Chapter 10~kirchhoff's Laws

Question 1

State kirchhoff’s first law :

Answer 1

The sum of the currents entering any point (a junction) in a circuit 💫 is equal to the sum of the currents leaving that same point (that same junction)

Question 2

Formula for kirchhoff’s first law

Answer 2

⚟ I (in) = ⚟ I (out)

⚟ - sum of

Question 3

Think out current I 1, I 2 and I 3 entering a junction and I 4 leaving that junction, what would the formula be?

Answer 3

I 1 + I 2 + I 3 = I 4

Question 4

Kirchhoff’s first law (the current law) is referred to as the conservative of….

Answer 4

Charge

Question 5

State kirchhoff’s second law :

Answer 5

The sum of e.m.f.s around a closed loop in a circuit 💫 is equal to the sum of p.d.s. Around that same loop.

Question 6

Kirchhoff’s second law (voltage ⚡ law) is referred to as the conservative of….

Answer 6

Energy ⛮

Question 7

The formula related to kirchhoff’s 2nd law :

Answer 7

⚟ E = ⚟ V

⚟ - sum of

Question 8

Applying kirchhoff’s laws.

Do you work it out clockwise 🔃 or anticlockwise 🔄 when going around a loop.

➖ And when do u get a minus

Answer 8

Anticlockwise 🔄

➖ If the p.d.f or E. M. F opposes it the it’s a minus (it’s going in a different direction)

Question 9

Why is kirchhoff’s current law referred to the conservation of charge?

Answer 9

Because current =Q/t

And if the first law states

⚟ I in =⚟ out
Than

⚟ Q/t in = ⚟ Q/t out
(and you can cancel the t because the time ⌚ is the same and on both sides of the equation)

Therefore : ⚟ Q in=⚟ Q out

Question 10

Why is kirchhoff’s voltage ⚡ law referred to as the conservation of energy ⛮?

(remember work done is a form of energy)

Answer 10

V= W/Q

Law states : ⚟ E = ⚟ V
E stands for e.m.f

Therefore :
E = V1 +V2+V3
So…
W/Q = W/Q + W/Q + W/Q

Cancel the Q and you are left with energy ⛮

Question 11

Formula for resistors in series

Answer 11

R(total) = R1 + R2 + R3

Question 12

How to derive the resistors in series formula :

3 steps

Answer 12

Step 1 : Take 2 resistors of resistance R1 and R2

kirchhoff’s 1st law, the current in each resistor is the same. The p.d. V across the combination is equal to the sum of the p.d.s across the 2 resistors

Step 2 :
Therefore V=V1 +V2
Since V = IR
We can write IR =IR1 +IR2

Step 3 :
Canceling out the common fact I gives
R = R1 +R2

Question 13

What is the formula for resistance in a parallel circuit?

Answer 13

1/R (total) = 1/R1+ 1/R2 +1/R3 +……..

Question 14

How to derive the formula for resistance in parallel circuit 1/R(total) = 1/R 1 + 1/R 2 + 1/R3

In 5 steps

Remember these use instances!!!

Answer 14

Step 1 : for 2 resistors with resistance R1& R2 connected in parallel, the current divides ➗ between them. We use kirchhoff’s 1st law

I = I1 + I2

Step 2 : we apply kirchhoff’s 2nd law
I1R1 - I2R2 = 0 V
(0 V bec no source of e.m.f around loop and minus bec in this example the current is flowing in opposite direction to the 🔄)

Step 3:
Since they equal the same V we can write 📝 
I=V/R
I1=V/R1 
I2=V/R2

Step 4:remember I (total) = I1+ I2
So just sub in these new values for I

V/R= V/R1 +V/R2..

Step 5:
The V is the common factor so you can cancel it out and get your final equation

1/R = 1/R1 + 1/R2 +….

Question 15

3 important things to remember when components are connected in parallel :

1. p.d
2. Current
3. Resistance

Answer 15

1. All have same p.d. Across their ends
2. the current is shared/ divided between them
3. We use the reciprocal formula to calculate their resistance

Question 16

When 2 or more resistors are connected in parallel, their combined resistance is lower than their individual resistance, why?

Answer 16

By connecting resistors in parallel, you are providing extra pathways for the current.

Question 17

Since the combined resistance is lower than the individual resistances, it follows that connecting in parallel will increase/ decrease the current drawn by the supply?

Answer 17

Increase

Question 18

When components are connected is parallel, their p.d. Are?

Answer 18

The same.

So ignore parts of circuit which arent relevant to your calculation

Question 19

Ammeters are connected in

Answer 19

Series

Question 20

Because ammeters are connected in series, do they have a high or low resistance and why?

Answer 20

Low resistance so that as little energy ⛮ as possible is dissipated in the ammeter itself

Question 21

What do voltmeters measure?

Answer 21

The potential difference between two points in a circuit

Question 22

Voltmeters are connected in?

Answer 22

Parallel

Question 23

Why do voltmeters have a resistance?

Answer 23

Bec they are connected in parallel in order to measure the p.d between two points and need a high resistance to take as little current as possible