3.3 Circuits Introduction

Question 1

What is a circuit?

Answer 1

A closed loop that contains a power source (e.g. wire) and something for the electrons to flow through (e.g. wire)

Question 2

What is the current in electrical circuits?

Answer 2

• A measure of the flow of electrons around the circuit
• It is measured in Amps (A)

Question 3

What is the potential difference in electrical circuits?

Answer 3

• Potential difference is the driving force that pushes the charge around.
• It uses the unit of volt (V)

Question 4

What is resistance in electrical circuits?

Answer 4

Resistance is anything that resists the flow of charge

Question 5

What is the meaning of conventional current?

Answer 5

The idea that the current flows from the positive terminal to the negative terminal

Question 6

What are two things that could be a power source for an electric circuit?

Answer 6

• Cell
• Battery

Question 7

What is the force that drives the flow of electrons around a circuit?

Answer 7

Potential difference

Question 8

What is a diode? (3)

Answer 8

• A diode is a special device made from semiconductor material such as silicon.
• It lets current flow freely through circuits in one direction, but not in the other (i.e. there’s a very high resistance in the reverse direction).
• This turns out to be really useful in various electronic circuits, e.g. in radio receivers.

Question 9

What is an LDR? (3)

Answer 9

• An LDR is a resistor that’s dependent on the intensity of light. At a constant light level, its resistance is constant.
• In darkness, the resistance is highest. As light levels increase the resistance falls so the current through the LDR increases.
• They have lots of applications including automatic night lights, outdoor lighting and burglar detectors.

Question 10

What is a thermistor?

Answer 10

• A thermistor is a type of temperature-dependent resistor.
• In hot conditions, the resistance of a thermistor drops.
• In cool conditions, the resistance goes up.
• They’re used as temperature detectors, in e.g. thermostats, irons and car engines.

Question 11

How can you test a thermistor?

Answer 11

By placing it into a beaker of hot water and taking measurements as the water cools down and the temperature of the thermistor decreases

Question 12

What can sensing circuits be used for?

Answer 12

Sensing circuits can be used to turn on or increase the power to components depending on the conditions that they are in.

Question 13

How can a fan, fixed resistor and thermistor be used in a sensing circuit to increase the power of a fan?

Answer 13

• Set up a wire with a thermistor and fixed resistor in a series circuit and put a fan parallel to the resistor.
• As the room gets hotter, the resistance of the thermistor decreases and it takes a smaller share of the p.d. from the power supply. So the p.d. across the fixed resistor and the fan rises, making the fan go faster.