4.3 - Electrical Circuits

Question 1

What is Kirchhoff’s second law?

Answer 1

For any path (loop) of a circuit, the sum of all of the potential differences must equal to total emf of the circuit.

Question 2

How do you find the total resistance in a series circuit?

Answer 2

Rₜₒₜₐₗ= R₁+ R₂+ R₃ + …

Question 3

How do you find the total resistance in a parallel circuit?

Answer 3

1/Rₜₒₜₐₗ= 1/R₁+ 1/R₂+ 1/R₃ + …

Question 4

True or false: Adding a resistor to a circuit in parallel will always reduce the total resistance.

Answer 4

True.
Total resistance drops the more paths there are for the current to take.

Question 5

How can you measure the current in a circuit?

Answer 5

You can measure the current in a circuit with an ammeter connected in series with the component.

Question 6

How do you measure potential difference of a component?

Answer 6

Using a voltmeter, connected in parallel over the component being measured.

Question 7

True or false: In parallel circuits the total voltage in each loop is the same.

Answer 7

True
Vₜₒₜₐₗ = V₁= V₂ = V₃

Question 8

How do you calculate the total voltage in series circuits?

Answer 8

Vₜₒₜₐₗ = V₁+V₂+V₃+…

Question 9

How does the current vary between each component of a series circuit?

Answer 9

The current through all of the components is the same so the current does not vary.

Question 10

Is the current in each component of a parallel circuit the same?

Answer 10

No, each branch of a parallel circuit has different currents.
Because the voltage across each branch is the same - the higher the resistance of a branch, the lower the current flow through that branch will be.

Question 11

If you connect two cells in series what will be to total emf?

Answer 11

εₜₒₜₐₗ = ε₁+ ε₂

Question 12

In a series circuit, if two cells are connected negative to negative, would their emf add up or cancel out?

Answer 12

They will cancel out. The total emf would be equal to:
εₜₒₜₐₗ = ε₁- ε₂

Question 13

True or false: The e.m.f of a source and its terminal voltage will always be the same.

Answer 13

False.
Not all of the energy given to the charges in the source (e.m.f) makes it out of the cell.

Question 14

Define internal resistance.

Answer 14

Internal resistance is the resistance within a power source due to the materials and components which make it up.

Question 15

What are ‘lost volts’?

Answer 15

‘Lost volts’ refer to the difference in voltage between that supplied by the source and the amount available to the circuit.
They are ‘lost’ due to the internal resistance of the source.

Question 16

Give an equation which relates e.m.f to load and internal resistance.

Answer 16

ε = I(R+r)
Where ε = e.m.f, I = current, R = load resistance, and r = internal resistance.
In other words: e.m.f = IR + Ir = V + v
Where V = terminal voltage (ie. volts used in the circuit), v = lost volts (ie. volts used in the source).

Question 17

Describe an experiment to determine the internal resistance of a source.

Answer 17

Set up a circuit with a source, ammeter and variable resistor in series, and a voltmeter in parallel.
Vary the resistance and measure the voltage and current (take several readings).
Plot a V-I graph.
V = -rI + ε corresponds to y=mx+c
The internal resistance = -m (-gradient).

Question 18

What is the purpose of a potential divider?

Answer 18

To provide variable potential difference, or to provide a constant specific potential difference.

Question 19

How does the voltage across a component in a potential divider correspond to its resistance?

Answer 19

The proportion of the total voltage which is dropped across the component is equal to the proportion of the circuit resistance which it contributes.
In other words: V/Vₜ = R/Rₜ

Question 20

How can a potential divider circuit be used as a sensing circuit? (eg. to switch on a fan when it gets warm)

Answer 20

Put a resistor that varies with the desired conditions (eg. a thermistor) in the potential divider. Insert the responding load (eg. fan) over either the variable or fixed resistor depending on how the circuit should work.
E.g. In this case as temperature increases, the resistance of the thermistor decreases and so the voltage across it will decrease and the voltage across the other resistor will increase. Therefore, put the fan in parallel with the fixed resistor and as temperature increases it will turn on.

Question 21

Where should a bulb go to turn this circuit into a light-sensing circuit which switches the light on when it’s dark?

Answer 21

It should be added in parallel with the LDR.

Question 22

What equation can be used to calculate the voltage out of a potential divider in terms of the input voltage and the resistances in the circuit?

Answer 22