2.3 D.C Circuits

Question 1

How can you measure the current in a circuit?

Answer 1

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

Question 2

How do you measure the potential difference across a component?

Answer 2

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

Question 3

What is a diode?

Answer 3

Question 4

Unless stated in the question, should you assume that voltmeters have zero resistance or infinite resistance?

Answer 4

You should assume they have infinite resistance. Current takes the path of least resistance, so if the voltmeter has infinite resistance, when applied in parallel to the appliance, no current will flow through it and all the current will flow through the appliance.

Question 5

Why should you assume that an ammeter has zero resistance unless
stated otherwise?

Answer 5

This assumption means that there would be 0 potential difference across the ammeter and no energy is lost across the it. It does not affect the circuit.

Question 6

What is an Light Dependent Resistor (LDR)?

Answer 6

A semiconductor that is sensitive to light.
As the light intensity increases its resistance decreases.

Question 7

How does a thermistor work?

Answer 7

Similar to a LDR, but as the temperature increases, the resistance decreases.

Question 8

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

Answer 8

RTotal= R1 + R2 + R3 + …
Add the resistances of each component.

Question 9

If 6 cells each of voltage 5V are arranged in parallel, what is the voltage in the circuit?q

Answer 9

5V

Question 10

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

Answer 10

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

Question 11

Is the current in parallel components the same?

Answer 11

No, each branch of a parallel circuit can have different currents through them.

Question 12

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

Answer 12

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

Question 13

What is Kirchhoff’s first law?

Answer 13

All of the current going into a junction is equal to the total current leaving the junction.

Question 14

What is Kirchhoff’s second law?

Answer 14

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

Question 15

What is the purpose of a potential divider?

Answer 15

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

Question 16

A circuit is set up with a cell providing a voltage of 12V to 2 resistors of 6Ω and 7Ω in series, what is the voltage across the 7Ω resistor?

Answer 16

Total resistance = 6 + 7 = 13Ω
(7/13) x 12 = 6.5V

Question 17

What is emf?

Answer 17

Electromotive force: the electrical energy transferred by a power supply per unit charge. It has the same units as pd (V).

Question 18

Rearrange the equation Ɛ = I(R+r) into the form of
y = mx + c

Answer 18

y = mx + c
Ɛ = IR + Ir
Ɛ = V + Ir
So for a graph of V against I
V = -rI + Ɛ

y = mx + c

Question 19

What is the total resistance of a parallel arrangement of 3Ω, 7Ω and 9Ω resistors?

Answer 19

1/RT = 1⁄3 + 1/7 +1/9 1/RT = 37/63
RT = 63/37 = 1.7Ω