Electrical Resistance

Question 1

How do you work our voltage with resistance?

Answer 1

voltage = current x resistance 
V = IR
V = A x Ω

Question 2

What is the purpose of a fixed resistor?

Answer 2

They control the sizes currents and voltages in a circuit e.g. can control current so bulb does not ‘blow’

Question 3

What can a variable resistor do that is special?

Answer 3

They can alter their resistance e.g. they control the operation of a component such as the brightness of a lamp or the speed of a motor

Question 4

Describe a thermistor

Answer 4

• It is a special resistor (gets what it wants it won’t’ resist it)
• Changes its resistance with temperature
• Made from a semi-conducting material such as silicon or germanium
• At room temperature, small number of free electrons so resistance is large, but as temperature increases more free electrons so its resistance decreases
• Used often in temperature sensitive circuits in devices such as fire alarms, and thermostats

Question 5

Describe a light-dependent resistor

Answer 5

• It is a special resistor (gets what it wants it won’t’ resist it)
• Changes its resistance with light
• In dark conditions, LDRs contain few free electrons and so have a high resistance, but when a light is shone on the LDR more electrons are freed and the resistance decreases
• Used often in light-sensitive circuits in devices such as photographic equipment, automatic lighting controls and burglar alarms

Question 6

Describe a diode

Answer 6

• They control the current flow direction
• When current flows through them in one direction in has very low resistance however in the opposite direction the diode has a very high resistance meaning that very little current can flow
• Used often in circuits where it is important that current flows only in one direction, for example in rectifier circuits that convert alternating current into direct current
• Light emitting diodes (LEDs) are diodes that glow when a current is flowing through them and so they indicate the presence of current in a circuit

Question 7

How can you measure Ohm’s Law?

Answer 7

1. Place a piece of wire or electrical component as one of the components, and place a ammeter in series and a voltmeter in parallel
2. When the switch is closed record the values on the ammeter and voltmeter
3. Then alter the value on the variable resistor which is connected in series and a new reading is taken
4. Repeat this process at least 6 times
5. Plot graph (current on y axis and voltage on x axis)

Question 8

What is Ohm’s Law?

Answer 8

The current that flows through a conductor is directly proportional to the potential difference across its ends, providing that its temperature remains constant

Question 9

What affects resistance?

Answer 9

• Type of material
• Length: the longer the more resistance as the electrons must travel further so chance of collision are much greater
• Thickness: the thicker the less resistance, as this increases the surface area and so decreases the chance of collisions, in thick materials the charge carrying particles are able to move through the conductor more easily
• Temperature: the hotter the more resistance as the particles have more kinetic energy and so the particles vibrate more vigorously and these impeded electron flow so there are more collisions which increases resistance

Question 10

What can lamps and LEDs be used for?

Answer 10

To indicate the presence of a current in a circuit

Question 11

What is an electric current in a solid metallic conductor?

Answer 11

A flow of negatively charged electrons