Electricity Flashcards
1
Q
What is current?
A
- Current is the rate of flow of electrical charge
- The rate at which charge flows over time
2
Q
What is one coulomb equivilant to?
A
- The amount of charge that flows in one second when the current is 1 ampere
3
Q
What equation links charge and number of electrons?
A
- Q = n x e
- Charge = number of electrons x charge of a single electron
4
Q
What is the charge of an electron?
A
1.602 x 10^-19
5
Q
Conventional current vs electron flow
A
- Conventional current is the flow of positive charge which is from the positive to the negative terminal of the cell
- Electrons flow from the negative to the positive terminal of the cell
6
Q
What is potential difference?
A
- Work done per unit of charge
7
Q
What does a PD of 1 volt mean?
A
- One joule of electrical energy is transferred for each coulomb of charge moving through it
8
Q
How do cells create a potential difference?
A
- Through the seperation of charge
- One terminal of the cell has an excess of positive charge while the other terminal has an excess of negative charge
- Negatively charged electrons are repelled by the negative terminal and attracted by the positive
- When a wire is connected, this allows electrons to flow from one terminal to the other
9
Q
What happens to electrons through a cell and circuit?
A
- As electrons flow through a cell, they gain energy (12V cell means every coulomb of charge gains 12J of energy)
- As electrons flow through a circuit, they lose energy
10
Q
How to relate potential difference to kinetic energy?
A
- When a charge is accelerated due to potential difference, it gains kinetic energy
- W = V x e
- Work done = potential difference x charge of an electron
- W = 0.5 x m x v^2
- Work done = 0.5 x mass of electron x velocity
- Kinetic energy = potential difference x charge of electron
11
Q
What is ohm’s law?
A
- For a conductor at a constant temperature, the current flowing through it is directly proportional to the potential difference across it
- Constant temeprature implies constant resistance
- V = I x R
12
Q
IV characteristics of a resistor / ohmic conductor
A
- Straight line through the origin
- Follows ohm’s law
- Steeper the gradient, lower the resistance
13
Q
IV characteristics of filament lamps
A
- Line through origin which plateaus at both ends
- As the filament temperature increases, positive ions in the metal vibrate more vigorously
- This results in more collisions for electrons
- Electron movement becomes challenging
- Resistance of the filament increases
14
Q
IV characteristics of diode
A
- Line that increases in gradient in positive axis
- Resistance is extremely high when voltage is negative, current is almost negligable
- When PD is positive, resistance drops significantly above a threshold of approximately 0.6V
- Beyond this, current flow increases rapidly
15
Q
Why do materials have resistance?
A
- All materials have resistance to the flow of charge
- As free electrons flow through a metal wire, they collide with ions which get in there way
- As a result, they transfer some, or all, of their kinetic energy on collision, which causes electrical heating
- Since current is the flow of charge, the ions resisting their flow cause resistance
16
Q
What does the resistance of a wire depend on?
A
- Length of the wire
- Cross sectional area through which the current is passing
- The resistivity of the material
17
Q
What does the resistivity equation show us?
A
- The longer the wire, the greater its resistance
- The thicker the wire, the smaller its resistance
18
Q
What is resistivity?
A
- A property that describes the extent to which a material opposes the flow of electrical current through it
- It is a property of the material, and is dependent on the temperature
- Resistivity is measured in ohm metres
19
Q
What is the relation between resistivity and resistance?
A
- The higher the resistivity of a material, the higher the resistance
20
Q
What is an ohmic conductor?
A
- Materials that obey Ohm’s law at steady temperatures
21
Q
Why and how does temperature effect resistance?
A
- All materials are made up of vibrating atoms. The higher the temperature, the faster these atoms vibrate
- Electrical current is the flow of free electrons in a material. The electrons collide with the vibrating atoms which impede their flow, hence the current decreases
- So if current decreases, resistance will increase
22
Q
What is a superconductor?
A
- If a material is cooled below a temperature called its critical temperature, its resistivity disappears completely
- A superconductor is a material with no resistance below its critical temperature
23
Q
What is the critical temperature?
A
- The temperature at which a material becomes a superconductor
24
Q
Where are superconductors useful?
A
- They are useful in applications that require a large electric current:
- The production of strong magnetic fields
- The reduction of energy loss in power transmission
25
What is Kirchhoff's first law?
- The sum of currents entering a junction is equal to the sum of currents leaving the junction
- Current is not consumed or lost as it moves around the circuit
26
What is Kirchhoff's second law?
- The total EMF in a closed loop is equal to the sum of potential differences across each component in that loop
- The energy input is equal to the energy output
27
Current, voltage and resistance in a series circuit
- The current is the same through all parts of the circuit
- The EMF is divided amongst the components using the V=IR formula
- The total resistance is the sum of all individual resistances in the ciruit
28
Current, voltage and resistance in a parallel circuit
- The total current is the sum of the currents of each branch (current is shared amongst branches)
- The voltage of all the components in each branch is equal to the EMF of the power supply (voltage is the same at each branch)
- The total resistance is the reciprocal of the sum of the reciprocals of all the individual resistances
29
What are potential dividers?
- A simple circuit containing resistors in series, across which the source voltage is divided. It allows only a fraction of the total voltage to be used as the output voltage
30
How to calculate output voltage on potential dividers?
Vout = ( R2 / (R1 + R2) ) x total voltage
31
When are variable resistance components used in potential dividers?
- Variable and sensory resistors are used in potential dividers to vary the output voltage
- This means that a change in the surroudings (temperature) could cause a component to turn on (heater switch)
32
What are semiconductors?
- Semiconductors become more conductive when energy is added
- This happens because this process releases more charge carriers, thereby reducing resistance
- They are used to detect changes in the surroundings (thermistors, LDRs, diodes)
33
What is electromotive force?
- When charge passes through a power supply, it gains electrical energy
- The electromotive force (e.m.f) is defined as the amount of chemical energy converted to electrical energy per coulomb of charge when passing through a power supply
- e.m.f is equal to the potential difference across a cell when no current is flowing
- e.mf is the maximum voltage available to the circuit
34
What is the terminal potential difference?
- The potential difference across the terminals of the cell
- If there was no internal resistance, the terminal pd would be equal to the e.m.f
35
Why is the terminal pd always less than the e.m.f
- Due to internal resistance in the power supply
36
What are lost volts?
- The work done per unit charge to overcome the internal resistance of the power supply
37
What is internal resistance and what can it cause?
- The resistanace of the materials within the battery
- The internal resitance causes the charge circulating to dissipate some electrical energy from the power supply
- This causes the cell to heat up over a period of time
- This will cause a loss of voltage in the power supply over time
38
Why does the terminal potential differ from the EMF?
- The internal resistance causes a voltage drop via lost volts
- This means less potential is available to push charges externally than the total e.m.f
- Terminal potential difference is e.m.f minus lost volts
39
What is a capacitor and what do they look like?
- Electrical devices used to store energy in electronic circuits, commonly for a backup source of power
- They are made up of two conductive metal plates connected to a voltage supply (parallel plate capacitor)
- There is commonly a dielectric in between the plates
40
What is a dielectric and its role in a capacitor?
- A dielectric is a substance that is a poor conductor of electricity but a good supporter of electric fields
- This is to ensure that charge does not flow freely between the parallel metal plates
41
What is capacitance?
- The charge stored per unit potential difference (between the plates)
- The greater the capacitance, the greater the energy stored in the capacitor
42
How do you calculate capacitance?
- C = Q / V
- Q is the charge ON the plates NOT of the capacitor
- C is measured in F (Farads)
43
How to convert between nm, um, mm, cm, m
- nm = 10^-9 m
- um = 10^-6 m
- mm = 10^-3 m
- cm = 10^-2 m
44
What is a dielectric made up of?
- Made up of many polar molecules (molecules that have positive and negative end poles)
- When no charge is applied, there is no electric field between the plates and the molecules are alligned in random directions
45
What happens to the dielectric when charge is applied?
- One of the plates becomes negatively charged and the other becomes positively charged
- Hence, an electric field is generated (from positive to negative)
- The negative ends of the polar molecules are attracted to the positive ends of the plate and vice versa
- All of the molecules rotate and allign themselves parallel to the electric field
46
What is permittivity?
- A measure of how easy it is to generate an electric field in a certain material
- It is the ratio of the permittivity of the material to the permittivity of free space
47
How to calculate permittivity?
- Relative permittivity = permittivity of material / permittivity of free space
48
How do dielectrics increase the capacitance?
- When the polar molecules in a dielectric allign with the applied electric field from the plates, they each produce their own electric field which opposes that of the plates
- The larger the opposing electric field, the larger the permittivity
- The opposing electric field reduces the overall electric field, which decreases the potential difference, therefore the capacitance increases
49
How does a capacitor work?
- When connected to a power supply, electrons are pushed from the positive plate to the negative plate
- It therefore does work on the electrons and electrical energy becomes stored on the plates
- At first, a small amount of charge is pushed which then gradually builds up. Adding mroe electrons to the negative side is initally easy as there is little repulsion
- As the charge on the negative plate increases, the repulsion increases and more work must be done to push the electrons
- The difference in charge between the plates causes a potential difference across the plates
50
Why does moving a voltmeter mean that the sum of the voltages doesn't equal the total EMF?
- The current in the circuit changes as the position of the voltmeter changes because resistance in the circuit will change
51
How to calculate energy stored in a capacitor?
- Energy stored = area under graph = 1/2 x Q x V
- Q = C x V
- E = 1/2 x C x V^2
52
Why is energy lost in a capacitor?
- Half of the energy supplied is always lost in a capacitor to heat energy due to its resistance
53
What is the time constant of a capacitor?
- The time taken for the capacitor to charge to 63.2% of its maximum potential difference
- Time taken for the capacitor to discharge to 37% of its maximum potential difference
54
How do you calculate time constant of a capacitor?
- Time constant = R x C
- Resistance of resistor beingg discharged through x capacitance
55
Capacitors in series vs capacitors in parallel
- Capacitors in series: same charge stored on both, shared PD, total capacitance is the reciprocal of the sum of the reciprocals of the individual capacitances
- Capacitors in parallel: 2x charge stored on both, same PD, total capacitance is the sum of the individual capacitances
56
How to differentiate between conductors, semiconductors and insulators
- The charge density of a material (number of charged particles for a given volume) determines how well a material conducts electricity
- Conductors have a high charge density
- Semiconductors have a lower charge density however, their resistivity decreases at higher temperatures hence, the conductivity increases
- Insulator have close to 0 charge density
57
Why does the resistance of NTCs change?
- Made of semi-conducting materials
- Have a relatively low charge carrier density
- As the temperature increases, more electrons break free
- More charge carriers available hence the resistance decreases
