Topic 2 - Electricity Flashcards
Current
- 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
Potential Difference
- The driving force that pushes the charge around
- Unit of p.d: volt, v
Resistance
- The resistance is anything in the circuit which slows the flow down
- Unit of resistance: ohm
What does the current flowing through a component depend on?
- 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)
Definition of current
The size of the current is the rate of flow of charge
Formula for charge
Q = It
Charge (C) = current (A) x time (s)
Current measurement
Amperes (A)
Charge measurement
Coulombs (C)
Time measurement
Seconds (S)
Formula linking p.d, current and resistance?
V = IR Potential Difference (V) = Current (A) x Resistance (ohms)
Resistance and ohmic conductors
- 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
More facts on resistance and ohmic conductors?
Resistance of some resistors and components does change, e.g. a filament lamp or a diode.
Why does resistance of a filament lamp change?
- 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.
Why does the resistance of a diode change?
- 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.
Examples of ohmic conductors?
A wire or a resistor
Static electricity
- Static electricity is all about charges which are not free to move (e.g. insulating materials)
- Build-up of static is caused by friction
What happens when certain insulating materials are rubbed together?
- 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
Polythene and acetate rods being rubbed with a cloth duster
- With the polythene rod, electrons move FROM the duster TO the rod
- With the acetate rod, electrons move FROM the rod TO the duster
What moves during static electricity: Protons, Neutrons or Electrons?
- 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
Cause of a positive static charge?
- Always caused by electrons moving away elsewhere.
- Material that loses the electrons loses some negative charge, left with an equal positive charge
Mains supply
AC
Battery supply
DC
Two types of electricity supplies:
- Alternating current (AC)
- Direct current (DC)
What is AC?
- Current is constantly changing direction
- Produced by an alternating p.d. In which the positive and negative ends keep alternating
