4.2 Energy, Power and Resistance

Question 1

What is potential difference? (2)

Answer 1

A measure of the energy provided to the charge carriers in order for them to convert it into other forms of energy
The amount of work done per unit charge

Question 2

What is electromotive force?

Answer 2

The energy transferred from one type of energy to electrical energy per unit charge

Question 3

What is the difference between potential difference and electromotive force? (2)

Answer 3

PD - the transfer of electrical energy to other types of energy
EMF - the transfer of other types of energy to electrical energy

Question 4

What is 1 volt equal to?

Answer 4

1 joule per coulomb

Question 5

How is voltage calculated?

Answer 5

V (V) = W (J) / Q (C)

Question 6

How is the energy transfer for electrons and other charged particles calculated?

Answer 6

eV = 1/2 mv^2

Question 7

How is resistance calculated?

Answer 7

R = V / I

Question 8

What is Ohm’s law?

Answer 8

The current in an ohmic conductor is proportional to the voltage across it, provided that the temperature and other physical conditions are kept constant

Question 9

What is electrical resistance? (2)

Answer 9

The interference of metal atoms with the motion of electrons

The amount of volts per amp

Question 10

How does temperature affect resistance? (2)

Answer 10

As temperature increases, atoms vibrate faster

The likelihood of them impeding electron flow increases meaning resistance increases

Question 11

I-V Characteristics:
Fixed Resistor (4)

Answer 11

Straight line through (0,0)
Voltage is directly proportional to current
Obeys Ohm’s law
Resistance is constant

Question 12

I-V Characteristics:
Filament Bulb (4)

Answer 12

For the same change in voltage, current increases less as voltage increases
Voltage is not proportional to current
Does not obey Ohm’s law
Resistance increases as PD increases

Question 13

I-V Characteristics:

LED

Answer 13

Before the activation PD (0.7 - 1.5) there is infinite resistance so no current flows
After the activation PD resistance decreases rapidly
A large current flows with a small PD
Does not obey Ohm’s law

Question 14

I-V Characteristics:

Thermistor (4)

Answer 14

For the same increase in PD, current increases at an increasing rate
As current increases, temperature increases causing resistance to fall
Resistance decreases with increasing PD
Does not obey Ohm’s law

Question 15

How does the resistance of an LDR change with light intensity? (3)

Answer 15

Resistance decreases as light intensity increases
Photons of light hit a cadmium sulphide track, giving the electrons enough energy to jump into the conduction band
Approximately 1 mega ohm to 500 ohms resistance

Question 16

What is the equation for the resistivity of a metal?

Answer 16

Resitivity (ohm metres) = AR / L
A = cross sectional area (m^2)
R = resistance (ohm)
L = length (m)

Question 17

What is resistivity? (2)

Answer 17

An electrical property of a material that affects its resistance
Resistivity is constant at a constant temperature

Question 18

How does the resistance of a thermistor change with temperature?

Answer 18

As temperature increases, resistance decreases

Question 19

What is power?

Answer 19

The rate of doing work

Question 20

How is power calculated? (2)

Answer 20

P (W) = V (V) x I (A)
Or
Power = energy / time

Question 21

What is 1 watt equivalent to?

Answer 21

1 joule per second

Question 22

How is energy transfer calculated?

Answer 22

Energy transfer = voltage x current x time

Question 23

How is energy calculated? (2)

Answer 23

Energy (J) = Power (W) x Time (s)

Energy (kWh) = Power (kW) x Time (hr)

Question 24

What is 1kWh? (2)

Answer 24

The energy transferred by a device working at 1kW for 1 hour

1kWh = 3.6 MJ