Electricity and Magnetism Flashcards
1
Q
What is the symbol for charge?
A
Q
2
Q
What is charge measured in?
A
Coloubs (C)
3
Q
Def. Electric field
A
A region of space where electric charges will be forced to move
4
Q
How can you tell how strong a magnetic field is by looking at a diagram?
A
How close or far away the lines are from each other (each representing the movement of positive electric charges)
5
Q
What defines the direction of an electric field?
A
The direction a positive charge would move
6
Q
How do charges behave with each other?
A
Opposites attract, the same repel
7
Q
What angle do electric fields leave surfaces from?
A
Perpedicular
8
Q
What is the symbol for current?
A
I
That is a capital i, by the way
9
Q
How is current measured?
With wich tool and which unit?
A
Using an ammeter
In Amperes/Amps (A)
10
Q
Def. Current
A
A measure of the amount of charge passing a point per unit time
11
Q
What is the formula for current?
A
I = Q/t
I = Current (A)
Q = Charge (C)
t = Time (s)
12
Q
How many Milliamps are there in one Amp?
How many Microamps are there in one Amp?
Give the conversion rates for each one as well
A
1,000 (10^-3)
1,000,000 (10^-6)
13
Q
What are the basic components of a circuit?
A
- Switch
- Power supply
- Lamp
14
Q
In which direction do electrons flow in a circuit?
A
From - to +
15
Q
I which direction does the conventional current flow in a circuit?
A
From + to -
16
Q
What does the voltage provide?
A
The ‘push’ to move a current
17
Q
What is e.m.f
A
Electromotive force
Batteries have electromotive force which is the work done on the charge by the battery
18
Q
What is the formula for e.m.f?
A
E = W/Q
E = E.M.F (V)
W = Work (J)
Q = Charge (C)
19
Q
Def. Potential difference
A
Work done by a unit of charge on a component in a circuit
20
Q
What is potential difference measured in?
A
Volts (V)
21
Q
What is the formula for potential difference?
A
V = W/Q
V = Potential difference (V)
W = Work (J)
Q = Charge (C)
22
Q
What is resistance measured in?
A
Ohms (Ω)
23
Q
What factors affect resistance?
(in a wire)
A
- Length
- Area
- Resistivity
- Temperature
24
Q
How does the area of a wire affect it’s resistance?
Explain why
A
As the area increases, the resistance decreases
This is beacause if the area increases, there are less particles that inhibit the movement of electrons
25
How does the length of a wire affect its resistance?
| Explain why
As the **area increases**, the **resistance increases**
This is because the electrons have to travel further and therefore collide into more metal ions
26
How does the resistivity of a wire affect it's resistance?
As the **resistivity increases**, the **resistance increases**
27
How does the temperature of a wire affect it's resistance?
| Explain why
As the **temperature increases**, the **resistance decreases**
This is beacuse if the temperature increases, the metal ions in the wire will vibrate faster and obstruct/collide more electrons
28
What is the Voltage Law for Series Circuits?
The voltage of each component will add up to equal the voltage of the power supply
29
What is the Current Law for Series Circuits?
The current across the series is equal
30
What is the Voltage law for Parallel Circuits?
Each branch of a parallel circuit will recieve a voltage equal to the power supply
31
What is the Current Law for Parallel Circuits?
The current will split at parallel branches based on the resistance of each branch
32
What is Ohm's Law?
V = IR
## Footnote
V = Voltage (V)
I = Current (A)
R = Resistance (Ω)
33
What is the formula for the total resistance in a series circuit?
RT = R1 + R2 + R3...
## Footnote
R = Resistance (Ω)
34
What is the formula for the total resistance in a parallel circuit?
1/RT = 1/R1 + 1/R2 + 1/R3...
## Footnote
R = Resistance (Ω)
35
What is a shortcut rule to find the total resistance of parallel resistors with the **same resistance**?
RT = R/n
## Footnote
RT = Total resistance
R = Resistance of the Resistors
n = Number of parallel resistors
36
What does electrical power as a measurement give us?
A measurement of the rate at which energy is transferred within a circuit
37
What is power measured in?
Watts (W)
38
What is the formula for electrical power?
P = IV
## Footnote
P = Power (W)
I = current (A)
V = Voltage (V)
39
What two formulas can be derived from V = IR and P = IV?
P = I2R
and
P = V2/R
## Footnote
P = Power (W)
I = Current (A)
V = Voltage (V)
R = Resistance (Ω)
40
What is the formula for energy (used) in electrical circuits?
E = IVt
## Footnote
P = Power (W)
I = Current (A)
V = Voltage (V)
t = Time (s)
41
What can energy be measured in in electrical circuits as well as Joules, and why it it used instead?
kWh (Kilowatt hours)
It is better to represent larger quantities
| 1kWh = 3,600,000J
42
What are Kilowatt Hours (kWh)?
The amount of kW used in one hour
| It is a measurement of energy
43
In which direction i the magnetic field?
From North to South
44
Def. Field
A region of space where an effect can be felt
45
How does the Right Hand Grip Rule work?
Thumb points towards the current and fingers grip the imaginary wire. The magnetic field follows fingers around the wire.
46
What is the Right hand Grip Rule Used for?
To find the direction of the magnetic fied around a curent carrying wire
47
How can a compass find the poles on a bar magnet?
The north arrow points in the direction of the current, from north to south, so towards the magnetic south end.
48
How does electromagnetic induction work?
Due to the right-hand slap rule, the magnetic field causes a wire moving through it to have its electrons moving in one direction, creating a current. A current in a wire can also cause the wire to move.
49
Why is a current induced when a wire is passed through a magnetic field?
Because when a charged particle moves through a magnetic field at a right angle to it will experience a force, as shown by the Right Hand Slap rule, the electrons are forced to move along the wire, creating current.
50
Def. Solenoid
A coil of current carrying wire. The effect of the current is a large (but not necessarily strong) magnetic field.
51
Why is it easier to induce magnetism in soft metals?
Because soft metal domains are easier to rearrange in the same direction
52
Def. electromagnet
A solenoid with a soft iron core. The iron core becomes an induced magnet due to the solenoid. This creates a strong magnetic field overall.
52
What rule can you use to find the north and south ends to a Solenoid?
Use your fingers curved as if gripping the solenoid coil of wire in the direction of the current. If you point out your thumb, it points towards the North end of the Solenoid.
52
How does a relay circuit work, and what are they used for?
- A smaller, low voltage ciurcuit powers an electromagnet
- This electromagnet pulls another switch towards itself.
- This switch turns on a much higher voltage system
These are used as safety measures so if something goes wrong in the big circuit, it is not connected with the small circuit so the user can safely interact with the switch.
53
What are the main safety hazards with electricity?
- Overheating
- Electrocution
- Overloading plug sockets
54
What wires does the mains switch consist of?
- Live wire
- Neutral wire
- Earth wire
55
When does the fuse offer the most protection?
When its rating is the lowest value which is greater than the amount of current the appliance is designed to use
56
What are the 2 methods of making electrical appliances safe?
- Double-insulated with a non-conducting case
- Metal case that must be earthed
57
What are the hazards of damaged insulation?
Electrical wires can electrocute you if not insulated
58
What are the hazards of overheating cables and appliances?
Fire risk
59
What are the hazards of damp conditions?
You can get electrocuted through the insulator because water is a good conductor
60
What are the hazards of excess current for the overleading of plugs and extension leads?
The current goes to fast and can cause a fire
61
Why must a switch be connected to the live wire for the circuit to be switched off safely?
Switching of any other wires in the mains supply does not guarantee the electricity supply will be cut
62
What is the purpose of a fuse?
A fuse without an earth wire protects the circuit and the cabling for a double-insulated appliance
63
What are the uses of trip switches and fuses?
To cut the current of a circuit if thereis too much current to prevent electrocution or fire
64
What are the benefits of double insulation?
There are two insulating layers i.e. around wire and around appliance, minimising the chance of electrcity ground through you and electrocute you
65
What are the benefits of earthing an electrical appliance?
If there is a high voltage or curent, instead of electrocuting a human who touches the appliance, the eectricity is earthed though the wire because it is less resistant.
66
What is the purpose of the Live Wire?
| And what safety precaution should be made when setting it up?
- The live wire (brown) carries the current from the mains supply.
- An on/off switch would be connected to this wire because it is the source of current.
- Attaching an on/off switch to any other wire would not guarantee to break the circuit in case of a fault.
67
What is the purpose of the Earth Wire?
The earth wire (green and yellow) is a safety feature used to prevent electrocution
67
What is the purpose of the Neutral Wire?
The neutral wire (blue) completes the full circuit. It does not supply current
68
What are advantages of trip switches instead of fuses?
- They can be reused and reset
- They can break the circuit faster than fuses which need to heat up and snap
69
How do trip switches work?
It is the opposite of a relay circuit. When there is too much current flowing, an electromagnet opens the switch and breaks the current.
70
How do fuses work?
There is a thin strip of wire inside the fuse which melts and snaps under too high currents, breakin the circuit.
71
Explain how the loop in a DC motor moves
The current in the loop of the wire causes the loop to experience a force when it is in a magnetic field
72
Explain the purpose of a commutator in a DC motor
To make the loop spin in one direction, the force on each side of the loop must always be in the same direction.
To make this happen, a commutator is used. This ensures that current flows in one direction in a loop of wire regardless of what side the field it is on.
The role of a commutator is to change the polarity of the electromagnet (loop of wire) every half turn. This means it will always repel the permanent magnet/experience a force in the sae direction, and therefore keep rotating.
73
What are the benefits of AC
- Easier to produce than DC
- More efficient to transport over large distances - less energy loss
- Easy to step up/down voltage as needed
74
Compare AC current to DC
DC is a direct current that flows in one direcetion.
AC causes electrons to move back and forth.
75
What postion is the rotor in an AC generator where it generates the most EMF?
Perpendicular to the magnetic field
76
What postion is the rotor in an AC generator where it generates the least EMF?
Parallel to the magnetic field
77
What are slip rings used for?
They ensure a constant contact between the brushes, which lead to the rest of the (external) circuit, and the sides of loop of wire, while **allowing the coil to rotate freely**.
78
Why do motors heat up when they are operated for more than a few seconds?
This increases becasue the electrons move while the motor is on, increasing electron collisions with metal ions. This causes them to vibrate more and gain more energy. This energy is thermal energy, increasing the temperature of the motor.
79
Explain why a force acts on a wire perpendicular to a magnetic fieldwhen there is a current in it
The current in the wire produces a magnetic field around itself.
The magnetic field of the permanent magenet then interacts with it.
80
What are the 3 ways to increase the turning effect in a motor
- Increase current
- Increase strength of magnetic field
- Add more coils
81
What would happen to the e.m.f if you rotate the coil faster in an AC motor?
- Higher peak e.m.f
- Higher frequency
82
What are the 2 types of Transformers?
Step Up and Step Down
83
What is the purpose of a Step Up transfromer?
To increase the voltage of electricity
84
What is the purpose of a Step Down transformer?
To decrease the voltage of electricity
85
How do transformers work?
1. The alternating current in the primary coil induces a magnetic field in the iron core of the transformer
2. This magnetic field also alternates
3. The alternating field inducces an alternating current in the secondary coil
4. The ratio between the number of turns of wire in the primary and secondary coils determines the size of the change in voltage
86
What is the formula that shows the relation between Voltage and Number of Coils in a transformer?
Vp/Vs = Np/Ns
## Footnote
Vp = Voltage in Primary Coil (V)
Vs = Voltage in Secondary Coil (V)
Np = Number of turns in Primary Coil (A)
Ns = Number of turns in Secondary Coil (A)
87
What is the formula that shows the relation between Voltage and current in a **100% efficient** transformer?
| Does **not** work if the transformer is **not** 100% efficient
VpIp = VsIs
## Footnote
Vp = Voltage in Primary Coil (V)
Vs = Voltage in Secondary Coil (V)
Ip = Current in Primary Coil (A)
Is = Current in Secondary Coil (A)
88
Explain how rotating thhe coil in an AC generator causes a galvanometer attatched to a circuit to show an alternating current.
- As the coil rotates, it cuts the magnetic field between the magnets
- This induces an e.m.f/voltage in the coil
- This produces a current in the coil, which is transferred to the galvanometer via slip rings and carbon brushes
- The direction of the current changes every half spin/180 degree rotation
89
What is a Variable Resistor?
| Give an example of its use
A resistor where the **resistance is not fixed**, allowing it to be changed.
It is used, for example, as a volume control for a radio.
90
What is a LDR?
| Give an example of its use
**Light Dependent Resistor**
The resistance in the resistor is **dependent on the amount of light** shining on the resistor
As the brightness increases, the resistance decreases.
It is used, for example, for streetlights which light up in the dark
91
What is a Thermistor?
| Give an example of its use
A resistor with a resistance that decreases by large amounts over small increases in temperature changes and vice versa.
| Used, for example, as a warning light in a fridge.
92
What is a regular diode?
A circuit component that only allows the flow of electricity in one direction. It does not allow electricity to pass under around 0.7V, and may break, allowing current to flow in the wrong direction, when ecountering a current of around 50V in the wrong direction.
93
What is an LED?
**Light Emitting Diode**
A diode that emits light in addition to only letting current flow in one direction
94
What is an Ohmic Resistor?
A component that displays proportional voltage and current
95
What is a Non-Ohmic resistor?
A component that does not display a proportional relationship between current and voltage
96
What does a current-voltage graph look like for a diode?
There is no current until the is enough current to 'activate' the diode (usually ~0.7V)
After this, current increases at a very high rate compared to voltage.
If there is a voltage backwards, the diode will mitigate any current until ~50V in the wrong direction. at this point, the diode will break and current will be allowed to flow in that direction.
