Electricity

Question 1

define and calculate current

Answer 1

current - the rate of flow of charge per second

I = Q/T

Question 2

elementary charge

Answer 2

1.6 * 10^-19

Question 3

net charge of lithium atom

Answer 3

0C

Question 4

net charge of lithium 1+ ion

Answer 4

+ 1.6*10^-19

Question 5

Kirchhoff’s first law

Answer 5

sum of currents entering a junction is equal to the sum of the currents leaving the junction

Question 6

define mean drift velocity

Answer 6

average velocity that charged particle attains due to an applied electric field

Question 7

calculate current from drift velocity

Answer 7

I = Anev

n - free electron density
v - drift velocity
I - current in amps
A - cross-sectional area of the conductor
e - charge of an electron

Question 8

describe conductors in terms of charge carrier density

Answer 8

a material with a very high free electron density

metals have 10^29 electrons per m^3

Question 9

describe semi conductors in terms of charge carrier density

Answer 9

a material with a intermediate free electron density but the electron density will change when conditions change e.g. temperature

Question 10

describe insulators in terms of charge carrier density

Answer 10

a material with a very low free electron density

Question 11

define pd and states its unit

Answer 11

the work done by a component on each charge to decrease their energy

measured in volts (V)

Question 12

define E.M.F. and states its units

Answer 12

the work done by a power source on each charge to increase their energy measured in volts (V)

Question 13

define voltage and state its units

Answer 13

measurement of the electrical potential energy per unit charge between two electric fields

Question 14

describe the difference between E.M.F. and P.D. in terms of energy transfer

Answer 14

EMF sources increases the energy of the charges, P.D. decreases the energy of each charge

Question 15

calculate the work done by a component in a circuit

Answer 15

W = QV

Question 16

state and re-arrange an equation using conservation of energy to calculate the velocity of electrons accelerated by an electric field in a vacuum

Answer 16

eV = 0.5 * m * v^2

re-arrange for v. I cant write it here

Question 17

explain why drift velocity in a circuit is considerably less than this velocity

Answer 17

collision with atoms in an atoms electric fields provide a force opposite to the motion of electrons, decreasing the acceleration and therefore decreasing the final velocity achieved by electrons

Question 18

define and calculate resistance

Answer 18

the ratio of potential difference to current (measured in ohms)

R = V/I

Question 19

state ohms law

Answer 19

Potential difference is directly proportional to current when the potential difference is across a fixed resistance

(temperature can affect this)

Question 20

describe an experiment to investigate the I-V characteristics of a component

Answer 20

1. Adjust the variable resistor between its maximum and minimum settings, taking 7 measurements of potential difference and current.
2. Turn off the circuit when not taking measurements to ensure temperature does not change which would affect the I-V characteristics.
3. Plot a graph of current vs potential difference (the resistance is the reciprocal of the gradient of that graph)

Question 21

describe the key feature of an LDR

Answer 21

as the LDR absorbs photons, more free electrons are released, increasing the charge carrier density of the LDR. Therefore using I = Anev, if n increases, the current must also increase if light intensity increases. If the potential difference supplied is fixed and current increases, its resistance must have decreased as current and resistance are inversely proportional for fixed potential difference.

Question 22

calculate the resistivity of a material

Answer 22

ρ = RA/L

Question 23

describe an experiment to determine resistivity of a metal

Answer 23

1. Using a micrometre, measure the diameter at 3 different parts of the wire and calculate an average cross sectional area.
2. Measure the resistance at different lengths using an Ohmmeter.
3. Plot a graph of resistance vs length, determine the gradient and multiply by the area to get resistivity
   R = (ρ/L) x L

Question 24

describe how resistivity varies with temperature

Answer 24

as temperature increases, the atoms in the conductor vibrate with a larger amplitude. Therefore each electron is involved with more collisions with the atoms, decreasing current and therefore resistance increases.

Question 25

describe the key features of a thermistor

Answer 25

as temperature increases, more charge carriers are released in the material, increasing the electron density. Therefore using I = Anev, if n increases, the current must also increase if light intensity increases. If the potential difference supplied is fixed and current increases, its resistance must have decreased as current and resistance are inversely proportional.

Question 26

calculate the power dissipated by a component

Answer 26

P = IV
P = I^2R
P = v^2/R

Question 27

calculate the work done by a component using power

Answer 27

P = W/T
re arrange to get:
W = PT
W = IVt

Question 28

calculate the cost of running components using the unit of kilowatt hours for energy

Answer 28

1KWH = 13p

Power x Hours x 0.13

Question 29

Kirchhoff’s second law

Answer 29

sum of the EMFs around a series loop is equal to the sum of the PDs.

Question 30

calculate the total resistance of resistors in series

Answer 30

add resistors to find total

Question 31

calculate the total resistance of resistors in parallel

Answer 31

reciprocal of resistance to find total

Question 32

EMF source in series

Answer 32

same direction: add

opposite direction: subtract

Question 33

Identical EMF sources in parallel

Answer 33

Same EMF as one on its own

Question 34

Different EMF sources in parallel

Answer 34

Average of the EMFs

Question 35

internal resistance

Answer 35

a model for work done to decrease the energy in each charge occurring inside a power source

Question 36

Terminal PD

Answer 36

the remaining energy in each charge when they have left the terminals of the battery (already decreased from internal resistance)

Question 37

calculate terminal PD

Answer 37

E = IR
E = I(R + r)

Question 38

describe an experiment to measure EMF and internal resistance

Answer 38

1. keeping the power source on a fixed setting, varying the external variable resistor and take 7 readings of terminal voltage and current.
2. ensure the circuit is turned off between readings to ensure any temperature change is minimised.
3. Plot a graph of terminal voltage against current where the gradient x - 1 is the internal resistance and the y - intercept is the EMF

Question 39

describe a potential divider circuit

Answer 39

a potential divider is created when 2 or more components are connected in series so that any charge that passes through one component must also pass through the other

Question 40

describe the impact on the ammeter and voltmeter readings due to temperature of the thermistor increasing

Answer 40

as temperature increases, the resistance of the thermistor will decrease so as the circuit EMF is constant, ammeter reading of current will increase. If the current increases and the resistance of the fixed resistance is constant, according to Ohms law, the voltmeter reading of potential difference must increase.

Question 41

explain how a potential divider circuit that can be used as a sensor circuit work

Answer 41

as light intensity decreases, the resistance of the LDR increases, increasing the total circuit resistance. Therefore the circuit current decreases as EMF is fixed.
If the current decreases and the resistance of the fixed resistance is constant, according to Ohms law, the potential difference across the fixed resistor must decrease and then using Kirchhoff’s second law if EMF is fixed, the potential difference across the lightbulb must increase.