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Electricity - Currents in Electrical Circuits

Electricity is a major topic in GCSE Physics. In this quiz we look specifically.at currents formed by the flow of electrons in electrical circuits.

From your studies of static electricity, you should by now be familiar with the idea that electricity is something to do with electrons. It seems that the ancient Greeks knew about static electricity - rubbing a substance called amber created a small force that could attract small objects. That was about the limit of knowledge until the scientific revolution in Europe during the Seventeenth Century. The physician to Elizabeth I, William Gilbert, experimented and described static electricity in many substances.

The origin of the word electricity is uncertain but it is thought to originate from the ancient Greek word for amber, elektron, in honour of the Greek discovery 2,000 years earlier. Amber is the fossilised resin of trees that is found in many places around the world. The largest source of amber is around the shores of the Baltic Sea and it is this amber that is used for jewellery. Occasionally amber contains beautifully preserved fossils of insects.

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Eventually, scientists became aware that there was another form of electricity - current electricity. We now understand electricity much batter and know that electrical currents are formed by the flow of electrons through electrical circuits. An electrical circuit consists of a closed loop of conductors connected to a source of a potential difference such as a battery or generator. A potential difference has two aspects, positive and negative; it can sometimes help to think about it as being an electron pump. Electrons have negative charge and so will flow away from the negative pole and be attracted to the positive pole. Just to confuse things, when current electricity was first discovered, scientists had no idea bout what it was. They realised it was a flow of something and decided that it went from positive to negative - we call this conventional current and this idea is still used when designing and talking about electrical circuits.

The ability to control the currents in electrical circuits has allowed us to create an astonishing variety of electronic products. The current flowing in a circuit depends on two variables, the potential difference (voltage) and the resistance. The relationship between the two is governed by Ohm's law and is expressed as the formula V = I x R (V = IR) where V is the potential difference in volts, I is the current in amps and R is the resistance in ohms. In a circuit, where it reaches a junction, the current will divide and flow down each of the parts of the circuit linked to the junction. The amount of current flowing through each part can be calculated using Ohm's law. For your GCSE, you are expected to be able to use this to calculate current, potential difference and resistance in all or part of a circuit. To do this, you need to be able to rearrange the terms of the equation.

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  1. What is the definition of current?
    Metals are the best conductors of electricity because of metallic bonding
  2. Two 2 Ω resistors are placed in parallel in a circuit. If the potential difference supplied to the circuit is 10 V, what size is the current which passes through each resistor?
    Since the resistance is the same in the two parts of the parallel circuit, the current splits evenly. You can prove this to yourself by calculating from V = I R
  3. What is the current through a circuit which contains three resistors of size 3 Ω, 5 Ω and 7 Ω respectively which are placed in series when a potential difference of 12 V is applied?
    V = I multiplied by R so rearranging the equation gives I = V divided by R. The resistors are in series so the total resistance to use in the equation is the sum of the three individual resistors
  4. What is the total resistance in a circuit if the potential difference is 12 V and the current flowing through the circuit is 3 A?
    V = I multiplied by R so rearranging the equation gives R = V divided by I
  5. Calculate the current in a circuit when the voltage of the circuit is 5 V and the circuit has a total resistance of 5 Ω.
    V = I multiplied by R so rearranging the equation gives I = V divided by R
  6. If two components are connected in series, how is the potential difference across each split?
    Current, potential difference and resistance are closely linked. Each one depends on the value of the others
  7. Two circuits are set up and both circuits contain two bulbs. One circuit is set up so that the bulbs are connected in series, whilst the other is set up so that the bulbs are connected in parallel. In which circuit will the bulbs be brightest if both circuits have the same resistance and the same voltage applied?
    Placing components in parallel reduces the resistance which each electron has to travel through to get to the positive anode of the cell, resulting in a larger current through each component
  8. If the voltage in a circuit remains constant but the resistance is increased, what will happen to the current?
    The current and resistance are inversely proportional
  9. If the voltage in a circuit is increased whilst the resistance remains constant, what will happen to the current in the circuit?
    Current and voltage are directly proportional
  10. Why does a 60 W equivalent fluorescent bulb use less energy than a 60 W filament bulb?
    Fluorescent bulbs create a large potential difference between two electric plates. The space between the plates is filled with an inert gas and when an electron gains enough energy to travel from one plate to the other, it can collide with the gas in the bulb. This creates an 'avalanche effect' where electrons are knocked out from the outer electron shells of the inert gas. The electrons are then absorbed by another atom and this process emits a photon of light. When the photons of light strike the coating on the inside of the bulb, the coating fluoresces, giving out light

Author: Martin Moore

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