4.2 - Energy, Power and Resistance

Question 1

What is potential difference (p.d.)?

Answer 1

The energy transferred from electrical to other forms per unit charge

Measured in volts, V

Question 2

What is the volt?

Answer 2

The potential difference across a component when 1J of energy is transferred per 1C passing through the component

Question 3

What is the equation for potential difference?

Answer 3

V = W/Q

Where:
• V = potential difference, V
• W = work done, J
• Q = charge, C

Question 4

What is a voltmeter?

Answer 4

Device used to measure potential difference

Connect in parallel across a particular component

Ideally should have infinite resistance so that no current passes through the voltmeter itself (not possible in reality)

Question 5

What is electromotive force (e.m.f.)?

Answer 5

The energy transferred from another form to electrical per unit charge

Symbol ε
Measured in volts, V

Question 6

What is the equation for e.m.f.?

Answer 6

ε = W/Q

Where:
ε = e.m.f, measured in V
W = work done, measured in J
Q = charge, measured in C

Question 7

What are some similarities and differences between e.m.f and p.d.?

Answer 7

SIMILARITIES:
• both measured in volts
• both energy transferred per unit charge

DIFFERENCES:
• e.m.f = energy transferred from another form to electrical
• p.d. = energy transferred from electrical to another form

Question 8

What is the electron gun?

Answer 8

• a metal filament is heated by an electric current
• the electrons in the filament gain kinetic energy, and some gain enough to leave the metal surface (thermionic emission)
• if the heated filament is placed in a vacuum with a high potential difference applied between the filament and an anode, the filament acts as a cathode
• the freed electrons accelerate towards the anode, gaining kinetic energy

Question 9

Describe the energy transfer in an electron gun

Answer 9

• as electrons accelerate towards the anode, they gain kinetic energy
• from the definition of p.d., the work done on a single electron travelling from the cathode to the anode is eV
• by considering the law of conservation of energy, we can derive an equation:

eV = 1/2mv²

Where:
•e = elementary charge of an electron
• V = potential difference 
• m = mass
•v = velocity of the electron

This assumes that electrons have a negligible kinetic energy at the cathode

Question 10

What is resistance?

Answer 10

The ratio of potential difference across and a component and current flowing through it

Question 11

What is the equation for resistance?

Answer 11

R=V/I

Where:
• R = resistance,Ω
•V = p.d., V
• I = current, A

Question 12

What is an ohm?

Answer 12

The resistance of a component that has a potential difference of 1V per 1A of current

Question 13

What is Ohm’s Law?

Answer 13

The potential difference across a component is directly proportional to the current flowing through it as long as physical conditions remain constant (eg temperature)

Question 14

What happens to resistance in a wire as temperature increases?

Answer 14

As temperature increases, resistance increases

• at higher temperatures, positive ions in the metal wire have more kinetic energy so they vibrate with greater amplitude
• this means collisions between charge carriers (electrons) and positive ions are more frequent
• charge carriers do more work (transfer more energy) as they move through the wore,