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SP13

GCSE Separate Science Physics

QuestionAnswer
What does a changing magnetic field around a wire cause? It induces a potential difference.
What flows then there is a potential difference? A current .
What does the size of the induced potential difference in a coil dependent on? How fast the magnetic field is changing or moves past the coil
What happens to the induced p.d. if the magnetic field is reversed? The direction of the p.d. reverses
What does a generator consist of? A coil that is rotated in a magnetic field
What are the ends of the coil connected to? Slip rings.
What touches the slip rings so they are connected to the external circuit? Carbon brushes.
What type of current is produced by a coil rotating with slip rings in a magnetic field? Alternating current.
If the ring is connected to a commutator, what type of current does it generate? Direct current.
What is the name of a generator that produces direct current? Dynamo.
What is the national grid ? This is system of wires and cables that bring electricity from the power system to our homes and other buildings.
Where is energy lost in the national grid? By heating of the transmission lines.
What happens if the voltage (potential difference ) across the transmission wire is increased? The current decreases.
Why do we transmit electricity at low currents? Less energy is dissipated at heat to the surrounding air and it is more efficient.
What type of transformer increases the voltage (potential difference) as the electricity leaves the power station? A step up transformer.
What voltage ( potential difference change is there by the step up transformer? 25kW to 400kW
What type of transformer reduces the voltage (potential difference before it enters our homes? A step down transformer.
What happens to the current as electricity passes through a step down transformer? It increases.
What are the two coils in a transformer called? primary and secondary.
If the voltage (potential difference ) is to be increased what can you state about the ratio of the coils in the transformer? There are more coils on the secondary compared to the primary.
What is potential difference? It is a measure of the energy transferred by each coulomb of charge that flows through a wire.
What does the current measure? It measures the number of coulombs per second passing a point in a circuit.
What is electrical power? It is the energy transferred by the electric current each second.
What is the equation for electrical power? Electrical power = current x potential difference
What is the equation in symbols? P = I X V
What is the principle of conservation of energy? Energy cannot be created or destroyed only transformed into other forms.
What can you say about the power supplied by the primary coil compared to the power transferred away from the secondary coil? They are the same.
How can this be written in an equation in symbols? Vpx Ip = VsxIs
What is assumed in the transformer equation? It is assumed that the transformer is 100% efficient and there are no energy losses.
If Ip=2A, Vs=200V and Is=0.1A, what is Vp? "Vpx Ip = VsxIs Vp x 2= 200 x 0.1 Vp x 2= 20 Vp =20/2=10V
Current which regularly changes direction is called… alternating current
An example of alternating current is…? mains electricity
Current which flows in one direction is…? direct current
An example of direct current is… batteries
What is the potential difference of mains electricity in the UK? 230V
What is the frequency of the alternating current in UK mains electricity? 50Hz
The national grid consists of…? Cables and transformers
Are power stations par of the national grid? no
What does a step up transformer do? Increases p.d.
What does a step down transformer do? reduces p.d. to 230 V
Why are transformers used? reduce current so that less heat is lost in cables, increases efficiency
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