8.06 - 8.07 Resistance

Question 1

What equation is used to calculate resistance?

Answer 1

resistance = p.d. across conductor (V) / current through conductor (A)

Question 2

What is the SI unit of resistance?

Answer 2

The SI unit of resistance is the ohm.

Question 3

Factors affecting resistance

Answer 3

Length, Cross-sectional area, Material, Temperature

Question 4

Length

Answer 4

Doubling the length of a wire doubles its resistance

Question 5

Cross-sectional area

Answer 5

Halving the end on area of a wire doubles its resistance. So a thin wire has more resistance than a thick one.

Question 6

Material

Answer 6

A nichrome wire has more resistance than a copper wire of the same size.

Question 7

Temperature

Answer 7

For metal conductors, resistance increases with temperature. For semi-conductors, it decreases with temperature.

Question 8

Resistance and heating effect

Answer 8

There is a heating effect whenever a current flows in a resistance. The heating effect occurs because electrons collide with atoms as they pass through a conductor.

Question 9

What are resistors?

Answer 9

Resistors are specially made to provide resistance. In simple circuits, they reduce currenty. In more complicated ciruits, such as those in radios, TVs, and computers, they keep currents and p.d.s at the levels needed for other components to work properly.

Question 10

Variable resistors (rheostats)

Answer 10

Are used for varying current. In hi-fi equipment, rotary (circular) variable resistors are used as volume controls.

Question 11

Slide control variable resistor

Answer 11

Moving the slide control of the variable resistor to the right increases the length of resistance wire in the circuit. This reduces the current and dims the lamp.

Question 12

Thermistors

Answer 12

Have a high resistance when cold but a much lower resistance when its hot. They contain semiconductor materials.

Question 13

Light dependent resistors (LDRs)

Answer 13

Have a high resistance in the dark but a low resistance in the light. They can be used in electronic circuits which switch lights on and off automatically.

Question 14

Diodes

Answer 14

Have an extremely high resistance in one direction but a low resistance in the other. In effect, they allow current to flow in one direction only. They are used in electronic circuits.

Question 15

Ohms law

Answer 15

The current is proportional to the p.d.