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Electricity - Electrical Circuits 01

Electricity is an important topic in GCSE Physics. In this quiz we look at how the flow of electricity (the current) is measured in electrical circuits.

Electrical current flows in a closed path known as an electrical circuit. When a circuit is 'broken' it means that there is a gap which is stopping current from flowing. The break could be caused by a damaged component or wire, a component or connection that has become disconnected, or where there is a switch. Understanding the basic principles associated with electrical circuits is useful as it allows you to predict how circuits behave. If you learn a few simple equations, you will be able work out a wide variety of important values relating to circuits and components. By now, you should be very familiar with Ohm's law (V = I R) and have had plenty of opportunity to use it, but there are some other vital relationships related to electrical current and electrical circuits that you should know for your GCSE.

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Electrical current is a flow of electrons. Each electron carries a small amount of charge and if there are enough electrons on the move, the amount of charge is useful. The unit of electrical charge is the coulomb (C) and if you have one coulomb of charge passing a given point in an electrical circuit per second, then a current of one amp is flowing. Two coulombs per second is two amps, five coulombs in a second is a current of five amps and so on. If you have ten coulombs flowing past in five seconds, that boils down to two coulombs per second, so the current is two amps. The current is worked out from the quantity of charge flowing divided by the number of seconds it takes it to pass a defined point in a circuit.

Moving electrical charge through a circuit needs work to be done - the greater the resistance, the greater the energy required. The energy is supplied by the power source for the electrical circuit. As the charge moves through the circuit, some of the energy it carries is transferred. In the wires, it is transferred to heat and wasted. In a component such as an LED, it is transferred into heat and also directly into light, which is the useful energy. Electrical potential difference is a measure of the energy transferred as the charge passes through a component and has the units of volts (V). One volt of electrical potential difference represents one coulomb of charge gaining or losing one joule of energy.

When you plot a graph of the current flowing through a component against the electrical potential difference, you end up with a current-potential difference graph. When asked about electricity in the GCSE, you should be able to recognise the shapes of graphs for a resistor at constant temperature, a diode and a filament lamp. The examiners may put one in for a component for which you have never seen the graph, but you will be told what it is and be asked some simple questions about the information you can get from it.

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  1. What is the size of the current in a circuit if the charge, Q, is 100 C and lasts for 25 seconds?
    Remember the definition - current is a measure of how much charge flows past a given point in one second. This should help you to get the right answer even if you can't recall the equation
  2. What does the size of the electric current depend on?
    High currents = bigger flow rates
  3. What can be calculated from current-potential difference graphs?
    The gradient gives the resistance
  4. What is the work done in a circuit if the voltage is 10 V, the current is 4 A and the circuit is on for 25 seconds?
    You need to rearrange the equation V = WQ and work out the charge. Remember that one coulomb is one amp flowing for one second so if you have four amps flowing for 25 seconds, how many coulombs is that?
  5. What is potential difference?
    Strictly speaking, it should really be called electrical potential difference but at GCSE, potential difference is acceptable, when used in the correct context
  6. What is the unit of charge?
    The coulomb is the unit of charge. It is denoted by a C after a value of charge and is named for the French scientist Charles-Augustin de Coulomb who carried out a lot of pioneering work on electrical charge and magnetism
  7. What is the formula for potential difference?
    V is in volts when W is in joules and Q is in coulombs
  8. What is the unit of current?
    Amperes or Amps for short is the unit of current. It is usually denoted with an A after a value of current
  9. Which formula calculates the size of the current?
    If you didn't remember the equation, you can work it out. Whenever you need to calculate the rate of something in physics, it is expressed as something per second. To get the 'per second' bit, you know that you need to divide by time. That eliminates two of the alternative answers so the one that works out the current must be the correct one
  10. What is electrical current?
    Current is the flow of electric charge around a circuit. The charge is carried by electrons which are emitted by the negative electrode of the cell and travel to the positive electrode of the cell through the circuit

Author: Martin Moore

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