DC circuits

Question 1

Define electric current

Answer 1

Rate of flow of electric charge

Question 2

Explain e.m.f.

Answer 2

• Energy transferred from non-electrical to electrical forms, per unit charge in driving a unit charge round a complete circuit
• Exists as a property of the energy source regardless of current flow
• Refers to any 2 points across the e.m.f. source

Question 3

Explain p.d.

Answer 3

• Energy transferred from electrical to non-electrical forms, per unit charge in driving a unit charge through 2 points in a circuit
• Exists only if there is current flow
• Refers to any 2 points in a circuit
• Represents energy transfer due to motion of charge carriers

Question 4

Define power

Answer 4

Rate of energy transfer

Question 5

Formula for current

Answer 5

I = Q/t = nAvq

Question 6

Formula for number density of charge carriers

Answer 6

n = Q/q

Question 7

How does a bulb light up instantly

Answer 7

• Switch is closed
• Electric field set up almost immediately
• All free electrons drift towards higher potential simultaneously
• Motion of free electrons already in the filament of bulb are responsible for immediate lighting

Question 8

Explain why when there is no current, drift velocity = 0

Answer 8

• No current means no electric field
• No preferred direction of motion for charge carriers
• Random velocities of charge carriers equally likely to be positive or negative
• Drift velocity = 0

Question 9

Define drift velocity

Answer 9

Average velocity in one direction along the wire

Question 10

Explain why when current exists, drift velocity is not 0

Answer 10

• Current exists means presence of electric field
• Velocity of charge carriers is increased in direction of applied electric field
• Velocity of charge carriers is decreased in direction opposite to applied electric field
• Average velocity now slightly more than 0