Electricity Definitions

Question 1

define: the coulomb

Answer 1

the charge that flows (past a point) in 1 second when the current is 1A

Question 2

define: electric current

Answer 2

the charge that flows pasts a point in a circuit per unit time

Question 3

define: conventional current

Answer 3

the direction in which positive ions/charges move OR the OPPOSITE direction to the electron flow

Question 4

direction of conventional current

Answer 4

From positive to negative

Question 5

direction of electron flow

Answer 5

From negative to positive (remember that electrons are negative) (opposite to conventional flow)

Question 6

direction of electron flow within a cell

Answer 6

The electron flow continues in the same direction as the external circuit and so it flows from the positive terminal to the negative terminal (an exception to the norm)

Question 7

what does Q=It stand for

Answer 7

charge=current x time. Charge is measures in coulombs (C). Current it measured in amps (A) and time in seconds (s).

Question 8

what does I=nevA stand for

Answer 8

I=current 
n=number density  
e=elementary charge
v=average drift velocity(of the electrons) 
A= cross sectional area

Question 9

what does number density mean

Answer 9

the number of conduction electrons per unit volume (therefore measured in m^-3) for copper this value is 10^29

Question 10

what does elementary charge mean

Answer 10

The charge on each electron. Is always 1.6 x 10^-19 C

Question 11

define: Kirchhoff’s 1st law

Answer 11

sum of currents entering a junction = sum of currents leaving a junction. This law is an expression of the conservation of CHARGE

Question 12

define: Kirchhoff’s 2nd law

Answer 12

For a CIRCUIT LOOP sum of e.m.f. s = sum p.d. s.

This law is an expression of the conservation of ENERGY

Question 13

define: p.d.

Answer 13

Energy transferred from electrical energy to other forms per unit charge

Question 14

define: e.m.f.

Answer 14

(essentially the opposite of p.d.) Energy transferred from other forms into electrical energy per unit charge

Question 15

define: terminal p.d.

Answer 15

the potential difference across the terminals of a cell (N.B this may NOT be the same as the e.m.f. of the cell, due to internal resistance)

Question 16

Define: internal resistance (of a chemical cell)

Answer 16

electrical resistance due to chemical processes within a cell i.e the resistance from an e.m.f. source

Question 17

ohm’s law

Answer 17

the current is directly proportional to the p.d. (voltage) at a constant temperature

Question 18

Define: the ohm

Answer 18

Is the resistance of a component when a potential difference of 1 volt is produced per amp

Question 19

Ohmic

Answer 19

describes a component that obeys Ohm’s law

e.g a resistor

Question 20

Non-ohmic

Answer 20

describes a component that does NOT obey Ohm’s law e.g. a filament bulb or a diode

Question 21

define: electrical resistance

Answer 21

resistance = voltage/current

Question 22

resistivity equation

Answer 22

ρ=RA/L
where ρ=resistivity-measured in ohm metres 
R=resistance 
A=cross-sectional area
L=length

Question 23

define: kilowatt-hour

Answer 23

the energy transferred when a 1kW device operates for 1 hour

Question 24

potential divider equation

Answer 24

V out = R2/ (R1+R2) x by V in

Question 25

define: the volt

Answer 25

the energy transferred per coulomb charge that flows between two points in a circuit