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Electricity and magnetism ((Fuses protect components in a circuit from…
Electricity and magnetism
Magnets
induced magnetism - magnetism can be induced in an metals which have magnetic properties by simply placing the iron close to a strong magnet without touching.
pattern of fields around a bar magnet=
Magnets have magnetic fields around them them and when two magnet’s field lines intersect each other, they attract each other.
Distinguish between the magnetic properties of iron and steel=
Soft iron (iron):
Easy to magnetise
Cannot retain magnetism in the absence of magnets, i.e. its magnetism can be turned on and off by moving the magnet closer and farther from it. This property makes soft iron of more use in electromagnets
Hard iron (steel):
Hard to magnetise
Retains magnetism even in the absence of magnets, which is why steel is used to make permanent magnets whose magnetism will last a long time.
Distinguish between the design and use of permanent magnets and electromagnets. Electromagnets are objects that become magnetic when electricity is flown through it. They are temporary, as their magnetism can be turned on and off by switching an electrical circuit on and off.
Permanent magnets’ magnetism cannot be turned on and off, they are permanently magnetised.
Electrical quantities=Demonstrate understanding of current, potential difference, e.m.f. and resistance, and use with their appropriate units.
nb c
Electric Current:
is the flow of electric charge within a circuit.
is measured in amperes or amps (A).
represents how much electric charge is passing a single point in the circuit
is the same at the beginning and end of a circuit.
Potential Difference:
is the difference in the energy there is to drive a current through a wire, between two points
is measured in volts (v)
E.M.F:
electro-motive force (e.m.f) is the voltage (potential) that a battery will supply to the entire circuit
is the driving force that gives the electrons the energy to move around the circuit.
State that charge is measured in coulombs (C).
Charge is measured in coulombs (C).
Resistance:
is a measure of how difficult it is to push a current through a circuit.
is measured in Ohms (Ω)
se and describe the use of an ammeter and a voltmeter.
Ammeter is a device that measures the amount of current flowing through a circuit in amperes (amps).
Voltmeter is a device that measures the potential difference between two points in a circuit.
Electric charge
Describe and interpret simple experiments to show the production and detection of electrostatic charges.
Electrostatic attraction can be produced and observed in a simple experiments:
Inflate a balloon and rub it quickly on any dry surface e.g. a carpet. Then open a tap and hold the balloon next to it (without touching the water). You should see that the water bends towards the balloon!
Tear up a piece of paper into small bits. Then take a ruler, rub it on your hair and place them just above the bits of paper, without touching them. You should see the paper get attracted to the ruler.
This kind of attraction is called electrostatic attraction. This happens because of the charges on the materials.
Describe an electric field as a region in which an electric charge experiences a force.
Just like how magnets have magnetic fields around them, electricity carrying wires have electric fields around them. They are created when an electrically charged object is placed near another charged object.
State that there are positive and negative charges.
Since all materials on earth are made up of atoms which contain the positively charges protons and negatively charged electrons, any imbalance in the number of protons and electrons will cause the material to be charged or ionised. A loss of electrons will make it positively charged and a gain of electrons will make it negatively charged.
When you rub the balloon, for example on the carpet, electrons (with a negative charge) build up on the surface of the balloon (they are transferred from the carpet to the balloon). This is called static electricity, which means “non-moving electricity”.
State that unlike charges attract and that like charges repel.
Yep, just like magnetic poles, electric charges also attract when they’re unlike and repel when they’re like.
Distinguish between electrical conductors and insulators, and give typical examples.
Conductors are materials in which current can flow. Examples: copper, aluminum, gold and silver.
Insulators cannot conduct electricity. Examples: glass, wood, paper, air and rubber.
Current, Electromotive Force and Potential Difference
State that current is related to the flow of charge.
Current is the flow of charge (electrons) within a circuit.
State that the current in metals is due to a flow of electrons. Metallic bonding is the strong attraction between closely packed positive metal ions and a ‘sea’ of delocalised electrons. These delocalised elecetrons can carry charge and move freely through giant the metal structure, thus making metals conductive of electricity.
Use the term potential difference (p.d.) to describe what drives the current between two points in a circuit.
Potential represents how much energy there is to drive a current through the wire and is measured in volts (v).
Distinguish between the direction of flow of electrons and conventional current.
At first scientists believed that electricity from the positive to the negative terminal of a battery, but then found out that it was actually the other way around- electrons flow from the negative to the positive termianl. In order to not confusing, they called the initial assumption ‘conventional current’.
Demonstrate understanding that e.m.f. is defined in terms of energy supplied by a source in driving charge round a complete circuit.
Demonstrate understanding that a current is a rate of flow of charge, and recall and use the equation I = Q /t.
Current is the rate of flow of charge in a given point of the circuit.where I=current (amperes), Q=charge (coulomb) and t=time (seconds/minutes)
Resistance
State that resistance = p.d./current and understand qualitatively how changes in p.d. or resistance affect current.
The electrical resistance is a measure of the difficulty to pass an electric current through a conductor.
Resistance is calculated by dividing the voltage between two points by the current flowing through the points.
R = V ÷ I
From this formula, we can conclude that an increase in voltage will increase resistance. Reducing voltage will reduce the resistance.
Recall and use the equation R = V / I
If the voltage is 6 volts and the current is 2 amps, the resistance = 6 ÷ 2 = 3 ohms
This formulas is known as Ohm’s law and rearranged to find each of the values
I = V ÷ R
V = I * R
Describe an experiment to determine resistance using a voltmeter and an ammeter.
Voltmeter measured voltage/p.d. and ammeter measures current, so you can use these devices to determine the resistance between two points in a circuit.
Set up an ammeter somewhere in the series circuit; this will give you the amount of current flowing in the circuit.
Next set up a voltmeter in parallel to the object, in this case a light bulb, to find the potential difference across it.
Using the equation R = V/I , we can find the resistance.
Relate (without calculation) the resistance of a wire to its length and to its diameter.
As the length of the conducting wire increases, the resistance of the current flowing increases. Resistance and length of a wire are directly proportional
The greater the diameter of the wire, the smaller the resistance. Resistance and cross-sectional area of a wire are inversely proportional.
Recall and use quantitatively the proportionality between resistance and length, and the inverse proportionality between resistance and cross-sectional area of a wire.
We now know that resistance is directly proportional to length, denoted as R α length.
Resistance is also inversely proportional to cross-sectional area, denoted as
R α (1 ÷ cross-section area)
Combining these two we get:
R α length ÷ cross-section area OR R α l ÷ A
we can add a constant ρ (pronounced as rho, in greek alphabet) to rewrite it as:
R = ρ * (l ÷ A)
In order to find the constant (by what proportion resistance changes with respect to length and cross-section area), we can rearrange the formula to:
ρ = R * (A ÷ l)
Recall and use the equations P = IV and E = IVt
Power is a measure of how quickly energy is transferred. The unit of power is the watt, W. The more energy that is transferred in a certain time, the greater the power of the appliance. It is calculated using the formula:
power = current * voltage
P= I * V
where P=power, I=current and V=voltage/p.d. This can be rearranged to find the current and voltage if the other two values are given.
I = P ÷ V
V = P ÷ I
In an earlier unit, we learned that power = energy ÷ time. Rearranging it with energy as the subject, we get energy = power
time. We now know that power = current
voltage. So, energy can be written as:
energy = current
voltage
time
E = IVt
Dangers of Electricity
Identify electrical hazards, including:
• damaged insulation
• overheating of cables
• damp conditions.
Damaged Insulation:
In a circuit, insulation is the plastic sheath that covers wires. If you have damaged insulation, it means that metal wires inside the cable are exposed.
If a person touches the exposed wires, they could be electrically shocked, which may lead to death.
Overheating of cables:
When a very high current is run through a cable, there is a risk of overheating the wire due to too much energy. This overheating could lead to electrical fires.
Damp conditions:
Since water is a conductor, during damp situations such as inside a bathroom, electricity from the electrical appliance may electrocute nearby people through the conductive water. Even skin, if wet, will electrocute the person if they touch a socket.
Demonstrate understanding of the use of circuit-breakers.
A circuit breaker is a safety device that forces a circuit to open (switch off) when an extremely high level of current flows through the circuit.
circuit breaker
Electricity flows in the circuit breaker through the metal contacts.
If an extremely high current flows through the circuit breaker, the electromagnet gets stronger and pulls the iron catch towards it.
This causes spring to pull the metal contacts apart, causing the circuit to open/break.
Demonstrate understanding of the use of fuses.
Fuses protect components in a circuit from overheating by breaking circuit. A high level of current flowing through the circuit causes the wires inside the circuit to heat up. The fuse, a metal wire with a low melting point, will melt and cause the circuit to break. fuse
