P8. Electricity Flashcards
Electric field
A region in which an electric charge experiences a force
Current
Rate of flow of charge
Electromotive force (e.m.f)
The energy supplied by a source per unit charge in driving charge round a complete circuit
Potential difference (p.d.)
The work done per unit charge moving between two points in a circuit
Ohm’s Law
The current is directly proportional to the p.d. provided the temperature of the metal conductor is constant
What is the difference between current and electron flow?
Current flows from the positive terminal to negative of cell, while electrons flow from the negative terminal to positive of cell
Why is there a heating effect when current flows in a resistance?
A heating effect occurs because electrons collide with atoms as they pass through a conductor; electrons lose energy, atoms gain energy and vibrate faster, resulting in higher temperature
Variable potential divider (potentiometer)
Used for varying the resistance and current in a circuit
eg. control brightness in lamp. control volume in radios
Thermistor
Temp. increases, resistance decreases;
temp. decreases, resistance increases
eg. in temperature-operated alarms (switch on when cold)
Light-dependent resistor (LDR)
Bright, resistance decreases;
Dark, resistance increases
eg. light-sensitive switch (switch on when dark)
Relays
Electromagnetically operated switches
Diodes
Allow current to flow in one direction only, because they have a very high resistance in the other direction
eg. used as a rectifier (convert a.c. to d.c.)
Hazards of damaged insulation
Contact with the wire could cause an electric shock and could burn someone if they touch the wire
Hazards of overheating of cables
Could melt the insulation and cause a fire
Hazards of damp conditions
Water can conduct electricity, so wet electrical equipments can cause an electric shock
