Electricity

Question 1

Find current (using charge)

Answer 1

Charge passing a point/time

Q/t

Question 2

Find voltage (using work done)

Answer 2

Work done/charge

W/Q

Question 3

Charge of an electron in coulombs (or e)

Answer 3

1.6 x10^-19

Question 4

Kirchhoff current law

Answer 4

Sum of current into a point = sum of current out
(ΣIin = ΣIout)
Because conservation of charge

Question 5

Number density

Answer 5

Number of free electrons per cubic metre of material

Question 6

Find current (using mean drift velocity)

Answer 6

Current = cross sectional area*number density*e*mean drift velocity
(I = Anev)

Question 7

Find voltage (using current)

Answer 7

Voltage = current*resistance
(V = IR)

Question 8

Find resistance (using resistivity)

Answer 8

Resistance = resistivity*length/cross sectional area
(R = ρL/A)

Question 9

Find resistivity

Answer 9

Resistivity = resistance*cross sectional area/length
(ρ = RA/L)

Question 10

Resistivity

Answer 10

How much a material opposes electron flow

Question 11

Resistance

Answer 11

How much a component opposes electron flow

Question 12

Current

Answer 12

Rate of flow of charge

Question 13

Potential difference (p.d.)

Answer 13

Energy lost (work done) by each unit of charge flowing through a component

Question 14

Electromotive force (e.m.f.)

Answer 14

Energy gained by each unit of charge passing through a cell

Question 15

How to measure p.d. of a component?

Answer 15

Connect a voltmeter in parallel

Question 16

How to measure current through a component?

Answer 16

Connect an ammeter in series

Question 17

Power

Answer 17

Rate of work done by a component

Question 18

Find power (of a component)

Answer 18

Power = voltage*current
(P = VI)

Question 19

Find power (using only current and resistance)

Answer 19

Power = resistance*current^2
(P = I²R)

Question 20

Find power (using only voltage and resistance)

Answer 20

Power = voltage^2/resistance
(P = V²/R)

Question 21

Find kinetic energy of an electron from an electron gun

Answer 21

Kinetic energy = e*accelerating p.d.

0.5mv² = eV

Question 22

Diode

Answer 22

• only allows current in one direction
• very high resistance until threshold p.d. is reached
• non ohmic component (V not proportional to I)

Question 23

Thermistor

Answer 23

• can have positive or negative temperature coefficient
• NTC decreases resistance with temp rise
• PTC increases resistance with temp rise
• non ohmic

Question 24

Photoresistor (or LDR)

Answer 24

• decreases resistance as light intensity increases

- non ohmic

Question 25

Find total resistance in series

Answer 25

Rt = R1 + R2 + ...

Question 26

Find total resistance in parallel

Answer 26

1/Rt = 1/R1 + 1/R2 + ...

Question 27

Kirchhoff voltage law

Answer 27

In any loop, Sum of e.m.f. = sum of p.d. (Σε = ΣV) Because conservation of energy

Question 28

Find p.d. across R1 of a potential divider

Answer 28

V1 = Vt*R1/Rt

Question 29

Find e.m.f. of a cell (using internal resistance r)

Answer 29

e.m.f. = terminal p.d. + p.d. of internal resistance (ε = V + Ir or ε = I(R + r))

Question 30

On a terminal p.d. vs current graph, how do you find: - e.m.f. - internal resistance

Answer 30

``` ε = V + Ir (because of voltage law) V = -rI + ε so: - e.m.f. = y intercept - internal resistance = negative of gradient ```

Question 31

Terminal p.d.

Answer 31

p.d. across a cell (same as p.d. across rest of circuit)

Question 32

Direction of current flow

Answer 32

Positive to negative

Question 33

Direction of electron flow

Answer 33

Negative to positive

Question 34

n values for different materials

Answer 34

Conductors - high Semiconductors - lower Insulators - very low

Question 35

I-V graph of a resistor

Answer 35

- resistance stays the same - straight line through the origin - ohmic conductor

Question 36

I-V graph of a filament lamp

Answer 36

- resistance increases with V (because of heating) | - curve with gradient that reduces to 0

Question 37

I-V graph of a thermistor

Answer 37

- resistance decreases with V (because of heating) | - current curves upwards like a tan graph

Question 38

I-V graph of a diode

Answer 38

- resistance goes from high to low at threshold voltage | - curves sharply upwards at V = 0.7 (different for LEDs)

Question 39

Explain the effect of temperature on resistance / resistivity

Answer 39

High temperature causes ions in the metal to vibrate, making them more likely to block electron flow

Question 40

Find work done by a component

Answer 40

Work done = voltage*charge (W = VQ or W = VIt)

Question 41

Kilowatt hour

Answer 41

Unit of energy used to calculate energy costs

Question 42

Ratio of voltages in a potential divider

Answer 42

Ratio of voltages = ratio of resistances | V1/V2 = R1/R2

Question 43

Potentiometer

Answer 43

Resistor with three terminals and a sliding contact - can be used as a variable resistor or a potential divider

Question 44

Ohm's law

Answer 44

Voltage is proportional to current at constant temperature