electrical currents

• Electric current is when charge flows. The charge in an electric circuit is carried by electrons. The unit of current is the ampere (amp, A)
• In circuit diagrams, current flows from the positive terminal of a cell or battery to the negative terminal. This is known as conventional current.
• In a single closed loop, the current has the same value at any point in the circuit
• Metals are good conductors of electricity because they contain delocalised electrons, which are free to flow through the structure.
• Potential difference (p.d.) is a measure of how much energy is transferred between two points in a circuit
• . The unit of potential difference is the volt (V)
• • The p.d. across a component is the work done on it by each coulomb of charge that passes through it
• • The p.d. across a power supply or battery is the energy transferred to each coulomb of charge that passes through it
• An atom has no charge because it has equal numbers of positive protons and negative electrons
• When electrons are removed from an atom it becomes positively charged.
• When electrons are added to an atom it becomes negatively charged.
• When electrons move through a circuit, they collide with the ions and atoms of the wires and components in the circuit. This causes resistance to the flow of charge
• The unit of resistance is the ohm (Ω).
• A long wire has more resistance than a short wire because electrons collide with more ions as they pass through a longer wire.
• If one component in a series circuit stops working the whole circuit will stop working.
• Components with a higher resistance will transfer a larger share of the total p.d. because V = IR
• A parallel circuit is made up of two or more loops through which current can flow.
• If one branch of a parallel circuit stops working, the other branches will not be affected.
• The total resistance of two or more components in parallel is always less than the smallest resistance of any branch.
• Current is directly proportional to the p.d. in an ohmic conductor at a constant temperature
• The current through a diode only flows in one direction – called the forward direction
• As more current flows through the filament, its temperature increases
• The resistance of a thermistor decreases and temperature increases
• The resistance of a light dependent resistor (LDR) decreases as light intensity increases