When electrons are pumped across a circuit, a charge is produced. The charge is given the unit of coloumbs. The movement of the charge per second is called current. It is measure in Amperes or amps (A) and is measure by an Ammeter. To find the amount of current passing through a circuit we use the equation:
I = Q/t
I is for current, Q is for coloumbs of charge, and t is for the amount of time in seconds for the charge to pass.
These symbols are used to represent components or conductors that are normally involved in a circuit. The circuit is drawn in symbols with lines to represent wires becuase this way the circuit is understood by all and is not much time consuming. Here are a few electronic symbols used.
An ammeter is a device used to measure current passing through a circuit. There are two types of ammeter, one is analogue which has a needle and a scale on it but a digital ammeter is more commonly used due to its diverse use. The digital ammeter can be connected to a multimeter for other uses such as measuring Volts. A digital ammeter may have a range of 0 – 10A which can by modified by a shunt. A shunt has thin wire in it which can divert the current to flow through it allowing the ammeter itself to measure a small amount of current, this is how the range of an ammeter increases. For example a shunt has been set at 90%, then the range of the ammeter will be increased to 0 – 1A. Ammeters are connected in a series circuit.
Prefixes used for Amperes and Volts
The current that flows through a circuit is normally below 1 which is why special prefixes are used to calculate the Amperes of currents that flow through.
- MilliAmpere, mA is the unit which is the 1/1000th of an ampere therefore a 1000 mA is equal to 1 A.
- MicroAmpere, µA, is 1/10000th of an ampere so 10,000 µA is equal to 1 A.
Although millilivolts and micro volts are not used much but have the same have value as the prefixes for Ampere.
- MilliVolt , mV, is 1/1000th of a Volt therefore 1000 mV is equal to 1 V.
- MicroVolt, µV, is 1/10000th of a Volt therefore 10,000 µV is equal to 1 V.
Cells can be combined to make up 1 battery. Cells may be placed in same direction causing an increase in potential difference, for example two cells having p.d of 1.5 V will make up 3 V if placed in same direction.
If cells are placed in opposite direction to each other then the resultant p.d will be the difference between the p.d of both cells, for example two cells having p.d of 1.5 V will make up total p.d of 0V.
When a cell is connected to a circuit it forms two terminals, one is positive while the other is negative. These terminals have a difference in energy and this is because this difference is called potential difference. This difference causes the electrons to flow from the negative terminal to the positive terminal. The p.d is measured in Volts which is why it is sometimes called the voltage. The voltage is the reason a charge flows through the circuit. The greater the p.d the greater the charge flows throught the circuit. The p.d also affects the rate at which the electrical energy changes into other forms of energy.
The greater the p.d the faster the rate at which the electrical energy is transformed. For example a circuit with a bulb in it has p.d of 240 V will be brighter and hotter than a bulb in a circuit having p.d 12V. To calculate this work done, i.e; electrical energy changed into a different form of energy, we use this equation: W = I x t x V. The voltage or p.d is measured by a voltmeter which may have a range from 0 – 10 V. The Voltmeter can be connected to a multimeter to serve various purposes. It can be connected with a multiplier to increase is range. The voltmeter is connected in parallel to a conductor to measure the p.d across it.
A circuit may have a lot of conductors in it which is why different currents flow through them. This is because each conductor has different resistance towards the flow of the electrons. Resistance is basically the ease at which electrons can flow through a conductor. The easier it is for the elctrons to flow the less resistance there is against it. Resistance is measured by ohms, Ω.
Resistance is used to control the flow of current and is done so by resistors. There are two types of resistors fixed resistors and variable resistors. The fixed resistors have a fixed resistance but variable resistors can have a varied amount of resistance. A Light Dependant Resistor (LDR) depends upon light. If it is placed in front of a light source, its resistance drops while placing it in the dark will increase its resistance.
Resistors in Parallel and in Series circuits
Resistors can be connected in a series. This results in various different p.d to be formed between each resisitor. The Voltage can be calculated as V = V1 + V2 + V3. Resistance in such a circuit is calculated by R = R1 + R2 + R3. To find the resistance the formula used is R = V/I. V is for Volts, R is for Resistance and I is for current. Resistors can also be connected in parallel circuits. The total resistance in this instance is calculated
by 1/R = 1/R1 + 1/R2 + 1/R3. The voltage across the parallel circuit remains the same. The total current will have to be calculated by the equation: I = I1 + I2 + I3.
The Ohm’s Law says that the current through a metallic conductor is directly propertional to the p.d across it, if other factors such as temperature is constant. We can prove this by increasing the temperature of a circuit which has a ohmic conductor in it, as the temperature increases the resistance of the ohmic conductor also increases. If a bulb filament is heated its resistance will decrease showing that all semiconductor or insulator will lose its resistance once heated.
Resistance of a Wire
Resistance of a wire is directly propertional to its length and is indirectly propertional to its cross section area. If a wire is long it has more resistance than a short resistance and if a wire is thin it has more resistance than a thick wire. Resistors more commonly have a short length of carbon in them, which in some cases have a long wire winding around the ceramic rod in a casing. This makes it cheaper.
Electromotive force of a source of electrical energy, such as a cell, is the total p.d of the cell when a circuit is open. For example a cell having a p.d of 1.5 volt on an open circuit will be its e.m.f but when the circuit is closed and if there is a conductor such as a bulb in the circuit ther p.d will decrease due to the internal resistance of the cell. Therefore e.m.f is the work done ÷ charge.
Since 1 V is equal to 1 J of energy, and not all the emf is used then the difference in the p.d of an open and closed circuit will be the wasted energy. The equation to find out EMF is E = V + Ir. Ir is for internal resistance, and E is for EMF, and V is for Volts. When cells are connected in a series then the total resistance of the cells is R = r + r + r. If the cells are placed in parallel to each other then the total resistance of the cells is R = r/3. The lost p.d can be found by the equation, Lost p.d = I x r. Then the result can be subtracted from the EMF.