DC circuits

Question 1

Define electric current

Answer 1

Electric current is the rate of flow of charge

Question 2

Define e.m.f of a source

Answer 2

e.m.f. of a source is the energy transferred from non-electrical to electrical forms, per unit charge, in driving a unit charge round a complete circuit

Question 3

Define potential difference

Answer 3

p.d. across 2 points in a circuit is the energy transferred from electrical to non-electrical forms, per unit charge, as a unit charge passes from a point to another

Question 4

Compare e.m.f and p.d.

Answer 4

e.m.f.
- 2 points across e.m.f. source
- Transfer of non-electrical energy to electrical energy per unit charge passing through e.m.f. source
- Exists as property of energy source, regardless of whether current is present

p.d.
- Any 2 points in a circuit
- Transfer of electrical energy to non-electrical energy per unit charge passing through 2 points
- Exists only if there is current flowing
- Represents energy transfer due to motion of charge carriers

Question 5

Define power

Answer 5

Power is the rate of energy transfer

Question 6

Why does a bulb light almost the instant the switch is flipped when drift velocity of charge carriers is so slow

Answer 6

• Switch is closed
• Electric field set up almost immediately
• All free electrons drift towards higher potential simultaneously
• Motion of free electrons already present in the filament of bulb are responsible for the lighting of bulb
• Electrons in e.m.f. source not responsible for immediate lighting

Question 7

Why is average velocity slightly more than zero

Answer 7

• Electric field applied across conductor
• Velocity of charge carriers is increased in direction of applied electric field
• Velocity of charge carriers is decreased in direction opposite applied electric field