 Basic Electricity Chapter 17

Battery  A battery converts stored chemical potential energy into electrical energy.  A battery consist of two unlike metal electrodes in an electrolyte, a solution that conducts electric charge.  When a complete circuit is formed, electrons from the more negative electrode will move through the circuit.

Battery  The result is a flow of electrons in the wire.  The battery has a negative terminal and a positive terminal.  The Anode is the positive terminal  The Cathode is the negative terminal  Electrons move from negative to positive, or from the cathode to the anode.

Battery  So how much energy a battery or power source can put off is referred to as its voltage.  Voltage (V) – Electric Potential Energy. It is the energy contained in a moving charge. Its units are J/C or V-volts.

Current  Current (I) – the amount of charge that flows times time. Units A – amperes.  Sustaining a current, or the flow of a charge requires a voltage source and a complete circuit.  Complete Circuit – the name given to a continuous conducting path.

Current  Electric current (I) is given by I = q/t I = current q = charge t = time

Example  Suppose there is a steady current of 0.50 A in a flashlight bulb lasting for 2.0 minutes. How much charge passes through the bulb during this time? How many electrons does this represent?

Resistance  If a voltage is placed across any conducting material in a complete circuit, what factors determine the resulting current?  The greater the voltage, the greater the current.  However, another factor also influences current.  Internal friction or resistance of the material will affect the flow of charge.

Resistance  Resistance (R) – an object that restricts the flow of current. This is a ratio of voltage to current and has units of ohms – Ω  Any object that offers significant resistance to electrical current is called a resistor.

Resistance  Resistance is a ratio of voltage to current, so R = V/I

Ohm’s Law  For some materials, the resistance may be constant over a range of voltages. A resistor that exhibits constant resistance is said to obey Ohm’s Law. V = IR

Example  Any room in the house that is exposed to water and electrical voltage can present hazards. For example, suppose a person steps out of a shower and inadvertently touches an exposed 120 V wire with a finger thus creating a complete circuit through the body to the ground. The human body, when wet, can have an electrical resistance as low as 300 Ω. Using this value, estimate the current in that person’s body.

Example Continued  While this is a small current by everyday standards, it is a large current for the human body. A current over A can cause severe muscle contractions, and currents on the order of 1 A can stop the heart.

Example  When the human body is dry, its resistance can be as high as 100 KΩ. Under these conditions, what voltage would produce a current of 1 mA?

Factors that Influence Resistance  The type of material of which an object is composed  The objects length  The objects cross-sectional area  Temperature

How are length and cross sectional area related to resistance?  If cross-sectional area increases, resistance decreases. Inversely related  If length increases, resistance increases. Directly related R is proportional to L/A

Power  Power (P) – The amount of energy used per time. Units W – Watts or J/s. P = IV = V 2 /R = I 2 R

Example  A digital video disk (DVD) player is rated at 100W at 120 V. What is its resistance?