2. Electrical Circuits

What is Kirchhoff’s second law?

The sum of e.m.f.s = the sum of p.d.s in a closed loop

Why does Kirchhoff’s second law exist?

Due to the conservation of energy

How to do a Kirchhoff’s laws calculation question?

1. Label different currents on circuit as I₁, I₂ etc

2. Use Kirchhoff’s first law to form your first equation

3. Use Kirchhoff’s second law to form your other equations - split the circuit into loops (to find sum of e.m.f. add source e.m.f.s + to find sum of p.d.s use V = IR)

4. Solve using simultaneous equations

5. If current is negative then direction is wrong

How to find the total resistance of a series circuit?

Add the individual resistances of the components together

How to find the total resistance of a parallel Circuit?

1/R_T = 1/R₁ + 1/R₂ + 1/R₃ etc (where R₁ is the resistance of a branch in the circuit)

What is the e.m.f. of a circuit with more than 1 source?

• e.m.f. of circuit = sum of e.m.f. of sources

• If a source is the wrong way then subtract its e.m.f for the e.m.f. of the circuit

What is a potential divider + equations?

• Potential divider - 2 resistors in series used to split p.d.

• V_out = R₁ / (R₁ + R₂₎ x V_in

• V₁ / V₂ = R₁ / R₂

How to investigate the potential divider?

1. Setup a circuit with a battery, fixed resistor, LDR/thermistor + voltmeter across the LDR/thermistor

2. Vary light intensity (e.g by putting tracing paper over the LDR) or temperature (e.g by putting thermistor in boiling water + then adding cold water)

3. Record reading on voltmeter