Topic 2 - Electricity

Question 1

Current

Answer 1

• Electric current is the flow of electric charge round the circuit
• Will only flow around a complete (closed) circuit if there’s a p.d.
• Can only flow if there’s a source of p.d.
• In a single closed loop, the current has the same value everywhere in the circuit
• Unit of current: ampere, A

Question 2

Potential Difference

Answer 2

• The driving force that pushes the charge around

- Unit of p.d: volt, v

Question 3

Resistance

Answer 3

• The resistance is anything in the circuit which slows the flow down
• Unit of resistance: ohm

Question 4

What does the current flowing through a component depend on?

Answer 4

• The potential difference across it and the resistance of the component
• Greater the resistance across component, smaller the current that flows through it (for a given p.d across the component)

Question 5

Definition of current

Answer 5

The size of the current is the rate of flow of charge

Question 6

Formula for charge

Answer 6

Q = It

Charge (C) = current (A) x time (s)

Question 7

Current measurement

Answer 7

Amperes (A)

Question 8

Charge measurement

Answer 8

Coulombs (C)

Question 9

Time measurement

Answer 9

Seconds (S)

Question 10

Formula linking p.d, current and resistance?

Answer 10

V = IR
Potential Difference (V) = Current (A) x Resistance (ohms)

Question 11

Resistance and ohmic conductors

Answer 11

• Resistance of ohmic conductors doesn’t change w/ the current
• At constant temp, current flowing through an ohmic conductors is directly proportional to the p.d across it
• R is constant in V = IR
• Ohmic conductors will only have a constant resistance at a constant temperature

Question 12

More facts on resistance and ohmic conductors?

Answer 12

Resistance of some resistors and components does change, e.g. a filament lamp or a diode.

Question 13

Why does resistance of a filament lamp change?

Answer 13

• When an electrical charge flows through filament lamp, it transfers energy to the thermal energy store of the filament (which is designed to heat up)
• Resistance increases with temp, so as current increases, filament lamp heats up more and the resistance increases.

Question 14

Why does the resistance of a diode change?

Answer 14

• For diodes, the resistance depends on the direction of the current.
• They’ll happily let it flow in one direction but have a very high resistance if reversed.

Question 15

Examples of ohmic conductors?

Answer 15

A wire or a resistor

Question 16

Static electricity

Answer 16

• Static electricity is all about charges which are not free to move (e.g. insulating materials)
• Build-up of static is caused by friction

Question 17

What happens when certain insulating materials are rubbed together?

Answer 17

• Negatively charged electrons will be scraped off and dumped on the other
• This will leave materials electrically charged:
• Positive static charge on one material
• An equal negative static charge on the other
• Which way electrons are transferred depends on the two materials involved

Question 18

Polythene and acetate rods being rubbed with a cloth duster

Answer 18

• With the polythene rod, electrons move FROM the duster TO the rod
• With the acetate rod, electrons move FROM the rod TO the duster

Question 19

What moves during static electricity: Protons, Neutrons or Electrons?

Answer 19

• ONLY ELECTRONS MOVE (never positive charges)
• Both +ve and -ve electrostatic charges are only ever produced by the movement of electrons. Positive charges DO NOT MOVE

Question 20

Cause of a positive static charge?

Answer 20

• Always caused by electrons moving away elsewhere.

- Material that loses the electrons loses some negative charge, left with an equal positive charge

Question 21

Mains supply

Answer 21

AC

Question 22

Battery supply

Answer 22

DC

Question 23

Two types of electricity supplies:

Answer 23

• Alternating current (AC)

- Direct current (DC)

Question 24

What is AC?

Answer 24

• Current is constantly changing direction

- Produced by an alternating p.d. In which the positive and negative ends keep alternating

Question 25

UK mains supply

Answer 25

An AC supply at around 230 V

Question 26

What is the frequency of the ac mains supply?

Answer 26

• 50 cycles per second

- 50Hz

Question 27

What supplies DC?

Answer 27

Cells and batteries

Question 28

What is direct current?

Answer 28

• A current that’s always flowing in the same direction created by a direct p.d.

Question 29

What connects electrical appliances to the mains supply?

Answer 29

• Three-core cables, meaning they have three wires inside them w/ a core of copper and a coloured plastic coating

Question 30

What does the colour of the insulation tell you?

Answer 30

Shows it’s purpose so you can tell them apart

Question 31

Neutral wire

Answer 31

• Blue
• Completes the circuit
• Carries away the current
• Electricity normally flows in through the live wire and out through the neutral wire
• It’s around 0V

Question 32

Live wire

Answer 32

• Brown

- Provides the alternating p.d. at around 230V from the mains supply

Question 33

Earth wire

Answer 33

• Green and yellow
• For protecting the wiring and for safety, stops the appliance casing from becoming live
• Doesn’t usually carry a current - Only when there’s a fault
• Also at 0V