Electricity

Question 1

Electric current

Answer 1

The amount of charge passing a point
per second

Question 2

Electric current is measured in…

Answer 2

Amps

Question 3

Charge (current) =

Answer 3

current x time

Question 4

Charge

Answer 4

is a property of some particles, e.g. protons and electrons

Question 5

Unit for charge

Answer 5

Coulumbs (C)

Question 6

Electrons have a charge of

Answer 6

1.6 x10^-19 C

Question 7

Charge (electrons) =

Answer 7

number of electrons x electron charge

Question 8

1C of charge =

Answer 8

6.25 x10^18 electrons

Question 9

Conventional current

Answer 9

defined as moving from positive to negative terminal. This is what is marked on all diagrams and used in calculations.

Question 10

Electric field lines show:

Answer 10

• Which way a positive charge will move
• The direction of the electric field

Question 11

Objects have gravitational potential energy due…

Answer 11

• to the gravitational force acting on mass

Question 12

Objects have electric potential energy due…

Answer 12

to their position in an electric field, and the electric force acting

Question 13

Potential difference (or voltage)

Answer 13

is the difference in electric potential energy between two points for 1C of charge

Question 14

When current passes through components:

Answer 14

energy is transferred from the charges:
- The charge lose electric potential energy
- The potential difference drops

Question 15

When current passes through batteries/cells:

Answer 15

energy is transferred to the charge:
- The charges gain electric potential energy
- The potential difference increases
- We say the batteries/cells provide an electromotive force e.m.f (it just means a positive change in p.d.).

Question 16

P.D.

Answer 16

is the energy transferred per coulomb from
charge carriers to components.

Question 17

E.M.F.

Answer 17

is the energy transferred per coulomb to
charge carriers from sources (batteries/cells)

Question 18

Resistance:

Answer 18

• Charge carriers collide with the materials
  they flow through, whether it’s a lattice or
  a liquid.
• Each collision transfers KE from the
  carriers to thermal energy in the material

Question 19

Ohm’s law:

Answer 19

• The current through a conductor at
  constant temperature is directly
  proportional to the voltage supplied
• For each conductor this constant is its
  resistance.

Question 20

Resistance and temperature

Answer 20

• Resistance increases with temperature
• As temperature of a material increases, its
  atoms/molecules vibrate more.
• This causes more frequent and disruptive
  collisions with charge carriers, causing
  greater transfer of energy.

Question 21

Ohmic conductors:

Answer 21

Anything with a fixed resistance - which therefore follows Ohm’s law:
- Fixed resistors
- Wires
- Most metal components

Question 22

Non-ohmic conductors

Answer 22

Anything with a variable resistance - which therefore shows non-linear relationships on I-V graphs:
- Filament lamps
- Diodes

Question 23

Resistance of a wire depends on:

Answer 23

• Length (more wire to travel through, more resistance)
• Area (more charge carriers (or more pathways), less resistance)
• Resistivity

Question 24

R =

Answer 24

ρL/A

Question 25

Resistivity

Answer 25

Measure of the intrinsic resistance of amaterial, due to electron density, electron structure and lattice structure

Question 26

Temperature dependance

Answer 26

- Regular conductors (as resisitivity increases temp increases), increase in energy goes to vibrations in the lattice - Semiconductors (as resistivity increases temp decreases), increase in energy frees more electrons for current

Question 27

free electrons:

Answer 27

- Free electrons behave like a gas inside conductors - The more free electrons, the more conductive a material

Question 28

electrons drift

Answer 28

- Electrons drift through circuits at about 0.1 mm s-1. - Electrons bump into the lattice and each other – they don’t travel in straight lines, so their drift velocity varies. - Electric signals travel faster than this because once a potential difference is applied to a circuit, all electrons move at once

Question 29

current (drift velocity) =

Answer 29

I = nqvA

Question 30

Semiconductors

Answer 30

- semiconductors are materials (including the elements on the right) that have a conductivity less than conductors but greater than insulators - as temperature rises lattice ions release electrons, increasing number density (n) and so decreasing resistivity

Question 31

Holes

Answer 31

the positive 'holes' left behind by free electrons themselves move (the opposite way to electrons), also increasing the current

Question 32

In semiconductors electrons absorb energy, moving from the...

Answer 32

valence band to the conduction band of an atom, leaving behind a hole.

Question 33

valence band

Answer 33

is the last electron shell of atom

Question 34

conduction band

Answer 34

contributes to the sea of delocalised electrons that cause current

Question 35

Insulators (band theory)

Answer 35

the conduction band is quite empty and there is a large energy gap

Question 36

Semiconductors (band theory)

Answer 36

the conduction band is quite empty but the energy gap is small.

Question 37

Conductors (band theory)

Answer 37

the bands overlap, so the conduction band has lots of electrons

Question 38

Thermistor

Answer 38

Resistance decreases with temperature: - Heat energy transferred to free electrons, increasing charge carrier density - (Regular conductors have positive temperature coefficient: R ∝ T ) Used to monitor temperature, e.g. thermostats, air-conditioning, incubators, electrical devices, preventing overheating

Question 39

Light-dependent resistor (LDR)

Answer 39

- Made from a semiconductor material in which incident light frees more charge carriers, increasing charge carrier density - Used for automatic lighting

Question 40

Diodes

Answer 40

Non-ohmic component formed of two types of semiconductor that allows current in one direction only

Question 41

Superconductors

Answer 41

- For these materials at very low temperatures (<30K) resistance drops to zero. - They undergo a phase transition in which electrons form pairs (Cooper pairs) due to a quantum-mechanical attraction. - The electron pairs behave like a superfluid in the lattice, flowing without viscosity - not losing any KE to collisions.

Question 42

Why do electrical signs travel faster?

Answer 42

- All e- move at once - e- pass charge through collisions to move charge quickly

Question 43

Potential dividers

Answer 43

A component (resistor) that divides the potential energy

Question 44

Potential dividers equation:

Answer 44

v1 / v2 = r1/ r2

Question 45

3 electrical power equations

Answer 45

- P = V x I - P = I^2 x R - P = v^2 / R

Question 46

More brightness means

Answer 46

more power