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GCSE Physics OCR 21st Century (Triple) (9-1) > P3 - Electric Circuits > Flashcards

P3 - Electric Circuits Flashcards

1
Q

What is the charge on an electron?

A

Electrons are negatively charged

relative charge: -1

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2
Q

What happens when two objects are

rubbed together?

A 
They become charged because
negatively charged electrons are
transferred from one object (which
becomes positive) to the other (which
becomes negative).
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3
Q

Where are electric fields found?

A

Around every electric charge.

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4
Q

What is an electric field?

A

A region of space in which the effects of

charge can be felt.

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5
Q

Objects with the same charge…

A

…repel.

Objects with the same charge repel

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6
Q

Objects with opposite charges…

A

…attract.

Objects with opposite charges….attract.

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7
Q

Define an insulator

A

An object which does not conduct
electricity. Electrons cannot flow through
the material.

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8
Q

How is static electricity produced?

A

When two insulators are rubbed
together, transferring electrons, to form a
positive and a negative charge.

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9
Q

How does sparking occur?

A 
Sparking occurs when enough charge builds up
between two statically charged objects. A spark is
produced by the charge jumping through the air
from the negative to the positive object to restore
the charges (back to two neutral objects).
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10
Q

Define a conductor

A

A conductor is a material which can
conduct electricity; electrons are able to
flow through it.

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11
Q

Why can conductors not produce static

electricity?

A

Electrons flow through the materials
when they are rubbed together to cancel
out any charging effect.

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12
Q

Define an electric current

A

Current is the rate of flow of charge in an

electric circuit.

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13
Q

What is required in order for a

charge/current to flow?

A

● A potential difference

● A closed circuit

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14
Q

Describe the value of current across a

circuit

A

Current has the same value at any point

in a closed (series) circuit.

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15
Q

Give an equation linking charge and

current, giving SI units

A

charge (C) = current (A) x time (s)

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16
Q

Give an equation linking current and

voltage, giving all SI units

A

p.d. (V) = current (A) x resistance (Ω)

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17
Q

Define conductors in an electrical circuit

A

The components of the circuit (including
wires) which carry a charge and conduct
electricity.

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18
Q

Define resistors in an electrical circuit

A

Components such as resistors, lamps
and motors which resist the flow of
charge through them.

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19
Q

Why are wires not considered resistors?

A

Their resistance is so small it is

considered negligible.

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20
Q

How does resistance affect the current

flowing through a circuit?

A

The larger the total resistance in the

circuit, the smaller the current will be.

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21
Q

Describe an experiment to investigate

the resistance of a wire

A

● Use a length of wire connected to an ammeter (in series), a
voltmeter (in parallel) and a power supply.
● Connect two crocodile clips to the wire, one at each end, and
record the current and voltage.
● Vary the length of the wire (moving one of the clips),
recording V and I.
● Plot a graph of V against I; the gradient = resistance of wire.

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22
Q

Describe the key features of an ohmic

conductor

A

Resistance is constant, meaning the
conductor’s IV characteristic (graph of
current against voltage) has a linear
(straight line) gradient.

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23
Q

How does resistance change with

temperature?

A

As temperature increases, ions and electrons
have more kinetic energy. This results in more
collisions between the stationary metal ions,
making it more difficult for electrons to move
through the wire and carry a charge. Therefore
resistance increases with temperature.

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24
Q

What is a thermistor?

A

A resistor in which resistance decreases

as temperature increases.

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25
What is an LDR?
A resistor in which resistance decreases as light intensity increases... e.g. used in automatic night lights.
26
Draw the circuit symbol for a switch
----o-----o---- ------o__ o----- | closed open
27
Define potential difference
The work done per unit charge.
28
Give the equation linking potential difference and work done, with all SI units
p.d. (V) = work done (J) ÷ charge (C)
29
What is a series circuit?
A series circuit is one in which the same charge passes through all components, so the current through each component is the same.
30
What is a parallel circuit?
A circuit in which the charge is split | between branches.
31
Describe the current across a series circuit
Current is the same at all points in a | series circuit.
32
Describe potential difference in a series circuit
Potential difference is split between components, according to the ratio of their resistances.
33
Describe current across a parallel circuit
Current is split between the branches of | the circuit.
34
Describe potential difference across a parallel circuit
Each branch has a potential difference equal to the potential difference of the supply.
35
What is total resistance in a series circuit?
The sum of the resistances of each | component.
36
What is the total resistance in a parallel circuit?
``` The total resistance will be lower than that of the branch with the lowest resistance. This is because the charge is split; decreasing the charge to each component decreases its resistance. ```
37
What is a sensing circuit?
A circuit in which resistance is dependent on an environmental factor such as light or temperature, so therefore can be used to monitor environmental changes.
38
How can sensing circuits be constructed?
Using thermistors or LDRs.
39
What factors affect the energy transferred when charge flows through a component?
● Amount of charge. ● The potential difference across the component.
40
What is the power rating of an appliance?
It shows the power the appliance uses, | measured in Watts.
41
Define power
The rate of energy transfer, or the rate at | which work is done.
42
Give an equation linking power and | potential difference, giving all SI units
power (W) = current (A) x p.d. (V)
43
Give an equation linking power and energy, giving all SI units
power (W) = energy (J) ÷ time (s)
44
Give an equation linking energy and | potential difference, giving all SI units
work done (J) = p.d. (V) x charge (C)
45
Give equations linking power with | resistance, including all SI units
power (W) = current^2 (A) x resistance (Ω) P = I^2 x R power (W) = p.d.^2 (V) ÷ resistance (Ω) P = V^2 ÷ R
46
What is a step up transformer?
A transformer which increases voltage | decreasing the current
47
Why does current decrease when voltage is increased?
Because of the principle of conservation of energy; the output power cannot be greater than the input power because the energy cannot be increased.
48
Name the two coils in a transformer
Primary coil and secondary coil.
49
A step up transformer has more...
... turns on the secondary coil than on | the primary coil.
50
What is a step down transformer?
A transformer which decreases the | voltage (increasing the current).
51
A step down transformer has more...
...turns on the primary coil than on the | secondary coil.
52
Why is electricity transmitted through the national grid with a high voltage?
This results in a low current, resulting in less power dissipation (so less energy is wasted in transport).
53
When are step up transformers used?
Increasing voltage for transport across | the National Grid.
54
When are step down transformers used?
Decreasing voltage of the National Grid | supply to 230V for domestic use.
55
What is a magnetic field?
A region in which a magnetic object | experiences a force.
56
Where is a magnetic field strongest?
At the poles of a magnet.
57
Magnetic field strength decreases as...
Distance from the magnet increases.
58
Where does attraction occur?
``` Between opposite (unlike) poles of two magnets. ```
59
Where does repulsion occur?
Between like poles of two magnets.
60
Describe the key features of field lines in a magnetic diagram
● They have arrows pointing from north to south. ● The lines never touch, cross or overlap.
61
How is the strength of a magnet displayed in a diagram?
By the spacing of the field lines (the closer together they are, the stronger the magnet.)
62
What do the arrows on field lines represent?
The direction of the force that would be experienced by the north pole of a magnet placed in the field.
63
What is a permanent magnet?
An object which always has poles, and is | therefore always magnetic.
64
What is an induced magnet?
A magnetic material which does not have fixed poles. They can be induced to become magnets (giving them poles) by placing them in a magnetic field, but they lose their magnetisation when the field is removed.
65
Explain how compasses work
When placed in the Earth’s magnetic field, the north pole of a magnetic compass points towards the Earth’s north pole.
66
What do compasses provide evidence for?
That the core of the Earth is magnetic.
67
What is the difference between Earth’s magnetic and geographic poles?
Geographic north is at the top of a globe. Magnetic north is at the bottom of a globe, meaning magnetic south is at the top. This is why the north poles of compasses point towards the north pole (it is the magnetic south).
68
What is produced around a current carrying wire?
A magnetic field.
69
How can you determine the direction of a magnetic field around a wire?
``` Using the Right Hand Grip Rule. Produce a thumbs-up shape with your right hand and point your thumb in the direction of the flow of current. The field lines wrap around in the direction of your fingers. ```
70
What is a solenoid?
A coil of wire with a magnetic field, which | can be used as an electromagnet.
71
How does coiling a wire affect the magnetic field?
It increases the field strength, as the magnetic fields of each turn of wire are added together.
72
How can you increase the strength of a solenoid magnet?
● Using an iron core to carry field lines (as they travel more easily through metal than air). ● Increase the number of turns in the coil. ● Increase the current.
73
Describe how a loudspeaker works
● Current flows into a coil, producing a magnetic field. ● The field interacts with a field from a permanent magnet in the loudspeaker. ● The coil experiences a force, causing it to move. ● The movement of the coil causes the loudspeaker cone to move, producing pressure variations which produces sound.
74
What is the motor effect?
The force experienced by a current-carrying wire placed in a magnetic field, causing the wire to be forced out of the field.
75
Describe the mechanism of the motor effect
● When a current-carrying wire produces a magnetic field within the field of a permanent magnet, the two fields interact. ● The wire experiences a force pushing it away from the magnetic field, at right angles to the direction of the permanent field and the current.
76
How can you predict the direction of the motor effect?
How can you predict the direction of the motor effect?
77
What is conventional current?
A model for current which flows in the opposite direction to electrons. Conventional current flows from positive to negative.
78
What kind of current is used in Fleming’s rule?
Conventional current.
79
Which factors affect the strength of the motor force?
● The length of wire placed in the field ● The current in the wire ● The strength of the permanent field
80
Give an equation linking force with current, including all units
Force (N) = magnetic flux density (T) x current (A) x length of wire (m) F = BIL
81
Describe how an electric motor works
● A rectangular wire, or coil, lies between two permanent magnetic poles, so current flows up one side and down the other. ● The motor effect produces a couple on the coil, causing one side to move upwards and the other to move downwards, causing the wire to rotate.
82
How is mains electricity produced?
Using electromagnetic induction.
83
What is electromagnetic induction?
The production of a potential difference when there is relative movement between a conductor and a magnetic field.
84
In what direction is the potential difference induced?
In the opposite direction to the | movement which produced it.
85
When is a current produced?
When the ends of the coil are connected | to a complete circuit.
86
How are electrical generators set up?
``` In the same way as a motor, with a rectangular loop of wire between permanent magnets. The main difference is the presence of a turbine to spin the coil. ```
87
Describe how an electrical generator works
● A turbine spins the coil of wire between the magnets. ● The wire cuts through the magnetic field, experiencing a changing magnetic field. ● A potential difference is induced. ● A current is produced.
88
What kind of current is produced by an ordinary generator?
An alternating current.
89
How can a direct current be produced?
Using a split ring commutator; this disconnects and reconnects the wires every half rotation, switching the current so the motor spins continuously.
90
What is a dynamo?
A system which produces direct current by continuously spinning a coil in a permanent magnetic field.
91
How do transformers work?
● An alternating current flows through the primary coil, producing an alternating magnetic field. ● This causes the secondary coil to experience a changing magnetic field, inducing a potential difference, which produces an alternating current in the secondary coil.
92
Why do step up transformers increase voltage?
There are more coils experiencing the | change, so a larger p.d. is induced.
93
State one assumption used in | transformer calculations
The transformer is 100% efficient (the power is assumed to be the same in both coils).
94
Give the transformer equations linking | number of coils, p.d. and current
n_1/n_2 = v_1/v_2= 1_2/I_1
95
How do microphones work?
● A current is produced which is proportional to the signal strength of the sound. ● There is a fixed magnet in the centre, with the coil around it free to move. ● Pressure variations from the sound waves cause the coil to move, cutting the magnetic field lines and inducing a p.d. and therefore a current. ● The current is transmitted to a loudspeaker.

GCSE Physics OCR 21st Century (Triple) (9-1) (6 decks)

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