Changes in charge, current and potential difference during the charging and discharging of a capacitor are all exponential-type behaviours. Specifically, the current that transfers charge to or from a capacitor (during charging or discharging respectively) is always greatest at first and declines to zero as time increases. During discharging, the charge loss and the drop … Continue reading Charging a Capacitor (analysis)
Uncertainties and Error Bars
Measurements are always uncertain: there is no such thing as a perfectly precise measurement. At the very least, the instrument used will limit the precision of the value but in many cases other factors can introduce even greater variability. It is always the greater of these two numbers (the resolution of the instrument and variability … Continue reading Uncertainties and Error Bars
How to determine voltages inside a resistor network
Resistor networks sometimes look scary but they can be solved by taking things one step at a time and applying basic circuit rules. The rules concerned are the calculations for combining series and parallel resistors, together with Kirchoff’s two laws for circuits. To combine resistors, we add their values if they are arranged in series … Continue reading How to determine voltages inside a resistor network
Hypothesis and Analysis
When carrying out a practical investigation it is always helpful to know what sorts of results are expected. In other words, before starting an experiment it is a good idea to consider what theory tells us should be the outcome. The predicted findings form the basis of an hypothesis, which is developed by considering an … Continue reading Hypothesis and Analysis
Key Facts: Electric circuits
Electricity is one of the forms of energy transfer so it is not surprising that the definition of the volt (the unit for measuring potential difference) is the energy carried per unit charge. In symbols, where Q is the symbol for charge, this relationship is written as; V = E / Q It is important … Continue reading Key Facts: Electric circuits
Investigating I-V Characteristics
Many GCSE Physics courses, including AQA Trilogy, include a compulsory practical to investigate the current-voltage (I-V) characteristics of various electrical components. This experiment links back to the work done by Georg Ohm, whose results have previously been summarised as; "the current flowing through a conductor is directly proportional to the potential difference (voltage) across the … Continue reading Investigating I-V Characteristics
Ohm’s Law
Potential difference (voltage) and current are fundamental measurements for electric circuits. Multiply the potential difference across a device by the current flowing through it to get the power rating for that device.Multiply the power rating by the time for which the device is used to get the energy that the device has transferred. All of … Continue reading Ohm’s Law
Energy Transfers in Electric Circuits
The amount of energy transferred in an electric circuit can be calculated by multiplying the current, time and potential difference. This is expressed in the equation given below. Current is measured in amps (amperes), time is in seconds, and potential difference in volts. Remember that the symbol for current is the letter I - not … Continue reading Energy Transfers in Electric Circuits
Resistance theory
Often it is true in physics that if you can understand one thing then it will automatically help you to understand something else. A good example of this occurs in resistance, where the model of resistance in a wire links directly to the behaviour of circuits that have resistors arranged either in series or in … Continue reading Resistance theory
Current and Voltage rules
There is a clear pattern to the measurements of current and potential difference (voltage) in electrical circuits - which, for us, means series and parallel circuits. These two patterns were identified by Gustav Kirchhoff and are therefore known as Kirchhoff's Laws. Kirchhoff's current law states that the sum of the currents flowing into a junction … Continue reading Current and Voltage rules
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