4.2 Energy, Power And Resistance

Question 1

Define emf.

Answer 1

Of a supply, the energy gained per unit charge by charges passing through the supply, when a form of energy is transferred to electrical energy carried by the charges. Measured in v or jc

Question 2

Define pd.

Answer 2

Measured across a component, energy transferred per unit charge by the charges passing through the component.

Question 3

Define Resistivity.

Answer 3

The resistance offered by a unit length of wire made of a material having a cross sectional area of one unit.
=RA/l and define terms.

Question 4

Define power.

Answer 4

The rate at which work is done.

Question 5

Define electron drift velocity.

Answer 5

The average displacement travelled of the electron along the wire per second. On average they move slowly in one direction through the metal lattice, when a pd is applied because they collide constantly.

Question 6

Define elementary charge.

Answer 6

Charge on a single electron, -1.6x10^-19

Question 7

Conventional current

Answer 7

The direction of flow of current from positive to negative, opposite to the flow of electrons, original idea behind flow of charge.

Question 8

Diode

Answer 8

Allows current to only flow in one direction

Question 9

Forward bias

Answer 9

Forward bias is when current flows in the direction allowed by the diode.

Question 10

Reverse bias

Answer 10

Flow of current against the direction of the diode

Question 11

Internal resistance

Answer 11

The resistance within a voltage source that causes a lower volte supply that expected when current flows

Question 12

Current

Answer 12

Rate of flow of charge, the charge flowing through a given point per second

Question 13

What is Kirchhoff’s first law

Answer 13

As charge is always conserved, the sum of charges flowing into a junction must be equal to the sum of the charge leaving it, the same is true for current

Question 14

Equation for drift velocity and factors in equation?

Answer 14

I=nAve

Question 15

Conductors

Answer 15

Larger number of free charger carriers per unit volume

Question 16

Insulators

Answer 16

Very few free charge carriers per unit volume

Question 17

Ohms law

Answer 17

The current through a conductor is directly proportional to the potential difference across it, provided the temperature is constant

Question 18

Factors that affect electrical resistance

Answer 18

• Material of wire
• Length
• Cross sectional area
• Temperature of wire

Question 19

How does length of wire affect resistance?

Answer 19

p.d is more spread amongst electrons

Question 20

How does cross sectional area affect resistance?

Answer 20

More electrons to flow at same drift velocity

Question 21

How does temperature of wire affect resistance?

Answer 21

Resisting ions have more energy to vibrate

Question 22

I-V characteristics of a resistor

Answer 22

Directly proportional

Question 23

I-V characteristics of a filament lamp

Answer 23

Low current= ohmic conductor

High current, resistance increases due to more collisions and higher temperature

Question 24

I-V characteristics of a diode

Answer 24

Require a threshold voltage for conduction to occur, current only in forward bias.

Question 25

Define resistivity.

Answer 25

The measure of the resistance of a material for its cross-sectional area per unit length of wire.

Question 26

Equation for resistivity

Answer 26

RA/I

Question 27

Define temperature coefficient.

Answer 27

Value representing the relationship between the change in resistance and change in temperature of a conductor/semi conductor

Question 28

Define electrical power

Answer 28

Rate at which energy is converted to or from electrical energy to or from another form

Question 29

Power equations

Answer 29

Power = work done /time
Power = current^2 x resistance
Power = voltage^2 / resistance

Question 30

What is conservation of charge?

Answer 30

the law stating that charge can’t be created nor destroyed, it can only be transferred and the total will always be constant

Question 31

What is a coulomb?

Answer 31

Unit of charge, the current that pass through a point over a given amount of time. 1 amp for 1 second

Question 32

Define resistance:

Answer 32

The potential difference across a component per current through it

Question 33

Kirchoff’s second law

Answer 33

The sum of the EMFs in a closed loop circuit is equal to the sum of the pds, hence energy is conserved.

Question 34

Define watt

Answer 34

Joule per second

Question 35

Define electron drift velocity

Answer 35

The average displacement travelled of the electrons along the wire per second
On average, they move slowly in one direction through the metal
Because they collide constantly