P5 - Electricity - OCR

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P5 ­ Electricity - Answers
1. Some insulating materials become electrically charged when they are rubbed together. When the materials are rubbed against
each other negatively charged particles called electrons move from one material to the other.
The material that loses electrons becomes positively charged.
The material that gains electrons becomes negatively charged.
Both materials gain an equal amount of charge, but the charges are opposite. The electric charge is called static electricity.
2. If two charged objects are brought close together they are either attracted towards each other, or they are repelled away from
each other.
If they have the same type of charge they will repel and push away from each other. Two negative charges placed near
each other will repel each other, and so will two positive charges.
If they have opposite charges they will attract each other. When a positive charge and a negative charge are placed near
each other they will attract each other.
Electrically charged objects can attract small uncharged objects that are close to them. For example, an electrically
charged plastic comb can pick up small pieces of paper.
For example, if you move a positively charged Perspex rod near to another positively charged Perspex rod suspended on a
string, the suspended rod will be repelled. If you move a negatively charged ebonite rod near to positively charged suspended
Perspex rod, the suspended Perspex rod will be attracted.
3. Charge is a basic property of matter. All matter is made of atoms, which in turn are made of protons, neutrons and electrons.
In most materials there are an equal number of positive and negative charges, so the whole thing is equal. When you charge
something, you move some electrons to it or from it.
4. An electric current is a flow of charge. It is measured in amperes (amps). Amps are a measure of how much electric charge is
flowing round the circuit.
5. In an electric circuit...
The components and wires are full of charges that are free to move
The battery causes the free charges to move.
The charges are not used up but flow in a continuous loop
6. Metal atoms release some of their electrons, and these electrons are free to move through the structure of the metal. This is
why metals are good conductors of electricity. Insulators have no charges free to move, which is why they do not conduct
7. Chemical reactions inside the battery have the effect of separating electric charges so that positive charge collects on one
terminal of the battery and negative charges on the other. If the battery is connected into a circuit, the charges on the battery
terminals set up an electric field in the wires of the circuit. This makes free charges in the wire drift slowly along. However, even
though the charges move slowly, they all begin to move at once, as soon as the battery is connected. So the effect of their
motion is immediate.
8. The larger the voltage of the battery in a given circuit, the bigger the current.
9. Components resist the flow of charge through them. Examples of components are:
The connecting wires in the circuit have some resistance but it's so small that it's usually ignored.
10. An electric current flows when charged particles called electrons move through a conductor. The moving electrons can collide
with the atoms of the conductor. This is called resistance and it makes it harder for current to flow. These collisions make the
conductor hot. It is this that makes a lamp filament hot enough to glow. Resistance is measured in
ohms ().
11. As long as a component's resistance stays constant, the current through the resistor is directly
proportional to the voltage across the resistor. So, if you double the voltage, the current also
doubles. Therefore the greater the resistance in a circuit, the smaller the current will be. This is called
Ohm's Law.
The equation below shows the relationship between resistance, voltage, current:
Resistance = Voltage / Current
Ohms () = Volts (V) / Amperes (A)
12. LDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security
lights. Their resistance decreases as the light intensity increases:
In the dark and at low light levels, the resistance of an LDR is high and little current can flow through it.
In bright light, the resistance of an LDR is low and more current can flow through it.

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Thermistors are used as temperature sensors, for example, in fire alarms. Their resistance decreases as the temperature
At low temperatures, the resistance of a thermistor is high and little current can flow through them.
At high temperatures, the resistance of a thermistor is low and more current can flow through them.
15. When you add resisters in series the battery has to push charges through more resistors so the resistance increases.…read more

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Alternating current flowing through the primary coil creates an alternating magnetic field
This changing field then induces an alternating current in the secondary coil
Transformers only work if AC is supplied to the primary coil. If DC was supplied, there would be no current in the secondary coil.
25. It is not practical to generate large amounts of electricity by passing a magnet in and out of a coil of wire.…read more


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