4.2 Energy, Power and Resistance

Question 1

What is the name of the component whose circuit symbol is a rectangle with two arrows pointing at it?

Answer 1

Light Dependant Resistor (LDR)

Question 2

Should voltmeters be connected in series or parallel?

Answer 2

Parallel

Question 3

What is potential difference? State the units

Answer 3

The work done per unit of charge; the difference in potential between two points in a circuit

Measured in volts

Question 4

What does “Ę” represent on electricity?

Answer 4

Electromotive Force (e.m.f)

Question 5

What is the difference between voltage and emf?

Answer 5

Voltage is the energy transferred per coulomb across a load resistance

Emf is the total amount of work done by the battery per coulomb

Question 6

Give the equation relating energy transferred to voltage

Answer 6

W = VQ

W = work done (energy transferred)
V = potential difference
Q = charge

Question 7

True or false: The kinetic energy of an electron is equal to its charge multiplied by the voltage

Answer 7

True

Energy transferred = eV = 1/2 m v^2

Question 8

What is resistance?

Answer 8

How difficult it is for current to flow through an appliance

The potential difference required for each unit of current that flows through a resistor

R = V/I

Question 9

What is the definition of 1 Ohm?

Answer 9

When a resistor is subject to a voltage of 1V and allows a current of 1A through, it’s resistance is 1 Ohm

Question 10

What is meant by an ohmic conductor?

Answer 10

A conductor that obeys ohm’s law

Question 11

What does Ohm’s law say?

Answer 11

The current through an ohmic conductor is directly proportional to the potential difference across it (ie. resistance doesn’t vary with voltage or current).

(This is only true if temperature is constant)

Question 12

What does the gradient of a current potential difference graph represent?

Answer 12

1/R

Because the gradient = rise in current / change in potential difference ie. gradient = I/V

As resistance is V/I, gradient is 1/R

Question 13

Why does the current increasing on a filament lamp cause an increase in resistance?

Answer 13

The flow of current causes collisions between the electrons and the metal lattice
These collisions increase the temperature (vibration) of the lattice
As the temperature increases, more collisions occur, causing the resistance to increase

Question 14

What is a diode?

Answer 14

A diode is an appliance that only allows current to flow in one direction

Question 15

What is a Light Dependent Resistor (LDR)?

Answer 15

A semiconductor that is sensitive to light: as the light intensity increases, resistance decreases

Question 16

How does a thermistor work?

Answer 16

Similar to a LDR, but as the temperature increases, resistance decreases

(Note - thermistors that work in this way are called negative temperature coefficient (NTC) thermistors).

Question 17

What is resistivity?

Answer 17

The resistivity is a property of a metal which shows how easy it difficult it is for current to flow in the material (at a specific temperature).

p = RA/L

p = resistivity 
R = resistance 
A = cross-sectional area
L = length

Question 18

Describe an experiment to determine the resistivity of a metal:

Answer 18

1) Measure the diameter of the wire using a micrometer
2) Use this to calculate the area
3) Set up circuit with ammeter in series, and voltmeter in parallel to the metal wire with a certain length (should be variable)
4) Vary the length and record the voltage and current for each length
5) Use R=V/I to work out the resistance
6) Plot a graph of resistance against the length
7) The gradient = resistivity + are
8) Rearrange to work out resistivity

Question 19

True or false: The resistivity of both metals and semiconductors increases with temperature

Answer 19

False

The resistivity of a metal increases with temperature because as the metal ions heat up the vibrate more and electrons bang into them and slow down. Some semiconductors get less resistive as temperature increases, because supplying energy actually causes more charge carriers to be released, so current can flow more easily

Question 20

What is power? state the unit

Answer 20

The rate of energy transfer

Measured in J/s or Watts (W)

Question 21

Give an equation for power in terms of current and voltage

Answer 21

P=V/I

Question 22

Is a kilowatt-hour (kWh) a unit of power, potential difference or energy?

Answer 22

Energy because it’s a unit of power multiplied by time

Question 23

Why do electricity companies use “units” (kWh) rather than joules or watts?

Answer 23

Joules and Watts are both so small that everyone would use tens or hundreds of millions of them each month - it’s impractical to have such large numbers used

Question 24

How much would this customer pay for their daily electricity if the cost is 15p per unit?

morning reading: 03543kWh
evening reading: 03563kWh

Answer 24

Units used = 3563-3543 = 20

Total cost = units x price = 20 x 15p = 300p / £3