9. energy, power and resistance

Question 1

what is a battery

Answer 1

two or more cells connected end-to-end, or in series

Question 2

rules when drawing circuits:

Answer 2

1. only use the circuit symbols
2. do not leave any gaps between any components or wires

Question 3

what is potential difference

Answer 3

it is a measure of transfer of energy (work done) by charge carriers

Question 4

what is the volt?

Answer 4

potential difference across a component. 1J of energy is transferred per unit charge through a component.

Question 5

equation for voltage (when given charge and work done)

Answer 5

V=W/Q

Question 5

what does a voltmeter do?

Answer 5

measures potential difference

Question 6

what quality does the ideal voltmeter have?

Answer 6

infinite resistance. (to prevent any current from flowing through it)

Question 7

what is EMF

Answer 7

EMF = electromotive force
when work is done on charge carriers, which gain energy when passing through the battery/cell.

Question 8

what is an electron gun?

Answer 8

a device which is used to produce a narrow beam of electrons. used to ionise particles

Question 9

How do electron guns work?

Answer 9

1. small filament heated by current
2. electrons gain enough energy to escape atoms
3. If filament is in vacuum and high PD is applied between filament and anode, the filament becomes a cathode
4. free electrons accelerate towards the anode
5. anode has a small hole and electrons in line pass through the hole, creating a beam

Question 10

what is thermionic emmision?

Answer 10

when electrons are getting enough energy to escape their atoms

Question 11

energy transfer equation:

Answer 11

eV = 0.5 x m x v^2
work done on an electron = gain in kinetic energy

Question 12

what is a LINAC

Answer 12

LINAC= linear particle accelerator
series of cylindrical tubes to accelerate subatomic particles such as electrons.
There is a polarity in the tubes that alternates between positive and negative with precise timing
Every time an electron moved from one tube to another it gains energy (eV) where V is max emf of the alternating source connected to the tubes

Question 13

what does a large number of electrodes mean?

Answer 13

electrons can be accelerated to extremely high velocities

Question 14

application of LINACS:

Answer 14

-high energy X-rays
-cancer treatment (radiation therapy)
-industrial radiography
-sterilisation
-environmental remediation

Question 15

what is resistance

Answer 15

the opposition to the flow of electric current within a circuit component

Question 16

unit for resistance:

Answer 16

Ohm Ω

Question 17

Ohm’s law:

Answer 17

electric current (I) is proportional to voltage, and inversely proportional to resistance (R)

PROVIDING THE TEMPERATURE AND OTHER PHYSICAL CONDITIONS REMAIN CONSTANT

Question 18

what if the temperature of the wire were to increase?

Answer 18

resistance increases:
-the atoms within the wire vibrate more with the increased temperature
-causing the flow of electrons (current) to decrease
-thus increasing the resistance to increase

Question 19

what is an ohm?

Answer 19

unit of measurement for electrical resistance

Question 20

what is a IV characteristic

Answer 20

graphs which visualise the relationship between the current (y axis) flowing through different components and the voltage (x axis) applied through it.

Question 21

how to tell from a graph that something obeys ohm’s law:

Answer 21

straight line, through the origin of the graph

Question 22

what is resistivity?

Answer 22

measurement of how strongly a material opposes the flow of current (unit: Ωm)

Question 23

what is the relationship between resistance and length?

Answer 23

the resistance of a wire is directly proportional to its length.
R=ρL/A
ρ is resistivity

Question 24

what is the relationship between resistance and cross sectional area?

Answer 24

as the cross-sectional area of a conductor increases, its resistance decreases. this means they are **inversely proportional**

Question 25

equation for resistance

Answer 25

R=V/I

Question 26

techniques and procedures used to investigate the electrical characteristics for a range of ohmic and non-ohmic components:

Answer 26

1. set up a circuit containing a variable power supply, ammeter, voltmeter and component testing 2. take readings for the current and potential difference as the voltage of the power supply is changed 3. use these readings to plot an I-V graph for the component

Question 27

how does resistance vary with light intensity? (for an LDR)

Answer 27

resistance descreases as light intensity increases. this is because LDRs are made from semiconductor materials, and when light shines on them, it causes electrons to be knocked loose from their atoms, increasing the number of free charge carriers.