Electrical Circuits

State Kirchhoff’s second law.

The sum of the emfs equals the sum of the potential differences in a closed loop

What principle explains Kirchhoff’s second law?

Conservation of energy

What is the current like in a series circuit?

The same at every point

How is p.d. distributed in a series circuit?

Split between components

How does total p.d. compare to emf in series?

Total p.d. equals the emf

What is the total resistance in series?

Rt = R1 + R2 + …

Why does resistance add in series?

The same current passes through each resistor

What is the p.d. like across branches in parallel?

The same across each branch

How do currents behave in parallel circuits?

They add up to the total current

What is the total resistance in parallel?

1/Rt = 1/R1 + 1/R2 + …

Why is total resistance lower in parallel?

More paths for current

What is current like within one parallel branch?

The same at every point in that branch

How is p.d. distributed within a branch?

Split across components in that branch

What is internal resistance?

Resistance inside a source of emf

Why does internal resistance matter?

Some energy is lost inside the cell

What are lost volts?

The p.d. across the internal resistance

How do terminal p.d. and emf compare?

Terminal p.d. is less than emf when current flows

How is a cell modelled with internal resistance?

An emf in series with resistance r

What is the equation including internal resistance?

ε = I(R + r)

What does IR represent?

Terminal p.d. V

What does Ir represent?

Lost volts

When can internal resistance be ignored?

If not mentioned in the question

How is internal resistance measured experimentally?

Using V and I measurements

What components are used in the setup?

Cell ammeter variable resistor voltmeter

How are V and I used to find r?

Plot V against I

What is the y-intercept of the V–I graph?

The emf ε

What is the gradient of the V–I graph?

−r

What is a potential divider?

A series circuit that splits p.d.

Why does a potential divider work?

Current is constant in series

How is p.d. related to resistance in series?

p.d. is proportional to resistance

What is the ratio rule for a potential divider?

V1/V2 = R1/R2

What is the equation for output p.d.?

Vout = (R2/(R1+R2)) × Vin

What does Vin represent?

The supply emf

What does Vout represent?

The p.d. across the chosen resistor

Why use a variable resistor or sensor?

To change Vout

What happens to Vout if R2 decreases?

Vout decreases

What is a common use of potential dividers?

Sensor circuits

How does a thermistor affect Vout?

Lower temperature gives higher resistance and higher Vout

How does an LDR affect Vout?

Lower light gives higher resistance and higher Vout

How are potential divider circuits investigated?

By varying conditions and measuring Vout

What equipment is used in investigations?

Lamps water baths and a voltmeter