Cyberphysics

Basic electricity

Don't forget to play with the computer model on the network and download your own copy onto your home computer (applies only to students at WGHS).

Summary of Basic Circuit Knowledge

ammeter
cells in series
characteristic curves
circuit symbols
current
diode
Kirchhoff's First Law
parallel circuits
potential difference
resistances in parallel
resistances in series
series circuits
strands of a circuit
voltmeter An interactive PowerPoint presentation for you to use on the topic of resistance

A current will flow through an electrical component (or device) only if there is a voltage or potential difference (p.d.) across its ends. The bigger the potential difference across a component, the bigger the current that flows through it.

There must be a complete circuit for a current to flow. If there is a gap in the circuit then the whole strand that the gap is in will not have current flow through it.

Components resist a current flowing through them. The bigger their resistance, the smaller the current produced by a particular voltage, or the bigger the voltage needed to produce a particular current.

The p.d. across a component in a circuit is measured in volts (V) using a voltmeterconnected across (in parallel with) the component.

The current flowing through a component in a circuit is measured in amperes (A) using an ammeterconnected in series with the component.

When components are connected in series:

. their total resistance is the sum of their separate resistances
R_TOTAL = R₁ + R₂ + ..........R_N;
. the same current flows through each component;
. the total potential difference of the supply is shared between them.

When components are connected in parallel: . there is the same potential difference across each component;
. the current through each component depends on its resistance; the greater the resistance of the component, the smaller the current;
. the total current through the whole circuit is the sum of the currents through the separate components - this follows from Kirchhoff's First Law - see below.

The potential difference provided by cells connected in series is the sum of the potential difference of each cell separately (bearing in mind the direction in which they are connected).

A cell's potential difference between its terminals has a chemical source and that this can 'run down' with use or incorrect storage providing less of an electrical gradient for the current (i.e. the voltage stamped on a battery might not be correct).

You should be able to interpret and/or draw circuit diagrams using standard symbols. The following standard symbols should be known:

connecting wire
connection between two crossing wires
two crossing wires that are not connected to each other
switch (open)
switch (closed)
signal lamp
filament lamp
cell
battery
power supply
fuse
resistor
diode
variable resistor
thermistor
ammeter
voltmeter
L.D.R. (light dependant resistor)

Potential difference, current and resistance are related as shown:

V = I R
Where: V = potential difference (in volts, V)
I = current (in ampere, A)
R = resistance (in ohm, W)

Current-voltage graphs are used to show how the current through a component varies with the voltage you put across it.

These are called Characteristic Curves.

This characteristic is described by Ohm's Law - The ratio of the potential difference across the ends of a current carrying conductor to the current flowing though it is constant providing the temperature is constant. (Click here for an interactive experiment)

For details on an experiment to determine the characteristic curve for a filament lamp: click here

You should know these curves and be able to interpret them to explain how they show that:

The current through an ohmic conductor (e.g. a wire) (at constant temperature) is proportional to the voltage across the resistor. This is known as Ohm's Law.
The resistance of a filament lamp increases as the temperature of the filament increases.
The current through a diode effectively only flows in one direction only. It's resistance is very low as long as it has a potential difference of more than 0.6 volts across it. The diode has a very high resistance in the reverse direction therefore only a tiny current flows.Click here for more detail.
The resistance of a light dependent resistor decreases as the light intensity increases.
The resistance of a thermistor decreases as the temperature increases. (There are some thermistors which behave in the opposite way to this but all of your questions will be set on this version).

As an electric current flows through a circuit, energy is transferred from the battery or power supply to the components in the electrical circuit.

An electric current is a flow of charge.

When electrical charge flows through a resistor, electrical energy is transferred as heat.
The rate of energy transfer (power) is given by:

P = IV
Where: P = power (in watts, W)
V = potential difference (in volts, V)
I = current (in ampere, A)

1 watt is the transfer of 1J of energy in 1s.

The higher the voltage of a supply, the greater the amount of energy transferred for a given amount of charge which flows.

E = VQ
Where E = energy transferred (in joule, J)
V = potential difference (in volt, V)
Q = charge (coulomb, C)

The amount of electrical charge which flows is related to current and
time as follows:

Q = I t
Where: Q = charge (coulomb, C)
I = current (in ampere, A)
t = time (in seconds, s)

The total amount of energy transferred by an electrical device can be calculated as follows:

E = Pt
Where E = energy transferred (in joule, J)
P = power (in watts, W)
t = time (in seconds, s)

Using Domestic Electrical Appliances

Much of the energy transferred in homes and industry is electrical energy. This is because electrical energy is readily transferred as forms that are useful to us:

. heat (thermal energy);
. light;
. sound;
. movement (kinetic energy). You should be able: . to specify the energy transfers everyday electrical devices are designed to bring about;
. to give examples of everyday electrical devices designed to bring about particular energy transfers.

How much electrical energy an appliance transfers depend on:

. how long the appliance is switched on;
. how fast the appliance transfers energy (its power).

The power of an appliance is measured in watts (W) or kilowatts (1kW = 1000W).

The amount of energy transferred from the mains is measured in kilowatt-hours, called Units by the electricity boards

energy transferred (in kilowatt hour, kWh) = power (in kilowatt, kW) x time (hour, h)

You should be able, when provided with suitable diagrams of a digital domestic electricity meter, to calculate the number of Units used. (See KS3 page on this)

The cost of this energy can be calculated using: total cost = number of Units x cost per Unit.