CURRENT ELECTRICITY & DC CIRCUIT

Question 1

define current

Answer 1

current is the rate of flow of electric charge through a given cross-section of a conductor

Question 2

formula for calculating current

Answer 2

I = Q/t
si unit of current = ampere

Question 3

instrument to measure current
- series/parallel
- resistance?

Answer 3

ammeter
- connected in series
- very low resistance

Question 4

define conventional current

Answer 4

refers to the flow of positive charge
- opposite direction to electron flow

Question 5

define e.m.f

Answer 5

electromotive force of a source refers to the work done in driving a unti charge(e.g. 1C) across the whole circuit

Question 6

formula for calculating emf

Answer 6

E = W/Q
- si unit = volt (V)

Question 7

emf measured using

Answer 7

voltmeter
- the positive terminal is connected to the positive terminal of battery and negative terminal to negative terminal of batter

Question 8

define potential difference/ voltage

Answer 8

potential difference of a component refers to the work done in driving a unit charge (e.g. 1C) through the component

Question 9

formula for p.d.

Answer 9

V = W/Q

Question 10

p.d. is measured using
- series/parallel
- resistance

Answer 10

voltmeter
- parallel
- very high resistance

Question 11

define resistance

Answer 11

resistance of a component is the ratio of the potential difference V, applied across it to the current flowing through it I.

Question 12

si unit of resistance

Answer 12

ohm

Question 13

rheostat

Answer 13

variable resistor
- used to vary amount of current flowing through a circuit

Question 14

define Ohm’s Law

Answer 14

ohm’s law states that for a metallic conductor the potential difference applied across it is directly proportional to the current flowing through it provided that physical conditions like temperature remain constant.

Question 15

formula for resistance( ohmic conductor)
- shape of IV graph for ohmic conductor

Answer 15

R = V/I
- a straight line passing through the origin
- where the gradient represents the resistance

Question 16

explain resistance of filament bulb

Answer 16

as the current increases, the temperature increases, thus resistance increases, thus voltage increases

Question 17

formula of resistance in terms of resistivity

Answer 17

resistance = (resistivity x length )/ cross sectional area

Question 18

si unit of resistivity

Answer 18

ohm metre

Question 19

why is nichrome used to make heating coils

Answer 19

nichrome has a high resistivity
- thus a high resistance
- thus will generated more thermal energy when current passes through it

Question 20

current and pd in series and parallel circuit
- same/different

Answer 20

series
- current constant
- V different
- R different

parallel
- V same for every branch
- I different
_ 1/R = 1/R1 + 1/R2 +….

Question 21

advantages of parallel circuit over series
disadvantage of parallel circuit

Answer 21

advantage
- bulb glow more brightly
- if bulb doesn’t work, others still glow

disadvantage
- higher consumption of power

Question 22

use of potential divider

Answer 22

provide a fraction of voltage of source to another part of circuit

Question 23

formula potential divider

Answer 23

V out = (R1/R1+R2) x Ve

Question 24

define input transducer

Answer 24

input transducers are devices that convert non-electrical energy to electrical energy

Question 25

light dependent resistor and thermistor
relationship b/w resistance and temeprsture/light intensity

Answer 25

LDR
- light increase - resistance decrease
Thermistor
- temp increase - resistance decrease

Question 26

why cells with different emfs are not connected together in parallel

Answer 26

• excessive current flow
• very dangerous