Electric circuits Flashcards
1
Q
What is an atom made up of?
A
Charged particles, protons and electrons (and neutrons, which don’t have a charge)
2
Q
What is the charge on a neutral object?
A
No overall charge
3
Q
What sort of forces are there between charged objects?
A
Electrostatic.
4
Q
Like charges do what?
A
Repel
5
Q
Unlike charges do what?
A
Attract
6
Q
How does something become charged by rubbing?
A
The outer electrons are less strongly attracted to the nucleus and can be removed by rubbing
7
Q
What will an objects charge be if it has lost electrons?
A
+ve
8
Q
What will an object be if it has gained electrons?
A
-ve
9
Q
What sort of surfaces need to be rubbed together to generate a charge on the objects?
A
Insulating surfaces - any charge on a radiator, for instance, is passed on the the earth
10
Q
How is lightening believed to be generated?
A
Charge build up in the clouds due to friction of air/water particles/ice crystals.
11
Q
Why are metals good conductors?
A
They have free electrons
12
Q
Why are plastic insulators?
A
There are very few free electrons.
13
Q
If a bulb is lit in a circuit, what do you know about the circuit?
A
There is an electric current.
14
Q
Where does the energy in a circuit come from?
A
The moving electrons transfer their energy.
15
Q
Why does electricity flow in a closed circuit?
A
The electrons are attracted to the positive terminal.
16
Q
What is the flow of charge called?
A
The current
17
Q
What is the bulb doing?
A
Converting the energy carried by the electrons to light (and heat) energy
18
Q
What is the definition of the rate of flow of charge?
A
The current.
19
Q
What is electric current measured in?
A
Ampères, or Amps
20
Q
How does the current increase?
A
By putting more energy into the charged particles from the power supply
21
Q
What happens to charge in an electric circuit?
A
It is conserved.
22
Q
What happens to energy in an electric circuit?
A
It is transferred.
23
Q
How do you measure current?
A
An ammeter
24
Q
How is an ammeter connected?
A
In series
25
How is an ammeter shown in a circuit?
A capital A in a circle.
26
How do you measure voltage?
A voltmeter.
27
How is a voltmeter connected?
In parallel across a component.
28
How is a voltmeter shown in a circuit?
A capital V in a circle.
29
What is the unit of voltage?
The volt.
30
What is the voltage across the power supply a measure of?
How much energy is supplied to the circuit - the difference in energy between the two terminals.
31
What are the electrons in the circuit?
Energy carriers, they pick up energy at the power supply and transfer energy at the component.
32
What is the equation for power, voltage and current?
Power (W) = voltage (V) x current (A)
33
What is the scientific name for voltage?
Potential difference.
34
What does a potential difference of 1 volt mean?
1 joule of energy is transferred in or out of electrical form for each unit of charge.
35
In a circuit, higher resistance means?
Lower current.
36
What do all components have?
Some resistance
37
What is the circuit symbol for a resistor?
A rectangle.
38
What is resistance a measure of?
How much a conductor opposes the current.
39
What is resistance measured in?
Ohms. Symbol is omega, looks like an upside down jar.
40
What is the circuit symbol for a variable resistor?
A rectangle with an arrow across it, pointing to about 1 or 2 o'clock.
41
What is the equation that links resistance to current and voltage?
Resistance (Ohms) = voltage (volts)/ current (amps)
42
Ohm's law
Current through a fixed resistor is directly proportional to the voltage across (at constant temperature)
43
What is the implication of Ohm's law when you plot a graph of current v voltage across a fixed resistor?
You get a straight line through the origin. Voltage has to be the X axis, then resistance is equal to 1/gradient)
44
How do we account for the resistance of copper wires in a circuit?
We ignore it, as the resistance of copper is very low.
45
What does it mean when we say components are connected in series?
They are connected one after another,
46
What is the current through components in series?
The current remains constant
47
What is the potential difference of the whole circuit when all components are in series?
The sum of the potential difference of each component in series.
48
What will be the resistance of a circuit when all components are in series?
The sum of the resistance of each component.
49
What do we mean when we say components are connected in parallel?
The components are connected next to each other, the current has a choice.
50
How do we work out the total current in a circuit that is connected in parallel?
The total current is the sum of the current through each component.
51
What happens when 2 or more resistors are connected in parallel?
The choice for the current means that total resistance is lower.
52
What is the relationship between current and resistance when the resistors are connected in parallel?
It is inversely proportional, so the largest current goes through the least resistance.
53
What is the work done by a resistor?
It generates heat
54
If you connect cells (batteries) in parallel what happens to current and voltage?
Voltage remains the same, but the current doubles.
55
What is a thermistor?
A semiconductor whose resistance changes with temperature.
56
What is a light dependent resistor (LDR)?
A semiconductor whose resistance changes as the light falls on it - bright light makes low resistance.
57
What happens as temperature rises in a thermistor?
At high temperature resistance is lower.
58
What normally happens to resistance as temperature rises?
It goes up, the atoms are vibrating more and get in the way more often.
59
Why do semiconductors conduct better when they are heated?
As their thermal energy increases there are more free electrons.
60
What is a magnetic field?
The space around a magnet in which the magnetic force acts/ is strong enough to be detected.
61
Where is a magnetic field strongest?
Where the field lines are closest together.
62
What is induced when a magnet is moved near a piece of wire?
A voltage.
63
What is always induced when a magnet is moved near a piece of wire?
A voltage
64
What happens when the movement if reversed, or the magnet is turned around?
The current flows the other way.
65
What happens if you use a stronger magnet, or move the magnet faster, or you coil the wire?
You get more current.
66
How would you get a continuous current by moving a magnet?
The magnet must move continuously
67
What is the name given the the process of getting a current to flow by moving a magnet?
Electromagnetic induction
68
How would you increase the voltage induced by moving a coil of wire near a magnet?
Increase the strength of the magnet; increase the turns in the coil; use an iron core inside the coil; spin the coil faster.
69
What is the relationship between speed of rotation of the coil through the magnetic field, and voltage?
Faster rotation means greater voltage
70
How is mains electricity generated?
By using generators in power stations that produce alternating current (AC)
71
When the coil rotates, what happens that causes the voltage induced to vary?
It changes the rate at which it cuts the field lines, so the voltage induced varies.
72
What is direct current? (DC)
Electricity that always flows in the same direction.
73
What is alternating current? (AC)
Electricity that changes direction at regular intervals.
74
In the UK, what sort of current is mains electricity?
AC
75
What voltage is UK mains electricity?
230v
76
What is the frequency of UK mains electricity?
50Hz
77
How is the voltage of the mains electricity changed for domestic use?
A transformer is used at a substation
78
Why does the national grid use high voltage?
It is more efficient, less heat loss.
79
Why is the UK power supply AC?
It is easier to generate AC electricity in large amounts.
80
What is used in appliances to make things move?
Motors.
81
What shape is the magnetic field around the wire?
Round
82
What happens to the magnetic field when the wire is made into a coil?
It becomes similar to a bar magnet.
83
What is the name given to a coil of wire that generates a magnetic field like a bar magnet's when the current passes?
An electromagnet.
84
When a current flows in a wire that is in a magnetic field, and the wire is free to move, what happens?
It moves.
85
What is the movement of a wire in a magnetic field called?
The motor effect
86
When is the motor effect greatest?
When the current is at right angles to the magnetic field lines.
87
When is no motor force experienced?
When the current is parallel to the magnetic field lines.
88
What causes the motor effect?
The interaction of the magnetic field around a wire carrying current, and the magnetic field of the magnet.
89
What do transformers do?
Change the voltage of an AC power supply
90
What does a transformer consist of?
Two coils round an iron core.
91
Where does the input voltage go?
The primary coil
92
Where does the output voltage go?
The secondary coil
93
How does it work?
The AC current creates an alternating magnetic field, which the iron core channels to the secondary coil, the alternating magnetic field of the primary coil will cut the wires of the secondary coil, so an AC voltage is induced.
94
How would you calculate the transformers output?
voltage across primary coil/voltage across secondary coil = number of turns in primary/number of turns in secondary
