topic 6 - iknsphysicsib - home. the resistance of the cell is called the internal resistance as...
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Two types: ◦ Positive ◦ Negative
When will matter be neutral? Electrostatic force: force between
charges.. Why will u have this? How can we created charged object?
+ -
+ -
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+ -
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+ -
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Charging Up Rods
Acetate positive
Cloth Negative
Polythene Negative
Cloth Positive
It’s all about moving electrons
Conductors: allows a flow of charge to pass through them
Insulators: doesn’t allow a flow of charge to pass through them
e.g.: page 52 in study guide
The forces are equal because of Newton’s third law.
Law: the force between two point charges(q1 and q2) is directly proportional to the product of their charges and inversely proportional to the square of their separation.
A charge, or a combination of charges, is said to produce an electric field around it.
What will happen to a small test was placed in the field?
Definition: force per unit charge experienced by a SMALL POSITIVE TEST CHARGE placed in the field.
Unit NC-1
Vector quantity
What will be done if a larger force is applied making the test charge go towards the positive charge?
Electric potential energy: energy that a charge has as a result of its position in an electric field.
Since Work = ΔE Change in EPE = force x distance Change in EPE = Electric field strength x
charge x distance
The test charge will be pushed by the positive charges field in the opposite direction.
Loss in EPE = gain in kinetic energy
v
The test charge will be pushed by the positive charges field in the opposite direction.
½ mv2 = Eqd
v
In the previous example, what would the energy difference depend on? – test charge
If we double the charge, what would happen to the energy difference?
In the previous example, what would the energy difference depend on? – test charge
If we double the charge, what would happen to the energy difference? – double
What would remain the same?
In the previous example, what would the energy difference depend on? – test charge
If we double the charge, what would happen to the energy difference? – double
What would remain the same? – energy difference per unit charge.
This is called potential difference, p.d. Unit: J/C OR V Scalar quantity (does not depend on
direction taken by test charge) – electric field is conservative.
What’s the smallest negative charge? What’s the smallest positive charge? Voltage = potential difference Energy difference (gained) = p.d. x charge Can we change the above equation in
other terms?
What’s the smallest negative charge? What’s the smallest positive charge? Voltage = potential difference Energy difference (gained) = p.d. x charge Can we change the above equation in
other terms? Work = p.d. x charge So what’s the unit for energy? Is the unit too big or too small if we are
working at the atomic scale?
We therefore use a smaller unit called electronvolt (eV)
Electronvolt is the energy gained by an electron moving through a p.d. of 1 volts.
Energy difference (gained) = p.d. x charge ◦ 1 eV = 1 volts x 1.6 x 10-19 C = 1.6 x 10 -19 J
Current flows through an object when there is a potential difference across the object.
A battery or a power supply creates this potential difference.
The path the current flows through in is called its circuit.
Current flows from the positive (+ve) terminal of the battery to the negative (-ve).
This is called conventional current flow.
BIG PROBLEM: electrons are negatively charged, so they want to get away from the -ve and go to the +ve.
It is actually a measure of how hard it is for the electrons to travel through a part of the circuit (measured in Ohms (Ω))
High resistance will need a high pd in order to get a current to flow.
It is a collection of components wired to a battery or power supply, which pushes the small packets of charge around it.
Two things are important for a circuit to work: ◦ there must be a complete circuit ◦ there must be no short circuits
When more than one component is used in a circuit, there are two different ways of arranging them and these are called:
◦ Series
◦ Parallel
When components are connected one following another in a ring
Below is a series circuit shown with three different resistors.
The first disadvantage is that, if one component in a series circuit fails, then all the components in the circuit fail because the circuit has been broken.
The second disadvantage is that the more components there are in a series circuit, the greater the circuit's resistance*.
A parallel circuit is rather like two or more series circuits connected to the same energy source.
For example, here is a parallel circuit connecting a cell and two lamps (figure one) and a cell with three resistors (figure two):
Figure one Figure two
The first advantage of a parallel circuit is that a failure of one component does not lead to the failure of the other components.
The second advantage of parallel circuits is that more components may be added in parallel without the need for more voltage.
Ohm's Law is stated like this:
Current is directly proportional to voltage for a metal conductor at a constant temperature.
E.m.f. = electromotive force Definition: total energy difference per
unit charge available by an electric source. Energy difference in a cell is the energy
converted from chemical energy to electrical energy.
Units: V IT IS NOT A FORCE!!!!!
All cells are made of materials that have resistance. The resistance of the cell is called the internal resistance
As current flows through battery, some of the electrical energy is converted into heat.
Power is the rate of which work in being done… or energy transferred over time ◦ 1- Power delivered (from power supply): Energy change: chemical electrical EPE = Vq (V is the emf) = εq Power = εq / t = εI In real situations: power will be less since there is
internal resistance
◦ 2- Power dissipated (for resistor): Energy: electrical to heat Same proof as earlier but V will be voltage around
resistor. P = IV = I2R = V2/R
Thermistor: ◦ Semiconductor ◦ Higher temp less resistance higher
current
LDR: ◦ Higher light intensity less resistance
Strain gauge: ◦ Used to detect if parts of building are
stretching ◦ With extension the length will change
What are the factors that affect resistance? ◦ Length of wire ◦ Cross sectional area of wire ◦ Material of wire
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