Electricity

Question 1

what is current

Answer 1

the rate of flow of charge

Question 2

define potential difference

Answer 2

the energy transferred between two points from electrical energy to other forms per unit charge

Question 3

how can current be measured

Answer 3

connect an ammeter component in series

Question 4

what is Kirchhoff’s first law

Answer 4

the conservation of charge. The sum of currents entering a junction equals the sum of currents leaving

Question 5

what is meant by mean drift velocity

Answer 5

the average velocity of electrons as they move through a wire

Question 6

whats the average electron density of conductors, semiconductors and insulators

Answer 6

conductors- 10 ^29 m^-3
semiconductors- 10^19 m^-3
insulators- much less than 10^9 m^-3

Question 7

how can voltage be measured

Answer 7

connect a voltmetre in parallel

Question 8

define electromotive force (EMF)

Answer 8

the work done on charge carriers per unit charge measured across a cell/battery. the energy is transferred from chemical to electrical

Question 9

define resistance

Answer 9

a property of components calculated as voltage/current through the component

Question 10

how is the total resistance in a parallel circuit found

Answer 10

the reciprocal of the total resistance equals the sum of the reciprocal resistances of each resistor

1/ R(total) = 1/R(1) + 1/R(2) …

Question 11

what is an ohmic conductor

Answer 11

a conductor that obeys ohm’s law. this means a current-voltage graph would be linear. this can change at high voltages, it stops being linear so it’s no longer ohmic

Question 12

what is a diode

Answer 12

a semiconductor. it only allows current to flow in one direction

Question 13

what is reverse bias and forward bias

Answer 13

reverse bias is the direction that a diode stops current from flowing. forward bias is the direction that current can flow through a diode

Question 14

what is the breakdown voltage

Answer 14

the voltage required for current to travel in reverse bias

Question 15

what is the threshold voltage

Answer 15

the voltage required for current to travel in forward bias (through a diode)

Question 16

why are filament bulbs, non-ohmic conductors

Answer 16

as current flows through the bulb it produces heat. increasing temperature increases resistance limiting the current that can flow through. therefore an increase in the voltage doesn’t produce the same increase in current

Question 17

define resistivity

Answer 17

a property of a material. its the product of the resistance and the cross-sectional area divided by the length

Question 18

what is a thermistor?

Answer 18

resistors that change resistance depending on their temperature

Question 19

whats an NTC thermistor

Answer 19

a negative temperature coefficient thermistor. increasing its temperature decreases resistance

Question 20

experiment to determine the resistivity of a wire

Answer 20

1. find the length and cross-sectional area
2. set up an ammeter and voltmeter in a circuit
3. use a low current and find the resistance for different wire lengths
4. plot a resistance-length graph and find the gradient
5. multiply the gradient by the area to get resistivity

Question 21

what happens to power in a circuit

Answer 21

when the current flow in a component with resistance power is dissipated

Question 22

what is Kirchhoff’s second law?

Answer 22

the total electromotive force in a series circuit equals the sum of the P.d across each component. it’s the conservation of energy

Question 23

how can you find the total resistance in a series circuit

Answer 23

the sum of the resistances of each resistor

Question 24

what is the internal resistance of a cell

Answer 24

it reduces the voltage output before the current reaches the circuit.

Question 25

experiment to find a cell's internal resistance

Answer 25

1. connect the cell to two wires and use a voltmeter to find the voltage (this is the electromotive force) 2. connect a resistor of known resistance and measure the voltage across it 3. use an ammeter to find the current in the circuit 4. calculate the internal resistance with an equation

Question 26

what is a potential divider circuit?

Answer 26

it uses the resistance from two resistors to change the potential difference across components. the greater the P.d of the first resistor the smaller the P.d across the component. it's used when the P.d it too high for the intended use

Question 27

what is a variable resistor

Answer 27

it's a resistor that can change resistance.

Question 28

what is a light-dependent resistor?

Answer 28

its resistance changes with the light intensity. greater light intensity causes lower resistance because light causes the release of electrons

Question 29

define voltage

Answer 29

the P.d produced when one coulomb flows and releases one joule of energy

Question 30

what is Ohm's law

Answer 30

the P.d is directly proportional to the current in a component if the temperature remains constant

Question 31

define coulomb

Answer 31

the amount of charge passing a point from a current of 1 amp flowing for 1 second

Question 32

which side of a cell (its symbol in a circuit) is positive and negative

Answer 32

the longer line is positive and the shorter one is negative

Question 33

what is conventional current

Answer 33

it's used to describe the current in a circuit. it travels from the positive terminal to the negative terminal of a cell. this is imaginary because electrons flow in the opposite direction.

Question 34

what does the symbol 'n' mean

Answer 34

the number density. it's the number of free electrons per unit volume.

Question 35

how does an electron gun work

Answer 35

thermionic emission: a small metal filament is heated with a current. the electrons gain enough energy to escape the surface. these electrons are accelerated towards an anode

Question 36

why does resistance increase with temperature

Answer 36

as temp increases the positive ions increase in internal energy. this causes them to oscillate more. this increases the rate of collisions with electrons so the electrons have to do more work.

Question 37

how does resistivity change with temperature for a conductor

Answer 37

resistivity increases with temperature. this is because there are more collisions between the electrons and oscillating ions

Question 38

how does resistivity change with temperature for a semiconductor

Answer 38

resistivity decreases with temperature. as temp increases, more electrons are released so there number density increases. this makes it a better conductor

Question 39

why does the resistance of an NTC thermistor decrease with temperature

Answer 39

as the temp increases the number density of electrons increases so the resistance decreases

Question 40

uses of thermistors

Answer 40

thermometers, thermostats to control heating, monitor the temp in electrical devices,

Question 41

what is a kilowatt-hour

Answer 41

its a unit of energy used to measure large amounts of energy(joules). it's the energy transferred by a 1kW device in 1 hour

Question 42

how can you analyse a circuit if there are multiple cells in parallel

Answer 42

for each parallel part treat it as its own loop then combine the currents depending on the direction you labelled them and the junctions

Question 43

what causes internal resistance

Answer 43

the metal casing and materials in the cell. some energy is transferred to heat energy. this is wasted energy. this only happens when a current is flowing

Question 44

whats the terminal p.d of a cell

Answer 44

the p.d across the terminals of a cell when current is flowing