Chapter 9: Energy, Power, And Resistance

Question 1

What is Kirchhoff’s 2nd law?

Answer 1

In a loop, voltage in is voltage out. This is the case throughout the circuit in series and parallel circuits

Question 2

What is a volt?

Answer 2

A volt is one JC^-1 or joule per coulomb

Question 3

What is potential difference?

Answer 3

Potential difference is the amount of energy per unit of charge. It is the transfer of energy from electrical to other forms of energy

Question 4

How are voltmeters adjusted to prevent interference with the circuit?

Answer 4

The voltmeter is attached in parallel rather than series
Voltmeters have infinite (very high) resistance to prevent any current from going through the voltmeter’s loop and this limits its impact on the current

Question 5

What is the difference between an ammeter and a voltmeter?

Answer 5

An ammeter measures current in amps whereas a voltmeter measures voltage in volts
An ammeter is attached in series whereas a voltmeter is attached in parallel
An ammeter has no resistance whereas a voltmeter has incredibly high resistance

Question 6

What is electromotive force?

Answer 6

Electromotive force is defined as the energy transferred from chemical energy to electrical energy per unit of charge. The greater the e.m.f, the more energy per coulomb has been transferred into electrical energy

Question 7

What is the formula for electromotive force?

Answer 7

E.M.F = Energy Transferred///Charge

E = J/Q

Question 8

What is an electron gun?

Answer 8

An electron gun is an electrical device used to produce a narrow beam of electrons. These electrons can be used to ionise particles by adding or removing electrons from atoms

Question 9

Where are electron guns used?

Answer 9

Electron microscope, Mass spectrometer, Oscilloscopes

Question 10

How much does an electron weigh?

Answer 10

9.11 * 10^-31

Question 11

How do electron guns work?

Answer 11

All electron guns need a source of electrons which in most cases is a small metal filament that is heated by an electric current which allows it to gain kinetic energy. If the heated filament of an electron gun is put in a vacuum and a high PD is applied between the filament and an anode, the filament acts as a cathode, and the free electrons accelerate towards the anode, gaining kinetic energy. If the anode has a small hole in it, the electrons in line with the hole can pass through, creating a beam of electrons with specific kinetic energy

Question 12

What is the formula for the energy of an electron?

Answer 12

As electrons accelerate towards the anode they gain kinetic energy. EV is the amount of energy gained or lost by an electron between two points

eV = (1/2)(m)(v²)
or
eV = Volt*Charge

Question 13

What is resistance?

Answer 13

All components have a resistance that resists the flow of charge carriers through it. The resistance causes their to be energy to push electrons through the component

Question 14

How do you work out resistance from a voltage-current graph?

Answer 14

Gradient measured for the resistance is drawn from the origin to the specified point in the voltage-current graph

Question 15

How is temperature related to resistance?

Answer 15

As a component’s temperature increases, the resistance increases. This is only till the temperature stagnates where the cooling rate is equal to the heating rate

Question 16

Why does an increase in temperature decrease resistance in silicon?

Answer 16

Temperature increases and breaks the covalent bonds, allowing electrons from the covalent bonds to flow through, increasing the current and therefore decreasing resistance

Question 17

Why is the filament lamp IV graph the way it is?

Answer 17

In a filament lamp, as the voltage increases, the temperature increases, therefore increasing resistance and reducing the gradient. The filament lamp is a non-ohmic component

Question 18

What is the main purpose of a diode?
Why is this helpful with modern electronics?

Answer 18

A diode only allows current in a particular direction.
Most modern electronics can only work on DC, but the mains supply AC, so the diode helps to make the current appropriate for the appliance

Question 19

When do LEDs light up?

Answer 19

The diode function of an LED only allows current through the conventional current. LEDs are therefore very efficient as they use very little energy and only light up when current is in them

Question 20

What is the threshold PD?

Answer 20

The threshold PD is the PD at which current increases and so resistance starts to drop gradually increasing the gradient of the IV graph

Question 21

What is the relationship between resistance and cross sectional area?

Answer 21

They are inversely proportional. As cross-sectional area increases, current increases, and so resistance decreases

Question 22

What is the relationship between resistance and length?

Answer 22

Resistance is directly proportional

Question 23

What is the equation for resistivity?

Answer 23

p= Resistance x Area /// Length
Where ‘p’ is resistivity of a material at a given temperature

Question 24

What is resistivity?

Answer 24

The resistivity of a material at a given temperature is the product of the resistance of a component made of a material and its cross-sectional area, divided by its length (ohms metres)

Question 25

What is the difference between resistivity and resistance?

Answer 25

Different copper wires will have different resistance but copper components all have a unique resistivity

Question 26

What is resistivity in a resistance-length graph?

Answer 26

The gradient on a resistance-length graph is R/L so resistivity is the gradient multiplied by the cross-sectional area

Question 27

What is the setup for a circuit to measure the resistivity of a wire?

Answer 27

Attach a variable resistor and an ammeter in series to a circuit that includes the wire being measured. Attach a voltmeter in parallel to the wire. Measure the cross-sectional area. Attach the voltmeter at different lengths of the wire, and record voltage and current to calculate resistance. Use this to create a resistance-length graph, and use the cross-sectional area to work out resistivity of the wire

Question 28

How does a thermistor’s resistance change?

Answer 28

In a thermistor, resistance and temperature are inversely proportional which means it has a negative temperature coefficient

Question 29

When does a thermistor’s resistance decreases?

Answer 29

Increasing light or heat on a thermistor breaks its covalent bonds and releases more free charge carriers
This is the same as other semiconductors like silicon

Question 30

How does a positive temperature coefficient mean?

Answer 30

Some thermistors have a negative temperature coefficient whereas other thermistors have a positive temperature coefficient. Having a positive temperature coefficient means the thermistor responds conversely to an increase in heat and light by increasing in resistance