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unit 8 Flashcards

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1
Q

what is an electric circuit?

A

a path for electricity to flow

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2
Q

3 basic components of electric circuit

A
  1. source of electricity
  2. device using electricity
  3. wire connecting different parts of the circuit
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3
Q

2 conditions needed for electricity to flow

A
  1. presence of electric cell in the circuit
  2. the circuit is closed and complete
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4
Q

what is a circuit called when there is a gap in it

A

open circuit

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5
Q

energy conversion when electricity flows through the circuit?

A

chemical energy stored in the electric cell -> electrical energy

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6
Q

what are electrical conductors?

A

materials that allow electricity to flow through them

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7
Q

4 examples of electrical conductors

A

metals, graphite, acids, alkalis

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8
Q

what are electrical insulators?

A

materials that do not allow electricity to flow through them

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9
Q

3 examples of electrical insulators

A

plastic, glass, wood

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10
Q

function of switches

A

used to open or close a circuit

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11
Q

what are circuit diagrams used for?

A

to represent an actual circuit

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12
Q

structure of an atom

A

a nucleus (positive charge) surrounded by tiny electrons (negative charge)

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13
Q

what are free electrons?

A

some electrons that escape from the atom and move freely in metals

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14
Q

how is an electric current formed by free electrons?

A

the free electrons are driven to flow from the negative pole to the positive pole

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15
Q

what is the conventional current

A

a flow of positive charges from the positive pole to the negative pole

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16
Q

what is the size of an electric current?

A

number of free electrons that flow through a point in the circuit every second

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17
Q

how is the size of an electric current measured?

A

using an ammeter

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18
Q

what is the unit of an electric current

A

ampere (A)

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19
Q

how should an ammeter be connected

A

that the electric current flows into the ammeter at its positive terminal and leaves at its negative terminal

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20
Q

what is the heating effect of electric current?

A

when an electric current flows through, some of the electrical energy changes to thermal energy

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21
Q

how does the nichrome wire change after being connected to an electrical circuit with a high current

A

turns red hot and glows

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22
Q

energy conversion in the nichrome wire that turns hot and glows

A

electrical energy -> thermal energy + light energy

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23
Q

how is the magnetic effect of an electric current shown

A

when an electric current flows through a wire, the needle on a compass nearby changes direction (deflects)

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24
Q

what is an electromagnet?

A

a coil that acts like a magnet when an electric current flows through

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25
how is an electromagnet useful in 2 points?
- produces a magnetic effect only when a current flows through -> can switch on or off - strength can be adjusted
26
2 devices that electromagnets are commonly used in
electric door locks, electric bells
27
what is voltage?
amount of energy supplied to the free electrons by an electric cell
28
how is voltage measured?
voltmeter
29
unit of voltage?
volt (V)
30
what is the total voltage across connected cells?
sum of the voltages of all the electric cells
31
how does voltage affect the current in a circuit?
the current increases with the voltage
32
how does the increasing voltage affect the brightness of the light bulb in the circuit?
the brightness increases because of the larger current
33
what is resistance of a material?
the opposition of a material to the flow of electric current
34
what is the unit of resistance
ohm (Ω)
35
resistance between conductors and insulators?
conductors have low resistance while insulators have very high resistance
36
resistance between different materials
different resistance
37
how does the resistance affect the current
when the resistance of a circuit is higher, the current will be smaller
38
why does the resistance of a circuit increases when the current decreases?
a circuit of high resistance has stronger opposition to the flow of electric current
39
3 factors affecting resistance
1. material 2. length 3. thickness
40
resistance of a thicker wire compared to a thinner wire
a thicker wire has lower resistance and larger current
41
resistance of a shorter wire compared to a longer wire
a shorter wire has lower resistance and larger current
42
what is a resistor?
a circuit component with a fixed value of resistance
43
what is a resistor used for?
control or limit the sizes of currents in the circuits of electrical appliances
44
what is a rheostat?
a circuit component whose resistance can be adjusted
45
2 types of rheostats
sliding rheostat and rotary type rheostat
46
3 components of a rheostat
1. resistance wire 2. metal bar 3. sliding contact
47
how does the brightness of the light bulb change when the sliding contact is moved as the current flows through a longer length of the resistance wire?
the resistance of the rheostat increases -> smaller current -> light bulb becomes dimmer
48
2 applications of rheostats
1. dimmer switch of a lamp 2. volume control of hi-fi systems
49
what is a series circuit?
light bulbs that are connected one after the other to form a single path circuit -> connected in series
50
what is a parallel circuit?
light bulbs that are connected in a circuit with branches
51
why does the other light bulbs light up when one light bulb goes out in a parallel circuit?
if one of the branches is open, the current can still flow through the other branch
52
why does the other light bulbs goes out when one light bulb goes out in a series circuit?
if the circuit is open at any point, the circuit will be incomplete
53
why are parallel circuits useful?
the device in each branch operates independently
54
how are the electrical appliances in our homes connected?
in parallel circuit
55
current of a series circuit
same at every point
56
current of a parallel circuit
- current in the main loop equals to the sum of the currents in the branches - larger in the branch with lower resistance
57
voltage of a series circuit
not the same across different circuit components
58
voltage of a parallel circuit
same across every branch and equals to the voltage across electric cells
59
how do electrical appliances use the heating effect of electric current?
have heating elements, when current flows through, electrical energy -> thermal energy for use
60
how does a filament light bulb use the heating effect of electric current?
it contains a long thin tungsten filament that becomes very hot when the current flows through, making the filament produce light
61
how are electrical appliances as energy converters?
electrical energy can be converted into different forms of energy useful for different purposes in the electrical appliance
62
how is the main electricity transmitted to our homes?
mains electricity is generated in power stations and transmitted to our homes through electric cables
63
mains electricity in hk
220 V
64
what are the 3 pins in the three-pin plug?
live pin, neutral pin, earth pin
65
what are the 3 holes in a main socket?
earth hole, neutral hole, live hole
66
colour of the live wire
brown
67
colour of the neutral wire
blue
68
colour of the earth wire
green and yellow
69
function of the live wire
carries high voltage that drives the current flowing through the appliance
70
function of the neutral wire
provides a return path for the current to complete the circuit
71
function of the earth wire
a safety device that connects the metal casing of an electrical appliance to the ground -> earthing
72
what is an alternating current?
the current direction that changes periodically and sometimes flows in the opposite direction
73
how are the mains sockets and electrical appliances connected?
in parallel circuits
74
why are the mains sockets in a domestic circuit connected in parallel?
so each electrical appliances can operate independently
75
what is overloading?
if too many electrical appliances are connected to a universal adaptor
76
effect of overloading
the current from the mains socket becomes very large -> the circuit will become very hot -> accidents such as fire or explosion may result
77
what is a short circuit?
when a wire is connected in parallel across the light bulb
78
why is a short circuit dangerous?
the wire has low resistance and provides an easier path for the current to flow -> nearly all current flows through the wire -> current becomes very large -> circuit becomes very hot and may catch fire
79
where may short circuits occur?
- inside an electrical appliance if its wires are loose or worn out - circuit with dust built up on it - faulty electronic devices (eg defective mobile phones)
80
where are fuses found in? (4)
electric plugs, universal adaptors, extension units and electrical appliances
81
how does a fuse protect a circuit?
the thin metal wire inside the fuse will break / melt if the current becomes too large -> breaks the circuit -> flow of current stops
82
why should a fuse be installed in the live wire and not in parallel?
because a large current may pass through the heating element without blowing the fuse
83
what is a fuse rating?
the maximum current that can flow through the fuse without blowing it
84
which fuse with a fuse rating should we use?
a fuse with a fuse rating slightly higher than the current in the circuit under normal operation
85
what is a circuit breaker?
an automatic switch that has a function similar to a fuse and has a rating showing the maximum current that can flow through it
86
what will happen if the current exceeds the rating of a circuit breaker?
the circuit breaker will switch off by itself to break the circuit
87
difference between a fuse and a circuit breaker
- the fuse needs to be replaced with a new one once the fuse is blown - the circuit breaker need not be replaced after breaking the circuit. it can be reset (switched on again) to restore the circuit
88
what will happen if we touch the metal casing of a faulty appliance without an earth wire?
the current will flow through our body to the ground -> we will get an electric shock
89
what will happen if we touch the metal casing of a faulty appliance with an earth wire?
the current will flow through the earth wire to the ground instead of our body -> protects us from getting an electric shock
90
what is the function of the cable grip in the three-pin plug?
secure the wires in position
91
why shouldn't the fuse be installed in the neutral wire?
we may get electric shock even after the fuse melts

sci (6 decks)

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