4.2 - Energy, Power and Resistance

Question 1

Give the names of each of the following symbols:

Answer 1

1. Open switch
2. Closed switch
3. Cell (NOT battery)

Question 2

True or false: Both of these symbols represent resistors:

Answer 2

False. The bottom symbol is actually a fuse.

Question 3

Draw the symbol for a variable resistor.

Answer 3

Question 4

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

Answer 4

Light Dependent Resistor (LDR).

Question 5

Should voltmeters be connected in series or parallel?

Answer 5

Parallel

Question 6

What is potential difference? State the units.

Answer 6

The work done per unit of charge; the difference in potential between to points in a circuit.
Measured in volts.

Question 7

What does ε represent in electricity?

Answer 7

Electromotive force (e.m.f).

Question 8

What is the difference between voltage and emf?

Answer 8

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 9

Give the equation relating energy transferred to voltage.

Answer 9

W = VQ
Where W = work done (energy transferred),
V = potential difference,
Q = charge.

Question 10

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

Answer 10

Question 11

What is resistance?

Answer 11

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 12

What is the definition of 1 Ohm?

Answer 12

When a resistor is subject to a voltage of 1V and allows a current of 1A through, its resistance is 1 Ohm.

Question 13

What is meant by an ohmic conductor?

Answer 13

A conductor that obeys Ohm’s law.

Question 14

What does Ohm’s law say? (In words)

Answer 14

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 15

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

Answer 15

1/R.
Because the gradient = rise in current/change in potential difference i.e. gradient = I/V.
As resistance= V/I, the gradient = 1/R

Question 16

Does this graph represent an ohmic conductor?

Answer 16

Yes, as the line has a constant gradient and
passes through the origin. This shows that voltage is directly proportional to current

Question 17

Which graph represents a appliance with higher resistance?

Answer 17

B.
The gradient = 1/R so a smaller gradient means a higher resistance.

Question 18

What common appliance could this curve represent?

Answer 18

A filament lamp.
As current increases, resistance increases.

Question 19

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

Answer 19

● 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 20

What is a diode?

Answer 20

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

Question 21

What is a Light Dependent Resistor (LDR)?

Answer 21

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

Question 22

How does a thermistor work?

Answer 22

Similar to a LDR, but as the temperature increases, the resistance decreases.
(Note - thermistors that work in this way are called negative temperature coefficient (NTC) thermistors).

Question 23

What is resistivity?

Answer 23

The resistivity is a property of a metal which shows how easy or difficult it is for current to flow in the material (at a specific temperature).
⍴=RA/L
Where ⍴ = resistivity, R = resistance, A = cross-sectional area, and L = length.

Question 24

Describe an experiment to determine the resistivity of a metal.

Answer 24

• Measure the diameter of your wire using a micrometer.
• Use this to calculate the area
• • Set up circuit as shown
• Vary the length and record the voltage and current for each length
• Use R=V/I to work out the resistance
• Plot a graph of resistance against the length
• The gradient = resistivity ÷ area
• Rearrange to work out resistivity

Question 25

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

Answer 25

False.
The resistivity of a metal increases with temperature because as the metal ions heat up they vibrate more and the 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 26

What is power? State the unit.

Answer 26

The rate of energy transfer.
Measured in J/s or Watts (W).

Question 27

Give an equation for power in terms of current and voltage.

Answer 27

P=VI

Question 28

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

Answer 28

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

Question 29

Why do electricity companies use ‘units’ (kWh) rather than joules or watts?

Answer 29

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 30

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

Answer 30

Units used = 3563-3543 = 20
Total cost = units x price = 20 x 15p = 300p = £3