Module 4 Fundamental electricity

Question 1

Definition of electric current

Answer 1

the rate of flow of (negative) charge

Question 2

Unit for current?

Answer 2

Amperes, A

Question 3

Unit for charge

Answer 3

Coulombs, C

Question 4

Which letter is current in equations

Answer 4

I (i)

Question 5

Definition of a coulomb

Answer 5

1 coulomb is the electric charge flowing past a point in 1 second when there is a current of 1 ampere

Question 6

In terms of base units, what does 1C equal?

Answer 6

1As

Question 7

What are charge carriers in a metal wire?

Answer 7

electrons

Question 8

Formula for number of electrons using charge?

Answer 8

n=Q/e where Q is the charge and e is the elementary charge

Question 9

In a metal where there is no current, describe the positioning of ions and electrons

Answer 9

Ions vibrate around their fixed positions - delocalised/free electrons move in random directions.

Question 10

Describe the movement of electrons when a current flows in a metal?

Answer 10

electrons drift from negative to positive terminal (repelled by -ve, attracted by +ve)

with drift velocity

Question 11

In an electrolyte, what are charge carriers?

Answer 11

positive and negative ions

Question 12

Describe the direction of flow of electrons in a circuit

Answer 12

move from the negative terminal (repelled) to the positive side (attracted) of power supply

Question 12

What’s the direction of flow of conventional current?

Answer 12

from the positive to negative terminals

Question 13

State Kirchhoff’s first law

Answer 13

the sum of currents entering a junction at any point in a circuit is equal to the sum of currents leaving the same point

Question 14

What is conserved in Kirchhoff’s first law?

Answer 14

expression of conservation of charge

Question 15

Define drift velocity

Answer 15

average displacement of electrons along a wire per unit of time towards the positive terminal of the power supply

Question 16

What is number density?

Answer 16

number of conduction electrons per unit volume

(unit of number density m^-3)

Question 17

What is an insulator?

Answer 17

materials which have very few conduction electrons that we say it’s negligible

Question 18

Equation for current using number density?

Answer 18

I=Anev

I current
n number density of electrons
e elementary charge
v drift velocity

Question 19

Effect of doubling radius on drift velocity in a wire?

Answer 19

A=I/nev, I n e are constant,
A proportional to 1 /v,
area is formed from πr^2 – 2r^2 = 4r so area is increased by 4
therefore 4A=1/0.25v

velocity is multiplied by 0.25

Question 20

What is the unit of potential difference?

Answer 20

Volts V

Question 21

Definition of potential difference

Answer 21

energy transferred per unit charge when electrical energy is converted to another form of energy

Question 22

Equation for potential difference?

Answer 22

V=W/Q
V potential difference
W work done
Q charge

Question 23

Electromotive force definition

Answer 23

Energy is transferred per unit charge when a type of energy is converted into electrical energy

Question 24

Equation for EMF?

Answer 24

E=W/Q

E EMF
W work done
Q charge

Question 25

Difference and similarities between EMF and potential difference?

Answer 25

Both are energy per unit of charge (both are measured in volts)

In EMF energy is transferred to electrical energy, in PD electrical energy is transferred into another form

Question 26

Define what is a volt?

Answer 26

joule per coulomb

Question 27

Why are both potential difference and EMF’s measured in volts?

Answer 27

both are energy transferred per unit of charge

Question 28

What is electrical power defined as?

Answer 28

rate of energy transfer

Question 29

Formula for power using time?

Answer 29

P=W/t

P Power
W work done or energy transferred
t time

Question 30

How is the rate of energy transfer related to in an electrical component?

Answer 30

P=IV or P = I^2 R or P=V^2/R

P Power or rate of energy transfer
I current
V potential difference across a component
R is resistance of the component

Question 31

How is potential difference related to resistance and current?

Answer 31

V=IR

Question 32

What is the unit for resistance?

Answer 32

Ohm’s - Ω

Question 33

Derive the equation for energy transfer (W=IVt) using P=W/t

Answer 33

P=W/t

Substituting P=IV we get IV=W/t

Rearrange to get W=IVt

Question 34

What effects the cost of running an electrical device?

Answer 34

cost is per unit of energy transferred,
power of the device and
how long it has been left running for

Question 35

What is the unit used to measure electrical energy in homes?

Answer 35

kilowatt-hour (kWh)

Question 36

Definition of the kilowatt-hour

Answer 36

Energy transferred by a device with a power of 1kW operating for a time of 1 hour

Question 37

How many joules in 1kWh?

Answer 37

1kW=1000W=1000Js^-1 (W=J/t)
1hr = 3600s
1kWh = 1000x3600
=3.6MJ

Question 38

State Kirchhoff’s Second Law

Answer 38

it states that in any circuit, the sum of electromotive forces is equal to the sum of the potential differences around a closed loop

Question 39

Which quantity is conserved in Kirchhoff’s second law?

Answer 39

Energy is conserved

Question 40

How can you distinguish between a conductor, a semiconductor and an insulator?

Answer 40

Refer to number density of charge carriers
conductor - high number of charge carriers per unit volume
Semiconductors - fewer
Insulators - negligible