Direct Current Circuits

Question 1

State the rule for the current leaving a junction

Answer 1

At any junction in a circuit, the total current leaving the circuit is equal to the total current entering

Question 2

State the 2 rules for the current through components in series

Answer 2

1) The current entering a component is the same as the current leaving the component
2) The current passing through 2 or more components in series is the same through each component

Question 3

State Kirchhoff’s 1st law

Answer 3

The algebraic sum of the currents flowing through a junction is zero. Currents approaching the junction are + while currents going away from the junction are -.

Question 4

State Kirchhoff’s 2nd law

Answer 4

The algebraic sum of the potential differences in a circuit loop must be zero. Potential rises are + while potential drops are -.

Question 5

For a circuit with 3 components in series, give the equation for:

i) I₀
ii) V₀

Answer 5

i) I₀ = I₁ = I₂ = I₃

ii) V₀ = V₁ + V₂ + V₃

Question 6

For a circuit with 3 components in parallel, give the equation for:

i) I₀
ii) V₀

Answer 6

i) I₀ = I₁ + I₂ + I₃

ii) V₀ = V₁ = V₂ = V₃

Question 7

What is the total resistance for 2 or more resistors in series?

Answer 7

The sum of the individual resistors

R = R₁ + R₂ + R₃ + …

Question 8

What is the total resistance for 2 or more resistors in parallel?

Answer 8

The sum of the inverse of the individual resistors

R = 1/R₁ + 1/R₂ + 1/R₃ + …

Question 9

Describe the cause of the heating effect of an electrical current

Answer 9

Its resistance

Question 10

If a heating component is kept at a constant temperature, what is its rate of heat transfer to the surroundings?

Answer 10

Rate of heat transfer = I²R

Question 11

Give the equation for the heat energy transferred by an electrical current in time t

Answer 11

Energy transfer = power x time = I²Rt

Question 12

Define internal resistance

Answer 12

The loss of potential difference per unit current in the source when current passes through the source

Question 13

State the cause and effect of internal resistance

Answer 13

The internal energy of a source of electricity is due to the opposition of the flow of charge through the source. This causes electrical energy produced by the source to be dissipated inside the source when the charge flows through it

Question 14

Define emf (ε)

Answer 14

The electromotive force of the source is the electrical energy per unit charge produced by the source
If electrical energy E is given to a charge Q in the source:
ε = E / Q

Question 15

Give the equation for emf (ε)

Answer 15

ε = IR + Ir

Question 16

Give the equation for the power supplied by the cell in terms of emf

Answer 16

P = Iε = I²R + I²r

Question 17

State when the maximum power is delivered to the load in a circuit

Answer 17

Maximum power is delivered to the load when the load resistance is equal to the internal resistance of the source

Question 18

Describe an experiment to measure the internal resistance of a cell

Answer 18

By connecting the cell to a variable resistor and an ammeter in series with a voltmeter attached across the cell.
Changing the resistance of the variable resistor changes the current. Measure the pd across the cell at different currents and plot a graph of pd vs current
The gradient of the line will be equal to -r and the y-intercept is equal to ε since:
V = ε - Ir

Question 19

How would you calculate the net emf (ε) when 2 cells are combined?

Answer 19

The sum of the individual emf’s when you assign a sign (+ or -) for each direction of current

Question 20

For a circuit with n number of identical cells in parallel, what is the current through each cell?

Answer 20

Current through each cell, I = I(total) / n

Question 21

For a circuit with n number of identical cells in parallel, what is the pd dissipated at the internal resistance for each cell?

Answer 21

pd ‘lost’ = Ir / n

where r is the internal resistance of each cell

Question 22

For a circuit with n number of identical cells in parallel, what is the terminal pd across the each cell?

Answer 22

V = ε - Ir / n

Question 23

State 2 assumptions made for silicon diodes in a circuit

Answer 23

A silicon diode has:

1) a ‘forward’ pd of 0.6V whenever a current passes through it
2) infinite resistance in the reverse direction or at pd’s less than 0.6V in the forward direction

Question 24

Describe a potential divider

Answer 24

2 or more resistors in series with each other and with a source of fixed potential difference. The potential difference of the source is divided between the components in the circuit, as they are in series with each other

Question 25

Give 3 uses of a potential divider

Answer 25

1) To supply a pd which is fixed at any value between zero and the source of the pd
2) To supply a variable pd
3) To supply a pd that varies with a physical condition such as temperature or pressure

Question 26

For 2 resistors in series connected to a source of fixed pd, describe the relationship between the ratios of pd across each resistor and the resistance of each resistor

Answer 26

The ratio of the pd’s across each resistor is equal to the resistance ration of the 2 resistors
(V₁ / V₂) = (R₁ / R₂)

Question 27

Describe the set-up for a circuit to for a potential divider to supply a variable pd

Answer 27

The source is connected to a fixed length of uniform resistance wire. A sliding contact on the wire can be moved along to give a variable pd between the contact and one end of the wire

Question 28

State 2 uses for a potential divider to supply a variable pd in a circuit

Answer 28

• As a simple audio ‘volume control’ to change the loudness of the sound from a speaker
• To vary the brightness of a light bulb between zero and normal brightness

Question 29

Describe the advantage of using a potential divider rather than a variable resistor as a control for the brightness of a lightbulb

Answer 29

A potential divider can reduce the current of the circuit to 0 whereas there will always be a current through the variable resistor, even when at maximum resistance

Question 30

Define a sensor circuit

Answer 30

A sensor circuit produces an output pd which changes as a result of a change in a physical variable (such as temperature or light intensity)

Question 31

Give 2 examples of sensor circuits and describe the set-up for each

Answer 31

1) A temperature sensor - a potential divider made by using a thermistor and a variable resistor. The variable resistor allows the pd across the thermistor to be adjusted (calibrated) when at a constant temperature.
2) A light sensor - a potential divider made by using a light-dependent resistor (LDR) and a variable resistor. When the intensity of incident light changes, so does the pd dissipated across the LDR