Electric Circuits

Question 1

Kirchoffs law for current

Answer 1

total curent flowing into a point is equal to the current flowing out of that point. Conservation of charge principle.
curent is rate of flaw of charge, Q=It

Question 2

Kirchoffs law for potential difference

Answer 2

Around any closed loop in a circuit, the sum of the potential differences across all components is zero
Conservation of energy principle, V=W/Q

Question 3

Equations for power

Answer 3

P=IV
P=E/t
P=I2R
P=V2/R

Question 4

Interpreting resistance from an IV graph

Answer 4

Shallower gradient means greater resistance

Question 5

Why does resistivity of a metal increase with temperature

Answer 5

Charge is carried through metals by free electrons in a lattice of positive ions
As temperature increases, the lattice of ions vibrates more, meaning the electrons collide with them more frequently, transferring some of their kinetic energy into other forms
As kinetic energy is being lost by the electrons, their speed and therefore mean drift velocity decreases. Current is proportional to drift velocity (I=nqvA) so the current in the wire decreases so it’s resistance (and resistivity) increases.

Question 6

NTC thermistor

Answer 6

Resistance decreases as temperature increases
(Hence shape of curve)
Warming the thermistor gives more electrons enough energy to escape from their atoms, so there a more charge carriers available and the current increases (resistance decreases)

Question 7

Resistance of an LDR

Answer 7

Depends on light intensity
Greater intensity, lower resistance
Light provides the energy that releases more electrons= more charge carriers= higher current= lower resistance

Question 8

Determine the electrical resistivity of a material

Answer 8

Use micrometer to measure diameter of wire for at least 3 different points along the wire and take an average. Divide by 2 to get radius and record this. Calculate cross sectional area of wire using area of circle
Record length of wire and the voltage and current. Calculate resistance
Repeat for several different lengths across the length of the wire and plot results on graph of resistance against length, draw line of best fit
Find gradient and multiply by ares to get resistivity

Keep small currents flowing through wire to maintain constant temperature

Question 9

Use I=nqvA to explain the large range of resistivities of different materials

Answer 9

The higher the number of charge carriers, the higher the current at a given pd and so the lower the resistance and lower resistivity
Number of charge carriers varies greatly between different materials

Question 10

e.m.f

Answer 10

Total amount of work the battery does on each coulomb of charge

Question 11

Terminal potential difference

Answer 11

The work done when one coulomb of charge flows through the load resistance
If there was no internal resistance, the terminal pd would be the same as the e.m.f

Question 12

Lost volts

Answer 12

Energy wasted per coulomb overcoming the internal resistance

Question 13

Determine e.m.f and internal resistance of an electrical cell

Answer 13

Vary current by changing the load resistance using the variable resistor
Measure the pd for several different values of current
Plot V against I
y=mx+c
V=-rI+e
So the gradient of line of best fit is the internal resistance and the y intercept is the e.m.f

Question 14

Potential divider principles

Answer 14

Circuit with several resistors in series connected across a voltage source, used to produce a required fraction of the source of potential difference, which remains constant

You can make a potential divider supply a variable potential difference by using a variable resistor as one of the resistors in series

(The potential difference across the voltage source is split in the ratio of resistances
Can be used to supply a potential difference (Vout) between 0 and the pd of the source)

Question 15

Potential along a uniform current- carrying wire

Answer 15

Resistance of a wire proportional to length, uniform wire so directly proportional
Length increases, resistance increases uniformly
V=IR so increase resistance causes increase in potential difference

Question 16

Potential divider equation

Answer 16

Vout= (R2/R1+R2)Vs

Question 17

Potentiometer

Answer 17

Essentially a potential divider that has a variable resistor replacing R1 and R2
Move a slider to adjust relative sizes of R1 and R2
Used for when you want to change a voltage continuously (eg volume control in a stereo)

Question 18

The intensity of an LDR depends on the intensity of light on it
Explain how the change in resistance may be modelled in terms of conduction electrons

Answer 18

Increasing intensity of light on it increase the number of photons being emitted from the metal wire lattice in the LDR as more electrons absorb enough energy to be emitted
Increased number of conduction electrons in the wire per unit volume
Increase current
I=nqvA
Resistance decreases as V=IR

Question 19

Explain how the potential divider circuit can produce a range of values from 0 to 1.5
V on the voltmeter.

Answer 19

when slider at the bottom - reading on voltmeter is zero
Or minimum resistance - reading on voltmeter 15 zero
•
•
When slider at the top - reading on voltmeter is 1.5 V
Or maximum resistance - reading on voltmeter is 1.5 V
Potential difference split between top and bottom part of resistor (either side of slider)
Or reading on voltmeter depends on the ratio of resistances (either side of slider)
Or moving the slider changes the resistance that the voltmeter is across