Topic 3 - Electrical circuits

Question 1

Why does resistance in a bulb increases with temperature?

Answer 1

• As temperature increases, lattice ions vibrate with greater amplitude
• This means that they collide with electrons more frequently
• This decreases the drift velocity
• I = nqvA
• Decrease in current
• V= IR therefore greater resistance

Question 2

Explain the effect of temperature on the resistance in a negative coefficient thermistor.

Answer 2

• As temperature increases more charge carriers are released
• I = nqvA
• Thefore current increases
• V=IR therefore resistance decreases

Question 3

Describe a method to determine the internal resistance and e.m.f. of an electrical circuit.

Answer 3

• Draw the circuit diagram
• Record the p.d and current
• Use rheostat to change p.d
• Plot graph of p.d against current
• Y intercept = emf
• Gradient = -r

Question 4

Describe a method to determine the resistivity of a wire.

Answer 4

• Draw diagram
• At set distances across the wire record the resistance
• Plot a graph of resistance against length
• Resistivity = gradient x cross-sectional area

Question 5

Why does the brightness of bulbs decreases as you add more in parallel?

Answer 5

• Total resistance decreases as you add more bulbs in parallel
• This causes the total current in the circuit to increase (V=IR)
• Therefore, effect of internal resistance increases as V= ε - Ir
• Lower output voltage meaning dimmer bulbs
• Higher current means more work done thefore temperature increase ad more energy is wasted