3p1 Electric Circuits

Question 1

Write what the letters mean in the equation and the units
I = nqvA

Answer 1

I - current (amps)
n - charge carrier density - number of electrons per m^3 (m^-3)
q - charge (Coulombs)
v - drift velocity (m/s)
A - cross sectional area (m^2)

Question 2

What are semiconductors

Answer 2

components for which the resistance changes depending on external conditions

Question 3

What are superconductors

Answer 3

materials which have zero resistivity at and below a critical temperature

Question 4

equations with emf

Answer 4

ε = I R + I r
V = -rI +ε
ε = V + r I
y = mx + c

Question 5

why is emf never equal to terminal pd

Answer 5

because energy is dissipated within the source due to the cell’s internal resistance

Question 6

How to determine the electrical resistivity of a material

Answer 6

1) at various points along the wire measure the diameter and repeat at 90 degree angles at the same point to get about 6 readings and then find an average
2) find cross sectional area of wire
3) connect a circuit with a battery, variable resistor, ammeter and the wire with a voltmeter parallel to the wire
4) at a 10cm interval from crocodile clip attaching wire to circuit, touch wire with probe and record voltage and current
5) calculate resistance
6) measure length of wire measured from using a ruler
7) repeat with different lengths and plot a graph of R against L and draw line of best fit - R = p/A x L
8) so resistivity = gradient x A

Question 7

how to determine the emf and internal resistance of an electrical cell

Answer 7

1) connect a circuit with battery, resistor and ammeter and a voltmeter over the battery
2) measure pd across battery
3) vary current by changing the resistance of R using a variable resistor
4) repeat to get many readings of V and I
5) plot graph of V against I and draw line of best fit with equation V = ε - Ir
6) so gradient is negative internal resistance and y intercept is emf

Question 8

when a pan is placed on a coil connected to an ac supply why does the pan become hot

Answer 8

An alternating current in the coil produces a changing magnetic field through the pan, which induces an EMF, leading to a current in the pan and an associated heating effect