Electricity and Magnetism

Topics: Electric current, Resistor, Electromagnetism Pages: 6 (1210 words) Published: June 22, 2013
Electricity and Magnetism


Circuit symbols
Potential difference

The energy which is delivered to an electric circuit comes from the supply. The greater the potential difference of the power supply the more energy is delivered. Potential difference is measured in volts, which has the unit symbol V.

Cells in series

The more chemical cells that are connected in series the greater the potential difference that they supply. The potential difference of a number of cells connected in series is the sum of each cell separately (bearing in mind the direction in which they are connected.) .

Electric current

The energy supplied by the potential difference is delivered to the circuit by the electric current flowing through the circuit. The current in a metal is actually a flow of electrons (tiny particles that are part of an atom.)

The 1.5V cell supplies enough energy to the current so that it delivers enough energy for the bulb to be normally bright.

Series circuits

All of the current passes through each of the two identical bulbs, the energy supplied by the 1.5V potential difference is shared by the two bulbs, they are each less than normally bright.

Parallel circuits

The current splits at the junction in the circuit. The electrons deliver all of their energy to one or other of the bulbs. Both bulbs obtain the full 1.5V potential difference of the battery, they are each normally bright.

The effects of an electric current

An electric current can cause 1. a heating effect
2. a magnetic effect
3. a chemical effect

Measurement of electric current

Electric current flowing through a component is measured in ampère, A, using an ammeter which is connected in series with the component.

Electric current in a series circuit

The electric current at all points in a series circuit is constant.

Electric current in a parallel circuit

The electric current splits at a junction in a parallel circuit. The total current through the whole circuit is equal to the sum of the currents in the branches of the circuit.

Electrical resistance

Electrical components resist the flow of electric current.

The bigger the resistance of an electric component the smaller the current that is produced by a particular potential difference, or the bigger the potential difference that is needed to produce a particular current

Electrical Devices


An open switch is a break in a circuit which prevents current to flow.

A switch can be normally open or normally closed.

In the circuit shown in figure 1 bulb 1 is lit but not bulb 2. Switch A must be closed for current to flow through either bulb, but as switch C is open it does not flow through bulb 2.

The table below shows how this circuit would operate for all possible combinations of switch positions. This is known as a truth table for the circuit.

switch A| switch B| switch C| bulb 1| bulb 2|
open| open| open| off| off|
open| open| closed| off| off|
open| closed| open| off| off|
open| closed| closed| off| off|
closed| open| open| off| off|
closed| open| closed| off| on|
closed| closed| open| on| off|
closed| closed| closed| on| on|

Single-pole double-throw (s.p.d.t.) switches

A single-pole double-throw switch allows the current to be directed through one out of two possible routes through a circuit.

Figure 2 shows how such a switch can be used to switch on either bulb 1 or bulb 2, in this case bulb 1 is lit. If A was connected to Y then bulb 2, and not bulb 1, would be lit.

Figure 3 shows how two s.p.d.t. switches are used to allow a bulb to be switched on or off from two different positions (staircase lighting)

A force is a push or pull on an object.
The SI unit of force is the newton, N.

Free body diagrams

These show the nature, size and direction of all forces acting on an object.
Forces are...
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