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
Q
What is Kirchhoff’s first law?
A
- 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
Q
What is Kirchhoff’s second law?
A
- 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
Q
Current, voltage and resistance in a series circuit
A
- 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
Q
Current, voltage and resistance in a parallel circuit
A
- 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
Q
What are potential dividers?
A
- 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
Q
How to calculate output voltage on potential dividers?
A
Vout = ( R2 / (R1 + R2) ) x total voltage
31
Q
When are variable resistance components used in potential dividers?
A
- 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
Q
What are semiconductors?
A
- 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
Q
What is electromotive force?
A
- 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
Q
What is the terminal potential difference?
A
- 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
Q
Why is the terminal pd always less than the e.m.f
A
- Due to internal resistance in the power supply
36
Q
What are lost volts?
A
- The work done per unit charge to overcome the internal resistance of the power supply
37
Q
What is internal resistance and what can it cause?
A
- 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
Q
Why does the terminal potential differ from the EMF?
A
- 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
Q
What is a capacitor and what do they look like?
A
- 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
Q
What is a dielectric and its role in a capacitor?
A
- 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
Q
What is capacitance?
A
- The charge stored per unit potential difference (between the plates)
- The greater the capacitance, the greater the energy stored in the capacitor
42
Q
How do you calculate capacitance?
A
- C = Q / V
- Q is the charge ON the plates NOT of the capacitor
- C is measured in F (Farads)
43
Q
How to convert between nm, um, mm, cm, m
A
- nm = 10^-9 m
- um = 10^-6 m
- mm = 10^-3 m
- cm = 10^-2 m
44
Q
What is a dielectric made up of?
A
- 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
Q
What happens to the dielectric when charge is applied?
A
- 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
Q
What is permittivity?
A
- 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
Q
How to calculate permittivity?
A
- Relative permittivity = permittivity of material / permittivity of free space
48
Q
How do dielectrics increase the capacitance?
A
- 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
Q
How does a capacitor work?
A
- 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
Q
Why does moving a voltmeter mean that the sum of the voltages doesn’t equal the total EMF?
A
- The current in the circuit changes as the position of the voltmeter changes because resistance in the circuit will change
51
Q
How to calculate energy stored in a capacitor?
A
- Energy stored = area under graph = 1/2 x Q x V
- Q = C x V
- E = 1/2 x C x V^2
52
Q
Why is energy lost in a capacitor?
A
- Half of the energy supplied is always lost in a capacitor to heat energy due to its resistance
53
Q
What is the time constant of a capacitor?
A
- 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
Q
How do you calculate time constant of a capacitor?
A
- Time constant = R x C
- Resistance of resistor beingg discharged through x capacitance
55
Q
Capacitors in series vs capacitors in parallel
A
- 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
