2 Electrical resistance

Question 1

What is resistance?

Answer 1

It is the difficulty that current experiences in a circuit.

Question 2

What do we describe as having very low resistance?

Answer 2

Connecting wires.

Question 3

What is low resistance?

Answer 3

Low resistance is when there is nothing in opposition to the current flow. This means that it allows more current to flow through it.

Question 4

What is high resistance?

Answer 4

It is when the flow of charge is no tso easy and a large amount of energy is used to move the charges through them. This energy is transferred, usually as heat. Components like these are said to have high resistance.

Question 5

What do we describe as having high resistance?

Answer 5

Rubber.

Question 6

What is the equation for Voltage?

Answer 6

V (Voltage volts) =
I (Current Amps) x R ( Resistance ohms)

Question 7

What are the steps that we follow in the experiment for investigating how current varies with voltage for different components?

Answer 7

1) Set up the circuit (shown on next card).
2) Turn the variable resistor to its maximum value.
3) Close the switch and take the reading from the ammeter and voltmeter.
4) Alter the value of the variable resistor again and take a new pair of readings from the meters.
5) Repeat the whole process at least six times.
6) Place the results in a table, and draw a graph of current (I) against voltage (V).

Question 8

What is the circuit that we use in the experiment regarding how current varies with voltage for different components?

Answer 8

Question 9

What should the graph of current against voltage look like?

Answer 9

Question 10

What does the slope reflect?

Answer 10

It shows the resistance of the wire. If it has a steeper slope, the smaller the resistance of the wire is.

Question 11

What does the current against voltage graph look like for a wire or a resistor?

Answer 11

Question 12

Describe this graph.

Answer 12

The graph is a straight line. It has a constant slope, so the resistance of this component does not change.

Question 13

What does the current against voltage graph look like for a filament bulb?

Answer 13

Question 14

Describe this graph.

Answer 14

This is not a straight line. The resistance of the bulb changes. At higher currents and voltages the slope of the graph shows us that the resistance of the filament bulb increases - that is, as the temperature of the filament increases the current decreases.

Question 15

What does the current against voltage graph look like for a diode?

Answer 15

Question 16

Describe this graph.

Answer 16

This strangely shaped graph shows that diodes have a high resistance when the current is in one direction and a low resistance when it is in the opposite direction.

Question 17

What are fixed resistors?

Answer 17

They are included in circuits in order to control the sizes of currents and voltages.

Question 18

Why are fixed resistors used?

Answer 18

Without the resistor, the voltage across the bulb may cause too large a current and the bulb may ‘blow’ or break.

Question 19

What is a variable resistor?

Answer 19

They are called variable resistors as it is possible to alter their resistance.

Question 20

What happens if the resistance in a variable resistor is decreased?

Answer 20

There will be a larger current and the bulb would shine more brightly.

Question 21

What happens if the resistance in a variable resistor is increased?

Answer 21

The current will be smaller and the bulb will glow less brightly or not at all.

Question 22

How do variable resistors usually behave when being used in a circuit with a bulb?

Answer 22

They act as a dimmer switch.

Question 23

What else can variable resistors be used for?

Answer 23

They could be used in circuits containing electric motors, variable resistors can be used to control the speed of the motor.

Question 24

What is a thermistor?

Answer 24

It is a conductor whose resistance changes a lot when its temperature changes.

Question 25

Where would you place a thermistor?

Answer 25

On the left of the cell in the circuit.

Question 26

What is a graph which represents resistance against temperature?

Answer 26

Question 27

Explain the graph.

Answer 27

The thermistors resistance decreases when it senses a temperature increase.

Question 28

Where and how are thermistors utilised in day to day life?

Answer 28

In temperature-sensitive circuits in devices such as fire alarms.
- Freezers.
- Computers.

Question 29

What is a light dependent resistor (LDR)?

Answer 29

It has a resistance that changes when light is shone on it. In the dark its resistance is high but when light is shone on it its resistance decreases.

Question 30

What is a graph that represents resistance against light intensity?

Answer 30

Question 31

Explain the graph.

Answer 31

The LDR’s resistance decreases with an increase in light intensity.

Question 32

Where and how are LDR’s used in every day life?

Answer 32

In light sensitive circuits in devices such as:
- Photography exposure equipment.
- Automatic lighting controls.
- Burglar alarms.

Question 33

What are diodes?

Answer 33

They are very special resistors that allow charges to flow through them easily but only in one direction.

Question 34

Which way does the diode face when it conducts?

Answer 34

The left.

Question 35

Which way does the diode face when it doesn’t conduct?

Answer 35

The right.

Question 36

When are diodes used?

Answer 36

In circuits where it is important that electrons flow in only one direction.

Question 37

What kind of diodes glow when charges flow through them?

Answer 37

Light emitting diodes (LED’s)