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A level physics notes > 12/13. Electricity > Flashcards

12/13. Electricity Flashcards

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1
Q

Define current

A

The rate of flow of charge in a component/wire

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2
Q

What are charge carriers?

A

Delocalised electrons (from metal ions) that create the current (carry the electric charge)

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3
Q

How to check a material is a conductor?

A

Any non-zero reading on the meter (ammeter) in a circuit containing the meter, test material and a battery, indicates that the test material can conduct electricity

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4
Q

In which direction does current flow?

A

From the positive (longer) to the negative (shorter) terminal. While electrons flow in the opposite direction due to their negative charge, the convention for current to flow is the opposite

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5
Q

What is the unit of current?

A

A - ampere, in terms of the magnetic force between two parallel wires with the same I flowing through them

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6
Q

What is charge?

A

Symbol= Q, Unit = C (Coulomb), 1C id the charge flow in one second when I=1A

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7
Q

Give an equation linking I, Q and t

A

I=Q/t

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8
Q

How many electrons make up the current of 1A?

A

6.25x10^(18) as each electron contains 1.6x10^(-19)C

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9
Q

What is an insulator?

A

A material in which all electrons are bound to atoms so there are no charge carriers present

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10
Q

What is a semiconductor?

A

In a semiconductor, the number of charge carriers increases as temperature increases therefore resistance has decreased with temperature. A pure semiconductor is called an intrinsic semiconductor where more electrons break free as the temperature increase

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11
Q

What is potential difference?

A

The work done or energy transferred per unit charge

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12
Q

Give two equations for PD involving V, Q and W or E

A

V=E/Q or V=W/Q

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13
Q

What effect has current when flowing through a component with resistance?

A

Heating and magnetic effect

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14
Q

Describe the energy transfers in an electric heater device

A

The energy transferred by the electrons is converted into thermal energy as charge carriers collide with each other and vibrate more so temperature increases

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15
Q

Give an equation for power linking P, E and t

A

P=E/t

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16
Q

Give an equation linking P, V and I

A

P=IV

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17
Q

What is an ohmic conductor?

A

A component which obeys ohm’s law and a straight diagonal line is produced on an IV graph

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18
Q

What is ohm’s law?

A

It states that at a constant temperature, the pd and I through a component are directly proportional to each other

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19
Q

How does variable resistance control circuit allow the IV characteristics of a component to be investigated?

A

The variable resistor varies pd over component X as pd is shared over components in series

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20
Q

What does an ideal voltmeter mean?

A

A voltmeter with infinitely high resistance so no I flows through it, performance of circuit unaffected

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21
Q

What does an ideal ammeter mean?

A

An ammeter with 0 resistance, so that current flowing through it is unaffected, performance of circuit unaffected

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22
Q

How is using potential divider control better than using a variable resistor?

A

It allows a bigger range of pd values to be tested.

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23
Q

What is the purpose of a protective resistor?

A

When using the potential divider control for a diode. It limits the current flowing through the diode

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24
Q

Describe the IV characteristics of a resistor

A
  • ohmic conductor so straight diagonal line through the origin on an IV graph
  • PD is directly proportional to I
  • 1/gradient = Resistance so steeper gradient = lower resistance
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25
Describe the IV characteristics of a diode
- only allows current to flow in diode's forward direction - when PD in forward direction exceeds 0.6V, the I flowing increases rapidly and R decreases dramatically - in the reverse direction the R is very high so no/very little current flows
26
What happens in the diode's reverse direction?
(reverse biased) The resistance is very high so current is usually less than 1 microamp
27
What happens in the diode's forward direction?
(forward biased) When pd<0.6V, R is high. When pd>0.6V , R is low.
28
What is the turn-on voltage?
aka threshold voltage, the voltage at which current starts to flow. Different in different diodes.
29
Describe the IV characteristics of a filament lamp
- I is not directly proportional to V so NOT an ohmic conductor - As V increases, current increases. As I increases, temperature increases. As T increases, ions in the wire vibrate more causing resistance to increase. As R increases, the rate at which current increases, decreases
30
Describe a method of investigating IV (6 marks)
- describe the circuit used - Voltmeter measures PD over component X, ammeter measures I through X - vary setting through the potential divider or variable resistor to vary PD across X - record PD and I for various values in the positive direction - reverse battery, and record I and PD in opposite direction - plot a graph of V against I
31
Why does resistance increase with temperature?
As temperature increases, the ions in the metal wire vibrate more as they gain more kinetic energy from the passing current. Increased vibrations mean, charge carriers cannot pass as easily with a pd applied
32
What is a positive temperature coefficient?
Metals have a positive temperature coefficient meaning that as resistance increases, as temperature increases
33
What happens to the resistance of a pure semiconductor as temperature increases?
As temperature increases, resistance decreases as number of charge carriers increases with temperature so more current can flow
34
What is an intrinsic semiconductor?
It has a negative temperature coefficient (as T increases, R decreases) eg. thermistor. Also % change of resistance per kelvin changes of temperature is greater than that of a metal
35
Describe the graph for a thermistor of R against T
- same resistance at 0C - R of thermistor decreases non-linearly as T increases - R of metal wire increases less over same temperature range
36
Define resistance
Measure of the pushing force against the current making it difficult for charge carriers to flow and carry charge
37
What causes resistance?
Repeated collisions between charge carriers and each other as well as positive ions of metal in wire
38
Write an equation for resistance involving I and V
R=V/I
39
What circuit can be used to investigate resistance of a component?
The same as for investigating IV characteristics
40
How to find resistance from an IV graph?
Resistance=1/gradient
41
What is resistivity?
A property of a material defined as the resistance of material with a 1m^2 cross-sectional are and is 1m long
42
What us the symbol and unit of resistivity?
Symbol ρ and unit Ωm
43
What is the equation involving resistivity?
R=(ρL)/A
44
Describe method for determining the ρ of a wire
- measure diameter (d) using micrometer at several different points along wire and calculate the cross-sectional area (A=πr^2 and r=d/2) - measure R at different lengths (L) of the wire. Plot a graph of R against L where ρ=gradient x A
45
What is a super conductor?
A material with has 0 resistance below a certain temperature (critical temperature)
46
What happens when current passes through a super conductor?
There is no pd as R=0 so there is no heating effect
47
Why is it difficult to make and use superconductors?
The critical temperature is often very low (below 10K) so it is very expensive to ensure that the material is a super conductor. Copper oxides have the highest critical temperature discovered, about -123C which is still very low
48
List 3 uses of super conductors
- power cables - avoids loss of energy due to heating - strong electromagnets - MRI/bullet trains - electronic circuits - efficient
49
Define potential difference
Pd between two points in a circuit is the energy transferred per coulomb of charge that flows through the two points. It is electrical energy turned into other forms of energy.
50
Define electromotive force
It measured across anything that generates electrical energy. It is the amount of energy transferred
51
What is the name for the laws of current and pd in a circuit?
Kirchhoff’s laws
52
What is the rule about current at a junction?
Total current leaving junction is equal to the total current entering the junction- rates of Q flowing in and out are equal
53
What are the current rules for components in series?
- current leaving component is same as current energy component (not used up) - current passing through multiple components is same through each component
54
What are potential drops and rises?
If the charge carriers lose energy, there is a potential drop, if they gain energy there is a potential rise
55
What is the pd rule in series?
With multiple components in series, the total pd across all the components is equal to the sum of pd across each component
56
What is the pd rule for components in parallel?
Pd acrosss all components in parallel is the same
57
What is the pd rule for each loop of the circuit?
For each loop of the circuit, the sum of emfs around the loop is equal to sum of pd around the loop due to conservation of energy
58
What is the rule for resistance in series?
Total R is equal to the sum of individual resistances
59
What is the resistance rule in parallel?
Reciprocal of total resistance is given by total of reciprocals of all the individual resistances
60
Summarise the circuit rules in a series circuit
Pd: pd over individual components adds up to pd over the battery Current: current is the same through each component Resistance: adds up
61
Summarise the circuit rules in a parallel circuit
Pd: same on each parallel branch Current: current from battery is the sum of the currents on each branch Resistance: 1/RT: 1/R1 +1/R2
62
What causes heating effect in a component?
Resistance of the component. Charge carriers repeatedly collide with positive metal ions in the conducting material so there’s a net transfer of energy from charge carriers to ions’ internal energy store causing a heating effect
63
List 3 equations for power including V and I
P=IV P=I^2 x R P= V^2 /R
64
Give 2 equations involving energy transferred
``` E= P x t E= I x t x v ```
65
What does temperature rise depend on?
Power supplied to the component
66
What is electromotive force?
Aka emf, measured in V with symbol ℰ, the energy converted from other forms of energy into electrical energy per coulomb of charge
67
What is internal resistance?
Created by a source of electrical energy sure to opposition of the flow of charge through the source as the charge carriers move through the source from one terminal to the other
68
What causes internal resistance?
The materials the source is made from therefore it cannot be removed
69
What happens to the electrical energy in a source with internal resistance?
Some of it is dissipated inside the source as heat energy
70
What happens when the switch is open? (r)
No current flows so the pd of the source is E/Q delivered by the source
71
What happens when the switch is closed? (r)
Current flows, terminal pd is lower than emf whenever Current passes through the source and energy is lost over the j thermal resistance So: ℰ=V (over r) + V (over R)
72
Write the equation relating ℰ, I and resistances
ℰ= I(r+R)
73
What does the lost pd equal?
Difference between cell emf and pd across its terminals aka. Lost pd is E/Q dissipated due to r
74
Write an equation for power involving internal resistance
P=I^2r +I^2R
75
What does total power in the circuit euqal? (r)
Power supplied by source to R + power wasted by r
76
Describe a circuit used to investigate internal resistance
A cell with internal resistance r with voltmeter over r connected in series with an ammeter and a variable resistor R, with a voltmeter over R, and a switch.
77
What is the variable resistor R used for? (r)
It can be adjusted to vary the current through circuit which varies pd lost across r
78
What has to be recorded to determine emf and r?
Range of values for current and terminal pd
79
What is the purpose of the switch in the circuit? (r)
Avoids flattening battery and the variable resistor R overheating
80
What happens as current in circuit increases?
Volts lost in r increase so there is a smaller terminal pd produced
81
Describe graph of V(R) against I
Straight diagonal line with a negative gradient. The y-intercept is the emf and the gradient is the negative value of the internal resistance as VR=(-r)I+ℰ
82
What is a potential divider?
A circuit arrangement which consists of two or more resistors in series with a fixed source pd.
83
How is pd divided across the resistors in a potential divider?
In a ratio according to the ratio of their resistances
84
Write an equation for determining the pd of resistor 1 in a potential divider
V1=(V0 x R1)/R1+R2 | V0= pd over battery
85
What relates ratios of resistances and pds
Ratio of pd over R1 and R2 is equal to the ratio of the resistance of R1 and R2
86
What share of pd doesn the resistor with higher resistance take?
Greater share of total pd (V0)
87
How is a potential divider used to obtain a variable pd?
- source pd is connected to a fixed length of uniform resistance wire - a contact can be moved along the wire to give a variable pd - the longer the section before the contact, the higher the resistance over it so the greater the share of pd
88
Describe uses of potential dividers
- audio volume control: change loudness of a speaker. Signal pd (V0) is supplied to the potential divider. Variable output pd from potential dividers is supplied to the speaker - varying brightness of a lightbulb between 0 and normal brightness. potential dividers allow 0 brightness which would not be possible with a variable resistor
89
How can potential dividers be used in sensing circuits?
The output pd will change with the changes in physical conditions eg. temperature
90
Describe how a potential divider is used in a temperature sensor?
(thermistor and variable resistor connected in series) - when temp. of thermistor is constant, source pd is divided between it and the variable resistor - adjusting variable resistor adjusts the pd across thermistor - when temp. changes the pd across thermistor changes - if temp. rises, resistance falls (NTC) so the pd across it falls
91
Describe how a potential divider is used in a light sensor?
(LDR in series with a variable resistor) - pd across LDR changes a light intensity changes - if light intensity increases, the R of LDR falls and the pd across it falls

A level physics notes (1 decks)

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