Module 4.3 - Electricity: Electrical Circuits

Question 1

What is Kirchhoff’s first law?

Answer 1

The sum of currents ENTERING a junction in a circuit is EQUAL to the sum of currents LEAVING the junction - a form of conservation of charge.

Question 2

What is Kirchhoff’s second law?

Answer 2

The sum of e.m.f. is EQUAL to the sum of the P.D. in a closed loop - a form of conservation of energy.

∑e.m.f = ∑IR

Question 3

Describe Series circuits

Answer 3

> Current SAME at all points
e.m.f. is equal to the sum of p.d. across the resistors
Total resistance is equal to the sum of individual resistors

Question 4

How do you work out the total p.d. in a series circuit?

Answer 4

Vt = V1 + V2 + V3 + …

Question 5

How do you work out the total resistance in a series circuit?

Answer 5

Rt = R1 + R2 + R3 + …

Question 6

Describe Parallel circuits

Answer 6

> Total current is the sum of the individuals branch currents
The potential difference across the resistors in each branch is the same and equal to the e.m.f.

Question 7

How do you work out the total current in parallel circuits?

Answer 7

It = I1 + I2 + I3 + …

Question 8

How do you work out the e.m.f. of parallel circuits?

Answer 8

e.m.f = V1 = V2 = V3 = …

Question 9

How do you work out the total resistance of parallel circuit?

Answer 9

1/Rt = 1/R1 + 1/R2 + 1/R3 + …

Question 10

How do you find the output voltage, Vout, of the resistor, R2?

Answer 10

Vout = Vin x ( R1 / R1 + R2)

Question 11

When is the output voltage of R2 the largest?

Answer 11

When Vin and R2 are large, and also when R1 is smaller than R2

Question 12

Define Internal Resistance, r

Answer 12

The resistance of a SOURCE of e.m.f

Question 13

Why is some energy lost when charge carriers flow through a source of e.m.f?

Answer 13

Internal resistance causes the charge carriers to do work against them, therefore losing energy.

Question 14

What is terminal p.d.?

Answer 14

The actual p.d. across the terminals of an e.m.f source when it is connected to a circuit

Question 15

What happens to e.m.f. and terminal p.d. when no current is flowing across a cell?

Answer 15

They are EQUAL

Question 16

What happens to the terminal p.d. and e.m.f. when current is flowing across a cell?

Answer 16

Terminal p.d. will be lower than the e.m.f.

Question 17

How is the terminal p.d. lower than the e.m.f. when current is flowing? Why?

Answer 17

This is because the energy transferred to charge carriers (Eq) and energy transferred to the rest of the circuit (Ec) is different.

Eq > Ec

e.m.f = Eq ÷ Q
T p.d. = Ec ÷ Q

Therefore e.m.f > T p.d.

This is because energy is lost by charge carriers doing work against the internal resistance in a source of e.m.f.

Question 18

What is Lost Volts?

Answer 18

The difference between e.m.f. and terminal p.d.

e.m.f. - T p.d.

Question 19

How do you work out internal resistance?

Answer 19

Lost Volts ÷ Current

Question 20

How do you work out terminal p.d.?

Answer 20

= e.m.f. - [LOST VOLTS]

= e.m.f - [current x internal resistance]

Question 21

3 equation to work out e.m.f using internal resistance?

Answer 21

= T p.d. + [current x internal resistance]

= IR + [current x internal resistance]

= I (R + internal resistance)

Question 22

How do you deal with more than one e.m.f.?

Answer 22

Add them together, unless polarities are OPPOSITE then subtract