P9 - Energy, Power and Resistance

Question 1

Draw the 14 circuit symbols.

Answer 1

Look in folder.

Question 2

Define polarity.

Answer 2

Order of terminals in cells and batteries.

Question 3

Define Potential Difference.

Answer 3

Energy transferred from electrical energy to other forms per unit charge - charge carriers losing energy.

Question 4

Define a volt.

Answer 4

The P.D. across a component when 1J of energy is transferred per unit charge passing through a component. 1V=1JC^-1.

Question 5

What is the P.D. equation?

Answer 5

V = W/Q

Question 6

What is the emf equation?

Answer 6

E = W/Q

Question 7

Define EMF.

Answer 7

The energy supplied by the battery to each charge carrier passing through. The PD of a cell when no current flows. Voltage without charge? When charge carriers gain energy.

Question 8

What is the difference between P.D. and EMF?

Answer 8

P.D. is word done by charge carriers and EMF is work done on charge carriers.

Question 9

What is eV?

Answer 9

Energy gained when an electron accelerates through 1V of potential difference.

Question 10

How much is 1 electron volt equal to?

Answer 10

1eV = 1.6x10^-19 J
(derived from e = QV

Question 11

Define electron gun.

Answer 11

A device used to produce a narrow beam of electrons with precise kinetic energies.

Question 12

Define thermionic emission.

Answer 12

The process where free electrons are emitted from the surface of a metal when heat is applied.

Question 13

Describe the process of using an electron gun.

Answer 13

1. Electron source is small metal filament.
2. Electrons gain energy when filament is heated.
3. Some gain enough energy to escape the metal’s surface.
4. They enter a vacuum with a high P.D. applied between anode and filament.
5. They gain kinetic energy from P.D. as they travel through the vacuum.
6. They form a narrow beam through a hole in the anode.

Question 14

How do you derive eV = 1/2 mv^2

Answer 14

V = W/Q
W = QV
W = eV
W = 1/2mv^2
eV = 1/2mv^2

Question 15

Why do electrons have a greater velocity than protons?

Answer 15

Protons have larger mass so the velocity decreases (eV = 1/2mv^2)

Question 16

Define Ohm.

Answer 16

The resistance of a component when 1V is produced per ampere of current.
1 ohm = 1VA^-1

Question 17

Define electrical resistance.

Answer 17

Components resist charge that flows through it. Energy is needed to move charge carriers so the higher the resistance the more energy required.

Question 18

What do we know about ohmic conductors?

Answer 18

Fixed resistance - voltage and current are directly proportional. Straight line passes through origin.

Question 19

What us the resistance if an IV graph with low gradient

Answer 19

Hgh resistance

Question 20

What does the bar do in a diode

Answer 20

Stops conventional current flowing

Question 21

What do you call the point when resistance increases in a diode

Answer 21

Threshold voltage

Question 22

What are the key features of a diode

Answer 22

.Semiconductor
.Current flow one direction
.ones that emit light don’t get hot
.non-ohmic so v not dp to i
.resistance not constant
.dependent on polarity
Leds different colours due to different thresholds

Question 23

What is resistance proportional to?

Answer 23

Length and resistivity - inversely to area

Question 24

Define resistivity

Answer 24

Intrinsic property of how string a material opposed current flow whereas resistance is about one component

Question 25

What is the equation for resistivity

Answer 25

P =RA/L in Ohmeters

Question 26

What are the key features of a thermistor

Answer 26

.semiconducting material
.decrease resistance when temperature increases so inversely proportional in thermistor.

Question 27

What is an NTC

Answer 27

Negative temperature coefficient

Question 28

How do you measure resistivity of a wire?

Answer 28

Take a wire and measure area with a micrometer of vernier calliper. Then place on a ruler. Measure current and voltage across it to work out resistance and plot this against length. Multiply gradient by area to get resistivity.