Chapter 10

Question 1

What is kirchoffs first law?

Answer 1

Sum of the current entering a junction = total current leaving

Question 2

What is the difference between emf and pd?

Answer 2

Energy transferred per unit charge is emf
Energy transferred from a unit of charge is pd

E=VQ

Question 3

What is Kirchhoff’s 2nd law?

Answer 3

In any circuit the sum of emf = the sum of pd’s around a close loop

A closed loop is a possible path for current

Question 4

What happens in a series circuit?

Answer 4

There is only 1 path for current so current is the same everywhere
There is only 1 loop so the emf is shared between components.

Emf is shared equally if all components have the same resistance. Otherwise, the one with higher resistance a greater proportion of emf

Question 5

What happens in a parallel circuit?

Answer 5

There is more than 1 path for current to flow so current is split at junctions depending on resistance. If the path has higher resistance, less current flows through it.

Current in = current out
Each branch is a different loop so they don’t affect each other

Total PD across each branch = emf because of 2nd law. V is shared between components on that path according to resistance

Question 6

What is total resistance in a series circuit and how do I prove?

Answer 6

The sum of all the resistances

2nd law says that pd = sum of pd across all components so vtot=v1+v2…
And ohms law says that v=ir
So IRtot=IR1+IR2…

1st law says that current going through each resistor is the same so the current basically cancels

Question 7

What is total resistance in a parallel circuit?

Answer 7

More paths so more resistors means that there are more paths for current

In a parallel circuit Itot = I1+I2…
So v/r tot = v/r + v/r ….

And 2nd law means that that the 1/rtot = 1/r1 + 1/r2 …

Question 8

Define internal resistance

Answer 8

Resistance of a source of emf due to electrons having to do work when they move which causes the loss of energy

Question 9

What causes internal resistance?

Answer 9

When a current flows, work must be done by the charges as they move through a power source
Work is done for chemical reactions in a chemical cell
Also electrons collide with atoms inside the cell which causes resistance so electrons have to do work to get through.
So some energy is lost as a current flows through a power source(lost volts)

Question 10

Define emf

Answer 10

Work done on each colour of charge by the cell

Question 11

What is the formula of terminal PD

Answer 11

Terminal pd = emf - loss volts

Terminal pd < emf

Question 12

What happens to internal resistance as current increases

Answer 12

As current increases, more charge is travelling per unit time so more work is done by charge(also more collisions)
As current increases, lost volts increases so terminal PD decreases
Because V=IR

Current is directly proportional to lost volts

Question 13

What equation links EMF, current and resistances

Answer 13

EMF= I(Load resistance + internal resistance)

Question 14

What happens to emf in a series circuit?

Answer 14

Total emf is the sum of the individual emf’s. But the more cells in series increases the internal resistance which limits the current

Question 15

What happens to emf in a parallel circuit?

Answer 15

In parallel circuit, emf of 1 branch = emf of another branch because current is equally split between at junctions. Charge only gains emf from ONE cell so overall emf doesn’t increase
E1=E2=Etot

Internal resistance decreases so current is larger

Question 16

How do you measure internal resistance?

Answer 16

Vary the current by changing the resistance and measuring current
Plot a graph of V again I
Remember that Emf= V + lost volts
So Emf= V + ir(internal)
Y= mx+c V=-ri + Emf
So the y intercept is the emf and the gradient x-1 is the internal resistance

Question 17

How do graphs vary with different internal resistances and emfs?

Answer 17

If the gradient is less steep, the internal resistance is less. If the y int is higher, emf is higher

Question 18

What are potential dividers used for?

Answer 18

To divide the PD to give any pd up to the maximum supplied by source

Question 19

How do potential dividers work?

Answer 19

They have resistors in series with different resistances. PD is divided in ratio of the resistances. The one with higher resistance receives more PD

If you put a bulb across a resistor in parallel, the total resistance would be lower than R so Vout must be less than you calculate

Question 20

What is the formula for potential dividers?

Answer 20

V1/V2 = R1/R2

Vout = (R2/Rall) x Vin

Question 21

What is loading?

Answer 21

When you connect a component or circuit in parallel to R2 which lowers the overall resistance which lowers Vout

If Rload is high, Vout changes a but
Is Rload is low, Vout changes a lot

Question 22

What do you do if you want to vary Vout?

Answer 22

Put a variable resistor instead of a resistor so increasing the resistance increases Vout

Question 23

What happens if you replace the resistor with a thermistor in a potential divider circuit?

Answer 23

An NTC thermistor has a high resistance at low temperatures so as temperature increases, resistance decreases so Vout decreases

Question 24

What happens if you replace the resistor with a LDR in a potential divider circuit?

Answer 24

Vout varies with light intensity. There is higher resistance in the dark so as light intensity increases, resistance decreases so Vout decreases

You can flip this function if you replace the OTHER resistor with a LDR so that as light intensity increases, Vout increases

Question 25

What is a potentiometer?

Answer 25

Is a variable resistor with 3 terminals and a slider. Sliding the slider varies Vout between the 2 terminals

Basically as you increase the distance between the 2 terminals, resistance increases so Vout increases

See notes