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Electrical Currents

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on 24 February 2016

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Transcript of Electrical Currents

In cordless devices the source of the energy is the battery.

Inside the battery, chemicals react with each other to convert chemical energy into electric energy
We can't see charge flowing through a wire.
The unit of charge is the coulomb (C)
Electrical potential energy
Potential Difference
If we move a + charge from the negative end of the battery to the positive end, we need to do work
The Battery
In a circuit provides the push for the charges to move through a circuit.

1 V = 1 J/C

This is because the charge has a higher potential energy at the positive end than the negative end.
The amount of work done per unit charge is defined as the potential difference (p.d.) between the places or terminals
Is a measure of the flow of charge

I = Q/t

If a current of 10 A flows for 15 s, how much charge flows?
Conductors and Insulators
Conductor: Is a material that allows charge flow through it. Contains free electrons.
Insulators: Is a material that doesn't allow charge flow through it.
The rate at which charge can flow through a conductor depends on the size and the material
The resistivity of copper is 1.72 x 10^-8 Ωm. What is the resistance of a 1 m length of 2 mm diameter copper wire?
A conductor that does not let much current flow for a given p.d. is said to have a high resistance
A wire is subjected to a tension so that its length increases by 10% while the volume of the wire stays the same. How does thew resistance of the wire change?
Graphical treatment
Ohmic conductors are conductors that obey Ohm's law
Metallic conductors
Non - Ohmic conductors are conductors that don't obey Ohm's law
Diodes, light bulbs and LEDs, semiconductor p-n diodes, battery acid or alkaline solutions, alkali halide crystals, the ionized mercury vapor in a fluorescent lamp, and cathode ray tubes
What is Emf?
The emf of a cell is the amount of chemical energy converted to electrical energy, per unit charge. Unit is the volt (V)
Potential difference (p.d. or V)
The p.d. across a resistance is the amount of electrical energy converted to heat, per unit charge. The unit is the volt (V)
The reason for this is that when the light bulb gets hot, the metal atoms vibrate more.
More collisions between the electrons and metal atoms.
Increase resistance.
If the emf of a battery is 9 V, how much energy is converted from chemical to electrical when 2 C of charge flow?
What is the p.d. across a resistor if 24 J of heat are produced when a current of 2 A flows through it for 10 s?
Internal resistance of cells
The resistance of a cell is called internal resistance.
As current flows through the internal resistance, some energy is converted from electrical to heat inside the cell (so the cell gets hot).
Less energy to be converted to heat in the resistor.
A current of 0.5 A flows when a battery of emf 6 V is connected to an 11 Ω resistor. What is the internal resistance of the battery?
A 12 V battery with internal resistance 1 Ω is connected to a 23 Ω resistor. What is the p.d. across the 23 Ω resistor?
Rate at which energy is changed from one form to another
Power delivered
Power dissipated
Power delivered
Is the amount of chemical energy converted to electrical energy per unit time
Power dissipated
Is the amount of electrical energy converted to heat per unit time
If a current of 2 A flows through a resistor that has p.d. of 4 V across it, how much power is dissipated?
What power will be dissipated when a current of 4 A flows through a resistance of 55 Ω?
5 A flows through a 20 Ω resistor.

How much electrical energy is converted to heat per second?

If the current flows for 1 minute, how much energy is released?
If a battery has an internal resistance of 0.5 Ω, how much power will be dissipated in the battery when 0.25 A flows?
A current of 0.5 A flows from a battery of emf 9 V. If the power delivered is 4 W, how much power is dissipated in the internal resistance?
An electric car of mass1000 kg uses twenty-five 12 V batteries connected together to create a p.d. of 300 V. The car accelerates from rest to a speed of 30 m/s in 12 seconds.
What is the final kinetic energy the car?
What is the power of the car?
How much electrical current flows from the battery?
A light bulb for use with the 220 V mains is rated at 100 W.

What current will flow through the bulb?

If the bulb converts 20% of the energy to light, how much light energy is produced per second?
A 1 kW electric heater is connected to the 220 V mains and left on 5 hours.

How much current will flow through the heater?

How much energy will the heater release?
Measurement of p.d.
An ideal voltmeter has infinitely high resistance so that it does not take any current from the circuit
Measurement of current
An ideal ammeter has zero resistance so that it doesn't change the current in the circuit.
Calculate the current and potential difference measured by the meters in the circuit below. Assume the battery has no internal resistance and that the meters are ideal.
An electrical sensor is a device whose electrical properties change with changing physical conditions
Light sensor (LDR)
Strain gauge
Made from semiconducting material whose resistance decreases as temperature increases. As the thermistor gets hotter, more charge carriers are released, so the current can flow more easily.
LDR = light dependent resistor
Is a semiconducting device, but unlike the thermistor, it is light that releases more charge carriers, resulting in a lower resistance.
Strain Gauge
Is a thin metal wire. If it is stretched, its length increases and its cross-sectional area gets smaller. This results in an increase in resistance.
Calculate the current and potential difference measured by the meters in the circuit below. Assume the battery has no internal resistance and that the meters are ideal.
Calculate the output from the potential divider
Using the potential divider with sensors
When light stops shining on the LDR, its resistance increases, so increase in V out.
The increase in V o in turn activates the electronic switch that puts on the lights. The electronic switch needs a minimum p.d. to activate it, so it doesn't switch on the lights until V out is big enough
The battery has a emf of 12 V and no internal resistance. The p.d. required to activate the switch is 5 V. Find the value of R1 that will cause the lights to turn on when the resistance of the LDR rises to 200 k
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