Diode: only allows current to flow in one direction (high resistance backwards).
Ammeter: measures current (connect in series).
Voltmeter: measures potential difference (connect in parallel).
Thermistor: as temperature increases, resistance decreases automatically (thermostats)
LDR: as light intensity increases, resistance decreases automatically (street lamps)
Current: rate of flow of charge.
Q = It
Q: charge (C), I: current (A), t: time (s)
V = IR
V: potential difference (V), I: current (A), R: resistance (Ω)
IV Graph for Fixed Resistor
In a fixed resistor, resistance is constant, so the I-V graph is a straight line.
IV Graph for Filament Lamp
In a filament lamp, resistance increases as the lamp gets hot, so the current slows down.
IV Graph for Diode
In a diode, current doesn't flow backwards so the I-V graph doesn't go down.
Series Circuits: (one loop). Current is the same everywhere. Voltage adds up to battery. Resistance adds up.
Parallel Circuits: Voltage is the same everywhere. Current adds up from each branch.
Mains electricity is an ac supply. In the United Kingdom, the domestic electricity supply has a frequency of 50 Hz and is about 230 V.
Live wire: brown. Carries the alternating potential difference from the supply.
Neutral wire: blue. Completes the circuit.
Earth wire: green and yellow. Safety wire to stop the appliance becoming live.
P = IV
P = I2R
E = Pt
E = QV
P: Power (W), E: Energy (J)
The National Grid is a system of cables and transformers linking power stations to consumers.
Step-up transformers are used to increase the potential difference from the power station to the transmission cables. (P = IV so a high V allows I to be low. If I was high, that would cause heated wires, which is inefficient.
Step-down transformers are used to decrease the V for homes to use.
Triple Science Only
When certain insulating materials are rubbed against each other they become electrically charged. Negative electrons are rubbed from one material to the other.
When two charged objects are brought close together they exert a force on each other. Like charges repel. Opposite charges attract. These are non-contact forces.
A charged object creates an electric field around itself. The electric field is strongest close to the charged object. A second charged object placed in the field experiences a force. The force gets stronger as the distance between the objects decreases.
