9 Energy, Power And Resistance

Question 1

the volt

Answer 1

1V = 1JC^-1

Question 1

potential difference definiton

Answer 1

energy transferred from electrical energy to other form (heat, light ect) per unit charge.

V=W/Q

Question 2

The voltmeter

Answer 2

ideal voltmeter should have an infinite resistance

so no current passes through the
voltmeter itself. Whilst this is not possible in reality, most voltmeters have a resistance of several million ohms.

Question 3

EMF

Answer 3

Electromotive force is defined as the energy transferred from chemical energy (or another form) to electrical energy per unit charge

ε = W/Q

Question 4

thermionic emission

Answer 4

the emission of electrons through the action of heat.

Question 5

relating work done to the gain in kinetic energy of an electron

calculate the velocity of an electron

Answer 5

eV = 1/2 mv^2

assumes the electrons have negligible energy at cathode

Question 6

A linear particle accelerator

Answer 6

ses a series of cylindrical electrodes (drift tubes) to accelerate subatomic particles such as electrons. The polarity of the drift tubes is alternated between positive and negative with precise timing so that each time the electrons leave one of the tubes, the polarity changes in order to attract them to the next one.

Question 7

Resistance

Answer 7

1 Ω = 1V/A

a component wiht resistance of 1 omhs will have a p.d across it of 1V per ampere of current in it.

a component with a resistance of 500ohms would have pd of

Question 8

Ohm’s law

Answer 8

Ohm’s investigations into the resistances of metallic conductors led him to derive what is now referred to as Ohm’s law.

For a metallic conductor kept at a constant temperature, the current in the wire is directly proportional to the p.d. across its ends.

In other words, he found that when the p.d. across the wire (kept at constant temperature) doubled, the current in the wire also doubled.

Question 9

I-V Characteristics of resistors

Answer 9

directly propotional

obey Ohm’s law also descriubed as an ohmic conductor

resistor behave the same way regardless of polarity

resistance is constant

Question 10

I-V Characteristics of Filament lamps

Answer 10

non-ohmic componet

ressistance increases as the p.d across it increases

increase in resistance is caused by the wire getting so hot that it glows. As the current increases so does the rate of flow of charge
through the filament - more electrons per second pass through it, so more collisions occur between the electrons and the positive metal
ions per second. When the electrons collide with the ions they transfer energy to the ions, causing the ions to vibrate more, or in other words to increase in temperature, and to collide with still more electrons.

Question 11

LED

Answer 11

They emit light by a process very different from that in a filament lamp - electrical energy is transferred directly into light, and LEDs do not get hot, so they are much more efficient and draw much less power.

these light-emitting diodes (LEDs) emit light of a single specific wavelength

They are so energy efficient that sometimes it is used to indicate the direction of current flow in a particular part of the cirfuit

Question 12

Diodes

Answer 12

only allows current flow in one particular direction

Question 13

IV characteristics for a diode

Answer 13

The potential difference across a diode (or LED) is not directly proportional to the current through it. This means
* a diode does not obey Ohm’s law, and so can be described as a non-Ohmic component
* the resistance of the diode is not constant.

Diodes behaviour depends on the polarity

infinite resistance at negative v
no resistance until threshhold p.d, for silicon it’s around 0.7V

Above this value, the resistance drops sharply for every small increase in p.d. (at C). Above this point the diode has very little resistance.

Different LEDs have different values for their threshold p.d., related to the colour of the light they emit.

Question 14

Resistivity and resistance and relationships

Answer 14

used to describe the electrical property of a material

R∝L
R∝1/A

R = ⍴L/A

resistivity measured in ohmes meter

Question 15

Determining resistivity ⍴

Answer 15

see how resistance of a wire varies with its length.

gradient is resistivity/A

as R = ρL/a

Question 16

Thermistor

Answer 16

some semiconductors have a negative temperature coefficient, meaning their resistance drop ad temperature increases

this is because as the temperature increases the number density fo the charge carrier also increases

in terms of iv graph as v increases so does the temperature but unlike filiment lamp increase in temperature leads to less resistance

Question 17

Electrical energy