Ways to protect the user/appliance in a domestic setting
Insulation
Double insulation
EARTHING
Fuses
Circuit breakers
How insulation protects the user/appliance
Wires are insulated with plastic, to prevent an electric shock occurring when the user touches it
How DOUBLE insulation works
The appliance is completely covered in a layer of an insulating material, e.g. plastic, which means that there is no possibility of a current passing through to the user. This is usually used instead of an earth wire.
How earthing works
In addition to the (brown) live wire (that carries the current into the appliance), and the (blue) neutral wire (that carries the current out of the appliance) the (green and yellow) earth wire provides a path of very low resistance to the ground, so electrical energy flows through it rather than through the user.
How fuses work
The fuse is connected to the live wire. The value of the fuse must be slightly higher that what the appliance needs to function. When the current is too high, the fuse melts, breaking the circuit and protecting the appliance and user.
How circuit breakers work
Circuit breakers are resettable fuses. If the current exceeds a certain value, an electromagnet will separate a pair of contacts, breaking the circuit. They are more practical than fuses.
What happens when current flows through a resistor
The temperature increases, as a result of the electrons (that are flowing through the conductor as current) colliding with ions in the lattice and transferring some of their energy to these ions, which then vibrate more, causing the resistor to heat up. This is used in toasters, kettles, electric heaters, ovens.
What resists current
Ions in a lattice resist the flow of electrons
Equation - power, current and voltage
Power = current x voltage (P=IV)
Equation (don’t need to know) - energy, current, voltage, time
Energy transferred = current x voltage x time (E=VIT)
A.C.
Alternating current: current first flows one way, then the opposite way. Used to supply mains electricity.
D.C.
Direct current: current only flows one way. Used to supply current in cells and batteries.
Current in a series circuit
The current is the same throughout the entire circuit. As voltage increases, current increases. If there are more components in the circuit, current decreases, as there is more resistance.
Current in a parallel circuit
Current is shared between branches; current at source = sum of current in each brach
Voltage in a series circuit
Voltage is shared across the whole circuit; voltage at power supply = sum of voltage across each component
Voltage in a parallel circuit
Voltage is the same across each branch; voltage at power supply = voltage at each branch
Resistance in a series circuit
Total resistance = sum of resistance of each component. This is why more components increase resistance, therefore decreasing current.
How changing resistance affects current
Increasing resistance decreases current
Light Dependent Resistors
As light intensity increases, resistance decreases.
Thermistors
As temperature increases, resistance decreases
Equation - voltage, current and resistance
Voltage = current x resistance (V=IR)
Definition of current
Current is the rate of the flow of charge (measured in Amperes)
Equation - charge, current, time
Charge = current x time (Q=IT - we need to QUIT physics)
Electric current in solid metal conductors
The flow of (negatively charged) electrons
What happens to current at a junction
It is conserved - the current flowing into the junction is equal to the current flowing out of it, because current is the flow of electrons, and these cannot be created or destroyed, so the number of electrons flowing in the circuit must remain the same. This does not mean that the current will split equally along two branches - this depends on the resistance in each branch.
Voltage definition
Voltage is energy transferred per unit charge. This means that one volt is a joule per coulomb.
Equation - energy, charge, voltage
Energy transferred = charge x voltage (E=QV)
Current against voltage - resistor / metal wire
Directly proportional (goes through negative as well)
Current against voltage - diode
A diode is a component that only allows current to flow one way
Current against voltage - filament lamp
Flattening curve, opposite in the negative axis
the effect of a metal wire heating up
The metal ions vibrate more when temperature increases. This means that there is a greater chance of electrons colliding with them, making it harder for current to flow through, so the resistance increases.