9- Internal resistance, Circuit Types And Potential Dividers

Question 1

Conservation of energy

Answer 1

• energy cannot be created or destroyed, only transferred or lost
• charge out of a power supply = charge back into a power supply

Question 2

Internal resistance

Answer 2

• shown as r, actual resistance is R
• when current flows out of a power supply, r becomes work done against the power supply and voltage across the circuit becomes E.M.F-r
• sometimes referred to as lost volts

Question 3

Uses of internal resistance

Answer 3

• critical to performance of a power supply, as a large current would cause a massive increase in the temperature of the power supply
• High voltage lab experiments use high r to reduce the current to low levels and keep it safe

Question 4

Solar cells

Answer 4

• when light hits a photovoltaic cell it gives energy to free some electrons in the semi-conductor. An electric field within the cell produces the voltage and electron flow creates a current that works with it to generate power

Question 5

Ohm meters

Answer 5

-measure resistance using an internal battery with a known resistance, and the current of the circuit is compared to calculate it’s resistance

Question 6

Circuit types

Answer 6

• in series circuits, resistance adds up over all components
• in parallel it’s always smaller than the smallest resistor found using 1/Rt = 1/R1 + 1/R2 etc
• when a circuit is mixed between two types, do parallel calculations first to find the series resistance of that as one large ‘component’, then add up all the series resistances
• resistors in series divide resistance in a ratio, so the voltage will also be divided in that ratio, as I is constant in a series circuit

Question 7

Using thermistors

Answer 7

• thermistors can be used to control potential dividers, as temperature varies resistance.
• the voltage will be shared between two resistors proportionally to the ratio of the resistance of the two resistance
• getting a 50/50 split means the thermistor has raised its temperature to get the same resistance as a fixed resistor/set rehostat

Question 8

Potential divider circuits

Answer 8

• output voltage of a resistor in a series circuit = voltage into it * (Resistance of resistor/total resistance of a circuit)
• by using a rheostat as one of these resistors and connecting a branch of the circuit to it in parallel you can control the potential difference going into that branch of the circuit easily