Electricity Flashcards
1
Q
What does the circuit symbol for a switch look like
A
2
Q
What does the circuit symbol for a closed switch look like
A
3
Q
What does the circuit symbol for a cell look like
A
4
Q
What does the circuit symbol for a battery look like
A
5
Q
What does the circuit symbol for a diode look like
A
6
Q
What does the circuit symbol for a resistor look like
A
7
Q
What does the circuit symbol for a variable resistor look like
A
8
Q
What does the circuit symbol for an LED look like
A
9
Q
What does the circuit symbol for a lamp look like
A
10
Q
What does the circuit symbol for a fuse look like
A
11
Q
What does the circuit symbol for a voltmeter look like
A
12
Q
What does thet circuit symbol for an ammeter look like
A
13
Q
What does the circuit symbol for a thermistor look like
A
14
Q
What does the circuit symbol for an LDR look like
A
15
Q
Electric Current is….?
A
the rate of flow of electric charge
16
Q
Current is measured in what unit?
A
Amperes (Amps, A)
17
Q
What unit is charge measured in?
A
Coulomb ( C )
18
Q
Potential difference between two points in a circuit is….?
A
the work done when a couloumb of charge passes between the points.
19
Q
In a circuit the potential difference causes …..?
A
charge to flow
20
Q
Resistance is…?
A
caused by anything which opposes the flow of electric charge
21
Q
How much current you get for a particular potential difference depends on the ____________ of the component.
A
Resistance
22
Q
Particles which can be ‘charges’ in electric circuits are…
A
electrons or ions
23
Q
What is a series circuit?
A
A circuit with only one route for charge to flow
24
Q
What is a parallel circuit?
A
A circuit with more than one route for charge to flow
25
Are ammeters connected to a circuit in series or parallel?
Series
26
Are Voltmeters connected to a circuit in series or parallel?
Parallel
27
State the equation which links charge flow, current and time
Q=It
28
State the equation which links current, potential difference and resistance
V=IR
29
State the equation which links current, potential difference and power
P=IV
30
State the equation which links current, power and resistance
P=I2R
31
State the equation which links energy transferred, power and time
E=Pt
32
State the equation which links charge flow, work done and potential difference
W=QV
33
What is the unit of charge?
Coulomb ( C )
34
What is the unit of current?
Amps (A)
35
What is the unit for potential difference?
Volts (V)
36
What is the unit for resistance?
Ohms (Ω)
37
What is the unit for power?
Watts (W)
38
State the rule for current in a series circuit
the current is the same at every point in the circuit and in every component
39
State the rule for potential difference in a series circuit
the total potential difference of the power supply is shared between components
40
What is Ohms Law?
Ohms law states that provided the physical conditions, such as temperature, remain constant, the current through a ohmic conductor is directly porportional to the potential difference across it.
41
What happens to the resistance of an ohmic conductor if you double the potential difference across it? Give a reason for your answer.
The resistance remains constant. Ohmic conductors have a constant resistance.
42
Sketch the I-V characteristics for an Ohmic conductor
Straight line through the origin.
43
Sketch the I-V characteristics for a semiconductor diode.
In the forward direction, diodes require a voltage of around 0.6V, after which the the graph slopes upwards steeply. In the reverse bias, the resistance of the diode is very high and the current that flows is very tiny.
44
Sketch the I-V characteristics for a filiment lamp.
This is a curve that starts steep, but gets shallower.
45
Sketch the V-I characteristics for an ohmic conductor
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46
Sketch the V-I characteristics for a semiconductor diode
47
sketch the V-I characteristics for a filiment lamp
48
Explain the shape of the graph for the I-V characteristics of an ohmic conductor.
The current through ohmic conductors is directly proportional to the voltage through the conductor. This means you get a straight line graph through the origin as resistance is constant.
49
Explain the shape of the graph for the I-V characteristics of a semiconducting diode
Diodes only let current flow in one direction, so don’t allow current through in the reverse bias, but current is allowed through in the forward bias.
50
Explain the shape of the graph for the I-V characteristics of a filiment bulb.
For small voltages the I-V graph shows a proportional relationshop between current and voltage. But at larger currents, electrical energy is transferred to heat energy and causes the metal to heat up. This causes the metal ions in the metal to vibrate more. The more vibrations, the more difficult it is for the charge-carrying electrons to get through the resistor. The current cant flow as easily and the resistance increases.
51
Draw a circuit that could be used to determine the I-V characteristics of a component.
52
What resistance should an ideal ammeter have? Why?
0 resistance, so the current is not affected by the instrument measuring it.
53
What resistance should an ideal voltmeter have? Why?
infinate resistance. So no current will flow through the voltmeter and affect the voltage measurement being taken.
54
What component does this I-V graph belong to?
Filament lamp
55
What component does this I-V graph belong to?
Resistor at constant temperature
56
What component does this I-V graph belong to?
Diode.
57
Is a filament lamp an ohmic conductor? How can you tell this from its characteristic I-V graph?
A filament lamp is not an ohmic conductor. It's characteristic I-V graph is not a straight line, so voltage is not directly proportional to current.
58
What does a materials resistivity tell you?
The resistivity of a material tells you how difficult it is for current to flow through the material.
59
What three things does the resistance of a wire depend on?
Length of the wire, cross-sectional area of the wire, the resistivity of the material.
60
How does the length of a wire affect its resistance?
The longer the wire the greater the resistance. Resistance is proportional to the length of the wire.
61
How does the cross-sectional area of a wire affect the resistance?
The greater the cross-sectional area, the lower the resistance.
62
Define resistivity.
The resistivity of a material is the resistance of a 1 m length with a 1m2 cross-sectional area.
63
What unit is resistance measured in ?
Ohms
64
What unit is resistivity measured in?
Ohm meters
65
What is the formula which links resistivity, resistance, cross-sectional area and length?
*p* = RA/L
66
What happens to the resistance of a wire when it is heated? Why?
When a wire is heated, the resistance increases. This is because the metal ions within the wire are vibrating faster. There will be more collisions between the electrons and the metal ions in the wire, which will make it harder for the electrons to flow.
67
What is a semiconductor?
A semiconductor is a material that isn't as good at conducting as metals, but if enough energy is supplied to the semiconductor it can realease more charge carriers, decreasing the resistivity and allowing more current to flow.
68
Sketch the resistance against temperature graph for a thermistor.
69
What does NTC stand for?
Negative temperature coefficient
70
Describe how the temperature of a NTC thermistor varies with temperature.
As the temperature increases the resistance decreases.
71
Explain why the resistance of a NTC thermistor decreases as temperature increases.
The resistance decreases as the temperature of the thermistor increases because thermal energy is transferred to the electrons within the thermistor, giving them enough energy to escape from their atoms. This means there are more charge carriers and so lowers the resistance.
72
Sketch the resistance against light intensity graph for a LDR.
73
Describe how the resistance of an LDR varies with light intensity.
As the light intensity increases the resistance decreases.
74
Explain why the resistance of an LDR varies with light intensity.
As the light intensity increases the electrons are given enough enegry the be released from their atoms and therfore increases the number of charge carriers and reducing the resistivity.
75
What is a superconductor?
A superconductor is a material which when cooled below a critical temperature their resistivity becomes zero.
76
Below what temperature do superconductors have zero resistivity?
The critical temperature.
77
Name three applications of super conductors
Creating strong magnetic fields, which can be used in MRI scanners, particle accelerators or Maglev trains. Use in power cables for transmitting electricity without any loss of power. Use in fast electrical circuits as no resistance to slow the current.
78
Explain how superconductors can be used to reduce energy loss in transmission of electrical power.
In power transmission, energy is lossed in the wires due to resistance. If superconductors are used and the resistance is zero, then energy can be transmitted without any loss in power.
79
Give one dissadvantage of using superconductors.
Maintaining condutors at or below their critical temperature (generally around 10 kelvin) is difficult and very expensive.
80
Describe an experiment which could be used to investigate how the resistance of a thermistor changes with temperature.
The thermistor is connected to a power supply, which provides a constant potential difference, and an ammeter. The temperature of the thermistor is controlled by immersing it in the water bath. The temperature of the thermistor and the current are recorded at regular intervals as the temperature of the waterbath is reduced. The resistance of the thermistor at each temperature can be calculated and recorded.
81
State the equation used to calculate the cross-sectional area of a cylindrical wire, stating any assumptions you make.
Cross sectional ares = πr2 , where r is the radius of the wire in m. The assumption you are making is that the cross-sectional area of wire is a circle of uniform area for the entire length of the wire.
82
What instrument would you use to find the diameter of a wire?
A micrometer
83
Describe an experiment you could do to investigate the resistivity of a material.
Clamp a wire to a meter ruler. Measure its diameter using a micormeter in at least 3 different locations to find the cross-sectional area of the wire. Connect the wire to a circuit with a power source, ammeter and voltmeter. The current through the wire and the potential difference across the wire can be measured for a certain length of the wire. This would then be repeated to get an average resistance at that length. The length of the wire can then be changed and the process repeated. A graph of average resistance against length can be plotted. This graph can then be used to calculate the restisivity of the wire by finding the gradient and multiplying by the cross-sectional area of the wire.
84
Explain why it is important to keep the temperature constant when doing an experiment to find the resistivity of a material. Suggest how you could keep the temperature of the material constant.
The resistivity of a material is dependent on temperature, so the temperature must be kept constant so that it doesn’t affect the results of the experiment. The temperature can be kept constant by using small currents and by including a switch in the circuit so current is only flowing whist the measurements are being taken.
85
Define the term power.
Power is the rate of transfer of energy.
86
What is one Watt equivalent to?
A Watt is equal to one joule per second.
87
Give three equations you could use to calculate electrical power.
P= IV, P = I2R, P = V2/R
88
Give an equation for energy which relates current, potential difference and time.
E = IVt
89
What does E.m.f stand for?
Electromotive force
90
What is internal resistance?
Internal resistance is the resistance within batteries.
91
What is internal resistance caused by?
Internal resistance is caused by the electrons within the battery colliding with the atoms in the battery.
92
What is e.m.f?
The electromotive force is the amount of energy produced by the battery and transferred to each coulomb of charge.
93
What unit is e.m.f measured in?
Volts (V)
94
State the formula which links e.m.f, energy and charge.
e.m.f = E/Q
95
What is the terminal p.d?
Terminal p.d. is the potential difference across all of the components in the circuit, outside of the battery.
96
What causes batteries to warm up when they are used?
The internal resistance.
97
What is the load resistance?
The total resistance of all the components in the circuit, outside of the battery.
98
The energy wasted per coulomb overcoming the internal resistance is called the …………..?
Lost volts.
99
Give the equation which links e.m.f, current, load resistance and internal resistance.
e = I(R+r)
100
How can you express the equation for e.m.f in terms of terminal p.d and lost volts?
e= V+v
101
Give an equation for lost volts in terms of terminal p.d and e.mf.
v = e -V
102
Sketch a circuit which could be used to measure the internal resistance and e.m.f of a cell or battery.
103
What do the gradient and vertical intercept on a V-I graph for a power supply show?
The gradient shows -r, where r is the internal resistance, the y intercept shows e.m.f.
104
The total current flowing into a junction is equal to ………
The total current flowing into a junction is equal to the total current flowing out of that junction.
105
The total e.m.f around a series circuit = ………………..
The total e.m.f around a series circuit = the sum of the p.d.s across each component.
106
In a series circuit the total resistance is equal to……
In a series circuit the total resistance is equal to the sum of the resistance across each component in the circuit.
107
Give an equation which tells you the total resistance in a series circuit.
R = R1 +R2 +R3+…….
108
Give an equation which tells you the total resistance in a parallel circuit.
1/R = 1/R1 +1/R2 +1/R3 …..
109
How are the resistors R1 and R2 connected if their total resistance is equal to R1 +R2?
Series
110
Which type of circuit is 1/R = 1/R1 +1/R2 +1/R3 true for?
Parallel
111
What is a potential divider?
A potential divider is a circuit containing a voltage source and a couple of resistors in series. The voltage across one of the resistors is used as the output voltage. If the resistors aren't fixed, the circuit will be capable of producing a variable output voltage.
112
Give the equation you would use to work out the voltage output of a potential divider.
V out = (R2 / R1+R2) V
113
What are potential dividers used for?
Potential dividers can be used to supply a potential difference, V out, between zero and the potential difference across the power supply.
114
How can you make a light sensor using a potential divider?
You can make a light sensor using a potential divider by replacing one of the resistors with an LDR.
115
How can you make a temperature sensor using a potential divider?
You can make a temperature sensor using a potential divider by replacing one of the resistors with a thermistor.
116
How can you make a volume control using a potential divider circuit?
You can make a volume control from a potential divider by using a variable resistor in place of one of the resistors.
117
What is a potentiometer? When can it be used?
A potentiometer is a potential divider containing a variable resistor instead of two resistors in series. For example they are used in volume controls on a stereo.
118
For cells in a series circuit, how do you calculate the total e.m.f of the circuit when there is more than one cell?
The e.m.f's are added together.
119
For identical cells in a parallel circuit, what is the total e.m.f for the circuit?
For identical cells in a parallel circuit, then total e.m.f of the combination of cells is the same size as the e.m.f of each individual cells.
120
Explain why the combined resistance of two resistors in parallel is always less than either of the separate resistors.
In parallel there is less opposition to current because there is more than one path for the electrons to take.
121
Explain why two 1.5 V cells connected together in series can give an e.m.f of 3 V or 0 V. .
If they are both connected in the same direction, the e.m.f.s are added together. If they are connected in opposite directions the e.m.f is zero.
122
Explain why the terminal e.m.f of a cell, supplying current to a circuit, is always less than its e.m.f.
Because energy is wasted within the cell due to its internal resistance.
123
Give one factor which can affect the resistivity of a material.
Temperature
124
