P2 Current & Cicuit symbols

Electric current is the flow of electrical charge

Current is measured in units of amperes (A) or amps 

The size of the electric current is the rate of flow of electrical charge.  In other words how much charge passes through a point each second 

In metals, such as a copper wire the electrical charge that flows is electrons. Therefore the current in a circuit is a flow of electrons.

The unit of Charge is coulomb(C) 

The coulomb is equal to the quantity of electricity conveyed in one second by a current of one ampere i.e 1C = 1A (second)

Charge flow, current and time are related by equation:

Q = It

Charge(C) = Current(A) x Time(s)

Electric charge will only flow round a complete (closed) circuit is something is providing a potential difference, e.g. a battery

Potential difference is the 'driving force'  that pushes charge around the circuit.  

In a single closed loop the current is the same everywhere is the circuit

Resistance is anything that slows down electrcial charge

The current (I) through a component depends on both the resistance (R) of the component and the potential difference (V) across the component.

The greater the resistance of the component the smaller the current for a given potential difference (pd) across the component. 

Current, potential difference or resistance can be calculated using the equation:

potential difference = current × resistance,   V = I R

potential difference, V, in volts, V;

current, I, in amperes, A (ampere (amp))

resistance, R, in ohms, Ω

Voltage is never obstructed by the resistance. It is constant.

Resistance of an ohmic conductor is constant if their temperature doesn't change

Some components have changing resistance

• For a filament lamp, the higher the current, the higher the temperature. meaning the higher the resistance. 

• For diodes, resistance depends on the direction of the current. Current only flows through in one direction, but have a high resistance in the other direction. 

Pratical:  Resistance of the Length of a Wire at a Constant Temperature

• Indepenent variable = length of resistance wire, L
• Dependent variable = resistance, R
• Control variables:  
  • Potentail difference of the power supply
  • Temperature of the wire

Measure: Connect wire using crocodile clips to form circuit.  Record potential difference  using R = V / I to calculate resistance of witre.  Repeat for range of wire lengths

Record results:

Use equation  where 

Mains supply is ac, Battery supply is dc

Difference:  ac (alternating current) the current (flow of charge) is contrantly changing direction.  UK mains supply is an sc supply of around 230V

dc (direct current) current flows constantly in the same direction frequency of 50Hz

Most electrical appliances are connected to the mains using three-core cable. The insulation covering each wire is colour coded for easy identification:

live wire – brown           carries the alternating potential difference from the supply.

neutral wire – blue   completes the circui

earth wire - green and yellow stops the appliance becoming live. 

The potential difference between the live wire and earth (0 V) is about 230 V. The neutral wire is at, or close to, earth potential (0 V). The earth wire is at 0 V, it only carries a current if there is a fault.

Electrical shocks

Touching the live wire produces a large potential difference across our body (0 V). This causes a current to flow through our body, resulting in an electric shock.

A live wire may be dangerous even when a switch in the mains circuit is open. If a fault occurs where the live wire connects to the case, the earth wire allows a large current to flow through the live and earth wires. This overheats the fuse which melts and breaks the circuit

The amount of energy an appliance transfers depends on how long the appliance is switched on for and the power of the appliance. Different domestic appliances transfer energy from batteries or a.c. mains to the kinetic energy of electric motors or the energy of heating devices.

Work is done when charge flows in a circuit. The amount of energy transferred by electrical work can be calculated using the equation:

energy transferred = power × time,    E = P t

energy transferred = charge flow × potential difference,    E = Q V

energy transferred, E, in joules, J;
power, P, in watts, W;
time, t, in seconds, s
charge flow, Q, in coulombs, C;
potential difference, V, in volts

The National Grid is a system of cables and transformers linking power stations to consumers. Electrical power is transferred from power stations to consumers using the National Grid.

Step-up transformers are used to increase the potential difference from the power station to the transmission cables then step-down transformers are used to decrease, to a much lower value, the potential difference for domestic use.

This is done because, for a given power, increasing the potential difference reduces the current, and hence reduces the energy losses due to heating in the transmission cables.

National Grid system is an efficient way to transfer energy because it reduces energy loss during transmission.