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Physics A Level > 3 Electricity > Flashcards

3 Electricity Flashcards

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1
Q

Equation linking charge current and time (learn)

A

Q=It

Charge= current x time

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2
Q

What is one coulomb defined as

A

The amount of charge that passes in 1 second when the current is 1 amp

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3
Q

How do you attach an ammeter to a circuit?

A

In series

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4
Q

Equation for potential difference work done and charge

A

V= W/Q

Potential difference = work done/ charge

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5
Q

What is the maximum value that an ammeter or voltmeter called?

A

The full scale deflection

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6
Q

How should a voltmeter be used?

A
  • In parallel

* very large resistance

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7
Q

Does potential difference split or stay the same in parallel circuits?

A

Stay the same

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8
Q

Charge of an electron

A

-1.6x10^-19

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9
Q

What equation gives the mean drift velocity of electrons

A

I=nAve

Current=number density of charge carriers x cross sectional area x mean drift velocity x charge on each charge carrier

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10
Q

Why is drift velocity in semi conductors higher?

A

They have fewer charge carriers so the drift velocity needs to be higher to produce the same current

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11
Q

What is resistance?

A

A measure of how difficult it is to get a current to flow through it

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12
Q

Equation linking resistance voltage and current

A

V=IR

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13
Q

What things determine resistance?

A

Length- longer the wire, higher the resistance
Area- the wider the wire, the lower the resistance
Resistivity- depends on material and temperature

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14
Q

Equation for resistivity

A

R=pl/A

Resistance=resistivity x length/area

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15
Q

How can you determine the resistivity of a wire?

A

1) use a micrometer to measure diameter of wire in 3 different places
2) use diameter to calculate cross sectional area
3) the test wire should be clamped to a ruler and connected to a circuit using crocodile clips
4) record the length of wire connected in the circuit, the voltmeter readings and ammeter readings
5) use V=IR to calculate resistance for each length
6) plot graph of resistance vs length
7) multiply gradient by cross sectional area to get resistivity

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16
Q

Describe the circuit should you set up to measure current and voltage across a component

A

Power supply to variable resistor to component with voltmeter in parallel to ammeter

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17
Q

IV characteristics of a metallic conductor

A
  • ohmic device
  • directly proportional with constant resistance
  • gradient=1/resistance
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18
Q

IV characteristics of a tungsten filament wire

A
  • gradient starts steep but gets shallower as voltage increases
  • resistance increases with temp
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19
Q

How does temp affect resistivity?

A

1) charge is carried through metals by free electrons in a lattice of +ve ions
2) heating up a metal makes the ions vibrate more, meaning the electrons collide with them more, transferring KE into other forms
3) therefore the electrons drift velocity decreases, so current decreases and resistance increases

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20
Q

Why do semiconductors have higher resistivity than metals?

A

They have fewer charge carriers

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21
Q

When happens to a semi conductor when it is heated

A

More charge carriers are released so current increases and their resistance and resistivity decreases

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22
Q

How are semi conductors used as sensors?

A

When temp increases their resistance and resistivity decrease while their current increases

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23
Q

What are three types of semi conductors

A

Thermistors diodes and LDR’s

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24
Q

What happens to resistance in a NTC thermistor as temp increases

A

Resistance decreases

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25
Q

IV characteristics for a thermistor

A
  • gradient starts shallow but becomes steeper as voltage increases
  • gets steeper because it warms up so resistance decreases
26
Q

Why does resistance in thermistors decrease when temp increases?

A
  • more heat means more energy to electrons
  • this allows them to escape from their atoms
  • this means there are more charge carriers available
  • so current increases and resistance decreases
27
Q

How does light affect an LDR?

A

The greater the light the lower it’s resistance

28
Q

How does light affect resistance in an LDR?

A
  • light provides energy to electrons
  • this allows them to escape from their atoms
  • this means there are more charge carriers
  • so higher current and lower resistance
29
Q

IV characteristics of a diode

A
  • starts shallow gradient
  • as pd increases current shoots up to give a very low resistance
  • only allows current to flow in one direction
30
Q

Equation linking power voltage and current

A

P=VI

31
Q

Equation for power current and resistance

A

P=I^2R

32
Q

Equation linking energy voltage current and time

A

E=VIt

33
Q

Load resistance

A

The total resistance of all the components in the external circuit

34
Q

Equation linking emf charge and energy (learn)

A

EMF=energy/charge

35
Q

What is the emf

A

The amount of work the battery does to each coulomb if charge

36
Q

Equation linking emf pd current and resistance (learn)

A

Emf=V +Ir

37
Q

How can you work out the total emfs of cells in series?

A

Emf total =emf1 +emf2 +emf3

38
Q

What is true of the emf of cells in parallel

A

For identical cells the emf total= emf1=emf2

39
Q

How can you investigate internal resistance and emf with a circuit?

A
  • set up a circuit with a battery an ammeter and a variable resistor with a voltmeter across it
  • vary the current in the circuit using the variable resistor
  • measure the pd for several different values of current
  • plot v against I
  • the intercept of the y axis is emf
  • the gradient is -r
40
Q

Kirchhoffs first law

A

Current into junction= current out of junction

41
Q

Kirchhoff second law

A

The total emf around a series circuit= the sum of the pds across each component

42
Q

Characteristics of a series circuit

A

1) same current at all points
2) emf split between components so emf=V1+V2+V3
3) because I is constant R total=R1+ R2+ R3

43
Q

Characteristics of parallel circuits

A

1) current is split at each junction
2) same pd across all components
3) 1/R total= 1/R1+ 1/R2 1/R3

44
Q

How do potential dividers work?

A

Using two resistors in series to divide the voltage by sharing it between them

45
Q

How is pd split in a series circuit with two resistors

A

Split in the ratio of the resistances

E.g if R1 is 2ohms and R2 is 3 ohms, R1 get 2/5 of the pd across it and R2 get 3/5 of the pd across it

46
Q

Variations in resistance in metals

A
  • T increases = drift velocity decreases
  • therefore I decreases and R increases
  • positive temperature coefficient of resistivity
47
Q

What is the relationship of resistance in and out of a junction? (learn)

A

1/R=1/R1+1/R2+1/R3

48
Q

What does current do in a series circuit?

A

Stays constant

49
Q

How should an ammeter be used?

A
  • in series

* very small resistance

50
Q

Equation linking power voltage and resistance?

A

P=v^2/R

Power= voltage squared divided by resistance

51
Q

Ohms law

A

Current in a metallic conductor at a constant temperature is directly proportional to the potential difference (resistance is the same at all points)

52
Q

The ratio of voltage across two resistors (learn)

A

V1/V2=R1/R2

53
Q

Equation which can be used to find Vin and Vout in potential dividers (learn)

A

Vout/Vin = R2/(R1+R2)

54
Q

What are potentiometers used for?

A

To obtain a continuously variable output voltage

55
Q

How can potentiometers be set up?

A

Thermistors and LDRs can be used, or varying the length of the wire which gives Vout

56
Q

Equation for Vout in simple potentiometer (learn)

A

Vout= L2/L1 x Vin

57
Q

Variations in resistance in semiconductors

A
  • T increases= small decrease in drift velocity and large increase in number of charge carriers
  • therefore I increases and R decreases
  • negative temperature coefficient of resistivity
58
Q

Variations in resistance in LDRs

A
  • Light increases=number of charge carriers increases a lot and drift velocity decreases slightly
  • therefore current increases and resistance decreases
  • negative temperature coefficient of resistivity
59
Q

Explain why the resistance of a negative temperature coefficient thermistor changes as p.d increases

A
  • more energy to lattice ions
  • more charge carriers released
  • I=nAve
  • R=V/I so if I increases R will decrease
60
Q

Explain why the temperature of the filament in the increases when a p.d is applied

A
  • increased lattice ion vibrations
  • p.d causes a current
  • electrons have more frequent collisions
  • more energy is transferred when electrons collide with lattice ions
61
Q

What equation is used to work out the total resistance of two resistors in parallel?

A

1/R + 1/R = 1/Rtotal

62
Q

If 3 cells are charging is it quicker to charge them in series or parallel?

A

Series because current remains the same through all of the cells but splits in parallel. This means the value of W= IVT increases

Physics A Level (11 decks)

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