Chapter 9 Energy, Power and Resistance

Question 1

What is a battery?

Answer 1

It is a group of cells in series or parrallel

Question 2

LEARN THE SYMBOLS FOR ELECTRIC CIRCUITES

Answer 2

DO IT

Question 3

What is a volt?

Answer 3

It is the potential difference between two points on a circuit

Question 4

What is the unit and SI derivation of a volt?

Answer 4

The unit is Volts (V) and si units JC^-1

Question 5

What is the equation for a volt?

Answer 5

Volt = Work / Coulombs

Question 6

How is a volt meter used?

Answer 6

It is used in parallel with the circuit

Question 7

What is electromotive force? What is the symbol?

Answer 7

This is used to describe when work is being done on the charge carriers as they pass through a battery. It is the energy gained per coulomb passing through a battery so has the units Volts.
The symbol is ε

Question 8

What is the equation for electromotive force?

Answer 8

ε = W / Q

Question 9

How is energy transfer calculated from voltage?

Answer 9

Work = Voltage x coulombs
or
Work = Electromotive force x coulombs

Question 10

How does an electron gun work?

Answer 10

A hot filament in a vacuum is heated and given a negative charge (cathode) then an anode is put at the other end of the vacuum and the electrons released from the cathode are attracted to the anode and accelerate

Question 11

What is the equation for an electron gun?

Answer 11

eV = 1/2mv^2
e = elementary charge
V = potential difference of the cathode and anode
m = mass
v = velocity

Question 12

How can the acceleration of electrons be changed?

Answer 12

By changing the potential difference of the cathode and anode

Question 13

What is the most important equation for electricity?

Answer 13

V = IR

Question 14

What is resistance?

Answer 14

It is a measure of how hard it is for current to flow through a material

Question 15

What is the equation for resistance?

Answer 15

R = V / I

Question 16

What is the unit for resistance? and SI derivative and symbol?

Answer 16

The ohm
V A^-1
Ω

Question 17

What is ohms law?

Answer 17

As long as temperature remains constant, the current through a material is directly proportional to the voltage across it.

Question 18

How does temperature affect resistance?

Answer 18

The wire gets hot because of collisions between electrons and the positive ions in the wire. This heat causes to positive ions to vibrate more and this causes the frequency of collisions to increase causing more collisions. So as temperature increases so does the reisistance

Question 19

What material is used for fillaments?

Answer 19

Tungsten

Question 20

What does the IV graph for a resistor look like?

Answer 20

It is a straight line through the origin where the gradient is 1/resistance
It is ohmic because it obeys ohms law

Question 21

Filament lamps

Answer 21

The gradient starts even (middle e.g. 1 ) and then flattens out as the voltage increases. As the voltage increases the resistance increases
It is non ohmic because the gradient is not constant

Question 22

What is a diode?

Answer 22

It is a component that only allows current to flow in one direction. The resistance to current in the reverse direction is very high

Question 23

What is an LED?

Answer 23

It is light emitting diode that is made from a material the releases light when electricity passes through it.

Question 24

What is special about the light from an LED?

Answer 24

It is of a specific single wavelength

Question 25

What does the IV curve for a diode look like?

Answer 25

It starts flat (high resistance) and then past the threshold voltage the gradient increases and becomes much steeper (lower resistance) Then when the current is in the reverse direction the resistance is almost infinate.

Question 26

What are factors that affect the resistance of a wire?

Answer 26

Material Length of wire Cross-sectional area Temperature

Question 27

What are praportionalities that can be drawn from the resistance of a wire?

Answer 27

R ∝ L | R ∝ 1/A

Question 28

What is resistivity?

Answer 28

resistance refers to a specific component and resistivity is used to describe the electrical properties of a material It is the resistance of a component multiplied by its cross sectional area per unit length

Question 29

What is the symbol of resistivity? and what is its unit?

Answer 29

ρ and units Ωm

Question 30

What is the equation for resistivity?

Answer 30

R = ρL / A

Question 31

What affects resistivity?

Answer 31

The temperature

Question 32

What is the order of resistivity for conductors?

Answer 32

10^-8 Ω

Question 33

What is the order of resistivity for insulators?

Answer 33

10^16 Ω

Question 34

What is a negative temperature coefficient?

Answer 34

It means that as the temperature increases, the resistance drops

Question 35

What is a thermistor?

Answer 35

It is a component thats resistance changes with temperature

Question 36

What are thermistors made from?

Answer 36

They are made from semiconductors with a negative temperature coefficient

Question 37

What are uses for thermistors?

Answer 37

Thermometers Thermostats and air conditioning Inside computers Car engines

Question 38

What graph can be made from placing a resistor in hot water with a thermometer?

Answer 38

The Resistance of the thermistor can be calculated with a ammeter and a voltmeter and then plotted against temperature as the water cools

Question 39

What are the IV characteristics of a thermistor?

Answer 39

They are non ohmic and produce a graph oposite to a filament lamp. The gradient increases (so resistance decreases) as the voltage increases.

Question 40

What is an LDR and what does it do?

Answer 40

Light dependent resistor | A component thats resistance changes as light is shone on it

Question 41

What are the properties of an LDR?

Answer 41

In the dark they have a high resistance | In the light they have a low resistance

Question 42

How does distance from a light source affect an LDR?

Answer 42

The further the distance the lower the light intensity so the further the distance the higher the resistance

Question 43

What is the equation to work out electrical power?

Answer 43

P = VI

Question 44

What is the unit for power?

Answer 44

Watts (W)

Question 45

What are alternative equations for electrical power?

Answer 45

``` P = I^2R P = V^2 / R ```

Question 46

How is the P = IV derrived?

Answer 46

``` P = Work / Time P = Voltage x Coulombs / Time P = Voltage x Current ```

Question 47

How is energy transferred calculated?

Answer 47

Watts x time = Joules transferred (Work) | Work = VIt

Question 48

What is the definition of a kilowatt hour?

Answer 48

It is the energy transferred by a 1 kilowatt device operating for 1 hour

Question 49

What is the symbol for kilowatt hour?

Answer 49

kWh