Electricity

Question 1

What is electric current?

Answer 1

the rate of flow of charge, I=Q/t

Question 2

What is the value for elementary charge?

Answer 2

e=1.6x10^-19 C

Question 3

What is net charge?

Answer 3

the total sum of all positive and negative charges in a system calculated as the difference between them

Question 4

What is the formula for net charge?

Answer 4

Q= ±ne

Question 5

What is current?

Answer 5

the movement of electrons in metals and the movement of ions in electrolyte

Question 6

What are electrolytes?

Answer 6

liquids that can conduct charge

Question 7

What is conventional current?

Answer 7

the rate of flow of charge from the positive to the negative terminal, this is how ALL electric currents are treated

Question 8

What is Kirchhoff’s first law?

Answer 8

for any point in an electrical circuit the sum of currents into that point is equal to the sum of currents coming out of that point

Question 9

What is mean drift velocity?

Answer 9

the average velocity of electrons as they travel down the wire, colliding with positive metal ions

Question 10

What does the number density of a material represent?

Answer 10

the number of free electrons per unit volume

Question 11

What is the formula for mean drift velocity?

Answer 11

I= Anev

Question 12

What is the distinction between conductors, semiconductors and insulators in terms of n?

Answer 12

conductors have a very high number density, insulators have a much lower number density, and semiconductors have a number density in between the two

Question 13

What is potential difference?

Answer 13

the energy transferred from electrical energy to other forms per unit charge

Question 14

What is p.d. used to measure?

Answer 14

the work done by charge carriers which lose energy as they pass through the components in a circuit

Question 15

What is electromotive force?

Answer 15

the energy transferred from chemical energy to electrical energy per unit charge

Question 16

What is e.m.f. used to measure

Answer 16

the work done to charge carriers when they gain energy as they pass through a cell or power supply

Question 17

What is the difference between e.m.f. and p.d. in terms of energy transfer?

Answer 17

e.m.f is the energy transferred from chemical energy to electrical energy, p.d. is the energy transferred from electrical energy into other forms

Question 18

What are the equations for energy tranfer?

Answer 18

W = VQ AND W =EQ

Question 19

What is Ohm’s law?

Answer 19

the current in the wire is directly proportional to the p.d. across it and inversely proportional the resistance in an electrical circuit when the temperature is kept constant

Question 20

What are the I-V characteristics of a fixed resistor?

Answer 20

the p.d. is directly proportional to the current, the component follows Ohm’s law

Question 21

What are the I-V characteristics of a filament lamp?

Answer 21

non-linear curve passing through the origin, the component is non-ohmic

Question 22

What are the I-V characteristics of a thermistor?

Answer 22

its a non-ohmic component, as the current and temperature through the thermistor increases the resistance decreases

Question 23

What are the I-V characteristics of diode?

Answer 23

its a non-linear, non-ohmic component

Question 24

What are the I-V characteristics of a LDR?

Answer 24

LDR’s are non-ohmic components, when the light intensity incident on the resistor increases, the resistance of the LDR decreases

Question 25

What is the formula for resistivity?

Answer 25

p = (AR)/L

Question 26

What is the variation of resistivity of metals with temperature?

Answer 26

as temperature increases the resistivity increases

Question 27

What is the variation of resistivity of semiconductors with temperature?

Answer 27

as temperature increases the resistivity drops, increased thermal energy excites electrons from the valence band to the conduction band significantly increasing the density of charge carriers - an increase in the density of charge carriers directly increases its electrical conductivity, lowering its electrical resistivity

Question 28

What is the valence band?

Answer 28

the lower energy level of electrons

Question 29

What is the conduction band?

Answer 29

the higher energy level of electrons

Question 30

What techniques can be used to determine the resistivity of a metal?

Answer 30

1) Record the cross-sectional area of the wire 2) Set up a circuit using the recorded length of the wire with a voltmeter and ammeter 3) The values for p.d. and current can be used to calculate resistance, and used alongside length and cross-sectional area to determine the resistivity.

Question 31

What is power?

Answer 31

the rate of energy transfer

Question 32

What are the formulas for power?

Answer 32

P=IV P=(I^2)R P=(V^2)/R P=W/t

Question 33

What is the kilowatt-hour?

Answer 33

1 kilowatt-hour is defined as the energy transferred by a device with a power of 1 kilowatt when it is operated for a time of 1 hour

Question 34

What is Kirchoff's second law?

Answer 34

in any closed circuit the sum of e.m.f. is equal to the sum of p.d. in a closed loop

Question 35

How is Kirchhoff's second law applied to series circuits?

Answer 35

the current at every point in the circuit will be the same, and the e.m.f. will be split so that the total p.d. across each component is equal to the e.m.f.

Question 36

How are Kirchhoff's laws applied to parallel circuits?

Answer 36

the current in each loop adds up to give the total current, the p.d. across each loop is the same.

Question 37

What does it mean when a source of e.m.f has some internal resistance?

Answer 37

not all of the energy transferred to the charge carriers is available as some is transferred to the internal resistance of the cell

Question 38

What are lost volts?

Answer 38

the difference between the measured p.d. across the terminals of the power supply and the actual e.m.f of the cell due to internal resistance

Question 39

What are equations involving internal resistance?

Answer 39

E = I(R + r) and E = V + Ir

Question 40

What techniques can be used to determine internal resistance?

Answer 40

The cell with internal resistance is connected in series to an ammeter and a variable resistor. A voltmeter is connected in parallel around the cell. The resistance of the variable resistor is varied and the V and I readings are recorded. When a graph of terminal p.d. and current is plotted the y-intercept will be the e.m.f of the cell and the negative gradient will be the internal resistance of the cell.

Question 41

What is the equation to determine output voltage of a component?

Answer 41

Output Voltage = (R2/(R1 + R2)) * Input Voltage