divider circuits to change an output
Consists of a resistance wire and a sliding contact
Consists of A,B (fixed) and S (variable)

37
Q

What is a rheostat?

A

A VR used to control current
It connects AS, instead of AB
This shortens the length of the wire
The shorter the length of wire, the lower the resistance, so the higher the current

38
Q

What is a potentiometer?

A

A VR used to control the voltage
Uses all 3 - A, B, S
It turns the one VR into 2 different resistors.
V(out) is measured from AS
so the bigger AS, the longer the wire, so the bigger proportion of R, so the bigger V(out) is

39
Q

What is a light dependant resistor (LDR)?

A

An LDR is used to detect light
It turns on when there is not enough light
- By increasing its resistance on V(out) so it releases more V for a light source
So R is indirectly proportional to light intensity
The higher the light intensity, the less light is needed, so the lower R
E.g - security lights, patio lights

40
Q

What is a thermistor?

A

A thermistor changes it’s resistance depending on temperature
Theres 2 types:
- Negative Temperature Co-efficient
(NTC)
- Positive Temperature Co-efficient
(PTC)

41
Q

What is a Negative Temperature Co-efficient (NTC)

A

As the temp increases, their R decreases
so R is indirectly proportional to temp, hence Negative TC
Works with insulators and silicon
- At room temp, there are few electrons for conduction
- electrons are released from atoms at high temp
- R decreases as electrons join current
However, silicon breaks down permanently at around 150 C
More electrons released than lattice vibrations

42
Q

What is a Positive Temperature Co-efficient (PTC)

A

As the temp increases, their R increases
so R is positively coefficient with temp, hence Positive TC
Works with metal conductors
- They contain large numbers of free electrons in metallic sturcure
- they collide with vibrating metal ions
- oppose flow of electrons
- higher temp, more vibration
lattice vibrations are more intense than electrons released
so higher temp, higher R

43
Q

What is internal resistance?

A

internal resistance is the small resistance inside of the emf
called ‘r’
Electrons collide with atoms inside of battery
- leads to energy being lost before it leaves the battery
We put a voltmeter around the battery, so it doesn’t take internal resistance into account

44
Q

How can we find internal resistance?

A

As ε=I(R + r) (includes internal r here separate)
ε=V+Ir
V=-Ir+ε
can be used as y=mx+c
where ‘-r’ is the gradient and ‘ε’ is the y-intercept

45
Q

What is efficiency?

A

Efficiency = useful energy/total energy x 100
E.G Efficiency = GPE/Powerxtime
mgh/Pt

46
Q

How do electrons in LDR’s work?

A

They work very similar to NTC’s
When the light intensity increases, the electrons within the LDR gain energy from the light
These electrons then can leave the LDR and increases the number of electrons in conduction
Reduces the resistance in the LDR

47
Q

What is current (I)?

A

Current is the rate of charge per second
Q=It
I=Q/t

48
Q

What is charge, Q?

A

Charge is the total amount of electrons flowing through a circuit
- as electrons are the only thing carrying this negative charge
Q=ne
So Q is always the total it can hold
E.g the charge of a phone could be 6500C

49
Q

What is a photocell?

A

A type of LDR

🔱Physics (12 decks)

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