Physics current electricity

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.