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 dependent 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 ℰ represent in 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 coloumb across a load resistance

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

Question 6

Give the equation relating energy transferred to voltage

Answer 6

E=VQ
Work done(J) = Voltage(V) x charge(C)

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 = 1/2mv^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, its resistance is 1 Ohm

Question 10

What is meant by an ohmic conductor

Answer 10

A conductor that obeys Ohms law

Question 11

What does Ohms law say? (In words)

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 the temperature is constant)

Question 12

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

Answer 12

1/R

Question 13

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

Question 16

How does a thermistor work?

Answer 16

As the temperature increases the resistance decreases

Question 17

What is resistivity?

Answer 17

The resistivity is a property of a metal that shows how easy or difficult it is for current to flow in the material (at a specific temperature)
ρ=RA/L

Question 18

Describe an experiment to determine the resistivity of a metal

Answer 18

• Measure the diameter of your wire using a micrometer
• Use this to calculate the area
• Vary the length of the wire and record the voltage and current for each length
• Plot a graph of resistance against the length
• The gradient = resistivity/area
• 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 they vibrate more and electrons bang into them and slow down. Some semiconductors get less resistive as temperature increases, because supplying energy 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=VI

Question 22

Is a kilowatt-hour a unit of power, potential difference or energy

Answer 22

Energy because its a unit of power multiplied by time

Question 23

WHy do electrictiy 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 - 03543 kWh
evening - 03563 kWh

Answer 24

Units used = 3563 - 3543 = 20

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