Charging by friction.  Current.  Circuits.  Experiments.  Voltage.  Resistance.  Ohm's law.  Variable resistors.  Symbols. 

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  • Created by: Rianne B
  • Created on: 17-07-12 13:27

charging by friction

There are two types of electric charge - Positive and Negative. 

Only insulating materials can be charged by friction. 

If you take a polythene rod, initially it is uncharged this means it has equal numbers of positive and negative charges. When it is rubbed with a duster, the polythene rod becomes negatively charged.  This happens because electrons are transferred from the duster to the polythene.  The duster becomes positively charged.  This happens because there are more protons left than electrons. 

When acetate is rubbed with a duster, the acetate rod becomes more positively charged and the duster more negatively charged.  This happens because there are more electrons than protons on the duster because the electrons have transferred from the acetate rod to the duster. 

Two charges of the same type repel each other and two charges of different types attract each other.

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polythene rod and duster

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acetate rod and duster

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polythene rod - stream of water

Rub the polythene rod with a cloth then place the rod near the water.  The water curves towards the rod.

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polythene rod - pieces of paper

Rub the polythene rod then place the rod above the pieces of paper.  A few pieces should go up and stick to the rod. 

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polythene and acetate rod - hanger and clamp

Rub a polythene rod and stick it through the hanger.  Rub the 2nd polythene rod and place it near the other rod.  These rods will repel.  Then you try with the acetate rod near the polythene rod and it attracts to it.

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shuttling ball experiment

In this experiment a ball is coated in graphite paint(a conductor) and is hung by a nylon thread (an insulator) between two metal plates.  The plates are connected into a circuit so one is positively charged and one is negatively charged.  The ball touches the negatively charged plate and therefore becomes negatively charged.  Becasue the plate and ball have the same charge, the ball is repelled across the gap where it is attracted to the positive plate.  On touching the positive plate electrons flow off the ball leaving it positively charge, so it is now repelled by the positive plate and attracted to the negative plate.  The whole process is thn repeated over and over so that the ball moves rapidly back and forth between the plates, carrying a few electrons with it and therefore completeing the circuit, allowing a current to flow which is registered by the ammeter.  The ball is hung on by a nylon thread because it is an insulator and prevents the ball losing any of its charge apart from when it touches the plates.

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shuttling ball experiment diagram

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Voltage is measured using a Voltmeter in Volts (v).  The Voltmeter must be connected in parallel to the bulbs in the circuit.  Voltage is otherwise known as potential difference (p.d.).

Voltage is the energy per unit charge.  If a cell has a voltage of 3 volts it means each unit of charge gains 3J of energy as it passes through the cell.  As the electrons pass through the bulbs this energy is converted to heat and light energy, so voltage is used up.

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Electrons carry a charge.  The unit of charge is Coulombs (C).  One electron caries a charge of - 1.6 x 10    C.  When charge flows it creates and electric current.  Current is defined as the rate of flow of charge. 

current = charge/time        

current = I (A = amps)

charge = Q (C = coulombs)

time = t (s = seconds)

 I = Q/t   or Q = I x t  

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An ammeter is used to measure the current in  amps.  The ammeter must be placed in series in the circuit.

Conventional current

Conventional current flows from the positive side if the cell round to the negative side.

Electrons are negatively charged and in reality flow from the negative side of the cell round to the positive side.

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Series circuit

The current is the same at all points in the circuit.  The voltage is shared between the components.  The sum of the voltage across the components are equal to the voltage across the cells.

Parallel circuit

The current splits at a junciton.  The sum of the current entering the junction is equal to the sum of the current leaving.  The voltage is the same across the parallel branch.

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variable resistors - rheostats

  • The resistance of the coiled wire is high and the resistance of the bar is low.
  • With the sliding contact close to Terminal B the whole of the coil is included in the circuit and resistance is higher.
  • With the sliding contact at the other end, none of the coil is included and so the resistance is low.
  • The resistance of the rheostat can be varied by moving the sliding contact along the bar.
  • The inclusion of a rheostat in a circuit allows the current in that circuit to be varied.
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Ohm's Law

The effect of increasing the current on the voltage is that the current gets higer while the voltage gets higher as well.  Each time the current increases in 0.01 amp the voltage increases too.  The currrent should be directly proportional (constant relationship) to the voltage.  This is shown by the fact that the graph is a straight line passing through the origin. 

current       origin

            = directly proportional.

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Ohm's graph

The constant relationship between the 2 quantities is the resistance.

voltage = current x resistance

v = I x R

Ohm's Law states that the urrent through a conductor is directly proportional to the potential difference (p.d. = voltage) across its ends providing the Hmp remains constant.

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resistance - the effect of resistance on current

A fixed resistor of different calues is put in the circuit and the current is recorded.  The higher the resistance the lower the current.  The resistor trys to stop the current flowing through it.

As the resistance gets higher the current gets lower.  The resistor impedes the passage for the electrons to go round.  Resistance is the collision of protons and neutrons with electrons.

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