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Physics A-Level > Electricity > Flashcards

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
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2
Q

What is one coulomb equivilant to?

A
  • The amount of charge that flows in one second when the current is 1 ampere
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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
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4
Q

What is the charge of an electron?

A

1.602 x 10^-19

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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
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6
Q

What is potential difference?

A
  • Work done per unit of charge
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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19
Q

What is the relation between resistivity and resistance?

A
  • The higher the resistivity of a material, the higher the resistance
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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

Physics A-Level (6 decks)

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