Producing and Measuring Electricity

DC and AC DC stands for “Direct Current” – the current only flows in one direction: AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz) 1/50 th s 240V V V Time T

Types of Batteries Type of BatteryContainsUses Wet cell rechargeable Lead and acidCars, industry Dry cell rechargeable Nickel, cadmium, lithium Mobile phones, power tools Dry cell non- rechargeable Zinc, carbon, manganese, lithium Torches, clocks, hearing aids Why use rechargeable batteries? Long long-term expense Can be used many times Less energy to produce Why use standard batteries? No need for charger Less expensive Rechargeables contain carcinogens

Battery Capacity The capacity of a battery is measured in Amp Hours (Ah). Basically, a battery with a capacity of 1Ah will provide a current of 1A for 1 hour. Capacity (Ah) = Current (A) x Time (hours) 1)A battery provides a current of 2A for 2 hours. What was its capacity? 2)Another battery has a capacity of 10Ah. If it runs out after half an hour what current was being drawn? 3)A mobile phone battery has a capacity of 1100mAh. If it runs on a current of 250mA when being used in a phone call how long could the call last?

23/11/2015 Electric Current Electric current is a flow of negatively charged particles (i.e. electrons). Note that electrons go from negative to positive -+ e-e- e-e-

Basic ideas… Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____. Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______, a unit named after Volta. Resistance is anything that resists an electric current. It is measured in _____. It usually increases when a device gets hot. Words: volts, amps, ohms, voltage, ammeter, voltmeter

More basic ideas… If a battery is added the current will ________ because there is a greater _____ on the electrons If a bulb is added the current will _______ because there is greater ________ in the circuit

Electromagnetic induction The direction of the induced current is reversed if… 1)The magnet is moved in the opposite direction 2)The other pole is inserted first The size of the induced current can be increased by: 1)Increasing the speed of movement 2)Increasing the magnet strength 3)Increasing the number of turns on the coil

Generators (dynamos) Induced current can be increased in 4 ways: 1)Increasing the speed of movement 2)Increasing the magnetic field strength 3)Increasing the number of turns on the coil 4)Increasing the area of the coil

23/11/2015 Georg Simon Ohm Resistance Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me. The resistance of a component can be calculated using Ohm’s Law: Resistance = Voltage (in V) (in  )Current (in A) V RI

An example question: V A 1)What is the resistance across this bulb? 2)Assuming all the bulbs are the same what is the total resistance in this circuit? Voltmeter reads 10V Ammeter reads 2A

More examples… 12V 3A 6V 4V 2A 1A 2V What is the resistance of these bulbs?

Resistance Resistance is anything that opposes an electric current. Resistance (Ohms,  ) = Potential Difference (volts, V) Current (amps, A) What is the resistance of the following: 1)A bulb with a voltage of 3V and a current of 1A. 2)A resistor with a voltage of 12V and a current of 3A 3)A diode with a voltage of 240V and a current of 40A 4)A thermistor with a current of 0.5A and a voltage of 10V

Current-voltage graphs I V I V I V 1. Resistor 3. Thermistor 2. Bulb Current increases in proportion to voltage As voltage increases the bulb gets hotter and resistance increases Resistance goes down as the thermistor gets hotter (i.e. more voltage) Low R High R